So I’ve spent 5 nights working with auto focus, with it failing/giving erroneous results 9 times out of 10.
I rarely get a v curve, and when I do it is shallow and has “wings” often on one side only. Most of the time I get a jagged line that is nearly flat or slightly sloping.
I’ve tried a bunch of different settings; step size, cropping, threshold, nebulousness, exposure, center obstruction/autoscale etc. (and the various setting options keep changing with each beta).
For step size I used the reference calculator and my CFZ is 56 um. My focuser is giving me 2.2 um per step so my step size for autofocus is 24, but I’ve tried half that and double that and others as well.
I’m using the same exposure I use for platesolving, 10 seconds @ 12800 iso, but I’ve also tried 20 seconds. I image @ 480 seconds @ 1600 iso. All of this is through a CLS filter, and a clip-in IR/UV filter (The DSLR is full spectrum).
I don’t have working darks for autofocus yet, due to the file naming and format issues I’ve posted about in other threads, but I will try to make acceptable fits files next (is there a log that tells you when the dark files are found and used?).
Focusing with a bahtinov mask on a central bright star works fine and gives me a consistent starting point (in number of steps/focuser position).
I’m happy to make my focus packages available if that would help debug.
One thing I’m thinking might contribute to the problem (although cropping the focus runs doesn’t seem to help) is the curvature I have with my current optical train:
8" RC with a focal reducer AND a field flattener. According to CCDInspector this gives me the flattest field at 29% however there is kind of a double dip in the focus as you go out from the center. My guess is that this makes “whole field” focusing challenging, as different parts of the field come into focus with different focus positions. Except that, again, no amount of cropping seems to solve the problem.
Would FWHM with pinpoint be worth trying?
Any other ideas?
I do plan to experiment with removing the Field Flattener and/or the focal reducer just to eliminate those as the problem.