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The resolution of the SA is dependent on the distance from focus.
The more out of focus the star image is at the plane of the grating, the more lines are illuminated, the better the resolution.
The SA is 100lpm grating , so a 4mm OOF star would illuminate 400 lines, a 6mm image, 600 lines ( a 50% increase in resolution!) etc etc
You still loose efficency with large star images...
When the beam is collimated ( and a slit added) you have the potential of the full SA aperture 28mm, 2800 lines.... x7 the resolution.
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