As I sit here dreaming of a new moon phase without clouds and 90+% humidity, I'd like to resolve one of my latest niggling issues with my astrophotography rig, and I'm hoping someone here has a solution. I use a Taurus Tracker III OAG with a QHY5III178M guide camera configured as shown here:
The problem I'm seeing is that despite having used a Bahtinov mask several times to get the two cameras to the same focus point, it would appear that there's enough difference in path length and materials that the focus of the two cameras drift relative to each other as a function of temperature. The result is that while I can focus the main camera very well using SGP and my focus motor, the guide camera is not always in good focus. This probably wouldn't be a problem if it wasn't for the small pixel size of the camera and the fact that since I'm picking off the edge of the SCT, an out of focus condition results in a slightly crescent shaped star instead of round. The net result is that since PHD2 is apparently picking the brightest spot in the crescent rather than centering on the whole object, and since that fluctuates with atmospheric conditions, I end up with poor guiding along the direction of the crescent (RA in this case) even though it's excellent in the other direction (DEC). This of course results in slightly elongated stars in the RA direction in my image.
So, is there anyway to alter the star detection algorithm settings in PHD2 to better center on the out of focus star it's getting? I don't think there's much of a mechanical fix to be had. I've debated trying to design a second focuser specifically for the guide camera, but I'm not sure the pitch there is very compatible with my 3D printing abilities. I'm also not sure how I'd automate focusing the guide camera.
Any thoughts/suggestions are appreciated.