Powermates are the highly-evolved answer to the Barlow lens design from Tele Vue.
While barlow lenses increase the eye relief of the eyepiece in use, they also reposition the exit pupil of long focal length eyepieces far from the designer's intended position. This can cause vignetting and image degradation.
Using four elements, the Powermates leave the exit pupil where it was intended to be, restoring the full viewing aperture and optical quality of long focal length eyepieces. Have the best of both worlds; the long eye relief of the longer focal length eyepieces, but with higher magnification!
This Powermate is parfocal in all 1.25" diagonals & prisms, and is thus highly recommended over standard barlow lenses for refractors with 1.25" focusers.