From: Jan Bentz 
To: Steve Beccue 
CC:  atm{at}shore.net
Reply-To: Jan Bentz 


Steve -- As to your 1st question:  Yes, your glass selection is a good one.
 The lower the index the lower the spherochromatism. Any light crown is
suitable as long as the strain is OK.

2nd question:  f/2.5 is a reasonable ratio provided you live in a very dark
area or can get to one.
Try an ordinary camera with the ffilm you will be using for a series of
exposures to see when sky fog becomes unbearable.  Bear in mind that the
mechanical requirements become much stiffer when you decrease the focal
ratio.

Which brings us to the 3rd question:  4x5 film will probably wrinkle when
it is bent over a spherical surface w/radius equal to the focal length
(30").  The alternatives are to use 4x5 glass plates --expensive if
they are still availiable , or make a field flattener.  With a ray tracer
this should be fairly easy.  Watch the shape of the correcting plate though
as this should change to give a fully optimized system.
 The flattener will give a somewhat degraded image over the bare system
but will still give spot size well under what is needed.   Run various
decenterings and tilts through your ray tracer to get a feel for the
various  tolerances.  It ain't pretty.  About the only loose ones will be 
for the corrector.

Overall , the optical work will be easier than the mechanical work because
it is a Schmidt and because it is f/2.5.

Jan Bentz

Steve Beccue wrote:

>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?
>
>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.
>
>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?
>
>TIA
>
>Steve Beccue
>steve{at}beccue.com
>
>
>
>
>
>

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