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Originally Posted by Satchmo
Same.
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Well OK I accept that the result is much the same in either optic, if they depart their ideal figure by the same wavelength. But error doesn't tend to depart the ideal figure by the ideal amount. Error is error.
However I suspect just as with a spherical surface, the same zonal error in either optic, will produce different wavefront errors.
Well, I'm sure. But either the following opinion is correct or not.
http://poormanastro.tripod.com/page8.html
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That umpteen millionth of an inch error in the figure of an f4 scope might produce an error of 1/2 wavelength and create a "soft" or slightly blurry image (even when at best focus), but that same umpteen millionth of an inch error in the figure of an f15 scope might produce an error in the figure of only 1/8 wave and, thus, still produce a tack sharp image. It is simply cheaper to grind a good f15 glass versus a good f4 glass. Faults such as chromatic aberration and spherical aberration are much easier to control in a slow glass. We will touch on aberrations in more detail a bit later.
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