Earthshine blog

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.

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.