Quote:
Originally Posted by Satchmo
Absolutely. 0.5 wave of Spherical Aberration on an unobstructed system yields a Strehl ratio of around 60% regardless of F#.
Another factor that may contribute to the reputation of slower mirrors is that a fast one takes a lot more figuring . If the optician does not have good skills the surface roughness of the optic will build up. A well made fast optic is technically capable of just as fine planeatry images albeit with a slightly larger secondary , although for visual use this is only a slight increase.
Another way to look at it is that if you are capable of nailing the parabolic correction to within 10% then the mirror may be better than 1/4 wave for one with a 2.5 wave required correction. However a 10% error on a large fast mirror requiring 20 waves correction will result in a 2 wave error.
In my view the reason some large fast mirrors fail spectacularly is a residual under-corrected edge, that is often hard to read with a foucault test. This residual can be a steeply sloped as the remnant of the original sphere, causing a very poor star test. Such an error would have to polished onto a slower mirror but is much more likely to remain on a fast one as a job incomplete.
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Let's rephrase this. Are you saying that an F4 with a spherical mirror, will reveal no more final error than a F12 with a spherical mirror?