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Yeah, Ken... I did say I was ignoring seeing and imperfections. Seeing should be less of a problem at lower f/ ratio (the star image should be smaller) but the relative effect should be the same in terms of Å on the spectrum.
The defocussed star image at the SA is a full disc of 30mm diameter (that's my assumption for maximum dispersion) since if we go bigger than that then the SA becomes the field stop.
Al.
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