# Barlow lens

Also called: Barlow, Powermate, focal extender.

A lens that goes before the eyepiece and multiplies its magnification, usually 2× or 3×.

A 2× Barlow turns a 10 mm eyepiece into the equivalent of a 5 mm, effectively doubling your kit. Good Barlows add little light loss, and they keep the comfortable [eye relief](https://astropilot.space/glossary/eye-relief) of longer eyepieces. Tele Vue's Powermates are a refined (telecentric) kind.

A Barlow is most useful when it fills a gap — a power none of your eyepieces gives. If it only duplicates one you have, the eyepiece alone is simpler and has less glass.

**In AstroPilot:** AstroPilot suggests a Barlow only when it gives a power your eyepieces can't on their own.

**Related:** [Magnification](https://astropilot.space/glossary/magnification.md), [Eyepiece](https://astropilot.space/glossary/eyepiece.md), [Coma corrector](https://astropilot.space/glossary/coma-corrector.md), [Focal reducer](https://astropilot.space/glossary/focal-reducer.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/barlow