Televue PMT-4201 2 inch 4x Powermate with 1.25-Inch Eyepiece Adapter Vignetting, edge field aberrations and pupil movement - all introduced when using Barlows with long focal length eyepieces, are minimized. Telecentric operation (field rays leave parallel to optical axis, unlike Barlow lenses which diverge rays). This is ideal for Hydrogen-alpha filter use such as with Daystar models. High performance for image amplification, CCD or film (some of the finest Earth-based planetary imaging is being done with Powermates). Flexible for visual and imaging with all types of telescopes and eyepieces. Essentially Parfocal, and with nearly constant magnification regardless of image distance behind top surface (except 5x model which increases 1x for every 35mm of image distance increase). Higher magnifications are possible with high optical performance compared to "doubling up" Barlows. For smaller instruments, using heavy 2" eyepieces with the 4X POWERMATE may cause balance issues.