On the night 2456003 we happen to have a nice sequence of the Moon rising through a clear atmosphere and we can determine the instrumental alfa – i.e. the power of the PSF due to instrumental effects only.
Normally alfa depends on atmospheric and instrumental effects but on a steady night it is possible to gather data for several airmasses and then extrapolate to airmass zero.
The different colors are for different filters (more or less matching the wavelength – blue to red). The data were obtained from the fits of the EFM method.
We see a little bit of scatter in VE2 (red) but otherwise a remarkable agreement in intercept and slope. It would seem that the instrumental alfa is very near 1.724 for all filters. Since we have a ‘base PSF’ (with a power of 1.6) which we raise to alfa’th power to fit observed PSFs we see that the instrumental ‘real alfa’ is near 2.76. The diffraction limit is at 3. We are not saying we have a system that is diffraction limited – but any values of alfa larger than 3 would be impossible!
If we assume that the instrumental alfa is always fixed or decreases (large alfa means narrow PSF, small alfa means wide PSF) with time then measuring the alfa_instr on all good nights would allow us to perform some sort of quality check on the data for that night. Also, keeping an eye out for the largest alfa_instr ever will help us set better upper limits.
Such diagnostic testing as the above should be made a routine part of a future pipeline for reducing data.
Note: This is our second visit to night 2456003 – we have also looked at the same as the above but based on FFM, here. The value for alfa_instr is nearer 1.76 for the FFM, suggesting that there is method-dependence. However, FFM is currently our most troublesome method, of the FM methods.