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There is another factor involved in measuring the FWHM that nobody mentioned: the effect of the telescope used for the measurement.
Not only the quality of the optics and the thermal state of the OTA but even the size of the central obstruction will influence the measurement.
Take two instruments with perfect optics at thermal equilibrium, same aperture, but one with 50% central obstruction and have a look at the diffraction pattern of a star. The instrument with central obstruction will put more photons into the diffraction rings of the star therefore producing a larger spot.
I don't think the software that works out the FWHM takes into account the effect of the central obstruction.
Residual spherical aberration also has the effect of increasing the value of the FWHM reading.
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