Forgive the newb question, but what are the trade offs between these ~100mm scopes with longer focal ratios? Most of the 4" refractors I see come in at the f7 range. From my limited understanding a longer focal ratio means you get higher magnifications, but also means you get a narrower field of view and less light over all (slower for imaging). Practically speaking, what's the effect on viewing? If I had to guess planets would be much better with the longer focal length, as they are so bright, but you'd have trouble with DSOs like galaxies as you have a relatively small aperture for such a long length. Do I get it or not? Thanks!
A difference in focal ratio between two telescopes of identical aperture means nothing for visual observing, as long as you can get to the same magnification. 50x on a 4" is the same on a f/7, as it is on a f/11. The views will be practically identical, everything else equal. You just need two different eyepieces to get the same magnification. On the f/7, you need a 14mm, on the f/11, you need a 22mm. Image brightness is the same in both telescopes, if you keep the magnification the same.
In practice, the longer f/ratio will be much more friendly to the eyepieces and you can make do with considerably less advanced eyepieces and still get sharp stars across the entire field or very nearly so. But it doesn't stop there, as the performance of advanced eyepieces is also a lot better and some of the best ones can have truly mindblowing performance, when used in long-focus telescopes. In my 4" f/11 ED, my 17mm ES92 delivers the most incredibly tiny pinpoint stars across the entire field. Faint stars are literally pinpoints, when they slip behind the field stop.
Deep-sky performance is also not hurt at all, as long as you're not trying to look at the largest possible objects at the largest possible exit pupil. Here a shorter focal length scope will shine, but for all "normal" Messiers and bright NGC, an f/11 works perfectly fine. My 4" f/11 can reach a maximum possible field of 2.4°, easily large enough to frame most Messiers, except M31.
Personally, I really enjoy my 4" f/11 and don't find it to be a hassle to deal with at all, despite its length (which I find quite moderate, actually). The tube is very light and can be considerably shortened during transport. The views are really, really good. It somehow feels much more "relaxed" than the f/6 - f/8 refractors I've tried. Focus is easier and much less critical; my eyepieces work better; my binoviewers works better; prism diagonals work better; it just plain works.
Clear skies!
Thomas, Denmark
Edited by Astrojensen, 18 February 2020 - 02:14 PM.
