When we use the pinhole with the gas cell, we will get spots that depend on the size of the pinhole (designed to be 108 microns, but probably larger because of the etching process), the magnification from the slit plane to the detector plane (should be 1:1 and is probably very close to that), and diffraction, which will depend on wavelength.
I looked at this issue a couple years ago for a couple wavelengths, but today I ran my routine at 1 micron steps from 4.5 to 28.5 microns. The points are in the plot below. The fit is
sqrt(3.75^2 + ((lam-4)/2.2)^2)
Not particularly easy to remember, but the plot should help.
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