Posted By Jarad Schiffer
The issues that go into deciding on an f-ratio are:
1 -Tube length. Longer f-ratio, longer tube, more weight, more leverage on the tube requiring a beefier mount, etc.
2 - Maximum FOV. Shorter f-ratio give a wider maximum FOV for a given focuser size (1.25" or 2").
3 - Secondary size. The longer the f-ratio, the smaller the secondary can be made while giving adequate illumination. Note that there are limits to this, as discussed in another thread in this forum.
4 - Coma. As you get to fast f-ratios, coma increases. This can be dealt with by using a coma corrector down to f4 or so, but that adds expense.
5 - Expense. It costs more to make a fast mirror to high standards, and the fast mirror will need things like a coma corrector, and expensive eyepieces to handle the steep light cone without astigmatism, etc.
There are trade-offs either way. Most 8" newtonians I have seen are f6. I have seen a few at f8 that were optimized for planetary viewing, and a few at f4 that were optimized for wide field viewing and/or photography. I haven't seen any faster than f4, but that doesn't mean you couldn't make one. It would be pretty much a dedicated wide field scope, though, with a large secondary.
Hope this helps,