All kind of terrestrial viewing users, those missundertood (1 Viewer)

Certainly the Questar, the Celestron C90 and the Celestron C5 offer slightly better raw optical performance (again: resolution, contrast, color fidelity, and brightness) than any of premium 77-80mm prismatic refractors (i.e., the Kowa 77mm Fluorite, the Bausch and Lomb Elite 77 ED, the Optolyth 80mm Fluorite, or the Swarovski AT80). It is in comparing the Cats to the Tele Vue Pronto (the current reference standard in the 70 80mm refractor class) that things get complicated. Under identical test conditions, all three of the top Cats, the Questar, the Celestron C90, and the Celestron C5, yield higher resolution on the test chart than the Pronto. The C5, with its huge aperture, provides truly exceptional, one could almost say "amazing," resolution, nearly 85 times that of the naked eye. When it comes to brightness and contrast though, only the C5 can equal the Pronto at equivalent powers. Don't mistake me here. Both of the 90mm Cats provide very bright, very contrasty views ‹ better than any standard 80mm spotting scope...the kind that would satisfy even the most exacting birder...it is just that the Pronto is slightly brighter at any given power. All three of the top Cats provide better color fidelity, deeper color detail, and more intense color, than the Pronto

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In the real world, ED doublet and triplet (APO) refractors generally deliver more information to the eye and therefore an APO refractor can have a somewhat smaller clear aperture than a reflector (including catadioptrics) and still offer comparable performance. According to the results published in the article, "Aperture Comparison" by Jared Willson on Cloudy Nights, which compared 41 reasonably common astronomical telescopes, a 70mm APO refractor should provide similar low contrast resolution to a 90mm Questar 3.5 Mak-Cass catadioptric, which is about as good as mirror scopes get. Several 4" APO or ED refractors substantially out performed a Celestron C5 (5" CAT) and 6" APO refractors out performed the Celestron C6 XLT, a very good Schmidt-Cassegrain (SCT) catadioptric. A TEC 140 (5.5" APO refractor) achieved a low contrast number nearly identical to the very sophisticated Vixen VC200L 8" catadioptric.

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