Re: mounting guidescope on top of refractor, Losmandy G11
From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 08/28/04
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Date: Sat, 28 Aug 2004 14:15:47 GMT
On Sat, 28 Aug 2004 12:10:57 GMT, DL <dtlnew@yahoo.com> wrote:
>I want to mount my 5" refractor on my G11, and then mount a guide
>scope on top of the refractor. I currently use the side-saddle
>system, and I don't like having to balance on two axes. I've spoken
>to one vendor who has a solution, but I wanted to ask for input from
>anyone who's done this successfully (or not!) Whatever I do needs to
>be suitable for long exposure astrophotography.
I spent quite a while getting a guider system working solidly. Initially, I had
an ST80 guidescope mounted on a 12" LX200 using Guide Star adjustable rings.
These rings proved to be inadequate, with flexure that couldn't be eliminated- I
would definitely not recommend them. I switched to Losmandy dovetail plates and
rings, and that took care of flexure- they are very robust. I identified some
remaining image shift that was caused by the focuser and by the guide camera
rocking in the draw tube. The first was fixed with an extra lock screw, and the
second by shimming with Mylar tape.
The weakness in this system was that the FOV of the guidescope was still small
enough that I had to adjust its position to find guide stars. This was both
inconvenient and resulted in occasional image shift from the three-point screw
mount system. My current solution, which is completely hands-off, is a rigidly
mounted achromatic objective on one Losmandy plate, and a rigidly mounted camera
on another. I use a 200mm f/5.1 lens and an ST237A. The FOV is over 1 degree on
a side; the guider never needs to be aimed or focused. You can see this system
here: http://www.cloudbait.com/observatory/guider.jpg (with the floating tube
between the lens and camera removed for clarity).
I highly recommend a guidescope system that gives you a large enough FOV to
eliminate the requirement for positioning. I guide at 7 arcsec/pixel, and image
at 0.8 arcsec/pixel, and my guiding errors are smaller than I can measure. It is
very desirable to have a short focal length guidescope.
In your case, I'd keep it simple. A Losmandy plate and rings for the refractor,
and a rigidly mounted guider on separate Losmandy hardware on the that. For less
flexure, you might even try a custom solution. Make a new riser block for the
forward ring with a hole through it for the guider objective. Now you can place
a rigidly mounted guider camera underneath the piggybacked refractor.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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