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Earthshine blog

"Earthshine blog"

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

B-V and cloudiness variations

From flux to Albedo Posted on Feb 09, 2014 08:20PM

The amount of clouds on Earth changes from day to day, and as clouds reflect sunlight (with B-V=0.64) more clouds cause B-V of the earthlight to drift towards the solar value, while less clouds shows the Earth bluer. How much do we expect the terrestrial B-V colour to change day to day on the basis of changing cloud masses?

Inspection of GOES West full-disc satellite images of Earth shows that the local noon (i.e. disc is fully illuminated) brightness variations due to changing clouds amount to about 3.2% of the mean brightness. As the Earth below is dark (Pacific Ocean) almost all of the brightness is due to clouds so we are not too far off by saying that the amount of clouds varies by 3.2% around its mean value on a daily basis. With Earth on average being almost 67% cloudy we can re-use the Stam models of expected terrestrial spectra to see what the expected changes in B-V is due to cloud variations.

We find that B-V will vary, with a standard deviation near 0.005 around the observed value of B-V=0.44.

This is smaller than the total observational error we have. We have shown that the theoretical Poison-noise limited uncertainty would be 0.005 in B-V, but we cannot observe that well. Yet.

So – we should not expect to see values for B-V very different from 0.44, which helps explains why we get the same value as Franklin’s mean value using just a single (but precise) observation.

We may be able to see 2 and 3 sigma deviations in the cloud cover, however.

The purpose of our telescope was never to observe daily cloud variations – in the long term we hope to be able to qualify that we can set limits to climate-change induced changes in albedo.

Also, the above is just an investigation into whether we can use COLOUR changes to quantify albedo changes – we still have to quantify how well our direct-photometry measurements can see albedo changes. Paper II.



Brighter-Fatter effect

From flux to Albedo Posted on Feb 09, 2014 01:33PM

In an interesting paper by Antilogus, et al subtle effects of electron interactions inside the CCD material itself are discussed, which could be a tool for understanding and perhaps quantifying CCD nonlinearity.

We have tested our own Andor BU987 CCD camera for these effects and see them.

Plots of row- or column-neighbour means vs correlations. Top row: first panel: full set of values of means vs column-correlations
– we note the strong non-linearities that set in at about 50.000, and
some outliers.

Top row: second panel: for just the range of mean values below the onset
of non-linearity a robust regression is performed (red line).

Bottom row: same as top row but now for row-correlations.

We note the general correspondence to results in the ‘brighter-fatter’
paper: rows are more strongly correlated with their neighbours than are
columns – the ratio of slopes is about 2.0.

Can this be used quantitatively to correct for non-linearity in our CCD? The camera linearity was already tested for linearity while in Lund. See Figure 8 in the report.