https://bugs.kde.org/show_bug.cgi?id=500744
Bug ID: 500744
Summary: Guiding disrupted by focus offset
Classification: Applications
Product: kstars
Version: 3.7.5
Platform: Ubuntu
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: mutla...@ikarustech.com
Reporter: co...@mimas.ru
Target Milestone: ---
SUMMARY
When using focus offsets, guiding may be disrupted at the beginning of the
first exposure after focus or align, resulting in worse than normal guiding
performance.
STEPS TO REPRODUCE
1. Have a two optical trains setup with unfiltered OAG, filter wheel in the
main train, single focusser in both trains. Add R and O filters with different
offsets. Lock O filter to R for Focus. Set Align to use R as well. Have a
repeating job in the schedule with Focus, Align, Track, Guide enabled and
sequence with O exposures.
2. Start the schedule. First iteration may or may not hit the problem.
Subsequent ones will as they have just ended with O filter selected.
3. Focus switched the wheel to R, completes, switches to O and applies offset.
4. Align switches to R again and applies the offset (a bit excessive work but
fine). Runs successfully.
5. Guide starts (main train is still on R filter!), captures a couple images,
locks on a star.
6. CCD starts its sequence, selects the O filter, suspends guiding for focusser
action, applies the offset, resumes guiding.
7. Guide sees its star moved a bit by focusser shake and starts correcting.
OBSERVED RESULT
With my mount and focusser, likely, that shake is below configured tolerance,
so exposure continues but the image is worse than it could be.
Guide may take multiple iterations to move the star back to where it wants,
especially if significant Dec correction is required and that hits backlash. I
often see 3" swings in Dec over first 30s of a 2m exposure. That yields like
2×4" stars. I set guide tolerance to 2" which is already quite strict for my
weather.
EXPECTED RESULT
Multiple options could work better. Perhaps, restart guiding after moving focus
rather than suspend and resume. IOW, force Guide to lock on star in new
position. To me, this sounds much like what happens after dithering, so maybe
reuse that logic somehow.
SOFTWARE/OS VERSIONS
Ubuntu 24.04.1
KStars 3.7.5 from PPA
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