Tele Vue 2.5x Powermate

It turns a comfortable 10 mm eyepiece into the equivalent of a 4 mm while keeping the 10 mm's eye relief and field. That is the practical route to very high magnification without squinting into a short-focus eyepiece.

A Powermate sends light to the eyepiece in parallel rather than in a diverging cone, so the magnification stays at 2.5x no matter how far the eyepiece sits from it. An ordinary Barlow's amplification changes with spacing, which quietly alters your magnification.

Because it does not distort the light cone, the eyepiece keeps its full apparent field and its eye relief. Short eyepieces of the same effective focal length usually give up both.

Drops into any 1.25 inch focuser or diagonal, and it also accepts camera adapters, which is why planetary imagers use Powermates to reach the f/15 to f/20 range their cameras prefer.