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Originally Posted by Merlin66
Guys,
I note in this thread no one has mentioned a very easy quick test which will show the overall figure of the mirror, and identify Spherical abb ( much easier than the star test) collimation and tube currents....
The Rochi test using a 80 or 100 line per inch grating ( photographic copies available) when placed very close to the focus ( assuming you're looking at a reasonably bright star) should show perfectly straight and equal spacing; any distortion you in the lines see is coming from the optical system/ tube and atmospherics. Easy to photograph the outcome for comparison between scopes!!
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I'd need to add that this test has fairly poor sensitivity with an F5 scope and needs to be used with caution in declaring optics good or not. You need very good seeing conditions and view with no more than a few bands visible at most . The slightest bending of the bands in ward or outward , particularly out near the edge of the mirror can indicate significant spherical abearration to affect the airy disc at high power.
Most mirrors pass this test , looking superficially straight to the layman , but particularly with larger mirrors, the bands need to be knife edge straight right to the very edge. For example on a 20 " mirror , a 'kink' in the last 15 mm of a band at the mirrors edge edge , could mean you have the light of a 4 " mirror being thrown outside the Airy pattern ..not a good look at high power, Defects like this, masked superficially by the fuzzing of the seeing are often endlessly blamed on poor seeing.
Mark