Hi Keith,

I can see you don't have a flat field to the edges of the sensor (from your frames) and as your camera has a very big sensor the stars on the edges will likely focus at quite a different position to the centre of the sensor. You could run Aberration Inspector but it will likely only tell you that you don't have a flat field (which you already know).

You can apply a ring mask in Focus...

So, if for example your target covers just the centre of the sensor then if you have a ring covering this area then this would mean you have the best focus on the target. If your target covers more of the sensor then you will have to compromise on focus as best focus will change the further from the sensor's centre you go.

To get best advantage from your camera you probably need a field flattener. Although post processing techniques like BlurXterminator (if you use PixInsight) will make your corner stars round, you won't be able to get all of your sensor in focus at the same point.

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