Televue 5x Powermate 1.25 inch (1-1/4 in.) Brass Clamp Ring and Captive Screw Latest Version, Accepets the T-Ring 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).