John's Astronomy And Astrophotography Wiki - User contributions [en]

Main Topic Index

2019-05-21T03:00:55Z

Phpdevster:

*[[Telescope Basics]]
**[[Focal Ratio]]
**[[Aperture]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filters]]
**[[Barlow]]s

*[[Eyepieces]]
**[[Magnification]]
**[[Field of View]]
**[[Exit Pupil]]
**[[Eye Relief]]
**[[Barrel Size]]

*[[Observing]]
**[[Star Hopping]]
**[[Angular Size]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Main Topic Index

2019-05-21T03:00:11Z

Phpdevster:

*[[Telescope Basics]]
**[[Focal Ratio]]
**[[Aperture]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filter]]s
**[[Barlow]]s

*[[Eyepieces]]
**[[Magnification]]
**[[Field of View]]
**[[Exit Pupil]]
**[[Eye Relief]]
**[[Barrel Size]]

*[[Observing]]
**[[Star Hopping]]
**[[Angular Size]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Barlow

2019-05-21T02:57:13Z

Phpdevster:

==Overview==

A Barlow is a kind of telenegative lens that sits between the focuser and the eyepiece, whose purpose is to increase the effective focal length of the telescope. Common Barlow multiplications are 2x and 3x, but Barlows come in a variety of multiplication factors.

For example, when used in a 1000mm focal length telescope, a 2x Barlow would cause the telescope to have an effective focal length of 2000mm, which would then double the magnification of any eyepieces used in it. If a 10mm eyepiece produced 100x magnification, then with a Barlow it would produce 200x magnification.

However, the stated multiplication factor is typically only approximate and can vary from the actual multiplication factor of the product.

==Effects of Barlows==

===Effect on Magnification===

As stated above, a Barlow will multiply the focal length of the telescope, allowing for greater magnification with an existing set of eyepieces. However, there is no meaningful difference between say, a 10mm eyepiece and 2x Barlow, and a simple 5mm eyepiece. Assuming the Barlow is true to its stated multiplication factor, then the 10mm eyepiece + Barlow combination will effectively be like using a 5mm eyepiece.

===Effect on Exit Pupil===

The same rules regarding [[exit pupil]] apply to eyepieces paired with Barlows. If a 10mm eyepiece produces a 2mm exit pupil in an F/5 [[focal ratio]] telescope, then with a Barlow, the telescope effectively becomes F/10 and thus the exit pupil drops to 1mm, making the view darker as a trade-off of the increased magnification

===Effect on Eye Relief===

Simple Barlows will actually increase the effective [[eye relief]] of an eyepiece. In some situations this can be advantageous since not all eyepieces provide good eye relief. However, the downside to this is that the longer the eyepiece focal length, the more pronounced this effect is. This can be problematic since simple [[Plossl]] eyepieces already have adequate eye relief in the longer focal lengths where you would want to use a Barlow, so adding a Barlow may increase eye relief too much.

===Effect on Focus===

Barlows typically require additional inward travel of the focuser in order to come to focus. This means depending on the focuser and eyepiece, not all eyepieces will come to focus when using a Barlow (though this is rare).

==Types of Barlows==

===Telecentrics / Focal Extenders===

In addition to simple Barlows, there is also a class of telenegative known as a telecentric or focal extender, which has the same purpose as a Barlow, but doesn't change the converging angle of light from the telescope. This means that the eye relief of the eyepiece isn't affected. The additional glass elements of the focal extender also correct for astigmatism and chromatic aberration that can be found in simpler Barlow designs.

===Short vs Long Barlows===

Some Barlows are designed to be short to accommodate diagonals where there isn't a lot of room to insert the Barlow's barrel. Shorter Barlows can sometimes [[vignette]] the light before it reaches the eyepiece, creating a darker ring around the outer edges of the [[field of view]]. Short barlows may also introduce more chromatic aberration, and they also tend to increase the eye relief of eyepieces more so than longer barlows.

For the reasons above, long focus barlows tend to be preferred where possible.

===2-element vs 3-element Barlows===

Most standard Barlows have two elements (aka "doublets"). They exist in both short and long focus forms. Just like a refracting telescope however, the refracted light from the Barlow can introduce chromatic aberration. For this reason, Barlows with three elements (aka "triplets") are preferred, since the addition of another element can help reduce chromatic aberration. Sometimes triplet Barlows are labeled as "apochromatic" Barlows.

===2" vs 1.25" Barlows===

There is no inherent advantage to a 2" Barlow or focal extender vs a 1.25" variety, it simply allows 2" eyepieces to be used. However, because most 2" eyepieces are often low power eyepieces where you would want to use a Barlow, then it's generally advisable to get a 2" Barlow for the added flexibility of being able to use both 1.25" and 2" eyepieces with it.

==Advantages and Disadvantages of Barlows and Focal Extenders==

===Advantages===

Barlows can be a cost-saving measure when used intelligently with an eyepiece kit built around them. For example, often the cheapest eyepieces you can purchase are simple [[Plossl]]s, but Plossls have the disadvantage of extremely short eye relief at short focal lengths. Plossls with focal lengths shorter than 12mm can be challenging to look through because of their short eye relief. A Barlow is a good solution to this problem, since you can use longer focal length Plossls which have more comfortable eye relief.

For instance, suppose you had 130mm F/5 scope with a focal length of 650mm. To get reasonable planetary magnification (say, 150x), you would need a 4.3mm eyepiece. A 4mm Plossl would have eye relief so short you would essentially have to touch your eyeball to the lens housing of the eyepiece. So instead, a 3x Barlow could be paired with a 12mm Plossl to give you the same effective magnification, but with the longer eye relief of the 12mm Plossl, '''plus''' the increased eye relief from using the Barlow. This would be much more comfortable to look through.

A Barlow means you can double the focal lengths of your eyepiece kit provided you select focal lengths of eyepieces that won't cause redundancy. For example a set of 32mm 20mm and 12mm and a 2x Barlow would give you 32mm, 20mm, 16mm, 12mm, 10mm, and 6mm focal lengths to choose from. A 3x Barlow would be similar: 32mm, 20mm, 16mm, and 12mm eyepieces would give you 32, 20, 16, 12, 10.6mm, 6.66mm, 5.33mm, and 4mm focal lengths.

However, care has to be taken to ensure that you're not creating too much redundancy in your eyepieces, and that their focal lengths are a good fit for your particular telescope (e.g. they are not producing too much magnification when paired with the Barlow).

===Disadvantages===

As stated above, some Barlows can degrade optical quality a bit, and unless you are willing to pay extra for a high quality Barlow or focal extender, you are usually better off getting the equivalent focal length eyepiece instead. There are quite a few affordable options for short focal length eyepieces that also offer adequate eye relief, and the money you would save not buying a more expensive Barlow or focal extender could be put towards those eyepieces instead.

Barlow

2019-05-21T02:56:51Z

Phpdevster: Created page with "==Overview== A Barlow is a kind of telenegative lens that sits between the focuser and the eyepiece, whose purpose is to increase the effective focal length of the telescope...."

Focal Ratio

2019-05-20T22:38:03Z

Phpdevster:

==Overview==

The ratio of a telescope's [[aperture]] to [[focal length]] is known as its ''focal ratio''. The smaller this ratio, the "shorter" or "faster" the telescope is said to be. The higher this ratio, the "longer" or "slower" the telescope is said to be. Phrases like "fast scope" or "long focal ratio" or "telescope speed" all refer back to its focal ratio number.

The focal ratio itself isn't as important as the telescope's aperture or its focal length, but it does play a key role in describing the various aberrations that can be present when looking through the telescope.

==Focal Ratio Effects==

===Focal Ratio and Astrophotography===

The "speed" nomenclature of the focal ratio stems from photography and is analogous to the F-stop setting on a camera lens. A small F-stop number means a faster shutter speed can be used, while a large F-stop number means a longer shutter speed needs to be used.

This same concept applies to astrophotography. The faster the telescope is (e.g. the shorter its focal ratio), the less time you need to expose your camera in order to get a given signal level on the sensor. The slower the telescope (e.g. the longer its focal ratio), the longer you need to expose your camera to get the same signal.

This relationship between focal ratio and exposure time is based on the square of the focal ratio. An F/10 focal ratio requires four times the exposure time of an F/5 focal ratio to get the same signal.

This relationship also holds true across apertures. A 16" aperture at F/10 will still require four times the exposure length to get the same signal as even a 4" aperture at F/5. While this sounds counter-intuitive that a significantly smaller aperture can still require 4x shorter exposure, it's important to note that focal ratio is always relative to both the aperture and focal length of the telescope. F/10 always means that light will travel 10x farther than the size of the aperture, while at F/5 it means light will always travel only 5x farther than the size of the aperture. This means, relative to the aperture, F/10 will always spread the light out 4x more than at F/5, thus the amount of light that falls on a given pixel on a sensor is always 4x weaker.

So what role does aperture play in astrophotography? It allows for greater image scale a given focal ratio. Compare a 16" F/5 telscope against a 4" F/5 telescope. Each scope has the same focal ratio, so they will require the same exposure length to get the same level of signal. However, the 16" F/5 scope has a focal length that is 4x longer than the 4" F/5 scope, thus it can produce 4x the image scale. If the 4" scope tried to match that same image scale, a 4x [[barlow]] would be required and it would make the effective focal ratio of the 4" scope F/20, so it would require 8x the exposure length as the 16" scope.

It should be noted that the focal ratio isn't the *cause* of the increased or decreased exposure time (ultimately it's the aperture and how long of a focal length the light collected by the aperture has to travel), but it's merely a good ''descriptor'' for how much more quickly one scope will acquire good signal than another.

===Focal Ratio and Visual Astronomy===

When it comes to visual astronomy, the focal ratio doesn't have any direct effects like focal length does on magnification, or aperture does on brightness, but it does affect how well an [[eyepiece]] performs.

A short focal ratio means light rays converge to the [[focal plane]] more steeply than a long focal ratio. This steep angle of convergence must be straightened out or re-bent by the eyepiece in order to form a magnified, focused image. However, this process of refracting the light from the telescope can distort it, which can cause an aberration known as [[astigmatism]] to manifest near the edges of the [[field of view]]. The steeper this converging angle, the more aggressively it needs to be refracted, and thus the more extreme the aberration becomes. To eliminate this aberration requires eyepieces with more lens elements and elements with more complex shapes, which drives up cost and weight.

