Galaxies & clusters





Narrow band






Above is a picture of my set-up, a Celestron C8 with a William Optics 66 ZenithStar (doublet APO). The WO66 is used as a guide scope and the orion starshooter as a guide camera.

I noticed that my auto guiding (using MaximDL) still resulted in star drift. Typically I had 0.9 pixels/min (0.5 arcsec/min), which limited my exposures to max 1 min, which is a little too short when imaging at F10. After asking my friends at Digital_Astro (Yahoo user group), Rod Mckay gave me the tip that the reason was due to flexure in the set up. Even if MaximDL manage to guide perfectly on a star, the guide camera would move slightly relative to the imaging camera. Thus the guide star would be fixed in the guide scope but move in the imaging scope.

Doing the calculations right (I hope :-) I found the following. The drift 0.5 arcsec/min found in the images from the C8, corresponds to a drift of 0.25 pix/min of the guide scope. Since the guide camera pixel size is 6.5x6.3 microns this indicates that the guide camera moves some 1.6 to 1.7 microns per minute relative to the imaging camera. This corresponds to only 0.1mm in one hour!

The reason may be some very light drag from the cables as the scopes follows the sky, the weight of the cameras may cause the guide scope to tilt slightly etc.

There is mainly two ways to fix this; guide at the same FoV as the imaging scope (i.e. use an off-axis guider or another guide scope), or try to fix the flexure problem.

Rod (using a WO66SD as a guide scope) gave the tip that the focuser in his scope has some play in it. I tested it and I could make the guide camera move a few centimetres (!) relative to the C8, by just putting some pressure on it with my fingers. I tried to use a screw pushing the focuser tube from below upwards to fix it. This gave me an improvement and I decreased the star drift from 0.9 down to 0.3 pixels/min, which is still not good enough if you need 5-10 min exposures.

Finally I came up with the solution you see in the image above. The WO66 has a foot that can be used to attach it to a tripod and I have an adapter (normally used to piggy back my Canon 40D with a lens on the top dove tail). So I used this adapter to fix the WO66 foot to the main scope. I moved the backward ring and used it to clamp down the focuser tube really tight, while the forward ring stayed where it was fixing the front of the guide scope. This gave a substantial improvement and the drift went down to 0.08 pix/min - making it possible to do 5-10 min exposures.