In the EFM-method we determine the alfa values for every image. Is there a link between the alfa value for one filter and the rest in an interval of time? It is our understanding that alfa is determined by the amount of scattering in the optics plus the atmosphere. We therefore expect that on ‘bad’ nights the alfa values will tend to move in the same direction. We investigate this here.

We find all alfa values in all EFM-treated images. We sort them into half-hour bins. We calculate all the alfa values in each bin and plot the results. Below is a pdf showing all the plots between some filters, at different ‘zoom-levels’. The image shows the last zoom-level, highlighting the dense ‘clump’ of points:

There seems to be a general agreement that the alfa values are correlated – bad nights (i.e. ‘broad PSFs’) occur in all filters at the same time. Since VE1 is just about identical to IRCUT the scatter seen above means that the fitting routine is unable to make a perfect match – or that observing conditions, during the half-hour bins used, changed.

Using 15 minute bins does not improve matters:

I therefore suspect that the fitting method does not find the best fit each time.

Very interesting indeed!We didn’t quite finish the study of the correlation of alpha with airmass — there was a weak trend there that I picked up — I’ll dig that out for a blog posting. I am concerned about the IRCUT and VE1 filters, because they show greater scatter (about double) in the difference between the MLO measured apparent magnitude of the moon and the JPL model – for no apparent reason at this stage. Perhaps the CCD response is a doing something strange to the red — or flat fielding out there is more important — or the filters have some IR or blue leaks in them that are not quite stable? We’ll have to look at this when we get the telescope back to DMI.