[APML] F-ratio to Star Image Size

[Glenn Ray <Glenn.Ray@aspentech.com>]

I should know the answer to this, but here goes:

If I increase the f-ratio of a lens--thereby reducing the aperture--and
increase the exposure length accordingly, will I be able to reduce star
image size significantly? 

I assume that reducing the aperture alone does exactly that (reduce star
image size), but a longer exposure means recording more atmospheric
turbulance and guiding errors.

While mathematical theorems are welcome, I'll trust just about anything you
guys say. :-)
See the following image for the motivation behind this question:
http://www.geocities.com/glennlray/Astro/Pleiades-20000826.jpg (400mm f/5.6;
30 minutes).

Glenn Ray
Cypress, TX
http://www.geocities.com/glennlray/Astro/

"Though my soul may set in darkness, it will rise in perfect light;
I have loved the stars too fondly to be fearful of the night."
from "The Old Astronomer to His Pupil", Sarah Williams (1837-1868)



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