Guys,
Covering 50% of the grating didn't work - wishful thinking!
When you think about it, the camera lens is still producing an image of the star field whether is 50% covered or not...
The "temporary" solution is to slightly tilt the grating/ camera to move the spectral image from the worst of the interfering background stars.
The aperture stop mentioned (Astronomical Spectroscopy for Amateurs, p 182) works with the grating "in the converging beam" and isolates the target star "image" (at the grating) from the background.....
I will however try the Baader grating for comparison.
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