With my new Celestron RASA 11 scope I have run into a problem that I never noticed before with any of my prior equipment configurations. The RASA architecture moves the primary mirror when focusing and that causes an image shift problem. The RASA has the camera in front of the scope so the only practical focusing mechanism is the provided primary mirror focusing. My focuser backlash compensation setting is IN for step size +1000. Temperature Compensation value is -110 steps/degree (F). These numbers seem high but the autofocus increment that works well is 350 steps. The gearing architecture requires these settings.
Everything works fine when the temperature is dropping steadily during the night, at lease steadily enough to keep the temp sensor from registering a false momentary upward tick. This causes the focuser to rack out as the temp drops which is what we want. No backlash compensation required.
So the problem occurs when the temperature increases a tiny amount during the night. Which happens regularly since even if the temperature is dropping steadily throughout the night, it is pretty common for the temperature sensor to register a brief uptick, even if other sensors nearby might not. This causes a swing in the focuser up by 1000+x then down by 1000, which totally ruins the image it was taking. Double stars over the entire image, with nice lines connecting each pair. Quite annoying. One example was a night when I was taking 4 minute exposures. Approximately 1 out of every 4 or 5 images was ruined. Temperature dropped about 12 degrees over the course of the night, starting at the highest value, and ending at the lowest.
Am I missing some setting that would let me set a minimum temp increase before the temp compensation move kicks in? If not, could this be added? One easy initial fix for this could be to just automatically set the minimum reverse focus move to some fraction of the autofocus routine’s step size. In my case a cumulative reverse of 175 steps (350/2) or about a 1.5 degree temp increase would be required to actually trigger a focuser movement. This would rarely occur unless a real warming trend occurred. And being off up to half a step size in focus would probably not be noticeable.