Barlow Lens

Specifications

Observing parameters

Overview

A Barlow sits between the focuser and the eyepiece and spreads the light cone so that it comes to focus further back, effectively lengthening the telescope. A 2× Barlow doubles the magnification of every eyepiece you own, which is the cheapest way to get a high-power set.

It also increases eye relief relative to a native short eyepiece, so a 2× Barlow with a 20 mm Plössl behaves like a 10 mm with comfortable viewing. The trade is slightly reduced transmission and a longer, more top-heavy stack.

Quality matters more than multiplier: a cheap 3× or 5× Barlow is usually a way to produce a dim, mushy image. Stick to 1.5×, 2× or 2.5× from a reputable maker, and prefer an apochromatic (ED) design for imaging.

Where it fits

Practical notes

• A Barlow plus a mid-length eyepiece beats a very short eyepiece for eye relief every time.
• Be realism about the seeing: doubling magnification on a turbulent night doubles the mush.
• For imaging, an apo (ED) Barlow avoids the colour fringing a cheap achromat introduces.
• A 2″ Barlow lets you use long-focal-length 2″ eyepieces at higher power without vignetting.

Pairs with

• Plössl eyepiece
• Wide-Field Eyepiece
• Star Diagonal
• CMOS Camera