HI Hy,
Your approach was my initial idea. Let clarify why it doesn't work.
1st - Actually for plate solving I use third camera installed on finderscope. Mainly because the FOV. The plate solution is quick and always successful on finder. But the finder and the main telescope are not exactly aligned.
2nd - I tried to use guiding camera on the main telescope for plate solving. Unfortunately the FOV is too small. The process fails often.
3rd - for mechanical reasons put the slit exactly in the center of the image is not an easy task. Moreover it moves slightly, few pixels, as the telescope is pointing at different parts of the sky.

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