A fast way to correct it Bert would be to set the scope on a large starfield so the image is full of stars then do continuous 5-10 second focus images and adjust the plate as you go to get the stars as evenly round in 4 corners as possible.
Of course if you have to remove the camera to get access to the adjustiing screws that won't work.
A short focus image though is all that is needed to verify the change you just did. It may only take half an hour (yeah right, famous last words!).
Greg.
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