Telescope Reviews: Why do I want to image at a faster f ?

No I can't It is far too complicated for a yes/no answer.

You can't talk about f ratio alone. It works in conjunction with aperture, pixel size, chip size, focal length, camera electrical characteristics etc. Change focal length alone and you get one result. Change aperture alone you get another result, change pixel size alone and yet another result etc. Change two or more and things get complicated fast!

Keep in mind pixel size is a very important part of this. A 6" f/8 scope with an 8 micron camera will be identical in all respects to a 6" f/4 scope with a similar camera that has the same number of 4 micron pixels. No difference. Sub exposure time will be the same. Total time will be the same etc.

If you keep everything but focal length the same then the shorter focal length will need shorter subs. At f/4 they would be one forth the time needed at f/8 to be photon (sky) limited. But for the same signal to noise ratio of the image the total time governs not focal length. Though the shorter focal length will hide some of the noise due to lower resolution of both noise and image detail. This loss of resolution is at the gain of field of view. This may or may not be a worthwhile trade depending on what you are trying to accomplish. Faster subs helps with poorer polar alignment and lesser mounts. This too may be worth the trade of lower resolution.You can't talk about f ratio alone. It works in conjunction with aperture, pixel size, chip size, focal length, camera electrical characteristics etc. Change focal length alone and you get one result. Change aperture alone you get another result, change pixel size alone and yet another result etc. So with a shorter focal length you need more but shorter subs to get the same total time. But a low read noise camera can similarly reduce the sub exposure time needed to be sky limited gaining both shorter subs and no loss of resolution.

Oh yes, since stars cover several pixels they react the same as extended objects as f ratio changes may or may not change the number of pixels their image is spread over, same as extended objects.

In many cases, not all, fast systems cover a wider field of view at lower resolution than slow systems. You gain field and lose resolution. It won't necessarily speed up total image time, stars or extended objects for the same signal to noise ratio. Aperture is usually the main controlling factor for exposure time, this is why observatories are building larger and larger mirrors, not necessarily faster ones. Large pro scopes tend to be faster because of mechanical issues as these monsters get longer.

No matter the f ratio or most other factors the more total time used to create an image the better the signal to noise ratio. So more time is always of benefit but you do reach a point of little gain. 4 minutes of total time has half the noise of 1 minute, 16 minutes halve it again, as does 64 minutes, then 4 hours, then 16 hours then 64 hours but going to 65 hours has no significant change, for that matter going from 16 to 17 had a lot bigger change but it too would likely not be visible. Only an imager can say when enough is enough.

Rick