If you choose to auto select a star to focus using the entire frame it can use an annulus for where the stars are used to perform the focus.

Is there any way when using a sub frame to make sure the star selected for the sub frame is within the defined annulus?

I believe this would avoid the issue, I have found, where it always seems to select a globular cluster as a star to attempt focus on, when I am imaging a globular that is relatively small compared with the frame size.

Paul