Thus cheaper eyepieces often work well on long focal ratio telescopes (F/10 and longer), but increasingly poorly on shorter focal ratio telescopes. Only the most expensive "premium" eyepieces are designed to work well with short focal ratio telescopes.

===Focal Ratio and Refracting Telescopes===

Refracting telescopes with short focal ratios can suffer from [[chromatic aberration]] without the use of additional, [[low dispersion]] glass elements in the objective lens assembly. An [[achromatic refractor]] often requires a long focal ratio in order to minimize the effects of chromatic aberration, while an [[apochromatic refractor]] can get away with shorter focal ratios since it uses special glass that does a better job of preventing light from splitting up into its constituent colors than standard achromats do. Some apochromatic refractors also make use a 3-element objective assembly to further shorten the focal ratio without causing too much chromatic aberration.

===Focal Ratio and Parabolic Newtonian Reflectors===

The [[parabolic mirror]] in a [[Newtonian reflector]] produces an aberration known as [[coma]], which gets worse the shorter the focal ratio of the mirror gets. This relationship is based on the cube of the focal ratio. An F/5 mirror produces 8x the coma as an F/10 mirror. Even an F/4 mirror produces nearly twice the coma as an F/5 mirror.

===Focal Ratio and Spherical Newtonian Reflectors===

Newtonian reflectors that use [[spherical mirrors]] are especially sensitive to focal ratio. The shorter the focal ratio, the more pronounced the spherical mirror's inability to focus light to a single point becomes.

Using the formula, <code>e = 22 D / F^3</code> where <code>e</code> is the wavefront error, <code>D</code> is the diameter of the telescope's aperture in inches, and <code>F</code> is the focal ratio of the telescope, you can see that the wave front error is based on the cube of the focal ratio. This means as the focal ratio goes down, the wave front error goes up by the cube.

'''Examples:'''

* 5" F/5 telescope produces 0.88 waves of spherical aberration.
* 5" F/10 telescope produces just 0.11 waves of spherical aberration.
* 10" F/10 telescope produces 0.22 waves of spherical aberration (as you can see, spherical mirrors require you to increase focal ratio when increasing aperture, in order to keep the wavefront error down)

For reference, barely passable diffraction-limited optics is 0.25 waves, while excellent optics are 0.1 waves. Thus the 0.88 waves of spherical aberration from 5" F/5 telescope is very bad, but the 8x smaller spherical aberration of a 5" F/10 scope is acceptable.

Note that spherical aberration is in addition to other errors present on the mirror, so just because a 5" spherical mirror might only have 0.11 waves of spherical aberration, that doesn't mean the total wavefront error is only 0.11 waves. That said, at F/10, the spherical aberration is effectively too insignificant to matter.

Main Topic Index

2019-05-20T22:33:27Z

Phpdevster:

*[[Telescope Basics]]
**[[Focal Ratio]]
**[[Aperture]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filters]]

*[[Eyepieces]]
**[[Magnification]]
**[[Field of View]]
**[[Exit Pupil]]
**[[Eye Relief]]
**[[Barrel Size]]

*[[Observing]]
**[[Star Hopping]]
**[[Angular Size]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Focal Ratio

2019-05-20T05:18:08Z

Phpdevster: Created page with "==Overview== The ratio of a telescope's aperture to focal length is known as its ''focal ratio''. The smaller this ratio, the "shorter" or "faster" the telescope is s..."

==Overview==

The ratio of a telescope's [[aperture]] to [[focal length]] is known as its ''focal ratio''. The smaller this ratio, the "shorter" or "faster" the telescope is said to be. The higher this ratio, the "longer" or "slower" the telescope is said to be. Phrases like "fast scope" or "long focal ratio" or "telescope speed" all refer back to its focal ratio number.

The focal ratio itself isn't as important as the telescope's aperture or its focal length, but it does play a key role in describing the various aberrations that can be present when looking through the telescope.

==Focal Ratio Effects==

===Focal Ratio and Astrophotography===

The "speed" nomenclature of the focal ratio stems from photography and is analogous to the F-stop setting on a camera lens. A small F-stop number means a faster shutter speed can be used, while a large F-stop number means a longer shutter speed needs to be used.

This same concept applies to astrophotography. The faster the telescope is (e.g. the shorter its focal ratio), the less time you need to expose your camera in order to get a given signal level on the sensor. The slower the telescope (e.g. the longer its focal ratio), the longer you need to expose your camera to get the same signal.

This relationship between focal ratio and exposure time is based on the square of the focal ratio. An F/10 focal ratio requires four times the exposure time of an F/5 focal ratio to get the same signal.

This relationship also holds true across apertures. A 16" aperture at F/10 will still require four times the exposure length to get the same signal as even a 4" aperture at F/5. While this sounds counter-intuitive that a significantly smaller aperture can still require 4x shorter exposure, it's important to note that focal ratio is always relative to both the aperture and focal length of the telescope. F/10 always means that light will travel 10x farther than the size of the aperture, while at F/5 it means light will always travel only 5x farther than the size of the aperture. This means, relative to the aperture, F/10 will always spread the light out 4x more than at F/5, thus the amount of light that falls on a given pixel on a sensor is always 4x weaker.

So what role does aperture play in astrophotography? It allows for greater image scale a given focal ratio. Compare a 16" F/5 telscope against a 4" F/5 telescope. Each scope has the same focal ratio, so they will require the same exposure length to get the same level of signal. However, the 16" F/5 scope has a focal length that is 4x longer than the 4" F/5 scope, thus it can produce 4x the image scale. If the 4" scope tried to match that same image scale, a 4x [[barlow]] would be required and it would make the effective focal ratio of the 4" scope F/20, so it would require 8x the exposure length as the 16" scope.

It should be noted that the focal ratio isn't the *cause* of the increased or decreased exposure time (ultimately it's the aperture and how long of a focal length the light collected by the aperture has to travel), but it's merely a good ''descriptor'' for how much more quickly one scope will acquire good signal than another.

===Focal Ratio and Visual Astronomy===

When it comes to visual astronomy, the focal ratio doesn't have any direct effects like focal length does on magnification, or aperture does on brightness, but it does affect how well an [[eyepiece]] performs.

A short focal ratio means light rays converge to the [[focal plane]] more steeply than a long focal ratio. This steep angle of convergence must be straightened out or re-bent by the eyepiece in order to form a magnified, focused image. However, this process of refracting the light from the telescope can distort it, which can cause an aberration known as [[astigmatism]] to manifest near the edges of the [[field of view]]. The steeper this converging angle, the more aggressively it needs to be refracted, and thus the more extreme the aberration becomes. To eliminate this aberration requires eyepieces with more lens elements and elements with more complex shapes, which drives up cost and weight.

Thus cheaper eyepieces often work well on long focal ratio telescopes (F/10 and longer), but increasingly poorly on shorter focal ratio telescopes. Only the most expensive "premium" eyepieces are designed to work well with short focal ratio telescopes.

===Focal Ratio and Refracting Telescopes===

Refracting telescopes with short focal ratios can suffer from [[chromatic aberration]] without the use of additional, [[low dispersion]] glass elements in the objective lens assembly. An [[achromatic refractor]] often requires a long focal ratio in order to minimize the effects of chromatic aberration, while an [[apochromatic refractor]] can get away with shorter focal ratios since it uses special glass that does a better job of preventing light from splitting up into its constituent colors than standard achromats do. Some apochromatic refractors also make use a 3-element objective assembly to further shorten the focal ratio without causing too much chromatic aberration.

===Focal Ratio and Parabolic Newtonian Reflectors===

The [[parabolic mirror]] in a [[Newtonian reflector]] produces an aberration known as [[coma]], which gets worse the shorter the focal ratio of the mirror gets. This relationship is based on the cube of the focal ratio. An F/5 mirror produces 8x the coma as an F/10 mirror. Even an F/4 mirror produces nearly twice the coma as an F/5 mirror.

===Focal Ratio and Spherical Newtonian Reflectors===

Newtonian reflectors that use [[spherical mirrors]] are especially sensitive to focal ratio. The shorter the focal ratio, the more pronounced the spherical mirror's inability to focus light to a single point becomes.

Using the formula, <code>e = 22 D / F^3</code> where <code>e</code> is the wavefront error, <code>D</code> is the diameter of the telescope's aperture in inches, and <code>F</code> is the focal ratio of the telescope, you can see that the wave front error is based on the cube of the focal ratio. This means as the focal ratio goes down, the wave front error goes up by the cube.

'''Examples:'''

* 5" F/5 telescope produces 0.88 waves of spherical aberration.
* 5" F/10 telescope produces just 0.11 waves of spherical aberration.
* 10" F/10 telescope produces 0.22 waves of spherical aberration (as you can see, spherical mirrors require you to increase focal ratio when increasing aperture, in order to keep the wavefront error down)

For reference, barely passable diffraction-limited optics is 0.25 waves, while excellent optics are 0.1 waves. Thus the 0.88 waves of spherical aberration from 5" F/5 telescope is significantly bad, but is 8x smaller if that focal ratio was F/10 instead.

Barrel Size

2019-05-20T04:21:32Z

Phpdevster: Created page with "==Overview== Most eyepieces conform to one of three standard barrel sizes: * 0.965 inches (obsolete, no longer made except for very cheap telescopes) * 1.25 inches * 2 inch..."

==Overview==

Most eyepieces conform to one of three standard barrel sizes:

* 0.965 inches (obsolete, no longer made except for very cheap telescopes)
* 1.25 inches
* 2 inches

The modern standard sizes are 1.25" and 2".

It should be noted that 2" barrels are not inherently an upgrade to 1.25" barrels the way say, a faster CPU or a camera with more megapixels might be. Different sized barrels are more akin to different sized paint brushes. Sometimes you need a larger paint brush, sometimes you don't.

