Barlow

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 Plossls, 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.