Marty Roesch wrote:

> I've got a question. I'm planning on getting a CCD system for my 12"
> scope and I'm trying to plan out which accessories I should be getting.
> One of the critical things I need to know is how much back focus my scope
> is capable of. Does anyone know off the top of their head, or could some
> one tell me how to measure it?
> I'm currently planning to get an SBIG ST-7 CCD camera in addition to
> a JMI NGF-S focuser and Optec Maxfield focal reducer. Later additions may
> include things like a flip mirror system, color filter wheel, and AO-7
> adaptive optics system. (Yes, I know you can't use the Maxfield with all
> this other stuff.) I know I'm going to run out of back focus at some
> point, but what's that magic number?

I once measured this value and think it came to 180 mm or so. You can measure
it by focusing on a distant object in the daytime and using a ground glass to
view the image. Or you can simply add tubes and look with an eyepiece.
However, Gene Horr's suggestions are great. You want to keep the "stuff" on
the back as short as possible for several reasons. One is that too much
tubing gets to flexible. Another is that the telescope is best corrected with
the mirror back.

I used to use a flip mirror but no longer. It adds too much distance. I now
use a well collimated finder scope to center the object. With my ETX or C-5
as a finder, after finding with the little 60 mm job, I can get the object
dead center on the chip every time.

It is possible to use the 0.63 reducer with the AO but just barely. The 0.33
reducer from Optec must go directly on the ST. Many of the necessary
dimensions are on my web site.

Doc G <http://www.mailbag.com/users/ragreiner>

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