My fastest scope is f/3.3 but it's a reflector. Of my refractors, the fast ones are f/4.9, f/5, f/5.3 and f/6. First, if you get a fast scope that is not a flat field quad, I suggest adding a flattener to a fast triplet for visual. Your eyepieces are not going to correct scope FC, unless by happenstance your eyepiece has some intrinsic FC that happens to cancel scope FC. You can't readily know that from specifications. There are other issues wrt the fast light cone and for that, TeleVue Delites, Naglers, Panoptics, Ethos do well. I assume Delos are equally effective in fast scopes; I've just never looked through one. But since TeleVue scopes are fairly fast, it's natural their eyepieces perform in them. Takahashi UW are outstanding. Baader and Leica zooms cope with fast scopes when they have the focal range for those eyepieces to reach focus. The Pentax XW range are more than competent. At short FLs, the Takahashi TOE are exemplary. But the best thing you can do is start by using a visual flattener to make it easier on the eyepiece, if you haven't chosen a fast, flat-field quad. On my f/6 & f/7.5 LOMO triplets, I use a TSFlat2.
Edited by 213Cobra, 10 November 2019 - 08:51 PM.