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A quick method the check the slit is to measure the FWHM width of a "good" exposure of a zero slit image.
I recommend setting the gap to 3 -4 pixel FWHM (this is not as rigorous as it should be, but seems to work pretty well for novices)
This will then at least match your pixel size better than working out the linear star size on the slit and setting to that.
You can see the impact when using a neon reference - the FWHM doesn't change much when the slit is adjusted when you have larger CCD pixels.
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