From flux to AlbedoPosted by Daddy-o Mar 20, 2013 11:09AM
I have located all 'good images' in B and V. That is, all B and V images, made from stacks of 100 images, that are on Chris' list of 'good images'. I have furthermore identified all pairs of B and V images that are taken less than 1 hour apart. For all these pairs I calculate a B-V image, by using Chris' calibration of flux against known standard stars. I allow for an extinction correction based on kB=0.15 and kV=0.10. I get airmasses from the Julian date of the image, and IDL software. I plot a colour contour plot as well as a 'slice' across each image. The slice is made up of an average of 40 rows centred around the row that goes through the centre of the B-V image. A total of about 33 image pairs have been plotted in this way. Here is a (large) pdf file containing the plots.
There are several strange things to see. Generally we get the profiles shown here. That is - the BS is somewhat flat, while the DS slopes towards the DS sky. Oddities include images where the DS is as flat as the BS - just at another level. Is that 'extremely good nights'? I think not. In some, the DS is much higher than the BS - that could be images in which the exposure time is incorrect due to shutter problems. Or filter-wheel problems! In many the BS is flat, but not near the 0.92 value we expect based on publications. For small differences with this 'canonical value' I think we could be talking about exposure time uncertainties. If we are at B-V=1.0 instead of at 0.92 we could have an 8-10% error in the exposure time. The times are short and it does not seem unlikely we have that big a problem. Sigh.
I will suggest that we 'correct the BS value' to 0.92 by simple shifting, and then consider how our DS B-V values look. With some 'deselection' of some obviously bad images we can perhaps arrive at a base set of good B-V images.
I think the result will be that few show any leveling off in the 'slope of B.V wrt distance to BS'.
We have few images at small lunar phase (i.e. small illuminated fraction). Chris has analyzed one good pair of B and V images at small phase, but it is not in my pipeline of good images - the automatically detected centre coordinates and radius are way off (because the sicle is so small that automatic methods do not work). Our new student Johanne is inspecting these images by hand and will adjust radius and centre coordinates and then we should have a few more images to play with at small phase. These may show us if we can get far enough away from the BS that the DS slope in B-V levels off and shows the true earthshine B-V color.
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