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Ronchi testing v's star test
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!!
BTW it also is a great focussing aid; when positioned exactly at the focal point the lines disappear and a uniform grey is seen.
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