Re: [ATM] Baffling a Gregorian and corrector plate fabrication.

[Mike Lockwood <melockwo@uiuc.edu>]

Doug,

Douglas S Angle wrote:
> One thing I can't figure out is how to baffle such that stray light
> can't get from the entrance aperture to the eyepiece.  There's an
> article in ATM about a newt-cass-gregorian telescope, but the
> article doesn't address baffling at all, and the pictures don't
> look like there's any attempt to baffle.  I did a full size 
> drawing, and I can't see how to catch more than about 1/2 the stray
> light. I would be interested in hearing from anyone who has built a
> gregorian, or advice on how to solve the baffling problem.

The only practical way to catch ALL of the stray light is to increase 
the central obstruction with a conical baffle around the secondary. 
(Yes I know about Ramsden disks, but I don't consider them terribly 
practical.)   The size of the secondary baffle depends on the geometry 
of the instrument and the focuser baffle dimensions.  The stray light 
is the price you pay for a conveniently sized instrument of long focal 
length with a reasonable central obstruction.

My 12.5" F/12.5 classical Cassegrain has no baffle around the 
secondary, and a good amount around the tertiary.  Using the tertiary 
to direct the light path to the focuser on the top of the mirror box 
in itself performs some baffling, because the view of the sky through 
the eyepiece is limited to the reflection off the tertiary. 
Otherwise, the eyepiece sees the bottom of the mirror box.

The tertiary assembly, as viewed looking down from the front of the 
scope, can be seen in Figure 16 of this article:
http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/12in_cass/cass_article/cass.html
or just the photo for those with dialup:
http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/12in_cass/cass_article/tertiary.jpg

Since that photo was taken, the tertiary holder assembly has been 
enclosed in a 3" pipe with the top open to allow the light to reach 
the focuser, which is at the top and slightly left in the photo.  That 
step, along with flocking the inside of the pipe (and don't forget the 
inside of the focuser!) really improved contrast to a very nice level, 
which I expect is the best I can do without increasing the 
obstruction.  (Since I'm at 32% now, I won't do that.)  I can live 
with the stray light.  I really only notice it when the moon is just 
out of the field of view.

I also took some time to carefully tune the size of the "ring baffle" 
seen at left in the photo, through which the light passes before it 
hits the tertiary.  This minimizes the amount of stray light that can 
get to the tertiary.

Hope that helps.

	Mike Lockwood

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