If you’re looking up the night sky and you want to see something twice its size, you may want to invest in a Barlow lens. The lens, which was created by an English engineer named Peter Barlow, is put between the telescope’s mirror and the telescope’s eyepiece so it can double the eyepiece’s focal length. If you’re a budding astronomer, a good Barlow lens is important to your eyepiece collection. Since it can double the magnification, it can save you the money of buying more eyepieces.

There are many advantages to buying a Barlow lens, such as being able to attain higher magnifications, having an effective increase to the focal ratio, and having an increased eye relief, which is the distance from the lens needed to achieve focus for the eyes. There’s barely any disadvantage to the lens, other than the decreased brightness in the produced image.

When choosing a Barlow lens, remember to look for one that fits the barrel size of your current eyepieces and the size of your focuser. You don’t want to end up buying the wrong thing for your telescope, after all. Some Barlow lens can also provide triple magnification. There are even adjustable kinds. Choose the lens that you think you’ll need astronomically.

Assembling a Barlow Lens