To check consistency we now look at V minus V images, where the two V images are chosen to be closer in time than 30 minutes but not taken at the same time. We correct for extinction; we convert the raw images (bias subtracted, of course) into instrumental magnitude images by calculating the flux from the nominal exposure times and taking -2.5*log10, and then align them and subtract. We shoul dget images of 0s since the fluxes, once corrected for extinction, should show the same flux - at least on the BS where the Sun is shining. We do this and get a royal mess:
In each frame the insert shows a color-contour plot of the V-V image. The graph shows the usual slice across the middle of the image, averaging over 40 rows.
We see in upper left panel a clear offset at the terminator - i.e. the DS has different level but the BS are similar. Upper right shows a failry decent pair of images - the terminator is giving some problems and the DS as well as the BS differences are offset from 0 by a small-ish amount which coul dbe caused by an error in exposure time of some 10% or so (not unlikely). Lower left shows that while the two BSs are at teh same level then the DSs differ violently. The lower right shows a really nice example of two images agreeing.
What is going on? We hand-inspect the above images and see that in the case of the lower left image the sky level is much higher in one of the two images used, although they are observed less than 30 minutes apart. The counts inside mare Crisium are 3 and 11 in the two cases - i.e. a factor of about 4 or a magnitude difference of 1.5 - well outside the plot frame. So, from this we learn that we have a method to detect stable sky conditions! It also tells us that using images from different filters should be done with great care - even small differences in sky conditions will cause the DS to shoot off!
As it is, these images are merely bias subtracted - there is no individual correction for a 'pedestal' due to sky conditions. Luckily we can still use these images for albedo work in that we expressly fit a pedestal term!
What else can we learn? Well, any error in actual exposure time will influence the DS and the BS with the same factor - hence the magnitude differences plotted above will become offsets for both DS and BS - hence, upper right is consistent with 'wrong exposure time' in one of the images. Having assigned one filter and received another will have the same effect as an error in exposure time - if the DS and BS are altered by the same factor. I wonder if 'wrong filter' could camouflage as 'wrong exposure time'? Since DS and BS have different colors I doubt it - but we should investigate this.
So, three things learned:
1) Wrong exposure time will lift or depress DS and BS by same amount.
2) More sky brightness in one image than in the other - affects DS only.
3) Image pairs like the ones used for lower right panel, above, are probably both OK.