I just bought a Newtonian on Astromart. In-focus star images are nice points. But out-of-focus images are ellipses rather than circles. Does this symptom point to a specific adjustment needed (ie primary mirror, secondary mirror) or might there be several causes?
Elliptical Out-of Focus Star Images
Started by dborenstein, 12/01/2011 07:18PM
Posted 12/01/2011 07:18PM
Opening Post
Posted 12/01/2011 08:11PM
#1
David, If they form ellipses elongated 90° in different directions, in and out of focus, I believe that indicates astigmatism. The problem might be with the diagonal mirror being too tight in it's cell or the primary, for the same reason. If neither is being held too tightly I'd first suspect the diagonal as astigmatic, then the primary.David Borenstein said:
I just bought a Newtonian on Astromart. In-focus star images are nice points. But out-of-focus images are ellipses rather than circles. Does this symptom point to a specific adjustment needed (ie primary mirror, secondary mirror) or might there be several causes?
Let us know what you find out.
Darian R.
Posted 12/02/2011 02:51PM
#2
According to the operating instructions for my Cheshire sight tube, elliptical out-of-focus star images is a first and definitive test for any kind of mis-collimation. Put another way, if out of focus star images are circular, chances are the scope is properly collimated and no further adjustment is necessary. I guess now I have to find out which mirror needs a tweak.
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