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

Synodic period of Moon

Post-Obs scattered-light rem.Posted by Daddy-o Aug 27, 2012 11:36AM
In order to precisely remove scattered light from the observed images we need to know the centre of the lunar disc in image coordinates, as well as the radius. These numbers are used by the BBSO method, while the EFM method can work without them. Extraction of fluxes from designated areas of the Moon also requires knowledge of the disc coordinates.

We have, as described elsewhere, found a fairly good way to estimate disc centre and radius, and have more than 5000 images (singles or sums of stacks) with know coordinate estimates.

We can check on the quality of these data by inspecting the time evolution of the disc radius in terms of the lunar synodic period (27.322 days).

Plotting the detected radius against observing time modolu 27.322 we get, for the 5 filters:

There seem to be some outlier groups, as well as a general scatter. The scatter is on the order of 2-3 pixels while the outliers reach 5. These outliers can be identified and the relevant images inspected.

We fit a general sine curve to the data, and get:

Filter: B
Offset Amplitude Period
141.169 7.98152 27.6367
+/- 0.0108026 0.0166443 0.00208776

Filter: VE1
140.663 7.81091 27.5796
0.0122441 0.0199136 0.00187041

Filter: V
140.647 -7.47896 27.5736
0.0413606 0.0837630 0.0111588

Filter: VE2
139.845 8.51341 27.5771
0.0245651 0.0383531 0.00435187

Filter: IRCUT
140.744 7.98575 27.6255
0.0358718 0.0596396 0.00708377

The period is not close to the expected 27.322 days. We expect this is due to a poor fit (in turn due to the outliers). We identify the outliers. 108 images are found that have radius more than +/-3 from the fitted sine curve.

Upon inspection, it turns out that not many of the identified outliers are obvious 'bad images'. The determination of radius and disc centre is therefore somewhat deficient.

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