What is the lowest F/# for an 8" mirror

Could someone tell me the lowest F/# recommended for an 8" mirror
and why you would not want to go below this #. I polish optics for a
living but it has always been to customer specs and I have never gotten
into lens design. So I don't understand what kind of problems can be
caused by making the mirror to fast.

Thanks..............

The most common f-ratio for 8" scopes is f6. I think this is just because it makes a nice 4 ft focal length, convenient to put on a dob mount and get a reasonable eyepiece height. I have also seen some 8" f4 scopes, mostly for use on an equatorial mount (f4 to get tube length down to make it easier to mount). There is no true "shortest" f-ratio - you could make it as fast as you want.
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,

Jarad

This becomes an issue at or below f5. A 35mm Panoptic will have a 7mm exit pupil at f5 (or with an f4.3 scope using a paracorr). At f4 (with paracorr), you might want to make your lowest power eyepiece either a 31 nagler or 27 panoptic or similar. On the other hand, once you go above f7 or so, the 2" focuser will limit your maximum true FOV instead of exit pupil (i.e. you will be unable to reach the widest TFOV with any commercially available eyepiece).

Jarad