==2" vs 1.25" Barrels==

The reason for the existence of 2" barrels is typically to provide a wider [[field stop]], which allows for a wider [[field of view]] at lower magnification. For example, a 40mm eyepiece in a 1.25" barrel can only have an apparent field of view of about 42 degrees, but in a 2" barrel it can have an apparent field of view of about 70 degrees.

But since not all eyepieces have such long focal lengths, it is not always necessary to house their optics in a larger 2" barrel. For example, the 13mm Tele Vue Ethos has an extremely wide 100 degree apparent field of view, but only requires a 1.25" barrel to achieve it. Conversely, the 17mm Ethos requires a 2" barrel in order to provide that apparent field of view at that focal length.

If you have a 2" focuser, don't select eyepieces based on their barrel size..The barrel size is merely a consequence of their optical design and characteristics. Instead, choose eyepieces for their focal length and apparent field of view, and don't worry about what the barrel size is.

==Common Maximum Apparent Fields of View By Barrel Size and Focal Length==

===1.25" Barrels===

* 40mm ~42 degrees
* 32mm ~52 degrees
* 24mm ~68 degrees
* 16mm ~82 degrees
* 13mm ~100 degrees

===2" Barrels===

* 40mm ~70 degrees
* 32mm ~82 degrees
* 25mm ~100 degrees
* 9mm ~120 degrees (technically could be a 1.25" barrel, but the size and weight requires a 2" barrel to hold securely)

===3" Barrels===
* 30mm ~100 degrees

==Misc==

===Dual Barrel Eyepieces===

It's common for some 1.25" eyepieces to also have an outer 2" barrel. The reason for this is so that they can be used in 2" [[focuser]]s without the need for an adapter. It has no bearing on the eyepiece's optical performance, it simply is a matter of convenience.

It should be noted however, that depending on the design of the barrel, using these dual barrel eyepieces in 2" mode might result in damage to a prism or mirror diagonal if it's too shallow to accept the eyepiece.

In some cases, the focuser might not have enough outward travel for the eyepiece to come to focus in 2" mode, since 2" mode puts the eyepiece's [[focal plane]] much deeper in the focuser than the 1.25" mode does.

===Other Sizes===

The only mass produced 3" eyepiece on the market right now is the Explore Scientific 30mm 100 degree eyepiece. Generally speaking, 3" eyepieces are not common as there is not a strong need for them. The only reason to have a 3" barrel is to achieve 100+ degree apparent fields of view in focal lengths longer than 25mm. However, the sheer size and weight of such eyepieces means they are only suitable for large [[Dobsonian]]s or strongly mounted refractors that have large focusers. However, large Dobsonians tend to have short [[focal ratio]]s, and 30mm eyepieces may result in [[exit pupil]]s that are too large to be usable.

Field of View

2019-05-18T03:07:54Z

Phpdevster: /* Apparent Field of View Considerations */

==Overview==

Any given eyepiece has a designed attribute known as its apparent field of view. The ''apparent'' field of view is how "wide" the view appears to your eye as you peer into the eyepiece. In contrast, a given eyepiece and telescope combination will also have a ''true'' field of view, which describes how much of the night sky is visible at once. The two kinds of field of view are loosely related, but are different things.

===Apparent Field of View===

An eyepiece with an apparent field of view of 50 degrees means the angle your eye has to move from one edge to see the other edge, is 50 degrees. That is, the circle of light you see has an apparent [[angular size]] of 50 degrees. Meanwhile a 100 degree eyepiece shows you a circle of light that is 100 degrees in angular size.

The wider the apparent field of view is, the more immersive the view is. Looking through a 40 or 50 degree apparent field of view eyepiece might feel like looking through a narrow cardboard tube, while looking through a 100 degree apparent field of view eyepiece gives the impression of looking through a giant window on space ship.

Eyepieces come in all ranges of apparent field of view, from narrow 30 degree monocentric eyepieces, to 120 degree hyper wide eyepieces. The wider the apparent field of view is, the larger, heavier, and more expensive the eyepiece tends to be.

Eyepieces with wide apparent fields of view are typically referred to as "wide angle" eyepieces, but there is no hard definition for what constitutes a wide angle. There are other terms like "extreme wide angle" "ultra wide angle" and "hyper wide angle" and "super wide angle". These are nothing more than marketing terms that have no bearing on the stated apparent field of view.

===True Field of View===

True field of view is not the same as apparent field of view. The true field of view is the angle of measurement of the sky you are seeing in the eyepiece. For example if you were able to just barely fit all of the Pleiades (Messier 45) into the eyepiece at once, then your eyepiece and telescope combination is giving you a true field of view of about 1.6 degrees, even if the eyepiece was measured at 100 degree apparent field of view.

While apparent field of view is a fixed value of the eyepiece, the true field of view depends on what magnification the eyepiece is operating it, meaning it depends on what the focal length of the telescope it is used in is.

Using the above example, you can surmise that if a 100 degree apparent field of view eyepiece is showing you a region of the sky 1.6 degrees across, the rough magnification would have to be 100 / 1.6 = 62.5x. Thus there is a rough relationship between an eyepiece's true field of view and its apparent field of view. If you had a 50 degree eyepiece that was also providing 62.5x magnification, it would only have a 0.8 degree true field of view.

==Calculating True Field of View==

Calculating true field of view can be done in one of two ways: one being more accurate than another.

===Apparent Field of View and Magnification Method===

As hinted above, to roughly calculate true field of view, you can divide the apparent field of view, by magnification:

<code>eyepiece apparent field of view / magnification</code>

'''Examples:'''

* 100 degrees / 35x = 2.87 degrees
* 50 degrees / 35x = 1.43 degrees
* 68 degrees / 200x = 0.34 degrees

It should be noted that this method is only approximate. The more accurate method is below:

===Using Field Stop===

Most eyepieces have a [[field stop]] or an effective field stop (depending on their design). If the manufacturer has supplied this field stop value, you can use it to more accurately compute the true field of view using the following formula:

<code>eyepiece field stop diameter in mm / telescope focal length in mm x 57.3</code>

'''Examples:'''

* 42mm / 1200mm * 57.3 = 2 degrees
* 6mm / 1800mm * 57.3 = 0.19 degrees

Unfortunately, not all manufacturers supply field stop information. In some cases, an eyepiece has a Smyth field lens (essentially a built-in [[barlow]]) that renders the given field stop value inaccurate. High end eyepiece manufacturers like Tele Vue or Explore Scientific will provide accurate field stop information for all of their eyepieces, regardless of design.

==Apparent Field of View Considerations==

===AFOV and Barrel Size===

Most modern eyepieces come in one of three standard [[barrel size]]s: 1.25", 2", and 3". The reason for this it accommodate different apparent fields of view at longer focal lengths. Since the apparent field of view is really governed by the [[field stop]] of the eyepiece, it means there are limits to how much apparent field of view you can get out of a barrel of a certain size.

For example, a 40mm [[Plossl]] eyepiece in a 1.25" barrel will typically have a maximum apparent field of view of just 42 degrees. A 24mm eyepiece in a 1.25" barrel can have a maximum apparent field of view of 68 degrees. A 16mm 1.25" eyepiece can have an apparent field of view of 82 degrees, and a 13mm 1.25" eyepiece can have an apparent field of view of 100 degrees. Thus the longer the focal length of a 1.25" eyepiece, the narrower the apparent field of view must be, just due to the inherent geometry of light passing through the eyepiece.

If you wanted lower magnification ''and'' wide apparent fields of view, a 2" barrel is required. For instance, a 40mm eyepiece in a 2" barrel can have an apparent field of view of up to about 70 degrees. A 30mm 2" eyepiece can have an 82 degree apparent field, and a 25mm 2" eyepiece can have an apparent field of view of 100 degrees. A 3" barrel would allow a 30mm eyepiece to have a 100 degree apparent field of view.

===AFOV and Abberations===

Very wide apparent fields of view can have downsides - they can manifest aberrations near the edges of the field of view more egregiously than eyepieces with narrower apparent fields of view.

====Coma====

The first aberration to consider is [[coma]]. Coma is an inherent aberration present in [[Newtonian reflectors]] with [[parabolic mirror]]s. While the eyepiece itself does not cause coma, it can ''reveal'' coma. The wider the apparent field of view is, the worse the coma will be revealed. This relationship is linear. A 100 degree AFOV eyepiece will show 2x larger coma at the edges than a 50 degree AFOV eyepiece will. This mean that if you have a short [[focal ratio]] Newtonian reflector (which has strong coma), you will notice it more easily in wide angle eyepieces than eyepieces with narrower fields of view.

====Astigmatism====

[[Astigmatism]] in an eyepiece is not the same as astigmatism in your vision. Telescopes with short focal ratios (regardless of their design), send light into the eyepiece at steeper angles than telescopes with longer focal ratios. A wide angle eyepiece then needs to accept these steep light rays and bend them to form the wide apparent field of view. The wider the field of view, and the steeper the entrance angle of light from short focal ratio telescopes, the more strongly this light has to be bent. This can cause astigmatism, where stars appear cross-like or streak-like the closer you get towards the edges of the field of view.

To correct for this astigmatism requires more glass elements and complex shapes, which increases the cost of the eyepiece. This is why some wide angle eyepieces are significantly cheaper than others. While they are able to form a wide apparent field of view, they don't always do so ''cleanly'', and stars near the edges look badly distorted. More expensive wide angle eyepieces are able to keep stars looking like points all the way to the edges, even in telescopes with short focal ratios (assuming a [[coma corrector]] is used where appropriate).

Main Topic Index

2019-05-18T02:27:33Z

Phpdevster:

*[[Telescope Basics]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filters]]

*[[Eyepieces]]
**[[Magnification]]
**[[Field of View]]
**[[Exit Pupil]]
**[[Eye Relief]]
**[[Barrel Size]]

*[[Observing]]
**[[Star Hopping]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Field of View

2019-05-18T02:26:52Z

Phpdevster: /* AFOV and Barrel Size */

==Overview==

Any given eyepiece has a designed attribute known as its apparent field of view. The ''apparent'' field of view is how "wide" the view appears to your eye as you peer into the eyepiece. In contrast, a given eyepiece and telescope combination will also have a ''true'' field of view, which describes how much of the night sky is visible at once. The two kinds of field of view are loosely related, but are different things.

