Astronomy Hacks

Use a Barlow

Double your eyepiece collection on the cheap.

A Barlow lens (named for the 19th-century British mathematician and optician Peter Barlow) is one of the most useful accessories you can have in your eyepiece case. A Barlow fits between the telescope’s focuser and the eyepiece, where it increases the magnification provided by the eyepiece [Hack #47]. A Barlow lens has negative focal length, which means it reduces the convergence of the light cone arriving from the primary mirror or objective lens, effectively increasing the focal length of the scope. Because the magnification provided by an eyepiece is directly proportional to the focal length of the scope it is used in, using a Barlow has the effect of increasing magnification.

Although it’s possible to describe a Barlow by its negative focal length—just as an eyepiece is described by its positive focal length—it’s more convenient simply to describe it by its strength or amplification factor. Barlow amplification factors range from 1.5X to 5X or more. A 2X Barlow, for example, effectively doubles the magnification of any eyepiece it is used with. (So-called “zoom Barlows” with variable amplification factors exist, but most of these are of low quality.) Figure 4-4 shows a selection of high-quality Barlows. From left to right are a Tele Vue 3X, an Orion Ultrascopic 2X, and a Tele Vue Powermate 2.5X.

Figure 4-4. Tele Vue 3X, ...