Re: [ATM] 3 Inch Off Axis Schmidt Camera?

[Aplanatic@aol.com]

Hi Mike,
 
I've made several Schmidt cameras and a concentric Schmidt  Cassegrain.
 
Most of the work is not in the primary mirror.  It's in the Schmidt  plate, 
the OTA and the other gadgetry around it.  Therefore, I personally  would not 
design a camera around a partially completed primary, but would design  a nice 
camera and make the primary for it.
 
I would design an on-axis system.  There is no good reason at  all for an 
off-axis camera.  Imagine trying to collimate it, for  instance.
 
I would design the camera to be used with a CCD sensor.  This will  require 
an achromatic field flattener as close to the CCD image plane as you can  get 
it.  Interestingly, the use of a CCD sensor also  eliminates two of the most 
problematic aspects of a Schmidt camera  --  focusing and maintaining focus over 
temperature.  With a CCD you can  focus periodically if necessary, and more 
importantly, you can see what you're  doing.
 
I would design the system to have an 8" aperture with an f/ratio of about  
2.2.  Plenty fast enough for CCDs, easy enough to build, and small enough  to be 
easily mounted.
 
I would consider using an uncooled CCD video camera that has been modified  
to take time exposures.  At f/2.2 the longest useful exposure should  be around 
30 seconds or so, and dark current from the uncooled CCD  should not be a 
problem for this short integration period.  For these  short integration times 
unguided exposures can be made to work well,  with some care.
 
Dave Rowe
 
In a message dated 1/19/2005 2:52:16 P.M. Pacific Standard Time,  
maheald@octa4.net.au writes:

Hello!  I've just finished my 11 inch classical cassegrain.   As I consider
my next project, I am considering what to do with a 112mm  blank.  Before
beginning the cassegrain project, for a separate  project that I have since
abandoned, I ground the blank and have 85%  polished it to f1.5.



The mirror is too small for  a classical Schmidt camera - the corrector
would be about 75mm, and 35mm  film would create up to 60% obstruction.



So, I  wonder if I could use this blank for an off-axis schmidt camera.
The  corrector plate would have to be polished as if the primary was  about
275mm, giving the corrector a size of about 180mm, which I could then  trepan
down to the size I would need.  An advantage to an off-axis  design would be
that it would bring the film out of the telescope tube for  easier handling
and smaller filters could be used.  Additionally, I  could focus the system
like a view camera, using a loupe-type magnifier to  focus bright stars or
planets on the film holder.



Questions:



1)       What would the  image circle of a 75mm Schmidt camera be?  Would it
even cover 35mm  film?



2)       Would a radius of curvature  of the film of about 175mm be too
severe - would the film crinkle in the  holder?



3)       What would the sky  coverage of such a system be?



4)       How  close to perpendicular to the light path can the corrector
plate and film  cassette be in order to avoid unacceptable astigmatism?  I
think I can  focus the   

primary onto  the film well enough, but I haven't satisfied myself
that I can align the  film holder cassette and the corrector plate to within
tolerances to avoid  

astigmatism.



5)       Most importantly, is  such a system reasonably doable?







Thank you for your help.  Best regards.




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