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dothead
super member
Reged: 08/13/08
Posts: 163
Loc: Heidelberg, Germany
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Re: FET-300/EM-3700
08/15/08 11:45 AM
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Dave,
Immaculate translation (but please don't tell the kids!).
More seriously, I totally agree with the points you made on the big Takahashi/Pentax vs TMB/APM apos.
As Daniel Mounsey very rightly pointed out in his thoughtful first post in this thread, once you go to 10 inches or bigger with 3 element apos, the thermal/cooldown issues become the dominant/limiting factors.
Markus Ludes has put his 12 inch TMB for sale because he discovered that it didn't deliver the exquisite views he expected! Reason: The massive 12 inch 3 element lens just couldn't cool down fast enough to keep track with the often rapidly falling post-dusk temperatures here in Germany. So the sperical correction just almost never was good enough under the real life obseving conditions at his observatory. Such a scope would fare much better at a location in the tropics with minimal temperature changes. The TMB/APM 21 inch APO (elements) just isn't a practical telescope almost anywhere in the world!
A Petzval system with two front and two rear elements would come much nearer to solve the cooldown/sperical correction issues.
Just my two cents!
Ralph
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