From: "hudsonjk" 
To: 
Reply-To: "hudsonjk" 


This is in reply to Steve Beccue's note in yesterday's digest.

My own experience with making a Schmidt is that 1/4 inch is too thin -
better to use perhaps 3/8 or 1/2 if you can get it. You can use any
low-dispersion glass, i.e., crown.  You should know accurately its
refractive index.  The index determines the depth of figuring you have to
do.  The 1/4 inch thickness in 12 inch diameter is too thin for any support
I know of, other than wringing it to a flat of the same size.

I had good luck making an f:4 Wright design (see ATM, Book 2, avail from
Wilmann- Bell).  Read all the stuff you can get on Schmidts before
plunging.  My plate was 1/2 inch Shadowgraph Crown, made by PPG. They were
extremely kind to me when I inquired about it, even tho' I only bought one
piece (10 inch).  It was nicely edged, and they gave its n and V values.  I
think it was about 1.52, but I forget the V. The V doesn't matter, so long
as it is high (~64 or 65).  (Hi V => low dispersion.)

As Bentz and May correctly point out, you will have a strongly curved focal
plane at f:2.5, and only a dark sky will allow you to exploit that f
number.  The Wright has a flat field, and it is also suitable for visual
observing, in fact it makes a magnificent RFT.  It also enjoys a short
tube, since the plate goes near the focus. One of these days I'll get
up a Web page and explain the project in better detail.

The Wright's field is too small for a 4x5, but good enough for 2x2.  Yes,
your ray trace program was probably telling the truth - the off-axis images
of the classical Schmidt are superb, in fact unbeatable by any other
design.  Only drawback is the curvature of field.

Good luck.

Jerry Hudson

--- BBBS/NT v4.00 MP