Hello ZWO team and fellow imagers,
I’m encountering a serious autofocus reliability issue on ASIAIR (latest version) using a high-end APO refractor at 910mm focal length. Here's my full setup and detailed description:
📦 Equipment Setup:
Telescope: TS-Optics Photoline 140mm f/6.5 Triplet APO with 1.0x flattener (910mm native focal length)
Camera: ZWO ASI2600MM Duo
Focuser: 4” Rack & Pinion, EAF-controlled
Mount: ZWO AM5n
Backfocus: Precisely tuned to 54.00mm
ASIAIR settings:
Backlash compensation: 54 steps
Step size: 34
Autofocus exposure: 5s
Bin1, ROI full frame
Star detection: typically 700–1000 stars per run
No guiding during AF test
🧪 Observed Issue:
Running three autofocus routines in a row produces focus positions that differ by 2–4 steps, even when no settings or conditions have changed.
The star shape degrades significantly if the focuser is moved just 1 step away from best focus.
Manual focus (using preview and Bahtinov or visual star profile) confirms that ASIAIR sometimes misses the optimal focus, even when the V-curve looks good.
❓ Main Questions:
Is this behavior a known issue with ASIAIR on long focal length APOs?
Does ASIAIR use a robust focus curve fitting method (e.g., parabolic or polynomial regression)?
Could I adjust advanced parameters (step size, ROI, star selection, etc.) to improve accuracy?
📝 Final Notes:
Optical train is extremely solid (carbon tube, no image shift, heavy EFW + tilt adapter + Duo camera)
Seeing conditions during testing were stable
I use a backfocus-adjusted imaging train and have consistent star roundness — when focused manually
Would appreciate any insight from ZWO or the community.
Thanks in advance!
Cosmin
