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

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

At 10:26 23-05-03 +0200, Bernard Maugoust wrote:


>- with a simple magnifying glass acting as an eyepiece I cannot easily locate
>the focal plane, as one would expect in that sort of test.
>
>- a smoothed piece of glass would not 'stop' and show the image either,
>hinting at some abnormality with it.
>
>- the best way to describe what my eyes painstakingly saw is that the image
>seemed to be contained in a tiny area, like the ball of a pen on which my eye
>had to 'roll'. I was most often aiming at a convenient TV mast, of which I
>could see just as much as my anticipated field of view, that is 15' of arc.
>This apparently correctly sized field is worth noting IMO.
>

I wondered if you would get around to expanding on your "weird image" 
complaint.

The first thing you should do is verify the spacing and alignment of all 
the components. Primary-secondary separation is especially critical. A 
change in that spacing shifts the cassegrain focal plane by an amount 
proportional to the *square* of the secondary magnification. In your case 
the secondary magnification is almost 5, so a 1 mm error in the 
primary-secondary separation will produce a nearly 25mm shift in the focal 
plane.

Another possibility if you've only tested in daytime with no baffling is 
that you're just being overwhelmed by stray light. That might cause you to 
have just a small central area of the field where focussed light 
"outshines" stray light.

You should also verify that the tertiary isn't a source of vignetting. You 
should be able to see the whole secondary when you place your eye at the 
position of the focal plane.

Field curvature is not going to cause the effect you describe and it 
doesn't sound like any other optical aberration.

Mike Peck

By the way that arrangement is called a Nasmyth focus. A coude focus is 
something different. Just a hint for more productive google or archive 
searches.


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Michael Peck
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