[Note added later: we have updated this posting using extinction corrections for B and V. kB=0.15 and kV=0.1 were used, and airmass=1.67. JD was near 2456016.82]
In preparing for the EGU 2013 conference, where we will have a poster on color results from the earthshine project, I want to show a ‘B-V’ image of the Moon – since we can, and may be the first to publish such a thing.
Using Chris’ calibration of our filters against standard stars in M41 and NGC6633 I can reduce observed images in any filter to ‘magnitude images’ in the same filters. By taking the difference between the B and V image I thus arrive at a ‘B-V image’. Here is a slice across the middle of that image:
Second image added later: We now correct fore extinction:
Here, the BS is to the right of the ‘cut’ in the disk, and the DS is to the left. It appears that the BS is higher than the published value for ‘moonshine’ of 0.92 [see e.g. Allen, “Astrophysical Quantities”, 4th ed, Table 12.14]. Added later: Even after correcting for extinction the B-V is still higher than Allen’s value – we now get B-V near 0.95-1.0. Perhaps now, not so much larger than that value?
I am not quite sure about the DS. There is probably a slope because the halo in B and V are not identical and thus do not cancel. But as the BS is approached the color of the halo does approach the BS value, since that is where the light comes from. So I understand that part.
At the extreme left of the DS there is least influence from halo. The value is a bit up and down, but appears to be lower than the BS by up to 0.3. If B-V is a smaller number in one place compared to another it means that B is smaller than V in the first place, relative to the second – i.e. on the DS B is relatively smaller there than on the BS – since these are magnitudes it means we have shown that earthshine is blue – the Earth is indeed ‘a blue marble’!
Here is the B-V image itself, with cleverly chosen colours to reinforce our message: Sky is masked out, BS is to the right, DS to the left.
I’ll try to redo this for a lunar phase with less halo.