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Old 18-05-2018, 08:18 PM
Wavytone
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Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
Billyboy,

I'd ask you think carefully about what you intend to use this for... I've used a 20" and 30" cassegrain before and apart from the planets there's photometry but not much else these were good for - the field of view was way too small, and the focal ratio too slow. I'm not sure if any amateurs do photometry anymore, either. It might be better left as a Newtonian. If you just want a planet-killer a 12" schiefspiegler offers more than sufficient aperture, adequate light grasp and is easier to make.

There is a reason why you will probably be disappointed by a gregorian - the monochromatic off-axis aberrations of the secondary mirror add to those of the primary - rather than to some extent correcting them as in the case of a classical cassegrain. Even if the spherical aberration is perfectly corrected on axis, the off-axis astigmatism and coma are much worse than in a classical cassegrain.

I would encourage you to make a classical cass instead, the secondary isn't impossible to make or figure with patience if you have the scope essentially finished to the point you can star test.

The Nasmyth idea is good, and on that size I expect you would be riding as a passenger on the mount and using electric slewing. The focal length of a 20" will be of the order of 6-7 metres which poses a significant challenge however ... I found non-tracking altaz mounts infuriating to use with a mak of focal length of just 2700mm, so at 6000-7000mm the problem just gets worse and you will have to make this track and probably GOTO just to be useable.

Last edited by Wavytone; 18-05-2018 at 08:30 PM.
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