During acquisition for my November 2013 photograph, immediately after focusing and prior to starting a 40 minute subframe, I captured a short, 20 second exposure with guiding disabled. I measured the median star full width at half maximum (FWHM) of each subframe, and plotted the long-to-short exposure FWHM ratio of each subframe pair in the chart below. The mean FWHM ratio, indicated by the horizontal line, equals about 1.01, or equivalently, the FWHM of the long exposure is about 1% larger than the short exposure, on average. This small increase in long subframe FWHM indicates that my setup's guiding and focus drift compensation systems are working well and are not limiting factors in achieved FWHM.
The chart below shows the FWHM measurements of each subframe pair in pixel units. The FWHMs of all subframes are below 1 pixel. Seeing conditions were good to excellent during acquisition. On my setup pixel size is the primary limitation on achieved FWHM in good seeing.