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