Exit Pupil - Revision history

Jmh921 at 18:10, 4 February 2020

2020-02-04T18:10:10Z

Phpdevster at 05:15, 17 May 2019

2019-05-17T05:15:10Z

Phpdevster: /* Exit Pupil Effects and Considerations */

2019-05-17T04:17:57Z

‎Exit Pupil Effects and Considerations

Phpdevster at 04:09, 17 May 2019

2019-05-17T04:09:03Z

Phpdevster: /* Exit Pupil and Filters */

2019-05-16T21:57:07Z

‎Exit Pupil and Filters

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

2019-05-16T21:54:07Z

‎Exit Pupil Effects on Different Objects

Phpdevster: Changing section heading

2019-05-16T21:53:47Z

Changing section heading

Phpdevster: WIP edits

2019-05-16T02:42:05Z

WIP edits

Phpdevster: Initial work in progress

2019-05-16T02:38:29Z

Initial work in progress

New page

==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 on Different Objects===
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.

← Older revision		Revision as of 18:10, 4 February 2020
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	===Exit Pupil vs Magnification===		===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 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.
		+
		+	==Further Reading==
		+
		+	[https://medium.com/@phpdevster/how-telescope-aperture-affects-your-view-24507147d7fc How Telescope Aperture Affects Your View]
		+
		+	[https://www.reddit.com/r/telescopes/comments/7o18zb/what_i_learned_about_observing_in_2017/ A 2017 Reddit post discussing observing and exit pupil]

@@ Line 47: / Line 47: @@
 [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.
+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===

← Older revision		Revision as of 04:17, 17 May 2019
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	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.		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.

@@ Line 32: / Line 32: @@
 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===
+==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.
@@ Line 54: / Line 54: @@
 Generally any exit pupil larger than 2mm will work with line filters, but the larger the exit pupil, the better.

← Older revision		Revision as of 21:57, 16 May 2019
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	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.		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 Filters===	+	===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.

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 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.
-===Optimal Exit Pupil===
+===Exit Pupil and Astigmatism===
 ===Exit Pupil and Filters===

← Older revision		Revision as of 02:42, 16 May 2019
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	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.		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.
		+
		+	===Optimal Exit Pupil===
		+
		+	===Exit Pupil and Filters===