===Apparent Field of View===

An eyepiece with an apparent field of view of 50 degrees means the angle your eye has to move from one edge to see the other edge, is 50 degrees. That is, the circle of light you see has an apparent [[angular size]] of 50 degrees. Meanwhile a 100 degree eyepiece shows you a circle of light that is 100 degrees in angular size.

The wider the apparent field of view is, the more immersive the view is. Looking through a 40 or 50 degree apparent field of view eyepiece might feel like looking through a narrow cardboard tube, while looking through a 100 degree apparent field of view eyepiece gives the impression of looking through a giant window on space ship.

Eyepieces come in all ranges of apparent field of view, from narrow 30 degree monocentric eyepieces, to 120 degree hyper wide eyepieces. The wider the apparent field of view is, the larger, heavier, and more expensive the eyepiece tends to be.

Eyepieces with wide apparent fields of view are typically referred to as "wide angle" eyepieces, but there is no hard definition for what constitutes a wide angle. There are other terms like "extreme wide angle" "ultra wide angle" and "hyper wide angle" and "super wide angle". These are nothing more than marketing terms that have no bearing on the stated apparent field of view.

===True Field of View===

True field of view is not the same as apparent field of view. The true field of view is the angle of measurement of the sky you are seeing in the eyepiece. For example if you were able to just barely fit all of the Pleiades (Messier 45) into the eyepiece at once, then your eyepiece and telescope combination is giving you a true field of view of about 1.6 degrees, even if the eyepiece was measured at 100 degree apparent field of view.

While apparent field of view is a fixed value of the eyepiece, the true field of view depends on what magnification the eyepiece is operating it, meaning it depends on what the focal length of the telescope it is used in is.

Using the above example, you can surmise that if a 100 degree apparent field of view eyepiece is showing you a region of the sky 1.6 degrees across, the rough magnification would have to be 100 / 1.6 = 62.5x. Thus there is a rough relationship between an eyepiece's true field of view and its apparent field of view. If you had a 50 degree eyepiece that was also providing 62.5x magnification, it would only have a 0.8 degree true field of view.

==Calculating True Field of View==

Calculating true field of view can be done in one of two ways: one being more accurate than another.

===Apparent Field of View and Magnification Method===

As hinted above, to roughly calculate true field of view, you can divide the apparent field of view, by magnification:

<code>eyepiece apparent field of view / magnification</code>

'''Examples:'''

* 100 degrees / 35x = 2.87 degrees
* 50 degrees / 35x = 1.43 degrees
* 68 degrees / 200x = 0.34 degrees

It should be noted that this method is only approximate. The more accurate method is below:

===Using Field Stop===

Most eyepieces have a [[field stop]] or an effective field stop (depending on their design). If the manufacturer has supplied this field stop value, you can use it to more accurately compute the true field of view using the following formula:

<code>eyepiece field stop diameter in mm / telescope focal length in mm x 57.3</code>

'''Examples:'''

* 42mm / 1200mm * 57.3 = 2 degrees
* 6mm / 1800mm * 57.3 = 0.19 degrees

Unfortunately, not all manufacturers supply field stop information. In some cases, an eyepiece has a Smyth field lens (essentially a built-in [[barlow]]) that renders the given field stop value inaccurate. High end eyepiece manufacturers like Tele Vue or Explore Scientific will provide accurate field stop information for all of their eyepieces, regardless of design.

==Apparent Field of View Considerations==

===AFOV and Barrel Size===

Most modern eyepieces come in one of three standard [[barrel sizes]]: 1.25", 2", and 3". The reason for this it accommodate different apparent fields of view at longer focal lengths. Since the apparent field of view is really governed by the [[field stop]] of the eyepiece, it means there are limits to how much apparent field of view you can get out of a barrel of a certain size.

For example, a 40mm [[Plossl]] eyepiece in a 1.25" barrel will typically have a maximum apparent field of view of just 42 degrees. A 24mm eyepiece in a 1.25" barrel can have a maximum apparent field of view of 68 degrees. A 16mm 1.25" eyepiece can have an apparent field of view of 82 degrees, and a 13mm 1.25" eyepiece can have an apparent field of view of 100 degrees. Thus the longer the focal length of a 1.25" eyepiece, the narrower the apparent field of view must be, just due to the inherent geometry of light passing through the eyepiece.

If you wanted lower magnification ''and'' wide apparent fields of view, a 2" barrel is required. For instance, a 40mm eyepiece in a 2" barrel can have an apparent field of view of up to about 70 degrees. A 30mm 2" eyepiece can have an 82 degree apparent field, and a 25mm 2" eyepiece can have an apparent field of view of 100 degrees. A 3" barrel would allow a 30mm eyepiece to have a 100 degree apparent field of view.

===AFOV and Abberations===

Very wide apparent fields of view can have downsides - they can manifest aberrations near the edges of the field of view more egregiously than eyepieces with narrower apparent fields of view.

====Coma====

The first aberration to consider is [[coma]]. Coma is an inherent aberration present in [[Newtonian reflectors]] with [[parabolic mirror]]s. While the eyepiece itself does not cause coma, it can ''reveal'' coma. The wider the apparent field of view is, the worse the coma will be revealed. This relationship is linear. A 100 degree AFOV eyepiece will show 2x larger coma at the edges than a 50 degree AFOV eyepiece will. This mean that if you have a short [[focal ratio]] Newtonian reflector (which has strong coma), you will notice it more easily in wide angle eyepieces than eyepieces with narrower fields of view.

====Astigmatism====

[[Astigmatism]] in an eyepiece is not the same as astigmatism in your vision. Telescopes with short focal ratios (regardless of their design), send light into the eyepiece at steeper angles than telescopes with longer focal ratios. A wide angle eyepiece then needs to accept these steep light rays and bend them to form the wide apparent field of view. The wider the field of view, and the steeper the entrance angle of light from short focal ratio telescopes, the more strongly this light has to be bent. This can cause astigmatism, where stars appear cross-like or streak-like the closer you get towards the edges of the field of view.

To correct for this astigmatism requires more glass elements and complex shapes, which increases the cost of the eyepiece. This is why some wide angle eyepieces are significantly cheaper than others. While they are able to form a wide apparent field of view, they don't always do so ''cleanly'', and stars near the edges look badly distorted. More expensive wide angle eyepieces are able to keep stars looking like points all the way to the edges, even in telescopes with short focal ratios (assuming a [[coma corrector]] is used where appropriate).

Field of View

2019-05-18T02:26:39Z

Phpdevster: Created page with "==Overview== Any given eyepiece has a designed attribute known as its apparent field of view. The ''apparent'' field of view is how "wide" the view appears to your eye as you..."

==Overview==

Any given eyepiece has a designed attribute known as its apparent field of view. The ''apparent'' field of view is how "wide" the view appears to your eye as you peer into the eyepiece. In contrast, a given eyepiece and telescope combination will also have a ''true'' field of view, which describes how much of the night sky is visible at once. The two kinds of field of view are loosely related, but are different things.

===Apparent Field of View===

An eyepiece with an apparent field of view of 50 degrees means the angle your eye has to move from one edge to see the other edge, is 50 degrees. That is, the circle of light you see has an apparent [[angular size]] of 50 degrees. Meanwhile a 100 degree eyepiece shows you a circle of light that is 100 degrees in angular size.

The wider the apparent field of view is, the more immersive the view is. Looking through a 40 or 50 degree apparent field of view eyepiece might feel like looking through a narrow cardboard tube, while looking through a 100 degree apparent field of view eyepiece gives the impression of looking through a giant window on space ship.

Eyepieces come in all ranges of apparent field of view, from narrow 30 degree monocentric eyepieces, to 120 degree hyper wide eyepieces. The wider the apparent field of view is, the larger, heavier, and more expensive the eyepiece tends to be.

Eyepieces with wide apparent fields of view are typically referred to as "wide angle" eyepieces, but there is no hard definition for what constitutes a wide angle. There are other terms like "extreme wide angle" "ultra wide angle" and "hyper wide angle" and "super wide angle". These are nothing more than marketing terms that have no bearing on the stated apparent field of view.

===True Field of View===

True field of view is not the same as apparent field of view. The true field of view is the angle of measurement of the sky you are seeing in the eyepiece. For example if you were able to just barely fit all of the Pleiades (Messier 45) into the eyepiece at once, then your eyepiece and telescope combination is giving you a true field of view of about 1.6 degrees, even if the eyepiece was measured at 100 degree apparent field of view.

While apparent field of view is a fixed value of the eyepiece, the true field of view depends on what magnification the eyepiece is operating it, meaning it depends on what the focal length of the telescope it is used in is.

Using the above example, you can surmise that if a 100 degree apparent field of view eyepiece is showing you a region of the sky 1.6 degrees across, the rough magnification would have to be 100 / 1.6 = 62.5x. Thus there is a rough relationship between an eyepiece's true field of view and its apparent field of view. If you had a 50 degree eyepiece that was also providing 62.5x magnification, it would only have a 0.8 degree true field of view.

==Calculating True Field of View==

Calculating true field of view can be done in one of two ways: one being more accurate than another.

===Apparent Field of View and Magnification Method===

As hinted above, to roughly calculate true field of view, you can divide the apparent field of view, by magnification:

<code>eyepiece apparent field of view / magnification</code>

'''Examples:'''

* 100 degrees / 35x = 2.87 degrees
* 50 degrees / 35x = 1.43 degrees
* 68 degrees / 200x = 0.34 degrees

It should be noted that this method is only approximate. The more accurate method is below:

===Using Field Stop===

Most eyepieces have a [[field stop]] or an effective field stop (depending on their design). If the manufacturer has supplied this field stop value, you can use it to more accurately compute the true field of view using the following formula:

<code>eyepiece field stop diameter in mm / telescope focal length in mm x 57.3</code>

'''Examples:'''

* 42mm / 1200mm * 57.3 = 2 degrees
* 6mm / 1800mm * 57.3 = 0.19 degrees

Unfortunately, not all manufacturers supply field stop information. In some cases, an eyepiece has a Smyth field lens (essentially a built-in [[barlow]]) that renders the given field stop value inaccurate. High end eyepiece manufacturers like Tele Vue or Explore Scientific will provide accurate field stop information for all of their eyepieces, regardless of design.

