Earthshine blog

Earthshine blog

"Earthshine blog"

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

First results

Data reduction issuesPosted by Daddy-o Aug 31, 2012 02:00PM
Here are some of the first results from applying the BBSO linear method - the reductions are slow so more will be on hand later! Sorry about the low quality in the image - there must be a way to do it better, but ...

Rows 1-5 give results for each filter. Ignore columns 1,2 and 3 for now - they are diagnostic. Column 4 shows the ratio of observed terrestrial albedo to modelled terrestrial albedo as a function of lunar phase.

We pick out the DS as either Grimaldi or Crisium depending on which is in the DS. We then calculate the observed DS brightness divided by the brightness of the whole disk. We then do the same for the synthetic model, and plot the ratio of the two.

This may seem a strange quantity to plot, but consider that in the (unlickely case) that we both had perfect observations and the model was correct in all aspects, then we would see a ratio of 1.0.

If the model is somehow wrong - for instance if the phase function it is based on is unrealistic then the ratio would have a phase dependence.

As it is, we do not have perfect observations and we see a fair bit of scatter. The scatter comes about for several reasons - first of all the observations have Poisson noise - we are extracting a small 4x4 pixel patch on what is the dark side where counts are probably on the average of 5 or so. Additionally we have noise from the alignment between the actual Moon and the coordinates we have calculated from which to extract information - there is a missmatch of up to several pixels here, so for a small area like Grimaldi a few pixels error in placement brings you into the bright surrounding areas. For Crisium, which is larger, this is less of a problem.

Finally there is a still un-solved problem with synthetic models and observations apparently being off by some small amount in terms of a small rotation. This may be from the days when the CCD camera was actually physically twisted by a few degrees in its placement on the telescope. In such cases the intensities of the pixels extracted in the synethtic model and observed image are even more different.

So, some things to work on are:

1) Use larger DS patches, so that the Poisson statistics are not as much of an issue.

2) Put the patches in uniform areas on the Moon so that missalignments do not cause acquisition of contrasting areas. Inside large, even Mares or on the brighter highlands.

3) Use better estimates of disc centre and radius.

4) Figure out a way of aligning the synthetic model and the observed image better.

PS: More data are available all the time so the figure above will update now and then.

Added later:

Have implemented 1 and 2, by enlarging the area inside Crisium that is used, and using a rectangle in the highlands south of Grimaldi instead of Grimaldi itself. Also trying 3.

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