ATM Re: ATM Cassegrainian coudé: weird image at focal plane

[Michael Peck <mpeck1@ix.netcom.com>]

At 20:24 27-05-03 +0200, Bernard Maugoust wrote:


>I'm going to investigate the baffling issue (as mentioned by Mike Peck). Step
>one will be to make up the baffles (for the secondary and the flat) and see
>if it brings any difference. I'll do the testing with the moon as well as on
>distant objects in full daylight. And see what a search on 'nasmyth baffling'
>brings up.
>

I think baffling for a cassegrain with a Nasmyth flat should be the same as 
for an ordinary straight-through cassegrain, except of course that you have 
to provide a hole in the side of the primary baffle for the light from the 
tertiary to get through. I've shown how to calculate baffle dimensions for 
a conventional cassegrain using simple algebra. Just follow the links.

As for the problems finding an image you described earlier -- the math 
doesn't lie. If you've worked out the design correctly and the component 
parameters match the design within reasonable tolerances you will get a 
real image where you expect it to be. If you can't find it and you're 
confident no parameter was mismeasured then it's most likely that the image 
is just being swamped by stray light. I don't think the problems you 
described could be due to poor optical quality in any of the components.

Reassembling the telescope with at least partial baffling and testing it at 
night on some bright & easy to find object seems like a reasonable plan

Mike Peck

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Michael Peck
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