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Jon,
With a A type star you should be able to "lock in" to the Balmer series..
As you get closer to focus these will become more defined...
If you can adjust the distance from the grating to the camera sensor to achieve close to 8-10 A/ pixel dispersion - this would give a width of around a 1000 pixel from the zero order to the end of the red spectrum - an easy fit across the 2500 (approx) pixel width of the DSLR.
Try to set the grating to give a horizontal image.
Your initial exposure looks OK...
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