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A Rotation Gauge for CCD Cameras

Mounted on Astro-Physics refractors

   It's often necessary to return to a target over two or more nights to obtain sufficient CCD exposure time. The problem (especially if guide stars are hard to find) is to orient the CCD camera in exactly the same way each time. Some scopes have a motorized precision instrument rotator (PIR) that provides hands-off, computer-controlled rotation. However, on the standard A-P focuser, only a passive circular barrel is available and it has no index marks. After trying to position my camera the same way over several nights using just the computer image, I decided to give myself a little help with a protractor on the barrel.

As seen above, I took a plain piece of copy paper, a compass, and a good protractor and just drew a pair of circles that covered the end diameter of the standard A-P 2 inch holder. I then marked lines at 10 degree intervals and short ticks at 5 degree intervals. Had I been in less of a hurry, I would have cut out the disk much more carefully and glued it on more precisely. As it was, I kludged it a bit. Ordinary white glue seems to hold it on the anodized end surface of the holder OK. Leave enough space between the inner diameters of the holder and the paper protractor so that the 2 inch eyepiece tube on the CCD/CFW doesn't rub on  the paper.

I then screwed the holder into the focuser tube snugly and made a small mark at the top (aligned with the extension of the DEC axis). This was translated to the paper circle as 0 degrees, and the rest of the 30 degree increments labeled as shown.

Finally, I made a short but visible scratch with a sharp awl at the edge of the SBIG CFW filter wheel and adapter T ring so that the mark butts up against the paper protractor circle.

Now it's very easy to achieve 1-2 degree repeatability in the positioning of the camera when I reassemble the set up.

(Wouldn't it be nice if Roland produced a nicely engraved 2 inch holder with all the protractor marks accurately placed around the end?)