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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.

B-V for JD2456034 – updated

Showcase images and animations Posted on Dec 13, 2012 10:06AM

In this – uipdated – posting we combine more images from JD2456034 – we construct B-V images in various ways.
The discussion refers to Chris’ originalposting:
http://iloapp.thejll.com/blog/earthshine?Home&post=275

The first images we look at are not the bias-reduced images Chris uses, but ‘EFM-cleaned’ images (i.e. scattered light has been removed). Both the B and V image used were co-additions of 100 images each. The two images were centered in the image frame. We look at tow sets of B and V images – the one at the top is cleaned with one setting of the EFM method; and the one below is done with another setting of the EFM method. Some of the features to the left on the sky are the effect of ‘cyclical overlap’ from the right side when the image is shifted.


We see the DS to the left and the BS to the right. The BS halo and the BS have become undefined (i.e. ‘NaN’) because either B or V is negative here (remember, it is not a bias-reduced image with full halo in place – these are EFM-reduced images so the BS and parts of the halo are now zero or negative. In overlap almost all of the BS and the BS halo have become NaNs!

Here is a ‘slice’ across the middle of the upper image:

And here is the same slice across the lower image:
And finally, here is the slice across the un-reduced B-V image – that is, the image formed from B and V images calculated from the images that were only boas-reduced (as in Chris’ plot).
The uppermost image has a large ‘dip’ when we get near the BS and its halo remnant . In the middle image the halo has apparently been better removed. STrangely, thereis least sign of a halo in the ‘raw’ images where nothing has been done to remove the ahlo. This needs to be discussed!

We used other airmasses than Chris. In my calculations the airmasses for the two images involved are:

B_am=2.545 image is 2456034.1142920MOON_B_AIR_DCR.fits
V_am=2.477 image is 2456034.1164417MOON_V_AIR_DCR.fits

I used kB=0.15 and kV=0.10, like Chris.

I did not solve for B and V by solving two equations with two unknowns – I iterated. Convergence was fast. I iterated on the whole images.

The first of the above EFM cleanups was done with a weighting of the mask used to define the area of the sky on which to reduce the sum of squares that favored the DS part of the sky. In the second attempt above equal weight was given to the RH and LH sides.



More colour mapping

Showcase images and animations Posted on Dec 13, 2012 04:42AM

Following on from the previous post, we have made a colour map based on two images (V and B) taken on JD2456034

2456034.1142920MOON_B_AIR_averaged.fits
and
2456034.1164417MOON_V_AIR_averaged.fits

Both are averaged results from stacks of 100 exposures, so the S/N ratio is 10 times better compared to the images in the last post.

V band exposure time was 72.5 ms, and the B band exposure was 222 ms, for a total of 7.3 seconds in V and 22 seconds in B. The observations were at an airmass of ~ 2.3.

The colour map looks like this:


with the colour scale (B-V) running from 0.2 to 1.3.

The BS (brightside) has a colour of around 1.0, which is still a little redder than the expected value of 0.9 (for BS on a full moon) by van den Bergh (1962).

There is more scattering of the light in B than in V, which accounts for the deep valley to the right of the BS, and the excess halo light beyond it. Colours here are not reliable. Colours just beyond the concave edge of the crescent on the DS are probably also affected by differences in the scattered light profile, so should be regarded carefully (this part of the moon can be examined when the crescent is on the other side). The left half should be pretty good!

The DS (darkside) has colours ranging from about 0.6 to 0.9 — with the lowlands redder than the highlands.