Re: ATM Cassegrainian coudé: weird image at focal plane

[Bernard Maugoust <maugoust@free.fr>]

In message <007901c323d0$841f0120$b3813f42@p120>, 
 "Bob May" <bobmay@nethere.com> wrote:

> 
> If you're getting an image of the moon nicely (as you have stated) on a
> piece of paper and adding in the tertiary mirror to that setup won't
> produce a proper image again, then the tertiary mirror has to be at fault.

We're talking about a Quartz flat (1" minor axis) from Protostar...

> That mirror at 45 degrees does nothing (or should do nothing) to the light
> cone but change its direction, most assuredly not changing the focal length
> or other condition of the light cone. In any optical system, one of the
> accepted things is that a flat surface can be inserted anywhere and the
> only restrictions to the optical system is that the flat surface will
> obstruct the light going from one place to another. Needless to say, the
> requirement for the flat surface is that it is indeed flat and not some
> gentle curve.  Even if the surface is a smooth spherical surface, the end
> result is astigmatism from the curvature acting on the bouncing of the
> light cone.

Well, I have checked it last fall in a newtonian setup together with the
primary. No problem at all with them. The focal plane is where expected and
there is definetely an image there too. I'll check again once what I say
below is done.

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.

Cheers,

Bernard

-- 
Bernard Maugoust 3°59'E 44°28'N