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Huh ?!? I'm using an iMac and the link is to a pretty ordinary page of HTML...
The gist of the article is that the expansion co-efficient of the glass types used in modern f/7 ED and fluoro objectives is about 18X the expansion coefficient of the glass types used in f/15 refractors up to the 1990's. The consequences of thermal in-equilibrium are pretty dire when the thermal expansion easily exceeds the thickness tolerance to achieve the "diffraction limit. The author even considered the differential expansion between the centre and edge of a typical element.
Basically this is one of the reasons why the modern short f/7 ED refractors don't perform anywhere near as well as the old-school f/15 ones do, at the same magnification.
If you're making a 150mm reflector stick to f/12, a Newtonian with small secondary will be superb.
Last edited by Wavytone; 17-04-2013 at 03:45 PM.
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