Re: [ATM] Baffling Wynne corrector

["Tom Krajci" <tom_krajci@tularosa.net>]

>From: Polaraligned <polaraligned@optonline.net>

>I would like to use this Wynne corrector with a 25" f/5 Galaxy mirror I
>have.
>http://www.astrooptik.com/   Click on Newton Wynne 3" corrector to see
>data on it.

>Problem is that the focal plane of the mirror needs to be set 43mm beyond
>the last lens in the corrector.  The corrector is 142mm long.
>This means I need to size and place the secondary so the primary mirror
>focal plane is 185mm  (7.25") outside of the secondary cage.  This distance
>makes the first lens of the corrector right at the inside edge of the
>secondary cage.

You can't change the 43mm spacing between focal plane and rear surface of
last lens, but you can increase the distance from secondary mirror to focal
plane so that the corrector assembly is not quite so close to the inside
edge of the secondary cage.  (Yes, you'll have a big secondary mirror to
handle this situation without vignetting...but that's acceptable for imaging
at Newtonian focus.

>I plan on using a Berry/Kreige style secondary cage.
>Big question is how much baffling does the first lens of this corrector
>need?

The more important question is how much baffling is needed to control stray
light at the focal plane.

Anyway, if you increase the spacing from secondary mirror to focal
plane...you'll move the corrector assembly farther from the inside edge of
the secondary cage, which will make 'shielding' the first lens easier.  I
don't call it baffling, I call it shielding because this situation is more
like a lens shade for the front element of a camera lens.

>My thoughts are to move the corrector further out of
>the secondary cage which would require quite a larger diagonal.  Phillip
>Keller says not to move it out.

Is Phillip Keller the owner of astrooptik?  Take a look at several scenarios
of moving the corrector out:

- not at all
- 10mm
- 20mm
- 30mm
- 50mm
- etc.....

...and I bet you'll see that your secondary size requirement won't change
all that much.  Then you could also make some drawings to analyze how much
shielding you can provide to the front lens of the corrector.  If you move
the corrector far enough away from the secondary mirror, and use a large
enough disk baffle opposite the secondary mirror on the secondary cage...you
can arrange things so that the front lens 'sees' only the mirror surfaces or
black baffle surfaces, and not any unfocused/direct sky at all.  (I have not
done this analysis.)

It would be helpful if you could post these drawings/layouts somewhere for
us to look at.  A picture's worth a thousand words.

Tom Krajci
Cloudcroft, New Mexico

PS.  A 25 inch scope for imaging?  What kind of beefy mount will you use?



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