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

[Bernard Maugoust <maugoust@free.fr>]

In message <5.1.0.14.0.20030523100509.00ab9ec0@popd.ix.netcom.com>, 
 Michael Peck <mpeck1@ix.netcom.com> wrote:

> 
> 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.
> >

> The first thing you should do is verify the spacing and alignment of all 
> the components. Primary-secondary separation is especially critical.

It has been checked. I knew the effect of the primary-secondary separation
before I started.

> 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.

This I have considered. When I next get on with another go at testing, I'll
try on the full moon. Stray light while present should be less of a problem.
I hope this will be part of the answer.

> 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.

It's OK.

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

And that's what really baffles me. Many thanks for your suggestions Michael.
The funny thing is that a similarly 'rough' setup for my Ritchey didn't
produce the same visual effect.

Bernard

-- 
Bernard Maugoust 3°59'E 44°28'N