Re: [APML]: SCT CONVERSION

[Wil Milan <wmilan@airdigital.com>]

Mike Regish wrote:

> Don't quote me, because I'm really not sure, but if it's an f10 scope I'm
> pretty sure the primary is f5. Somebody told me the secondary is f2 making
> the end result f10.
>  Don't know if it's ever been done or if it can be, but I'd be interested
> since I now have a 12.5" f6.3 Newtonian with essentially the same focal
> length as my 8" f10 SCT.

It's the other way around. The primary is an f/2 or so, the secondary
multiplies that by 5x or so to get f/10.

However, just having an f/2 spherical primary and an aspheric corrector is not
enough to make a Schmidt camera because the corrector is ground for an f/10
SCT, not an f/2 Schmidt camera. Also, the tube is wrong; on an f/10 SCT the
corrector is located inside the focus of the primary, whereas on a Schmidt
camera the corrector is at 2x the focal length of the primary. That's why a
Schmidt camera is so much longer than a typical SCT.

Still, it would be interesting to investigate what it would take to turn an
f/10 SCT into a Schmidt camera. You'd need a longer tube and the corrector
would be wrong, so it would be either over- or under-corrected for spherical
aberration, but I wonder what the images would look like. And if all it would
take is a different corrector maybe someone could be talked into making them.
Or perhaps a small corrector lens could be place just ahead of the film; I
believe that's what the new Celestron CCD scope does at its f/1.95 focus.

One other factor: the primary on an f/2 Schmidt camera should be about 1/3
larger than the corrector aperture in order to capture the divergent light
cone from the corrector. Thus an 8" SCT could be used to make a 6" Schmidt
camera, and a 10" SCT could make a 7.5" Schmidt camera.


Wil Milan
--
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