Hi,
When I first got my auto focuser last year, I used the default "Gradient" detection with the "iterative" algorithm (default settings). This worked in some cases and other times it failed (Deadlock or the need of a movement too large). When it worked it worked very fast.

In order to optimize my auto focus I change to SEP detection with Linear algorithm (default settings). This has been working perfectly for months until the other night* (see below). This method is, however, very slow, which results in my target moving pretty far in my FOV when Ekos refocuses eg. once every hour. To eliminate this I've disabled refocusing in the "CCD" (Capture) tab in Ekos. I'd love to have refocusing enabled once each hour or when a temperature change of eg. 2°C occurs.

What to do? Which algorithm and detection method for fast and reliable focusing?


*The other night I experienced my first failure with the SEP / Linear method. I wanted to try out the sequencer in Ekos to create a mosaic of the Andromeda Galaxy. Ekos should do the following in each panel of the mosaic:

Slew, Focus, Plate Solve, Guide and then capture 20 x 180s

The first panel was fine, the second failed due to failure to focus and the third panel was fine. This was my first time with a focus error with the SEP / Linear method. I had to throw my images away though, because M31 wasn't in the field captured at all! No idea why, but that's another problem.

Have you tried playing with the backlash settings ?

I built a diy autofocuser and, not having prime steppers at hand, I had to add some backlash compensation before autofocusing consistently succeeded.

Hi Anders,
I started using an AF3 with my setup (TS 115 f/7 reduced to f/5.6 -> 645mm focal length, ASI1600MM-Pro) a couple of weeks ago only and I've had good results using the polynomial method.

I use the full frame with an annulus between 25% to 70%, exposures are 1 second long, binned 2x2 (I've setup an average of 3 images for exposure, but I'm not sure if this parameter is used in the full-frame mode). Autofocusing takes only 8 exposures and the solution falls consistently within my critical focus zone. The whole process takes approximately 140 seconds.

I'm curious, why does your target move that much during focusing?

I leave auto-guiding on during focusing (I guide a non-moded HEQ5 with a 60mm guidescope at f/11 with an old ASI120MM and my guiding is consistently around 0.5" RMS*), but even if off it will move very little if I've got my PA right. 

* that is, when the ASI120MM is not glitching...

Ricardo
 

What do you mean with "I use the full frame with an annulus between 25% to 70%"? How are this annulus defined?