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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.

What next, for Earthshine observations?

Links to sites and software, Observation Resources, Optical design, Post-Obs scattered-light rem. Posted on Jul 21, 2021 10:53

We now take the next steps, after building the Mauna Loa system and operating it: into Space with NASA!

The reason is simply that, from space we can avoid the variability due to observing through an atmosphere and do not need a global network of earth-bound telescopes – one suitable instrument will do it all from orbit.

Even from NOAA’s Mauna Loa Observatory (MLO) — one of the best observatories anywhere on Earth — we could easily see variability in our results due mainly to very thin high-altitude clouds. Extinction at MLO is low of course, but variable. Sure, the ‘bad seeing nights’ can be eliminated by detecting the variability each night, but then you end up with not very many good data!

When we noticed a small Earth-Observing student satellite ( @flying_laptop on Twitter) from University of Stuttgart — we asked if they would try to catch some images of the Moon for us — and they did! The images were interesting and taught us the importance of optimized optical design — optimization for the sake of driving down ‘scattered light’ (really, a phrase covering aperture diffraction as well as various internal scattering processes). We worked with the students and reported on what we found at the 2019 annual European Geophysical Union meeting in Vienna, in the “Earth radiation budget, radiative forcing and climate change” session that Martin Wild, and others, organize each year. See our poster here.

Building on that experience, we are now looking at more ways to go into space, and also to improve on the earthshine instruments we can orbit.

One such effort is with the SAIL (Space and Atmospheric Instrumentation Lab) at Embry-Riddle Aeronautical University in Daytona Beach, Florida. With three friends there we are putting together a NASA Instrument Incubator Proposal (IIP) for development of an optical system optimized for the task at hand: observing high-contrast targets with quite extreme requirements for performance.

In our present ground-based approach we are never really observing the Moon without a contribution from the atmosphere, and must resort to various ‘subtraction schemes’ to get rid of either a ‘halo around the Moon’ (which is light scattered along the path to the image sensor) or a ‘flat sky level’ which can be due to such things as airglow, or the Moon-light scattering up from the ground onto particles in the atmosphere (this is not a ‘halo contribution’). Both kinds of contributions have to be removed before the faint earthshine can be used for terrestrial albedo studies. From space, both of these contributions would be omitted automatically, leaving only a faint contribution due to aperture diffraction — our goal is therefore to study how to build a telescope that has as little diffraction as possible.

With our group of experts in optics and satellite payloads at Embry-Riddle we are considering refractive optics, advanced sensors and ‘baffling’ to minimize unwanted light reaching the image sensor. Our IIP proposal is being submitted this week!

We have tried NASA proposals before, and this is the second try, building on reviews of our first attempt. The opportunities — the proposal call aims — vary, so emphasis is different this time: First time we focused on the sensors, now we are working on the optics.

Instrumental zero points

Observation Resources Posted on Jul 01, 2012 03:47

Peter and Chris observed NGC6633 – an open cluster in the Galactic plane — on 26th June 2012. The aim was to calibrate the zero points and colour dependencies of all the filters.

Coordinates are RA = 18h 37m and DEC = +06 34 (J2000) — which goes more or less overhead. We observed very close to the zenith, so the airmass was very close to 1.0.

Image above shows the FOV for the IRCUT filter (which is very similar to V). About 80 well measured stars are identified in the cluster and marked with blue circles.

We got great images for all 5 filters (first time we got all 5!).

Johnson V and B data were obtained from WEBDA:

Exposure times were:
V 25 sec
B 34 sec
VE1 12 sec
VE2 32 sec
IRCUT 12 sec

Counts were measured in the ~ 70 identified stars in a 2.5 pixel aperture (i.e. 7*2.5 = 20 arcsec radius aperture).

Transformations to V, B, VE1, VE2 and IRCUT were derived from ~ 70 stars, where the instrumental magnitudes for each filter are of the form:

Vinst = -2.5*log10(Vcounts/exptime)
Binst = -2.5*log10(Bcounts/exptime) etc…

V = Vinst + 15.07 – 0.05*(B-V)
B = Binst + 14.75 + 0.21*(B-V)
VE1 = VE1inst + 16.30 + 0.18*(B-V)
VE2 = VE2inst + 13.88 + 1.09*(B-V)
IRCUT = IRCUTinst + 16.43 + 0.16*(B-V)

The scatter in the derived relations is
V 0.02 mag
B 0.05 mag
VE1 0.04 mag
VE2 0.06 mag
IRCUT 0.06 mag

The transformed data for the stars are shown in this plot:

The V and IRCUT filters both transform to Johnson V, and the B filter transforms to
Johnson B, with relatively small colour terms (i.e. the dependency on (B-V) of the transformation).


Firstly we compare these relations to those derived at the last (only partially successful) attempt to calibrate the filters using M41.

The report on the M41 data is here:

The transformations from the two clusters are:

V = 15.15 + Vinst – 0.08*(B-V) : NGC6633
V = 15.07 + Vinst – 0.05*(B-V) : M41

B = 14.46 + Binst + 0.26*(B-V) : NGC6633
B = 14.75 + Binst + 0.21*(B-V) : M41

The colour terms are quite similar, but the zeropoints differ substantially, particularly for B. Since we were unsure about the quality of the M41 data, I think these these should be disregarded. We’ll do NGC663 a few more times over the next few weeks — it’s a very well placed cluster, and the stars are quite sparse — which is very good for us.

We need a set of images as the cluster goes to much higher airmass, so we can measure the extinction coefficients for each filter. I think we have at least some of these data already from 26th June 2012.

TO DO: apply these to lunar images to measure the colour of the brightside and earthshine light.

MOON table for 2012

Observation Resources Posted on Jan 01, 2012 10:39

The table gives times and positions when the Moon is suitable for observing from MLO. The columns are: JD, SEM angle, illumination fraction, Moon altitude, Sun altitude and UTC date. An asterisk at the end indicates the special SEM angle near 42 degrees. PDF table – click on the icon:

New category

Observation Resources Posted on Jan 01, 2012 10:34

Let us collect various tables, plots and links under the new category so that it becomes easier to find them when observing sessions need to be planned.