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Jon,
With the grating in the converging beam the distance to the sensor is a critical one. Sounds like your DSLR has pretty small pixels (and you can't binned them...) Reduce the distance to around 50mm to get 8-10A/pixel.
Anything with a higher dispersion is just "empty magnification" - no more detail or resultion.....
The faster the f ratio the smaller the star image ( down to f5 ) but at the expense of added chromatic aberrations....
I think you're getting there - just more practise.....
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