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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.

Effect of SKE on halo

Post-Obs scattered-light rem. Posted on Jan 22, 2013 01:39PM

We have some images of the SKE inserted in the ray path of our telescope, during Moon observations. We also have some images of the same from the BBSO telescope, kindly lent to us by Phil Goode.


BBSO images om two separate periods about a week apart. Top row shows histogram equalized imges of the BS (left and the DS (right) with SKE superimposed over the BS. Lower row shows the same – BS short exposure on left and DS long exposure with SKE on right.

Histogram-equalized image of the Moon with SKE inserted from our own telescope. SKE runs from upper left to lower right and is opaque above that line.

Our image shows a halo around the Moon that stretches behind the SKE – hence we know that halo is formed to a large degree in the secondary optics! On the BBSO images we do not see this as clearly – either because the BBSO telescope is of a different design or because the exposures with SKE (right column of images) covers much more of the BS. Assuming the latter we understand why there is no halo behind the BBSO SKE – the BS was effectively blocked and no bright light entered the secondary optics and could not cause a halo. On our image the SKE is inserted experimentally only, leaving a large part of the BS to shine into the secondary optics.

So, we learn that the halo is not from atmosphere or primary optics alone – apparently a large fraction of it comes from the secondary optics!

We also see the importance of having an SKE. While the BBSO group uses the ‘linear BBSO method’ to remove scattered light over the DS they have a much smaller problem than us because the halo from the BS is nowhere near as strong as ours is!

We now see how terribly important the SKE is.



Natural variability in albedo

From flux to Albedo Posted on Jan 22, 2013 09:09AM

Following on from post we now inspect satellite images one week apart in order to understand the natural variability found in satellite images of the same area.

Upper panel: average image pixel value for sequences of MTSAT images, 1 hour apart, for almost one day in March 2013 and a week later (red curve). Image pixel values is in arbitrary units but is proportional to pixel brightness. Lower panel: difference between upper panel black and read curves, expressed as a percentage of the mean of ther ed and black curves.

The difference between the two curves is on the order of 10% and varies from 8% to 18% during one day.

This tells us two things: Albedo (or something proportional to albedo) can vary by roughly 10% over a week. Albedo can vary during one day by almost as much.

This is useful information to have when we interpret the earthshine data.

We keep in mind that the smooth variations in the black and red curves in panel one are due to the day/night cycle – not intrinsic albedo variations: but the difference between the curves and the variability in the difference tells us about albedo variations.