==Apparent Field of View Considerations==

===AFOV and Barrel Size===

Most modern eyepieces come in one of three standard[[barrel sizes]]: 1.25", 2", and 3". The reason for this it accommodate different apparent fields of view at longer focal lengths. Since the apparent field of view is really governed by the [[field stop]] of the eyepiece, it means there are limits to how much apparent field of view you can get out of a barrel of a certain size.

For example, a 40mm [[Plossl]] eyepiece in a 1.25" barrel will typically have a maximum apparent field of view of just 42 degrees. A 24mm eyepiece in a 1.25" barrel can have a maximum apparent field of view of 68 degrees. A 16mm 1.25" eyepiece can have an apparent field of view of 82 degrees, and a 13mm 1.25" eyepiece can have an apparent field of view of 100 degrees. Thus the longer the focal length of a 1.25" eyepiece, the narrower the apparent field of view must be, just due to the inherent geometry of light passing through the eyepiece.

If you wanted lower magnification ''and'' wide apparent fields of view, a 2" barrel is required. For instance, a 40mm eyepiece in a 2" barrel can have an apparent field of view of up to about 70 degrees. A 30mm 2" eyepiece can have an 82 degree apparent field, and a 25mm 2" eyepiece can have an apparent field of view of 100 degrees. A 3" barrel would allow a 30mm eyepiece to have a 100 degree apparent field of view.

===AFOV and Abberations===

Very wide apparent fields of view can have downsides - they can manifest aberrations near the edges of the field of view more egregiously than eyepieces with narrower apparent fields of view.

====Coma====

The first aberration to consider is [[coma]]. Coma is an inherent aberration present in [[Newtonian reflectors]] with [[parabolic mirror]]s. While the eyepiece itself does not cause coma, it can ''reveal'' coma. The wider the apparent field of view is, the worse the coma will be revealed. This relationship is linear. A 100 degree AFOV eyepiece will show 2x larger coma at the edges than a 50 degree AFOV eyepiece will. This mean that if you have a short [[focal ratio]] Newtonian reflector (which has strong coma), you will notice it more easily in wide angle eyepieces than eyepieces with narrower fields of view.

====Astigmatism====

[[Astigmatism]] in an eyepiece is not the same as astigmatism in your vision. Telescopes with short focal ratios (regardless of their design), send light into the eyepiece at steeper angles than telescopes with longer focal ratios. A wide angle eyepiece then needs to accept these steep light rays and bend them to form the wide apparent field of view. The wider the field of view, and the steeper the entrance angle of light from short focal ratio telescopes, the more strongly this light has to be bent. This can cause astigmatism, where stars appear cross-like or streak-like the closer you get towards the edges of the field of view.

To correct for this astigmatism requires more glass elements and complex shapes, which increases the cost of the eyepiece. This is why some wide angle eyepieces are significantly cheaper than others. While they are able to form a wide apparent field of view, they don't always do so ''cleanly'', and stars near the edges look badly distorted. More expensive wide angle eyepieces are able to keep stars looking like points all the way to the edges, even in telescopes with short focal ratios (assuming a [[coma corrector]] is used where appropriate).

Exit Pupil

2019-05-17T05:15:10Z

Phpdevster:

==Overview==
An eyepiece's primary purpose is to magnify the view in a telescope, but how much magnification it provides, and the size of the aperture of the telescope, can influence how bright the view is. High magnification in a smaller aperture will result in a very dim view. Conversely, lower magnification will produce a brighter view.

The brightness of the view is best described by a property known as exit pupil. While strictly speaking, exit pupil is the diameter of a virtual aperture produced by the eyepiece in the telescope, it is easier to think of it as the size of the "beam" of light that leaves the eyepiece and enters the eye. The larger the exit pupil, the brighter the view will be. The smaller the exit pupil, the dimmer the view will be.

The useful size of the exit pupil is typically between 0.5mm and 7mm, though it varies from situation to situation. The upper size limit depends on how widely one's own pupils dilate. The average for most young adults is around 7mm. If the exit pupil exceeds this size, it means not all light from the telescope is entering the observer's eye, which reduces its effective aperture.

For these reasons, it can be just as important to consider the exit pupil that an eyepiece will produce, along with its magnification, since the two are inextricably linked. Too little exit pupil may render the view too dim for the object being viewed. Too much exit pupil and light is being wasted since it's not all fitting through the iris of the observer's eye.

==Calculating Exit Pupil==

===Using aperture and magnification===
One way to compute the exit pupil is to divide telescope aperture in millimeters, by magnification:

<code>telescope aperture in mm / magnification</code>

'''Examples''':

* 200mm / 50x = 4mm exit pupil
* 127mm / 150x = 0.85mm exit pupil

===Using telescope focal ratio and eyepiece focal length===
A slightly more convenient method for computing exit pupil is to divide the eyepiece focal length by the telescope focal ratio

<code>eyepiece focal length in mm / telescope focal ratio</code>

'''Examples''':

* 20mm eyepiece / F4.7 = 4.25mm exit pupil
* 5mm eyepiece / F10 = 0.5mm exit pupil

The interesting thing about this method is that it shows the same eyepiece will produce the same exit pupil in telescopes with the same focal ratio, regardless of aperture. A 1000mm aperture F/4 telescope produces the same view brightness as a 100mm aperture F/4 telescope. This demonstrates that it's ultimately the exit pupil, not the aperture, that governs view brightness.

==Exit Pupil Effects and Considerations==
Exit pupil generally influences the brightness of the view through the eyepiece + telescope combination, but not necessarily in the same way for all objects. Stars are unaffected by exit pupil since they are optical point sources. They cannot be magnified, so their light does not spread out. Thus star brightness is directly governed by telescope aperture.

Meanwhile any object that has a measurable surface area and can be magnified, does get dimmer with exit pupil. This includes the Moon, planets, nebulae, galaxies, and even light pollution and general sky glow.

Since exit pupil affects the brightness of light pollution, but not stars, one way to add contrast to star clusters is to increase magnification. By increasing magnification, the stars will remain the same brightness, but light pollution will get dimmer, thus the contrast of the view will increase.

However, this same trick does not work for galaxies and nebulae. Galaxies and nebulae will get dimmer equally as quickly as light pollution as magnification increases, so contrast does not change.

===Exit Pupil and Astigmatism===

If you have astigmatism in your vision, it typically becomes worse the larger the exit pupil is. Astigmatism in astronomy can be particularly annoying because it causes stars to have a spiky, irregular appearance. The larger the exit pupil, the more this problem will manifest itself.

[http://www.televue.com/images/TV3_Images/Images_in_articles/DioptrixAstigmatismVis.gif This chart from Tele Vue] indicates which levels of astigmatism will manifest at which exit pupils.

If seeing pinpoint-like stars is important to you, then consider choosing an eyepiece that provides enough [[eye relief]] to be used with glasses if that eyepiece will produce a large exit pupil in your telescope.

===Exit Pupil and Nebula Filters===

If you plan on using aggressive line filters like UHC/Narrowband, OIII, or H-Beta filters, then it's usually beneficial to have a large exit pupil. Line filters will dim the view quite a bit because they only permit a narrow spectrum of light to pass through them. If you are already starting off at a small exit pupil that is producing a dim view, adding a line filter could make the view too dim to be usable.

Generally any exit pupil larger than 2mm will work with line filters, but the larger the exit pupil, the better.

===Minimum and Maximum Exit Pupil===

The human eye is ultimately the limiting factor of how large or small an exit pupil can be. Though it depends on individual genetics, human pupils are typically around 7mm in diameter in our 20s, and then get smaller as we age. If an eyepiece and telescope combination produces an exit pupil larger than what our eye can accept, that light is effectively wasted. This can become especially problematic for reflectors and catadioptrics whose central obstruction starts occupying a larger and larger percentage of the light that does enter the pupil. The more oversized the exit pupil is in a telescope with a central obstruction, the more pronounced the secondary shadow will be, and it will often seem like there is a dark, out of focus blob floating in the center of the field of view. Refractors do not have this problem.

While there is some case to be made for breaking the largest usable exit pupil rule in order to achieve a wider true field of view in some cases, the general advice is not to exceed an exit pupil larger than what your own eyes can support. It is worthwhile getting your dilated pupils measured by an optometrist to get a better idea of what your personal maximum useful exit pupil will be.

For minimum exit pupil, it often depends on the target. The brighter the target, the smaller the exit pupil can be before the object becomes too dim. It should be noted that the dimmer the exit pupil, the less light the eye has to work with. This can rob the view of contrast and clarity. Given the rough rule of thumb of not to exceed 50x per inch of aperture, this translates to a minimum useful exit pupil of 0.5mm. However, this is not a hard and fast rule, and it depends entirely on the object. Some small faint objects can effectively disappear from your vision at 1mm exit pupil, while other small bright objects remain easily visible even as low as 0.3mm. Bright objects like planets and the moon can tolerate even smaller exit pupils, but at that point you would be pushing magnification well past the useful limit of the telescope's optics. You'd be sacrificing brightness for no additional detail.

===Exit Pupil vs Magnification===
Exit pupil and magnification are inextricably linked. Increasing magnification will decrease exit pupil, and vice versa. Thus a balance between magnification and exit pupil must be struck. It's generally beneficial to achieve higher magnification at the expense of exit pupil, since our eyes behave linearly with image scale, but non-linearly with brightness. That is, we can tolerate dimmer views better than we can tolerate smaller views. As an example, consider Messier 51 - the Whirlpool Galaxy. When looking at the night sky with the naked eye, you are effectively using a 7mm exit pupil since your eyes are fully dilated. When you look through say, a 6" telescope at 100x, the exit pupil is just 1.5mm - 22x dimmer. But despite the view being 22x dimmer than the naked eye, the fact that it is 100x ''larger'' is what makes M51 visible. Our eyes do not detect small, low contrast things very well, but do detect large low contrast things. Thus using magnification to enlarge low contrast objects is more beneficial than trying to preserve a bright exit pupil.

