From flux to AlbedoPosted by Daddy-o Feb 28, 2013 10:08AM
In this entry: http://iloapp.thejll.com/blog/earthshine?Home&post=304 we investigated the effects of using a lunar albedo map based on scaling the Clementine map to the older WIldey map vs. scaling the Clementine map so that lunar mare and highlands matched what is published in the literature. [Note: the point being that the 'Clementine map' we have available is just a picture file - jpeg! - so that pixel values have to be scaled to albedo values somehow.]
We were doing this fitting in the 'new way' which is to fit 'profiles' starting on the sky on the DS side and extending onto the DS itself, modelling the contribution from the scattered BS light. In doing this we saw that improved fits could be obtained if the lunar map albedo was 'stretched' so that dark and bright areas better matched our observations. We did this stretching by eye and were able to improve the formal fits.
We have now compared the fits on 535 images done using the 'Clementine scaled to Wildey' map and the 'Clementine stretched by eye' maps.
We have found that the RMSE (that is, the square root of the sum of the squared residuals [i.e. observed profile minus best fitted model profile]) is improved using the scale by eye' map - in two ways: a subset of images were poorly fit using the other map; they are now much better fit, and the mean RMSE of the finally selected images is lower.
We selected 'good fits' on the basis of alfa (the PSF-width parameter) having to be in a narrow range and that the relative uncertainty on the fitted albedo should be below a certain limit.
Mean log10(RMSE) is now near -1.1 in units of counts/pixel. We fit a profile that is 150 pixel columns long - 50 pixels on the sky and 100 on the DS.
Mean relative fit-uncertainty on the albedo is near 1.5% when using 'counting statistics errors' on the observation.
We note that the VE2 images more frequently have a larger 'pedestal' or sky offset after bias removal than the other filters. While most filters have an offset of near 0.2 counts (+/-0.5 or thereabouts) the VE2 offset is more often near 4 or 5 (+/- 1-2 counts). What is the cause of this? The bias frames surrounding the VE2 exposures have been spot-checked and seem OK - bias is near 400. A few, otherwise perfectly all right, VE2 images have a sky offset of 10-40 counts! Observations from the same night in other filters show nothing like this - some sort of nIR 'fog'? Does the sky emit nIR light?
Does this have a conseqeunce for the 'tunnel selection' of VE2 images, done by Chris? [A data-selection method designed to take into account shutter and filter-wheel problems by requiring that image total fluxes follow a known phase-curve.]
While the 'new fitting method using stretched lunar albedo maps' formally works best of all methods we have seen so far, it is very empirical. Later we may be able to use the method to produce a 'best fitting lunar albedo map' that improves on the original Clementine map. We may then also be able to compare to what the LRO people (and LLAMAS) are finding.
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