Earthshine blog

Earthshine blog

"Earthshine blog"

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

B-V images

Post-Obs scattered-light rem.Posted by Daddy-o Apr 29, 2013 10:29AM
We have previously considered B-V images of the Moon. This was done with 'raw' images - that is, images where the halo had not been removed. Since we have the BBSO linear method implemented and since it does clean up the DS we can also calculate B-V images for the Moon based on these.

We have 55 pairs of B and V images thata re close in time (about 1 houror less apart). Using the standard star calibration relationships that Chris worked out fropm NGC6633 standard stars, we can convert images to instrumental magnitud eimages and from there to calibrated B and V magnitud eimages. We also corrected for extinction since the images were not obtained at the same time.

SInce the calibration relationships depend on B-V we have to assume some B-V values and iterate (Chris solves algebraically). The iterations converge quickly. We use onlythe brightest pixels in each image - i.e. the pixels delineating the BS - for calculating the mean B and mean V values needed to update B-V in each iteration.

The values for BS B-V that we converge to have this distribution:

The mean B-V=0.989, and the S.D.=0.019. The accepted value - e.g. Allen (4th ed), Table 12.14, gives B-V=0.92 (van den Bergh observations?). We therefore have a significant discrepancy. It should probably be noted that our values come from phases near 90 degrees, while the Allen values may be from 'Full Moon' conditions.

If we accept the above B-V (BS) values at face value we can continue:

The Sun has B-V=0.642 (Holmberg et al, MNRAS 2006). One reflection off the Moon reddends this value by 0.989 - 0.642 = 0.347. This value will also apply to earthshine that is observed after one reflection off the Moon, even if it is the DS. [We ignore here any colour-dependencies in the lunar surface mare vs highlands!]

If we can estimate the B-V of earthshine as seen on the lunar surface, we can work backwards to what the B-V of that light was before it struck the Moon - it will be the observed value minus 0.347.

Before trying this we need to understand to which degree the use of BBSO linear images, as opposed to 'raw' images, has helped us observe the true colour of the DS - has an important amount of the BS halo been removed from the DS?

We generate centered B-V images and plot the average of 20 rows across the middle of the images:

We see two panels - each panel is the result of using a fixed B image and two different V images - all three taken a short time apart. The black curve is the run of B-V values in the 'raw' image - that is, the image where no effort has been made to remove the BS halo. The red curve is from images cleaned with the BBSO linear method. The deep jag in the middle is the BS/DS terminator. The DS is to the left of this and the BS to the right. Since the BS is not altered by the BBSO linear method the red curve covers the black curve on the BS.

On the DS we see that cleaning the image has resulted in a slight reddening of the DS - it was 'too blue' in the red images.

We also see that the 'linear gradient' in B-V across the DS is unaltered qualitatively by the cleaning of the image. Why?

If we push on, ignoring the not-yet-understood gardient, and assume that the part of the DS closest to the sky has an un-polluted B-V value then we can calculate the colour of earthshine before it strikes the Moon, as explained above. First we extracted DS B-V values for that part of the DS disk that is to the left of 90% of the vertical columns on the disk. These values were on average 0.29 +/- 0.05 below the BS value.

If the BS value is given the canonical B-V=0.92, then we have a B-V for the DS of 0.63.

Franklin (JGR 72, no 11, p.2963-, 1967) measured B and V repeatedly on the DS. The difference between his mean B and his mean V is 0.64. We are close, but we are worried about scattered light!

Subtracting the effect of reflection once on the Moon brings us to the value for B-V of earthshine, before it strikes the Moon, that is, as it would be seen in space:

B-V_ES = 0.28.

There is one published B-V value for earthshine, based on Mariner II data in the 1960s. The paper is http://adsabs.harvard.edu/abs/1964JGR....69.4661W by Wildey. Unfortunately I cannot make head or tail of that paper!

Playing a bit more with the above, we can consider the effect of Earth on light - the Sun has B-V=0.642 when it strikes Earth. If the earthshine has B-V=0.28, then the bluing effect of Earth is 0.28-0.642 = -0.36 in B-V.

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