Main Topic Index

2019-05-17T05:14:29Z

Phpdevster:

*[[Telescope Basics]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filters]]

*[[Eyepieces]]
**[[Magnification]]
**[[Field of View]]
**[[Exit Pupil]]
**[[Eye Relief]]

*[[Observing]]
**[[Star Hopping]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Main Topic Index

2019-05-17T05:07:50Z

Phpdevster:

*[[Telescope Basics]]
*[[History of the Telescope]]
*[[Types of Optical Telescopes]]
**[[Refractor Telescope]]
***[[Achromatic Refractor]]
***[[Apochromatic Refractor]]
***[[Reflector Telescope]]
***[[Newtonian Telescope]]
***[[Classical Cassegrain Telescope]]
***[[Ritchey Chretien Telescope]]
***[[Dall-Kirkham Cassegrain telescope]]
***[[Gregorian Telescope]]
**[[Catadioptric Telescopes]]
***[[Schmidt Cassegrain Telescope]]
***[[Maksutov Cassegrain Telescope]]
***[[Bird-Jones Telescope]]

*Accessories
**[[Light Pollution Filters]]

[[Star Hopping]]
[[Magnification]]
[[Field of View]]
[[Exit Pupil]]

*[[Astronomy Software]]
**PC-Based:
***[[Planetarium Software]]
***[[Telescope Control Software]]
***[[Astrophotography Software]]
****[[Astrophotography Camera Control And Capture Software]]
****[[Astrophotography Stacking and Processing Software]]
***[[Other Astronomy Software]]

Users may feel free to suggest pages below. Of course, you can also always add your own pages at any time. These may, however, end up getting edited or folded into others in time.

----
(add suggestions here)

Light Pollution Filters

2019-05-17T05:06:15Z

Phpdevster:

==Overview==
Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters will dim the view, but their purpose is to dim light pollution and sky glow more than the object of interest by trying to allow the light from the object of interest to pass more freely.

Filters break down into three categories:

* Broadband filters (also commonly called light pollution filters or deep sky filters)
* Narrowband or UHC (ultra high contrast) filters
* Line filters, such as Oxygen-III and Hydrogen-Beta filters

==Filter Types==

===Broadband / Light Pollution Filters===

Broadband filters are the most permissive, and seek to block common wavelengths of light pollution that come from pink or orange sodium vapor lights. These filters do not work well if light pollution is predominantly white LED, since most of the light from the white LED light will pass right through the filter.

===Narrowband / UHC Filters===

Narrowband or "UHC" filters attempt to "whitelist" only certain wavelengths rather than "blacklist" light pollution wavelengths. These filters have considerably higher contrast than broadband filters as a result, but they only work on emission nebulae since they permit only wavelengths commonly emitted by emission nebulae - Oxygen III and Hydrogen Beta.

===Line Filters===

Line filters are even more strict about what they permit than narrowband filters are. They allow transmission of only specific sepctral lines, such as Oxygen III at 501nm and 496nm wavelengths or Hydrogen Beta at 486nm wavelength. Thus line filters offer the highest possible contrast, but do so at the expense of the most view brightness. Certain nebulae emit light more strongly in the O-III or H-Beta part of the spectrum. For example, the ionized gas that acts as a backdrop behind the Horse Head Nebula emits light most strongly in the H-Beta part of the spectrum, and is very low contrast. A UHC filter is often not high enough contrast to show it, an O-III filter can block it, but an H-Beta filter can be just enough to reveal it.

==Filter Characteristics==

All filters have two primary characteristics: bandwidth (how broad a range of the visible spectrum they allow), and transmission (what percentage of light they allow within the spectral range they permit. The highest quality filters have the tightest bandwidth and the highest transmission.

===Bandwidth===

Bandwidth is a measure of the width of the spectrum allowed through by the filter, in nanometers. For example, the Astronomik UHC filter has a bandwidth of 24nm, while the Astronomik UHC-E (economy) filter has a bandwidth of 35nm. ([https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/ 2019 Nebula Filters Buyer's Guide]).

The wider bandwidth of the UHC-E filter means it will have lower contrast, as it will permit more light beyond the boundaries of its target wavelengths. Conversely, the normal UHC filter does a better job of isolating only the wavelengths it wants to permit, so it has higher contrast as a result.

===Transmission===

Transmission is the other important characteristic of a filter. It is desirable to pass as much of the isolated wavelengths of light through the filter as possible to ensure maximum brightness and contrast. Similarly, a filter should minimize the transmission of the blocked wavelengths of light as much as possible. It is common for cheaper filters to maximize transmission of the isolated wavelengths of light by also permitting too much of the blocked light to pass through. Thus it's important to understand not only the light transmission percentage of the desired wavelengths, but also the light transmission percentage of the undesired wavelengths.

===Filters and Galaxies===

Generally speaking, galaxies do not benefit from filters since galaxies are full spectrum objects and do not emit light in any one particular part of the spectrum. In some cases, the emission nebulae of other galaxies (such as NGC 604) may benefit from filters, but the galaxy itself will not. A broadband / light pollution filter in an area with predominantly sodium vapor lighting may give "cooler" spiral galaxies a slight contrast enhancement, but older "warmer" elliptical galaxies may suffer as a result of the filter blocking the warmer parts of the spectrum. UHC and line filters would be detrimental to galaxy observing.

==Useful Resources==
* [https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/ The Prairie Astronomy Club filter performance comparison]
* [https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/ 2019 Nebula Filter Buyer's Guide by Don Pensak]

Light Pollution Filters

2019-05-17T05:05:47Z

Phpdevster:

==Overview==
Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters will dim the view, but their purpose is to dim light pollution and sky glow more than the object of interest by trying to allow the light from the object of interest to pass more freely.

Filters break down into three categories:

* Broadband filters (also commonly called light pollution filters or deep sky filters)
* Narrowband or UHC (ultra high contrast) filters
* Line filters, such as Oxygen-III and Hydrogen-Beta filters

==Filter Types==

===Broadband / Light Pollution Filters===

Broadband filters are the most permissive, and seek to block common wavelengths of light pollution that come from pink or orange sodium vapor lights. These filters do not work well if light pollution is predominantly white LED, since most of the light from the white LED light will pass right through the filter.

===Narrowband / UHC Filters===

Narrowband or "UHC" filters attempt to "whitelist" only certain wavelengths rather than "blacklist" light pollution wavelengths. These filters have considerably higher contrast than broadband filters as a result, but they only work on emission nebulae since they permit only wavelengths commonly emitted by emission nebulae - Oxygen III and Hydrogen Beta.

===Line Filters===

Line filters are even more strict about what they permit than narrowband filters are. They allow transmission of only specific sepctral lines, such as Oxygen III at 501nm and 496nm wavelengths or Hydrogen Beta at 486nm wavelength. Thus line filters offer the highest possible contrast, but do so at the expense of the most view brightness. Certain nebulae emit light more strongly in the O-III or H-Beta part of the spectrum. For example, the ionized gas that acts as a backdrop behind the Horse Head Nebula emits light most strongly in the H-Beta part of the spectrum, and is very low contrast. A UHC filter is often not high enough contrast to show it, an O-III filter can block it, but an H-Beta filter can be just enough to reveal it.

==Filter Characteristics==

All filters have two primary characteristics: bandwidth (how broad a range of the visible spectrum they allow), and transmission (what percentage of light they allow within the spectral range they permit. The highest quality filters have the tightest bandwidth and the highest transmission.

===Bandwidth===

Bandwidth is a measure of the width of the spectrum allowed through by the filter, in nanometers. For example, the Astronomik UHC filter has a bandwidth of 24nm, while the Astronomik UHC-E (economy) filter has a bandwidth of 35nm. ([https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/ 2019 Nebula Filters Buyer's Guide]).

The wider bandwidth of the UHC-E filter means it will have lower contrast, as it will permit more light beyond the boundaries of its target wavelengths. Conversely, the normal UHC filter does a better job of isolating only the wavelengths it wants to permit, so it has higher contrast as a result.

===Transmission===

Transmission is the other important characteristic of a filter. It is desirable to pass as much of the isolated wavelengths of light through the filter as possible to ensure maximum brightness and contrast. Similarly, a filter should minimize the transmission of the blocked wavelengths of light as much as possible. It is common for cheaper filters to maximize transmission of the isolated wavelengths of light by also permitting too much of the blocked light to pass through. Thus it's important to understand not only the light transmission percentage of the desired wavelengths, but also the light transmission percentage of the undesired wavelengths.

===Filters and Galaxies===

Generally speaking, galaxies do not benefit from filters since galaxies are full spectrum objects and do not emit light in any one particular part of the spectrum. In some cases, the emission nebulae of other galaxies (such as NGC 604) may benefit from filters, but the galaxy itself will not. A broadband / light pollution filter in an area with predominantly sodium vapor lighting may give "cooler" spiral galaxies a slight contrast enhancement, but older "warmer" elliptical galaxies may suffer as a result of the filter blocking the warmer parts of the spectrum. UHC and line filters would be detrimental to galaxy observing.

==Useful Resources==
* [https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/ The Prairie Astronomy Club filter performance comparison]
* [2019 Nebula Filter Buyer's Guide by Don Pensak https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/]

Light Pollution Filters

2019-05-17T05:05:22Z

Phpdevster:

==Overview==
Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters will dim the view, but their purpose is to dim light pollution and sky glow more than the object of interest by trying to allow the light from the object of interest to pass more freely.

Filters break down into three categories:

1. Broadband filters (also commonly called light pollution filters or deep sky filters)
2. Narrowband or UHC (ultra high contrast) filters
3. Line filters, such as Oxygen-III and Hydrogen-Beta filters

==Filter Types==

===Broadband / Light Pollution Filters===

Broadband filters are the most permissive, and seek to block common wavelengths of light pollution that come from pink or orange sodium vapor lights. These filters do not work well if light pollution is predominantly white LED, since most of the light from the white LED light will pass right through the filter.

