Earthshine blog

Earthshine blog

"Earthshine blog"

A blog about a system to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

For the poster at EGU: comparing methods' results

From flux to AlbedoPosted by Peter Thejll Mar 29, 2013 03:21PM
[edited versions:] We have discussed many ways of extracting albedo from our data. We first considered the 'ratio over ratio' methods - they consisted of extracting counts from patches on areas of the scattered-light cleaned images in the DS and the BS - the ratio of DS/BS observed to DS/BS model is proportional to the albedo of the Earth. The other method, used more recently, is based on 'profile fitting' near the DS sky edge.

We have now arrived at comparing the 5-color albedos derived from these two methods:

Right
: Albedos for positive and negative lunar phases (Full Moon is at 0 degrees) from the DS/BS method where the "BBSO log" method has been used to remove scattered light. Left: Same but for the profile fitting method. Note different vertical scales on axis. The same nights were considered for both plots, but not all are present in both, due to outlier removal, etc.

We see, for both methods, a rise in the derived albedo as phases nearer Full Moon are considered. This is possibly due to effects of scattered light from the BS which has been incompletely handled by the respective methods. The values found with the two methods are quite similar - apart from increased scatter and less colour-separation in the DS/BS method. The data for the positive and negative branch of the phase diagram are not similar, in either case.

From tests shown elsewhere in this blog we do not expect the halo to interfere with the DS for large absolute lunar phases - i.e. near new Moon. The above diagrams shows lowest values for phases near 110 degrees. There is a slight increase in values larger than this - what can that be due to, if the halo is interfering less and less? Well, we must again remember that the above results are model-dependent and the Model may be adding its own fingerprint. For instance, it may be that the synthetic images we use have a phase-dependent error in their representation of lunar and/or terrestrial albedos. Note that the same synthetic models are used in the DS/BS method as in the profile-fitting method.
This question can be addressed by studies of the effect on the 'bend' seen above of different BDRF models.

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