Mirror F/L Question

[mick pinner]

why is it that we don't see more short f/l mirrors in the 300mm range? lets say 300mm f/3.5. are there any specific issues that would preclude these sort of focal lengths for large aperature fast imaging scopes? any comments from the mirror gurus appreciated.

[Wavytone]

I had a 32 cm f/3.7 20 years ago. It was originally intended to be used as a Nasmyth classical cassegrain but I used it in Newtonian configuration in a homemade truss tube.

However I concluded it was too short for its own good:

a) It needed a 100mm diagonal. Huge central obstruction and it really did detract from the image.

b) It was useless with eyepieces longer than about 20mm, which gave only about 60X, and a 12mm only gives 98X. It's kind of dumb to be using short eyepieces and still only getting 100X.

c) The highest power I could use it at was limited by my eyepieces - I had a 6mm (200X) - but you don't want to look through a 6 for very long, most are uncomfortable on the eye and impossible to keep clean. In theory I needed a 3mm to get to its limit, but with the big fat secondary obstruction it wasn't really worth trying.

d) Off-axis field curvature and coma are savage at this focal ratio - the useful (sharp) field of view was about 6mm diameter. Widefield eyepieces don't tolerate these aberrations well. Someone once put a Pretoria eyepiece in it (these were designed specifically for f/4 with coma correction) and this was the only eyepiece I ever saw that worked well in it. However you won't find one these days.

e) They are very hard to figure to acceptable limits.

After the scopes I've over 35 years, I still think f/7 is the sweet spot for Newtonians - small secondary, can reach the resolution limit, and can fill a 50mm eyepiece.

At the time mine was made there was a twin at f/3.6, I think I know who has it, too. But it hasn't seen light in a scope.