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		<description>Diskussion </description>
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			<title>Alistair Gutcher schreibt:</title>
			<link>http://r2.astro-foren.com#comment-42</link>
			<description><![CDATA[Very interesting to see the TSC225 tested. But why was the Foucault test on this TSC225 done whilst the primary mirror was still cooling? I can see a rather obvious thermal plume in the image? The mirror's shape is usually different whilst it is cooling, and should not be tested until fully at ambient temperature (i.e. no thermal plume visible at all, at 700x for example, on an artificial star). Also I can see a thermal plume on the Phase-Contrast type test on the right of the Foucault test image. Was there still a thermal plume when you did the Ronchigram? In which case all of these tests that I mention become null and void, which is a great pity. Was the Ronchigram done in AutoCollimation (where the errors are doubled) or with a point source from far in front of the instrument (no reference flat mirror used) ? I am a little surprised and disappointed at Takahashi's optics (assuming that you tested in Autocollimation for the Interferogram when the primary mirror no longer showed ANY thermal plume AT HIGH MAGNIFICATION e.g. 700x, and also assuming that you are taking into account - in your software - the doubling of the errors through the instrument because of testing in Autocollimation for the Interferogram) since Tak's optics are supposed to be 1/20 lambda PV or better ! "The precision of optical surfaces is at least of λ/20" - see quote here ;- http://www.takahashi-europe.com/en/mewlon.features.php . And these Tak SCT's were 4,000 USD when on sale in the 1990's (they only made 100 of them), way more expensive than my C11 SCT. From this test I see no reason to think that Tak's optics are any better (in smoothness or surface accuracy) than a good Celestron Edge HD or Meade ACF. Tak's instruments are way overpriced, for not much or any better optics quality IMHO. Regards, Alistair G.]]></description>
			<dc:creator>Alistair Gutcher</dc:creator>
			<pubDate>Sat, 20 Sep 2014 21:33:56 +0200</pubDate>
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