Subject: Star trails, failed frames, possible causes and solutions.
Date: 12/3/23
ZWO S50 number: S50_f40d9ac7
Observed issue. I was imaging the wizard nebula NGC7380. It was just left of the meridian at 7:30 pm MST, and headed north west. I’ve been looking at the subframes and can see many with vertical star trails. Most of these are ignored correctly, when the streak has uniform intensity between start and stop. I’ve even seen the system incorrectly accept frames where tracking failed, especially if the star image moved suddenly to a new position during exposure. This is likely because the image looks like twice as many stars with a faint streak between the start and stop of the streaks, rather than a smooth continuous streak.
Clue 1. Most of the star trailing issues were in the vertical direction. This suggests the altitude tracking bearing is slipping or the altitude tracking gear train has excessive backlash. I have yet to confirm if this vertical axis error holds on other targets depending on their position in sky. Addendum: as the object got lower, finally a few frames showed streaks in horizontal direction. But most of them are vertical.
Clue 2. These seem to happen often, but not always, at multiples of 5 frames +1. So for example at 11, 21, 36 etc.
Hypothesis. I started the experimental menu and noted that dithering was already active, and set to 50 (pixels?, steps?) every 5 frames. When dithering often the next frame or two (or more) fail because of star trails. I suspect these trails are due to backlash in altitude and azimuth and the system is not taking up backlash (especially after dithering) for tracking. This would result in dithering not working as expected when dithering involves switching directions relative to tracking direction. In addition, when dithering involves moving opposite to normal tracking direction, not accounting for backlash could also result in failure to resume normal tracking after dithering. It appears the star streak dropped frame errors are most common after dithers. The result in more lost time for 10 s exposures since dithering occurs more often by the clock. Less time wasted for 20 s exposures since more light accumulated from 5, 20 sec exposures prior to dithering. Note: I still see all these same issues with default 10 s exposures.
Recommendation to fix tracking and star trailing errors. If you’re not doing it already, I recommend a pre-run execution of a routine for measuring backlash in altitude and also in azimuth and in + and - directions for each. Then include backlash calculations and take up backlash when switching directions of motion. So for example, if you’re moving west to track and dither to the east, you’ll need to apply the backlash take up to get the dither motion to work properly. And then you’ll need to apply the backlash again to get the system to resume tracking west. This last step is very important to avoid star trailing dropped frame errors. If you can fix this, many folks will be appreciative!
P.S. I’m glad to share some images.