Autofocus with a reduced number of stars


#1

Hello everyone,

To test the functioning of SG pro in multi-target mode, I created 2 targets (M45 and M81).

I selected autofocus at each start sequence. At the end of the second target, I programmed the Park position of the scope.

For the automatic realization of flats after M45 and M81, I created a third target (leaving the coordinates to zero). To this target, the event was preceded by the ignition of the flat panel by a VBS script. At the end of the sequence, another VBS script allows the extinction of panel flats

I launched the sequence and overall, everything went well. There is just a small problem with the autofocus. For M45, there are many stars, the V curve was well done and the point of focus was good. By cons, for M81, there are few stars around the galaxy, and SG Pro finally set wrong focus. The curve of HFR was a real zigzag.

At present, Autofocus is programmed bin1 mode, with a duration of 5 seconds. In the few cases I have tested, everything was fine, except for M81!

What are the good practices to use in this situation? Increase the duration of the shooting? Switch to bin2 or more? Use darks?

Thanks for your advices.

Jean-Pierre


#2

1x1 is almost never a good idea for AF. Go with 2x2. Increase your exposure length to 8 sec.

Finally, the more difficult part… SGPro can effectively focus with less stars, you just need to tell SGPro that this target has fewer real stars. You can do this with the AF sample size slider. It is best to do this while on the target in real time. @freestar8n had an idea that may resolve the need to do this (dealing with minimum HFR), but that would be in a future version.


#3

OK Ken

I will repeat tests on M81 with bin 2 and increase the exposure length.

I hope that the time needed to get a good curve in the M81 area will also apply to areas where there are a lot of stars.


#4

The latest versions of SGP will retain your AF Sample Size and AF Nebulosity Rejection settings on a per-target basis.

You can take a sample focus image using the Frame and Focus module. Select the same exposure and binning that you will use for AF.

Click the Star button show the stars that SGP will identify:

Adjust the sliders in the Focus Control module and watch the stars that are selected in real-time until you have the maximum number of actual stars, with no false stars or nebulosity selected.

Now when AF runs for that target you’ll have the optimal settings. You can repeat this for your other targets and SGP will keep the slider settings you selected for each target.

Andy


#5

Thanks @Andy. Your responses are way more awesome than mine.


#6

Andy, thank you for your advice.

When the sky is clear, I repeat testing by modifying the proposed parameters.

It is true that there are few stars in the M81 area with a value of HFR. In addition, for certain values of HFR, there was no star visible in the photo.

Merry Christmas to all forum participants.


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