[peter_4059 (Peter)]

I've been playing with auto focus settings in SGP trying to improve the reliability of the auto focus routine. The variables are: exposure length, binning, filter selection, number of points on the V-curve and number of steps between points. I've been trying various combinations of these and comparing the V-curve results with the focus point as indicated with a Bahtinov mask.

With the SN10/QSI683, the calculated critical focus zone is around 40um which is about 10 steps on the Moonlite focuser. What I'm finding is the V-curve is quite flat around the best focus point, especially if I use 9 points of 10 steps each, so the auto focus routine finds it difficult to achieve repeatable results. If I increase the number of steps between points to 23 I get better definition of the V-curve because the stars are a long way out of focus at each end of the curve, however this means each point on the curve is 92um apart which is more than double the critical focus zone width.

I've also found it is better to have more points on the curve as it is quite common for one or more points to be outliers causing the curve fitting to be thrown out. The down side is it takes longer to run the focus routine as you add more points.

SNR clearly plays a big part in calculating the focus metric (HFR in SGP) and I've found I'm getting better results with 2x2 binned 5 sec exposures than 1x1 binned 10 sec ones. The impact of SNR is quite noticeable if I use CCD Inspector to calculate FWHM of the autofocus images and compare this to HFR determined by SGP. I get quite good correlation between the two metrics when the SNR is better.

In calculating the critical focus zone I notice this changes with filter (wavelength) - 51um for red vs 37um for blue. I'm now wondering if there is any advantage using the red or blue filter rather than the Lum to perform the focus routine and use filter offsets pre-determined with the Bahtinov mask when changing to a different filter to image.

Has anyone played with using the RGB filters for focusing? Are there other auto focus routines that would work with SGP?