Benefits of Barlows

One of the more useful and cost-effective tools in an amateur astronomer's accessory case is a barlow lens. Invented in the early 1800s by British mathematics professor Peter Barlow, it is a simple concave (negative) lens that, when placed between a telescope's objective lens or mirror and the eyepiece, amplifies the magnifying power of the instrument.

If the standard eyepiece in your scope yields a magnification of 50x, a 2x barlow will boost that to 100x. If a high-power eyepieces in your scope yields 150x, insertion of a 2x barlow lens before the eyepiece will boost that to 300x.

A barlow works by reducing the convergence of the light cone heading toward the eyepiece. In this way it increases the focal length of the telescope. Since magnification is determined by dividing the telescope's focal length by the eyepiece's focal length, you can see that by doubling the telescope's focal length, a barlow lens doubles the magnification of the system for a given eyepiece.

A barlow lens is useful in several ways. First, it allows you to reach a higher maximum power. This is not always very beneficial, however, because the maximum useful power is generally limited not by the telescope or eyepiece but by prevailing atmospheric conditions ("seeing"). If atmospheric turbulence renders the image wavy and fuzzy at 150x, doubling the power to 300x is not going to help; it'll only degrade the image even more.

Barlows are particularly useful with telescopes that have short focal lengths. Such scopes often do not reach high powers even with fairly short focal-length eyepieces. A barlow can increase the magnification and allow the scope to achieve its maximum useable power.

Perhaps the biggest benefit of a barlow is that it doubles the number of magnifications available to you, effectively doubling the number of eyepieces in your repertoire. If you have 26mm, 18mm, and 10mm eyepieces, for instance, adding a 2x barlow will allow them to function as 13mm, 9mm, and 5mm eyepieces — like getting three more eyepieces for the price of one barlow (which often costs less than one eyepiece).

A less obvious but very nice perk of barlows is that they can make high-power viewing more comfortable. High-power (short-focal-length) eyepieces often have very little eye relief, which requires that you position your eye very close to the lens to see the image comfortably. But a barlow allows you to achieve the same magnification with a lower-power eyepiece, which typically has more eye relief. This can be a real benefit for eyeglass wearers, enabling them to see the whole field of view at higher powers, when normally they cannot.

Lastly, a precision-made barlow with antireflection coatings can actually improve eyepiece performance, providing sharper images and reducing edge-of-field optical aberrations at the expense of only a two or three percent reduction in image brightness. In fact, many deluxe high-power eyepieces contain built-in barlows to achieve their high magnifications.

Choices in Barlows
When selecting a barlow, the first thing you need to determine is the barrel size of the eyepieces you intend to use with it. (The barrel size is the diameter of the eyepiece tube that drops into the focuser. The most common barrel size is 1.25"; some larger telescopes and some refractors can also use 2" eyepieces, and some inexpensive telescopes use smaller 0.965" eyepieces.) Whatever the barrel size of your eyepieces, you'll want to select a barlow of the same diameter.

Barlows also come in different magnifications. The most common barlow magnification is 2x, which means that it doubles the power of any eyepiece with which it's used. There are also 3x, 4x, and 5x barlows, and even some in-between magnifications such as 1.5x and 2.5x. Some barlow lenses feature variable magnification, usually from 2x to 3x. They provide an even greater range of effective focal lengths from your eyepiece set. However, because variable-power barlows require extra focus travel, they may not work with all telescopes.

Make sure you get a quality barlow lens that is fully antireflection coated or fully multi-coated. The cheap barlows included with low-end department-store telescopes are virtually worthless.

Using a Barlow Lens
A barlow is very simple to use: It is inserted in place of the eyepiece, and the eyepiece is then inserted into the barlow. When the barlow is inserted into the optical path you will need to refocus, and often the change of focus required is quite large. To avoid excessive refocusing, therefore, you may find it easiest to sequence through your eyepieces without the barlow, then, if the object and conditions warrant higher power, insert the barlow and run through your eyepieces again starting with the longest focal-length (lowest power) eyepiece. As with all visual observing, don't increase magnification to the point that the image becomes fuzzy. Once the image becomes fuzzy there is no more detail to be seen, and such empty magnification makes it more difficult to discern detail.

Use of a barlow should be factored into your choice of eyepieces. If you're just starting out, you'll find that most telescopes come with a single low-power eyepiece, often of about 25mm focal length or thereabouts. A good strategy is to purchase a 2x barlow and a second eyepiece of about 1/3 the focal length of your first eyepieces. (If your first eyepiece is a 25mm eyepiece, your second eyepiece would therefore be in the range of 8mm to 10mm). Those two eyepieces and a single barlow will give you a range of four different magnifications, from low power to relatively high power — a very good way to get started.