Properly designed Barlows do wonderful things: They amplify power, slow the telescope's f/# (improving eyepiece sharpness), and give designers the opportunity to compensate for eyepiece aberrations. Tele Vue Barlows use multi-coated high index glasses for optimum aberration correction, exceptional contrast with virtually no light loss. Observed performance is aberration free, even when tested with f/4 scopes. They even improve our competitor's eyepieces and telescopes!For astro-imagers, the above chart indicates the magnification for each Barlow (2x, 2x Big & 3x) as a function of distance from the Barlow top surface to the imaging surface (CCD or film). 1.25" Barlows Offered in 2x and 3x magnifications, these 1.25" Barlows perform identically. Simply choose either or both based on your eyepiece collection. Features include parfocal-ization, 1.25" filter threads, captive lock screws that won't fall out and full multicoatings. 2" Big Barlow Offered in a 2x magnification, the Big Barlow is ideal for Newtonian scope owners. It will be parfocal with both 2" and 1.25" eyepieces in this application. Features include threads for 48mm filters, captive lock screw and full multi-coatings.