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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.

Ghost and Dragging

Optical design Posted on May 21, 2013 07:58

Here are example sof a ‘ghost’ and ‘dragging’:

The dragging is due to no shutter being used – i.e. readout was only way to terminate exposure and hence frame was illuminated as the image was shifted to the ‘hidden register’ We hav a frame-transfer camera. The image is from the test phase in Lund and no shutter was installed then.

The BS ghost is faintly vissible: It is a copy of the BS and the lower cusp is seen poking out slightly down and to the left of the real BS. This was due to the CCD camera being aligned almost perfectly along the optical axis of the system – the ghost arises as the shiny CCD surface reflects light back into the optical system and reflections are produced at all optical surfaces – the back of the second secondary lens, the front of the second secondary lens, the back of the first secondary lens, the front of the first secondary lens and the back of the primary objective and its various surfaces. At MLO the camera was tilted at an angle so that the reflection from the CCD did not go back up the system. This tilting did not appear to have any adverse effects on focus etc.

Images from lab hard disc recovered from MLO.

More on SKEs

Optical design Posted on Jan 26, 2013 10:06

In this post:
the importance of the SKE for scattered light was discussed. The images shown, though, were not shown fairly – with intensities scaled to comparable levels. I therefore extracted a line across the BBSO image and a line across our image, at right angles to the SKEs, rescaled the intensities, aligned the plots and get this:
The black curve is from our image (whichis a sum of 10 well-exposed images). The red curve shows the cut across a single BBSO image.The BBSO image has only the DS peeking our behind the SKE, while our image has the BS in full view.

We see entirely comparable ‘halos’! The BBSO image ha a more pronounced sloping ‘tail’ onto the black side of the SKE than we do, and more noise. If that sloping tail is ‘halo’ we had a better system than the BBSO!

What does the above mean? It does NOT mean that we have less ‘halo problems’ than BBSO does – because the BBSO expose their DS so that the halo from the BS is not allowed to be formed. Yes there is a similar halo from the DS on their images as there is from the BS on our images – but the halo from our BS is very much stronger than their DS halo.

When the BS halo is small – i.e. near New Moon – we have minimal effect of the BS halo.

Telescope spectral report

Optical design Posted on Dec 30, 2011 09:38

This is Ahmad and Rodrigo’s report on the spectral properties of the telescope:

Colour filters

Optical design Posted on Dec 27, 2011 14:52

The filter transmission as measured by Rodrigo for the B, V, VE1 and VE2 filter up to 800 nm and the IRCUT filter for a much wider range as given by the manufacturer.

ND0.9 filters do nothing

Optical design Posted on Dec 21, 2011 20:43

On the lamp in the dome we can test the densities of the various ND filters by calculating and comparing the observed fluxes with and without the filters. So far I have tested most of the ND0.9 filters. They appear to be blanks – or have not rotated into position.

Filter Without ND With ND0.9
B 3511 3512
V 26500 26500
VE1 210700 209700
VE2 268200 273000
IRCUT 196050 no data

The numbers are in counts/second and are based on the measured mean counts in the various frames and the exposure time MEASURED and reported in the FITS header.

The Spur

Optical design Posted on Oct 18, 2011 22:21

Ahmad pointed out that ‘the spur’ seen in coadded images of stars, and interpreted as an effect of the shutter staying open while readout is performed, is due to something else – not readout; because the readout direction is at right angles to the spur.

Diffraction-limited optics

Optical design Posted on Oct 03, 2011 14:03

According to theory, the aberration for a lens in a circular aperture, like ours, is given by the Airy function which is proportional to (BESELJ(r,1)/r)^2.

For a single wavelength the function looks like this:

Theory also predicts that assymptotically the envelope of the Airy function should go as 1/r^3, which is also plotted on the graph above.

For mixed-wavelengths as in our case the curve becomes smooth(er) and we should expect to see our PSF as a 1/r^3 envelope. Steeper envelopes than this should not be observed in correctly analysed optical systems. This has bearing on the 1/r^2 ‘King profile’.

Scattered light analysis

Optical design Posted on Mar 18, 2008 10:34

Here is Mette’s analysis of the BBSO telescope vs. the Lund design, in terms of Ghost intensities and scattered light from front lens surface.