===Narrowband / UHC Filters===

Narrowband or "UHC" filters attempt to "whitelist" only certain wavelengths rather than "blacklist" light pollution wavelengths. These filters have considerably higher contrast than broadband filters as a result, but they only work on emission nebulae since they permit only wavelengths commonly emitted by emission nebulae - Oxygen III and Hydrogen Beta.

===Line Filters===

Line filters are even more strict about what they permit than narrowband filters are. They allow transmission of only specific sepctral lines, such as Oxygen III at 501nm and 496nm wavelengths or Hydrogen Beta at 486nm wavelength. Thus line filters offer the highest possible contrast, but do so at the expense of the most view brightness. Certain nebulae emit light more strongly in the O-III or H-Beta part of the spectrum. For example, the ionized gas that acts as a backdrop behind the Horse Head Nebula emits light most strongly in the H-Beta part of the spectrum, and is very low contrast. A UHC filter is often not high enough contrast to show it, an O-III filter can block it, but an H-Beta filter can be just enough to reveal it.

==Filter Characteristics==

All filters have two primary characteristics: bandwidth (how broad a range of the visible spectrum they allow), and transmission (what percentage of light they allow within the spectral range they permit. The highest quality filters have the tightest bandwidth and the highest transmission.

===Bandwidth===

Bandwidth is a measure of the width of the spectrum allowed through by the filter, in nanometers. For example, the Astronomik UHC filter has a bandwidth of 24nm, while the Astronomik UHC-E (economy) filter has a bandwidth of 35nm. ([https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/ 2019 Nebula Filters Buyer's Guide]).

The wider bandwidth of the UHC-E filter means it will have lower contrast, as it will permit more light beyond the boundaries of its target wavelengths. Conversely, the normal UHC filter does a better job of isolating only the wavelengths it wants to permit, so it has higher contrast as a result.

===Transmission===

Transmission is the other important characteristic of a filter. It is desirable to pass as much of the isolated wavelengths of light through the filter as possible to ensure maximum brightness and contrast. Similarly, a filter should minimize the transmission of the blocked wavelengths of light as much as possible. It is common for cheaper filters to maximize transmission of the isolated wavelengths of light by also permitting too much of the blocked light to pass through. Thus it's important to understand not only the light transmission percentage of the desired wavelengths, but also the light transmission percentage of the undesired wavelengths.

===Filters and Galaxies===

Generally speaking, galaxies do not benefit from filters since galaxies are full spectrum objects and do not emit light in any one particular part of the spectrum. In some cases, the emission nebulae of other galaxies (such as NGC 604) may benefit from filters, but the galaxy itself will not. A broadband / light pollution filter in an area with predominantly sodium vapor lighting may give "cooler" spiral galaxies a slight contrast enhancement, but older "warmer" elliptical galaxies may suffer as a result of the filter blocking the warmer parts of the spectrum. UHC and line filters would be detrimental to galaxy observing.

==Useful Resources==
* [https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/ The Prairie Astronomy Club filter performance comparison]
* [2019 Nebula Filter Buyer's Guide by Don Pensak https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/]

Light Pollution Filters

2019-05-17T05:05:00Z

Phpdevster: Created page with "==Overview== Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters wi..."

==Overview==
Light pollution filters are special filters that permit or block certain wavelengths of light in an effort to improve contrast of deep sky objects. All filters will dim the view, but their purpose is to dim light pollution and sky glow more than the object of interest by trying to allow the light from the object of interest to pass more freely.

Filters break down into three categories:

1. Broadband filters (also commonly called light pollution filters or deep sky filters)
2. Narrowband or UHC (ultra high contrast) filters
3. Line filters, such as Oxygen-III and Hydrogen-Beta filters

==Filter Types==

===Broadband / Light Pollution Filters===

Broadband filters are the most permissive, and seek to block common wavelengths of light pollution that come from pink or orange sodium vapor lights. These filters do not work well if light pollution is predominantly white LED, since most of the light from the white LED light will pass right through the filter.

===Narrowband / UHC Filters===

Narrowband or "UHC" filters attempt to "whitelist" only certain wavelengths rather than "blacklist" light pollution wavelengths. These filters have considerably higher contrast than broadband filters as a result, but they only work on emission nebulae since they permit only wavelengths commonly emitted by emission nebulae - Oxygen III and Hydrogen Beta.

===Line Filters===

Line filters are even more strict about what they permit than narrowband filters are. They allow transmission of only specific sepctral lines, such as Oxygen III at 501nm and 496nm wavelengths or Hydrogen Beta at 486nm wavelength. Thus line filters offer the highest possible contrast, but do so at the expense of the most view brightness. Certain nebulae emit light more strongly in the O-III or H-Beta part of the spectrum. For example, the ionized gas that acts as a backdrop behind the Horse Head Nebula emits light most strongly in the H-Beta part of the spectrum, and is very low contrast. A UHC filter is often not high enough contrast to show it, an O-III filter can block it, but an H-Beta filter can be just enough to reveal it.

==Filter Characteristics==

All filters have two primary characteristics: bandwidth (how broad a range of the visible spectrum they allow), and transmission (what percentage of light they allow within the spectral range they permit. The highest quality filters have the tightest bandwidth and the highest transmission.

===Bandwidth===

Bandwidth is a measure of the width of the spectrum allowed through by the filter, in nanometers. For example, the Astronomik UHC filter has a bandwidth of 24nm, while the Astronomik UHC-E (economy) filter has a bandwidth of 35nm. ([https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/ 2019 Nebula Filters Buyer's Guide]).

The wider bandwidth of the UHC-E filter means it will have lower contrast, as it will permit more light beyond the boundaries of its target wavelengths. Conversely, the normal UHC filter does a better job of isolating only the wavelengths it wants to permit, so it has higher contrast as a result.

===Transmission===

Transmission is the other important characteristic of a filter. It is desirable to pass as much of the isolated wavelengths of light through the filter as possible to ensure maximum brightness and contrast. Similarly, a filter should minimize the transmission of the blocked wavelengths of light as much as possible. It is common for cheaper filters to maximize transmission of the isolated wavelengths of light by also permitting too much of the blocked light to pass through. Thus it's important to understand not only the light transmission percentage of the desired wavelengths, but also the light transmission percentage of the undesired wavelengths.

===Filters and Galaxies===

Generally speaking, galaxies do not benefit from filters since galaxies are full spectrum objects and do not emit light in any one particular part of the spectrum. In some cases, the emission nebulae of other galaxies (such as NGC 604) may benefit from filters, but the galaxy itself will not. A broadband / light pollution filter in an area with predominantly sodium vapor lighting may give "cooler" spiral galaxies a slight contrast enhancement, but older "warmer" elliptical galaxies may suffer as a result of the filter blocking the warmer parts of the spectrum. UHC and line filters would be detrimental to galaxy observing.

==Useful Resources==
* [https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/ The Prairie Astronomy Club filter performance comparison]
* [2019 Nebula Filter Buyer's Guide by Don Pensak](https://www.cloudynights.com/topic/655490-2019-nebula-filters-buyers-guide/)

Exit Pupil

2019-05-17T04:17:57Z

Phpdevster: /* Exit Pupil Effects and Considerations */

==Overview==
An eyepiece's primary purpose is to magnify the view in a telescope, but how much magnification it provides, and the size of the aperture of the telescope, can influence how bright the view is. High magnification in a smaller aperture will result in a very dim view. Conversely, lower magnification will produce a brighter view.

The brightness of the view is best described by a property known as exit pupil. While strictly speaking, exit pupil is the diameter of a virtual aperture produced by the eyepiece in the telescope, it is easier to think of it as the size of the "beam" of light that leaves the eyepiece and enters the eye. The larger the exit pupil, the brighter the view will be. The smaller the exit pupil, the dimmer the view will be.

The useful size of the exit pupil is typically between 0.5mm and 7mm, though it varies from situation to situation. The upper size limit depends on how widely one's own pupils dilate. The average for most young adults is around 7mm. If the exit pupil exceeds this size, it means not all light from the telescope is entering the observer's eye, which reduces its effective aperture.

For these reasons, it can be just as important to consider the exit pupil that an eyepiece will produce, along with its magnification, since the two are inextricably linked. Too little exit pupil may render the view too dim for the object being viewed. Too much exit pupil and light is being wasted since it's not all fitting through the iris of the observer's eye.

==Calculating Exit Pupil==

===Using aperture and magnification===
One way to compute the exit pupil is to divide telescope aperture in millimeters, by magnification:

<code>telescope aperture in mm / magnification</code>

'''Examples''':

* 200mm / 50x = 4mm exit pupil
* 127mm / 150x = 0.85mm exit pupil

===Using telescope focal ratio and eyepiece focal length===
A slightly more convenient method for computing exit pupil is to divide the eyepiece focal length by the telescope focal ratio

<code>eyepiece focal length in mm / telescope focal ratio</code>

'''Examples''':

* 20mm eyepiece / F4.7 = 4.25mm exit pupil
* 5mm eyepiece / F10 = 0.5mm exit pupil

The interesting thing about this method is that it shows the same eyepiece will produce the same exit pupil in telescopes with the same focal ratio, regardless of aperture. A 1000mm aperture F/4 telescope produces the same view brightness as a 100mm aperture F/4 telescope. This demonstrates that it's ultimately the exit pupil, not the aperture, that governs view brightness.

==Exit Pupil Effects and Considerations==
Exit pupil generally influences the brightness of the view through the eyepiece + telescope combination, but not necessarily in the same way for all objects. Stars are unaffected by exit pupil since they are optical point sources. They cannot be magnified, so their light does not spread out. Thus star brightness is directly governed by telescope aperture.

Meanwhile any object that has a measurable surface area and can be magnified, does get dimmer with exit pupil. This includes the Moon, planets, nebulae, galaxies, and even light pollution and general sky glow.

Since exit pupil affects the brightness of light pollution, but not stars, one way to add contrast to star clusters is to increase magnification. By increasing magnification, the stars will remain the same brightness, but light pollution will get dimmer, thus the contrast of the view will increase.

However, this same trick does not work for galaxies and nebulae. Galaxies and nebulae will get dimmer equally as quickly as light pollution as magnification increases, so contrast does not change.

