Thanks to all for your contributions. Since focusing correctly is one of the most demanding challenges in astrophotography, I really want to achieve it best possible.
I didn’t want to question that the focuser response is nearly linear if you only are away from focus sufficiently.
My problem seemed to be, that I didn’t manage to get HFR values in that region far away from focus because then no stars were found. Therefore I continued to do AF runs, following Buzz’s advice not to use AF binning with a DSLR. (Perhaps important, so I repeat it: I use a refractor FSQ 106N and a Canon EOS 600D.)
Extending the exposure time at ISO 800 from 8 s to 15 s at an AF binning of 1x1, I succeeded in getting decent AF runs with a step size of 27 (= 180 µm). Beyond that value star detection always failed. However, the HFR ratio of AF runs with 15 s decreased again to 3.0 (I already had reached 3.4 with ISO 800, 8 s, AF binning 1x1 before). So by extending the exposure time it even went worse. At a step size of 27 there was no indication of the response becoming linear either. Typical R^2 for the quadratic regression was in the region of 0.970 to 0.985.
In these focus run trials I didn’t crop autofocus frames. (Buzz proposed an exclusion zone of 10 to 15 %.) The ‘Minimum star diameter at 1x1 (px)’ was left at the default of 6. Nevertheless SGP does downsample the focus frame (excerpt from the logfile):
AF frame was too large… downsample = 0,25…
Star detection using min star size of 2px…
Star detection using max star size of 40px…
On further inspection of the very few (only 4 of 28) focus runs that were taken with an AF binning of 2x2, I detected that these delivered a higher HFR ratio than the shots with 1x1 binning, step size (= 25) being equal.
- Does this mean that the Bayer Matrix of the DSLR disturbs the HFR calculation?
- Have I to use 2x2 binning to get it right?