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the infamous star test...

Started by k4pdm, 12/02/2004 03:05PM
Posted 12/02/2004 03:05PM Opening Post
Hi all! I recently purchased a Hardin DSH-8 to replace my Coulter 10-inch that did very poorly on the star test--the image was fair on one side of focus, very rough on the other. My Hardin shows an excellent image on one side, but somewhat rough on the other, although better than the Coulter. Can a poor eyepiece or poor seeing cause this? (I wouldn't think they would affect one side of focus and not the other, but I don't really know.) I know that no inexpensive dob is perfect, but I'd just like to know if mine is up-to-par. How does everyone else's look? Thanks for any help you can give!-Paul M. :S
Posted 12/02/2004 03:28PM #1
Keep in mind it's getting to be winter and temperature can be a factor. A mirror that is not sufficiently cooled off will be somewhat the wrong 'shape'. Since it cools from the edges inwards, temperature differentials can mimic a turned edge, or undercorrection, or zones. The star test is a pretty stiff test, be sure that the scope has been outside for an hour or two before doing it.
Posted 12/03/2004 09:20AM #2
Paul, I had a 14" Pegasus mirror that I could never get to star test well inside of focus--the outside focus image was excellent, but inside always looked like a mess, so much so that I suspected a turned edge (gasp! on a Pegasus)--but I was completely wrong. A ronchi test showed the mirror had an excellent figure and no turned edge, and correspondence with Mr. Hall confirmed good specs on the mirror, which I had obtained second hand. More to the point, the mirror provided crisp beautiful views of Saturn and Jupiter. So I can only conclude the star test can be difficult to perform well--in particular it's almost pointless in poor seeing--and one should not give up on a mirror without other supporting evidence.

George Golitzin