Autofocus Setup newbie question using various Filters

Just starting to use SGP Autofocus . Confused about using different filters. I first get a rough focus with my Bartinov and then run an autofocus with my LUM. All is fine at this point. I then have it set to autofocus again when it changes to say a RED or even HA. Will the autofocus run as expected or is it absolutely necessary to use Offsets for each filter . If I do need offsets , how do I get them ?? Also, when using offsets…does SGP move to the offset THEN run an autofocus routine to double check focus ?? Is it absolutely necessary to use Offsets ??


It’s not “absolutely necessary” to use offsets, it just saves a ton of time. Autofocus per filter without offsets isn’t too bad with LRGB filters, but it becomes painful with narrowband filters.

If you are just starting out I would suggest you skip offsets for now until you get more comfortable with the autofocus routine and see what best practices are for your setup. So here’s what I’d do:

  1. Shoot all your Lum frames, then all your Red frames etc.

  2. Use autofocus and set it to run autofocus every 1C temp change and on a filter change. If you don’t have a temperature sensor then set it to run autofocus every hour or less. Use 9 data points, 7sec exposure for LRGB and 30s exposure for narrowband. Your step size will be determined by your equipment so I can’t tell you that.

Once you get comfortable with using autofocus and figure out what settings work best for your setup, then you can move on to calculating offsets. The simplest (and least accurate) way to get offsets is to wait until the ambient temp is stable and then do an autofocus run on the LUM filter. Write down the focus position (step position). Then switch to Red and run autofocus again. Write down the new focus position. The difference between the LUM step position and the Red step position is your offset for the red filter. Repeat this pattern (Lum then next filter, lum then next filter etc) for all your filters.

Once your offsets are entered in the filter wheel setup, any time SGP changes filter it will first move the focuser by the offset before doing anything else.

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thanks so much. That answers all the questions and more .:sunglasses:

If the least accurate way to determine filter offsets is to focus each filter independently and RE its the step differences between them, what is the most accurate way? I am about to undertake this exercise and would very much like it to be highly accurate. Currently losing a lot of time to narrowband focusing.

Determining filter offsets by focusing one filter, noting the focuser position, focusing the next filter and noting its focuser position and calculating the difference works fine, but it is only one data point and ignores potential temperature changes that happened while focusing.

Check out SGP Autofocus Log Viewer. It reads SGP logs and extrapolates the relevant data to more accurately determine average filter offsets over many autofocus runs and temperature changes.

Here’s what I’d do. When the moon is bright setup a sequence that rotates multiple times through all of your filters, and trigger an autofocus run on each filter change. After a few hours you will have lots of autofocus runs and SGP Autofocus Log Viewer will give you an average filter offset for each filter.

Also take a look at this post of Jerry: Autofocus Issue with - #6 by jmacon . SGP AF LogViewer is a great tool to determine the filter offsets.


Thanks! Wasn’t even aware of this tool. Will get on it.

Thanks. Much appreciated.

Is there a way to get SGP Auto Focus LogViewer to pull the data from all the logs? At the moment I can only figure out how to load one log at a time, and it seems a shame not to be able to pull all the historical focusing data from the many logs I have accumulated…

Yes you can, by simply selecting all those files at once in the open file dialog.

I posted an example of that here: SGP AF Logviewer v1.2 Release - #2 by mikaelA


Thanks much!