If you deselect "Full Field" then you will work with just one star, be it automatically selected or manually selected. However, if your field is changing during focus then you're probably not tracking very well, perhaps due to poor polar alignment or due to other mount issues.
If your field is not changing all that much then you'll still get a reasonable curve, as long as enough stars are detected, changing the annulus setting allows you to select the stars in the centre of your field, or in a ring between 25% - 75% of your field of view. What you chose for these settings will depend on your target and target type.

You might want to test focus on a non-nebulous target first to get the hang of it, then move onto nebulous targets which might interfere with the algorithms.

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