Mirror question

[TimmySand]

Could someone tell me about what f/ratio spherical aberration would be unnoticable or minimal for a spherical mirror?

Also, would spherical aberration be worse at lower magnifications?

[RickJ]

It depends on the size of the mirror. 4.5" is about f/8 though f/9 is better. 6" is usually said to be f/10 but my f/12 easily beats the f/10 mirrors I've seen. After that I can't remember as things get so long they are almost impossible to mount without folding the optical path.

If you work below these f ratios even at low power you will notice stars don't snap to focus and are soft looking. The higher the power the more objectionable this is of course. To me it is very obvious even at low power in a spherical 6" f/10 but even high power is fine at f/12. It depends on your tolerance for such things I suppose. I built that f/12 6" back in 1954 so over the years I've gotten rather picky.

Rick

[Dave Mitsky]

A 200mm spherical mirror with a blur size about equal to that of the Airy disc is generally considered acceptable visually if it is not any faster than approximately f/12. This mirror would exhibit only 1/10 wave of spherical aberration. (An f/8.9 mirror would have 1/4 wave of spherical aberration.) A 150mm f/12 mirror would have an extremely high Strehl ratio of 0.98.

Actually, it's high magnification performance that suffers with spherical aberration. In the case of coma, the opposite is true with a paraboloid.

I own a 150mm f/10 Dob with a spherical mirror that is really quite good visually, although it is somewhat awkward to use at times due to the long OTA.

Dave Mitsky

[RickJ]

Dave,
The formula I have in my notebook from decades back is:
22.55*K*D/(F^3)

K=1 for a sphere, D diameter in inches and F the focal ratio

You seem to be using 29.39 rather than 22.55. I have no idea where my formula came from. I'm not sure why the difference.

Rick