RE: [ATM] 8" Astrograph

["lou krajci" <loukrajci@comcast.net>]

>From: "Robert Turner" <rbturner@charter.net>

>I am interested in building an 8" F4 astrograph for 35mm film photography.
I
>have found a little information about coma correctors and specifically the
>Lumicon corrector. Reading I discovered that they leave some spherical
>aberration which can be corrected by overcorrecting the primary. I did not
>however find the specific overcorrection needed to fix the spherical
>aberration. Does anyone have some information that would help me design
such
>a system using some kind of coma corrector at F4?

Try this info...I reverse engineered it:
http://astro.umsystem.edu/atm/ARCHIVES/NOV96/msg00361.html
Plug that into raytrace software, add your mirror, fiddle with two
variables:

A.  Spacing of corrector to film.  (Start with the default/recommended
spacing given the in the above link.)
B.  Eccentricity of the main (hyperbolic) mirror.

Some trial and error will tell you how much you need to overcorrect the
mirror.

Remember that after you hyperbolize the mirror...it won't work well as a
visual instrument without the coma corrector.

There are other options, such as the Televue Paracorr and the (hard to find)
Celestron Baader coma corrector.  With the Paracorr you keep the mirror
paraboidal.  With the Celestron Baader corrector you also need to
hyperbolize the mirror a bit.

I hope this helps.

Tom Krajci
Albuquerque, New Mexico


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