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Earthshine blog

"Earthshine blog"

A blog about a telescopic system at the Mauna Loa Observatory on Hawaii to determine terrestrial albedo by earthshine observations. Feasible thanks to sheer determination.

Fluxes vs phases

Data reduction issues Posted on Aug 27, 2012 16:22

Since the setting of the colour filter (as well as the shutter) was unreliable we must find a way to detect which images are taken through which filters.

Here we plot the raw fluxes (counts divided by nominal exposure time):

Total lunar Fluxes plotted as magnitudes against the lunar phase (New Moon is at 0). Bright is up, faint is down.

The data for each image (black symbols), here expressed in magnitudes, are overplotted with a phase law (red) inspired by that in Allen “Astrophysical Quantities”, except we modify the coefficients in that and use instead:


Notably the coefficient on the linear term is about half of what Allen specifies.

Particularly in VE1 we note the presence of two sequences of data. We have ‘fitted’ (by eye) the sequence that is represented on both sides of the new moon to the Allen phase law. The orphan sequence is below the adopted data suggesting that a filter with less transmission was obtained here when VE1 was requested.

For B it seems that intermittency causes some fluxes to be higher, but they do not fall in a delineated sequence so cannot be identified with a filter.

The V filter seems to have the same problem, although less so.

VE2 is also somewhat ‘broad’ in its distribution.

IRCUT shows two sequences.

The VE1 and IRCUT filters are extremely similar in transmission properties and are quite similar in the plot above, including the presence of the ‘second sequence’.

The ‘second sequence’ is quite similar in flux to the V observations, and it is consistent to say that when IRCUT and VE1 failed to be chosen V was obtained instead.

This seems to also apply to some of the outliers in VE2 and B.

We thus suggest that a working hypothesis for the failure of the filter wheel is that when it failed the V filter was selected instead.

We next proceed to eliminate the outliers in each filter so that a dataset can be defined which will allow identification of the extinction laws in each filter. With that in hand it may be possible to ‘tighten up’ the data and move towards a ‘golden dataset’ from which also DS fluxes are worth extracting.

The presence mainly on the left side of the diagrams of a ‘second sequence’ of data implies that something like a mechanical problem is behind the FW failure – because the telescope is flipped over the meridian to observe mainly in the East or the West depending on whether the Moon is rising or setting (before or after New Moon).

Synodic period of Moon

Post-Obs scattered-light rem. Posted on Aug 27, 2012 11:36

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.