Quote:
Originally Posted by Satchmo
As I understand it now there is no point in paying for large fast highly corrected instruments . A 20" F5 with coma corrector will produce similar results to an exotic design 20" F2.5 in the same exposure time for 1/10 of the cost, with a narrower field of view.
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That's correct. The f/2.5 won't go any deeper.
As you know the rule doesn't apply to photographic emulsions. The f/5 will require 4X the exposure.
A good example of Quantum Mechanics at work.
Regards
Steven