Quote:
Originally Posted by Paul Haese
No scope larger than 10 inches will be at ambient inside 4 hours without active cooling. Even with fans the mirror gives off heat for hours.
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That is plain and simply not correct and to be honest it's "old school thinking". It certainly holds true for "full thickness mirrors" but things have moved on a long way from there.
I have an 18"/F4.5 Obsession with a 2" thick OMI mirror and a 14"/F4.5 SDM with a 1.3" thick Zambuto mirror. Both scopes are fitted with mirror cooling and laminar fans. The 18" scope because of it's thicker mirror can invariably take forever to cool and during summer nights at elevation with rapidly falling temperatures the mirror never catches up to the ambient temperature. After a few years experience with the 18" scope and it's cooling properties (or lack of them) I set about setting the specs for the 14" mirror and scope, to address this issue. The cooling properties of the 14" scope, with it's thinner mirror, are world's apart by comparison. The mirror cools so well in fact that I quite often don't bother to run the fans for more than about 30 minutes. Similarly, some of the larger conical mirrors produced by Mark Suchting and Bob Royce in the US address the cooling issue very well.
There were many opinions these thinner mirrors, which are becoming common place today, were astigmatic, particularly when viewing near the horizon. The truth of the matter is this is a non issue if the mirror cell is properly designed and built with a stainless steel cable sling. Further, when I do observe objects low in the sky which is very rare because the air is crud down there, the seeing and atmospheric effects clearly outweigh any astigmatism that might be introduced.
In short, if you design and build the scope properly mirror cooling becomes a much less significant issue. In addition, scope cooling can be further assisted by intelligent storage, transport and scope setup practices. The 14" SDM scope is "easily" the best planetary scope I have used. My planetary scope usage range covers refractors (apo and achro) from 50mm to 378mm, reflectors from 76mm to 915mm and catadioptric and compound scopes from 102mm to 400mm. The 14" SDM with thin Zambuto mirror and 18% central obstruction wins in a canter.
Cheers,
John B