From: Aplanatic{at}aol.com
To: atm{at}shore.net
Reply-To: Aplanatic{at}aol.com


Hi Steve,

I've made a couple of Schmidt cameras and a concentric Schmidt Cassegrain.
Here's my thoughts on your project:

> I wish to make a 12" schmidt camera.  I have some 1/4" thick
12.25"
> diameter plate glass for the corrector.  I searched the archives and
> found people suggest Nd=1.523.  Does everyone agree this is a good
> value?

Plate glass may have striae and inhomogenous regions.  I ground and
polished a piece of plate glass and Foucault tested it in front of the
spherical primary mirror for a 9" f/2.5 Schmidt camera.  It showed
inhomogeneous regions that had as much as 3 mils of longitudinal aberration
when compared with the average.  I felt that this was not good enough for a
high quality camera and ended up buying a BK7 plate.  I've been told that
plate glass can vary a lot, so your piece may be just fine.

> I was hoping to make a F/2.5.  Any comments on the utility of this
> focal ratio?  I only choose it because I'll have a 1/4 scale model
> of the palomar schmidt.

A 12" f/2.5 Schmidt is a very large camera.  What will you mount it
on?  How will you guide it?  To prevent significant vignetting for medium
format photography the primary should be 16" in diameter.  In other
words, it's a big sucker.

Have you made a Schmidt corrector?  If not, I'd recommend making a 9"
camera. F/2.5 would be a fine instrument with modern films.

> The raytrace I have done gives rediculously good spots 50mm off-axis.  Is
> this reasonable?  Is this design tolerant enough that real world
> tolerances give anything near this nice?   Does this, in
> fact, justify a
> 4" x 5" film holder?

Yes, Schmidt cameras have wonderful off-axis images.  A 9" Schmidt
will support a 3" image circle, and a 12" Schmidt will easily
support a 4" diameter image circle.  For large image diameters you
must make sure that vignetting won't be a problem.  I'd want less than 20%
vignetting at the edge of the image.  This will drive the diameter of the
primary.

It's a great project.  Best of luck.

-Dave Rowe-

--- BBBS/NT v4.00 MP