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Old 25-09-2012, 03:09 PM
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sopticals (Stephen)
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Join Date: Jun 2010
Location: Oamaru, New Zealand.
Posts: 226
Smile 25.2" mirror back to the figuring block.

Hi again,

Recent star testing of my 64cm mirror revealed two areas in the figure that need further work. From the 88% radius, some rolling down of the edge exists, and the central 55-60% of the mirror is still undercorrected. Interestingly enough, when I fitted a 22" aperture mask, (thus hidding the turned down 12% of the radius), I was intrigued by what was seen at the eyepiece, (ES82deg 24mm giving 125x). When viewing through the eye lens center, was unable to get a clear focus, but when viewing through the ocular nearer the edge of the eye lens, a sharp focus was possible, and star images were decidedly smaller than those of my 14" dob, with double stars being separated by a larger margin, when similar magnifications used in both scopes (and my 14" is no slug). Aperture does rule, even when the larger is considerably short of being perfect. Currently I rate this mirror as a 2/10. So some more work on the figuring block. Again I noted how much more sensitive the Ronchi (133 lpi) eyepiece star test is compared to the Ronchi (65 lpi) bench test at ROC.

PS. Currently would rate my other two mirrors: 22.2" f5.36 @ 3.5/10 and 22" f5.23 @ 5.5/10.

Last edited by sopticals; 26-09-2012 at 08:02 AM.
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