===Exit Pupil and Astigmatism===

If you have astigmatism in your vision, it typically becomes worse the larger the exit pupil is. Astigmatism in astronomy can be particularly annoying because it causes stars to have a spiky, irregular appearance. The larger the exit pupil, the more this problem will manifest itself.

[http://www.televue.com/images/TV3_Images/Images_in_articles/DioptrixAstigmatismVis.gif This chart from Tele Vue] indicates which levels of astigmatism will manifest at which exit pupils.

If seeing pinpoint-like stars is important to you, then consider choosing an eyepiece that provides enough eye relief to be used with glasses if that eyepiece will produce a large exit pupil in your telescope.

===Exit Pupil and Nebula Filters===

If you plan on using aggressive line filters like UHC/Narrowband, OIII, or H-Beta filters, then it's usually beneficial to have a large exit pupil. Line filters will dim the view quite a bit because they only permit a narrow spectrum of light to pass through them. If you are already starting off at a small exit pupil that is producing a dim view, adding a line filter could make the view too dim to be usable.

Generally any exit pupil larger than 2mm will work with line filters, but the larger the exit pupil, the better.

===Minimum and Maximum Exit Pupil===

The human eye is ultimately the limiting factor of how large or small an exit pupil can be. Though it depends on individual genetics, human pupils are typically around 7mm in diameter in our 20s, and then get smaller as we age. If an eyepiece and telescope combination produces an exit pupil larger than what our eye can accept, that light is effectively wasted. This can become especially problematic for reflectors and catadioptrics whose central obstruction starts occupying a larger and larger percentage of the light that does enter the pupil. The more oversized the exit pupil is in a telescope with a central obstruction, the more pronounced the secondary shadow will be, and it will often seem like there is a dark, out of focus blob floating in the center of the field of view. Refractors do not have this problem.

While there is some case to be made for breaking the largest usable exit pupil rule in order to achieve a wider true field of view in some cases, the general advice is not to exceed an exit pupil larger than what your own eyes can support. It is worthwhile getting your dilated pupils measured by an optometrist to get a better idea of what your personal maximum useful exit pupil will be.

For minimum exit pupil, it often depends on the target. The brighter the target, the smaller the exit pupil can be before the object becomes too dim. It should be noted that the dimmer the exit pupil, the less light the eye has to work with. This can rob the view of contrast and clarity. Given the rough rule of thumb of not to exceed 50x per inch of aperture, this translates to a minimum useful exit pupil of 0.5mm. However, this is not a hard and fast rule, and it depends entirely on the object. Some small faint objects can effectively disappear from your vision at 1mm exit pupil, while other small bright objects remain easily visible even as low as 0.3mm. Bright objects like planets and the moon can tolerate even smaller exit pupils, but at that point you would be pushing magnification well past the useful limit of the telescope's optics. You'd be sacrificing brightness for no additional detail.

===Exit Pupil vs Magnification===
Exit pupil and magnification are inextricably linked. Increasing magnification will decrease exit pupil, and vice versa. Thus a balance between magnification and exit pupil must be struck. It's generally beneficial to achieve higher magnification at the expense of exit pupil, since our eyes behave linearly with image scale, but non-linearly with brightness. That is, we can tolerate dimmer views better than we can tolerate smaller views. As an example, consider Messier 51 - the Whirlpool Galaxy. When looking at the night sky with the naked eye, you are effectively using a 7mm exit pupil since your eyes are fully dilated. When you look through say, a 6" telescope at 100x, the exit pupil is just 1.5mm - 22x dimmer. But despite the view being 22x dimmer than the naked eye, the fact that it is 100x ''larger'' is what makes M51 visible. Our eyes do not detect small, low contrast things very well, but do detect large low contrast things. Thus using magnification to enlarge low contrast objects is more beneficial than trying to preserve a bright exit pupil.

Exit Pupil

2019-05-17T04:09:03Z

Phpdevster:

==Overview==
An eyepiece's primary purpose is to magnify the view in a telescope, but how much magnification it provides, and the size of the aperture of the telescope, can influence how bright the view is. High magnification in a smaller aperture will result in a very dim view. Conversely, lower magnification will produce a brighter view.

The brightness of the view is best described by a property known as exit pupil. While strictly speaking, exit pupil is the diameter of a virtual aperture produced by the eyepiece in the telescope, it is easier to think of it as the size of the "beam" of light that leaves the eyepiece and enters the eye. The larger the exit pupil, the brighter the view will be. The smaller the exit pupil, the dimmer the view will be.

The useful size of the exit pupil is typically between 0.5mm and 7mm, though it varies from situation to situation. The upper size limit depends on how widely one's own pupils dilate. The average for most young adults is around 7mm. If the exit pupil exceeds this size, it means not all light from the telescope is entering the observer's eye, which reduces its effective aperture.

For these reasons, it can be just as important to consider the exit pupil that an eyepiece will produce, along with its magnification, since the two are inextricably linked. Too little exit pupil may render the view too dim for the object being viewed. Too much exit pupil and light is being wasted since it's not all fitting through the iris of the observer's eye.

==Calculating Exit Pupil==

===Using aperture and magnification===
One way to compute the exit pupil is to divide telescope aperture in millimeters, by magnification:

<code>telescope aperture in mm / magnification</code>

'''Examples''':

* 200mm / 50x = 4mm exit pupil
* 127mm / 150x = 0.85mm exit pupil

===Using telescope focal ratio and eyepiece focal length===
A slightly more convenient method for computing exit pupil is to divide the eyepiece focal length by the telescope focal ratio

<code>eyepiece focal length in mm / telescope focal ratio</code>

'''Examples''':

* 20mm eyepiece / F4.7 = 4.25mm exit pupil
* 5mm eyepiece / F10 = 0.5mm exit pupil

The interesting thing about this method is that it shows the same eyepiece will produce the same exit pupil in telescopes with the same focal ratio, regardless of aperture. A 1000mm aperture F/4 telescope produces the same view brightness as a 100mm aperture F/4 telescope. This demonstrates that it's ultimately the exit pupil, not the aperture, that governs view brightness.

==Exit Pupil Effects and Considerations==
Exit pupil generally influences the brightness of the view through the eyepiece + telescope combination, but not necessarily in the same way for all objects. Stars are unaffected by exit pupil since they are optical point sources. They cannot be magnified, so their light does not spread out. Thus star brightness is directly governed by telescope aperture.

Meanwhile any object that has a measurable surface area and can be magnified, does get dimmer with exit pupil. This includes the Moon, planets, nebulae, galaxies, and even light pollution and general sky glow.

Since exit pupil affects the brightness of light pollution, but not stars, one way to add contrast to star clusters is to increase magnification. By increasing magnification, the stars will remain the same brightness, but light pollution will get dimmer, thus the contrast of the view will increase.

However, this same trick does not work for galaxies and nebulae. Galaxies and nebulae will get dimmer equally as quickly as light pollution as magnification increases, so contrast does not change.

===Exit Pupil and Astigmatism===

If you have astigmatism in your vision, it typically becomes worse the larger the exit pupil is. Astigmatism in astronomy can be particularly annoying because it causes stars to have a spiky, irregular appearance. The larger the exit pupil, the more this problem will manifest itself.

[http://www.televue.com/images/TV3_Images/Images_in_articles/DioptrixAstigmatismVis.gif This chart from Tele Vue] indicates which levels of astigmatism will manifest at which exit pupils.

If seeing pinpoint-like stars is important to you, then consider choosing an eyepiece that provides enough eye relief to be used with glasses if that eyepiece will produce a large exit pupil in your telescope.

===Exit Pupil and Nebula Filters===

If you plan on using aggressive line filters like UHC/Narrowband, OIII, or H-Beta filters, then it's usually beneficial to have a large exit pupil. Line filters will dim the view quite a bit because they only permit a narrow spectrum of light to pass through them. If you are already starting off at a small exit pupil that is producing a dim view, adding a line filter could make the view too dim to be usable.

Generally any exit pupil larger than 2mm will work with line filters, but the larger the exit pupil, the better.

===Minimum and Maximum Exit Pupil===

The human eye is ultimately the limiting factor of how large or small an exit pupil can be. Though it depends on individual genetics, human pupils are typically around 7mm in diameter in our 20s, and then get smaller as we age. If an eyepiece and telescope combination produces an exit pupil larger than what our eye can accept, that light is effectively wasted. This can become especially problematic for reflectors and catadioptrics whose central obstruction starts occupying a larger and larger percentage of the light that does enter the pupil. The more oversized the exit pupil is in a telescope with a central obstruction, the more pronounced the secondary shadow will be, and it will often seem like there is a dark, out of focus blob floating in the center of the field of view. Refractors do not have this problem.

While there is some case to be made for breaking the largest usable exit pupil rule in order to achieve a wider true field of view in some cases, the general advice is not to exceed an exit pupil larger than what your own eyes can support. It is worthwhile getting your dilated pupils measured by an optometrist to get a better idea of what your personal maximum useful exit pupil will be.

For minimum exit pupil, it often depends on the target. The brighter the target, the smaller the exit pupil can be before the object becomes too dim. It should be noted that the dimmer the exit pupil, the less light the eye has to work with. This can rob the view of contrast and clarity. Given the rough rule of thumb of not to exceed 50x per inch of aperture, this translates to a minimum useful exit pupil of 0.5mm. However, this is not a hard and fast rule, and it depends entirely on the object. Some small faint objects can effectively disappear from your vision at 1mm exit pupil, while other small bright objects remain easily visible even as low as 0.3mm. Bright objects like planets and the moon can tolerate even smaller exit pupils, but at that point you would be pushing magnification well past the useful limit of the telescope's optics. You'd be sacrificing brightness for no additional detail.

Exit Pupil

2019-05-16T21:57:07Z

Phpdevster: /* Exit Pupil and Filters */

Exit Pupil

2019-05-16T21:54:07Z

Phpdevster: /* Exit Pupil Effects on Different Objects */

Exit Pupil

2019-05-16T21:53:47Z

Phpdevster: Changing section heading

Exit Pupil

2019-05-16T02:42:05Z

Phpdevster: WIP edits

Exit Pupil

2019-05-16T02:38:29Z

Phpdevster: Initial work in progress