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Originally Posted by Mark Suchting
Dennis
Unfortunately the book only shows simple cases ie spherical aberration. A depressed or raised zone around the secondary area can have the same effect superficially on th extra focal discs in the star test even though the rest of the mirror is good. A ronchi grating can help distinguish overall Spherical Aberration from more localised defects if you are not experienced at star testing.
Mark
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Thanks for the cautionary words of wisdom Mark, I guess sometimes a little knowledge can be dangerous in this area.
Cheers
Dennis