Earthshine blog

The recent total lunar eclipse gave an opportunity to apply photometric methods. It is possible that the light from the Sun, passing through the Earth’s atmosphere is coloured by processes that ultimately also have a climate effect on Earth. Recent volcanic eruptions may cause the eclipse to appear more red than usual.

Can this be measured accurately?

The Internet was overflowing with images and image sequences of the eclipse. Among them this composite image by K. Lewis (www.photosbykev.com):

Using the method described in the paper by Park et al, we can convert the RGB values from the image file above to Johnson B and V colours (the possibilities are limited to these two bands – more can be done with the NIR filter taken out of the camera).

We extracted the mean and median B-V colour from the sub-frames above, and plot these values in a sequence, starting from upper left and ending in lower right frame:

We see the reddening of the Moon as it is eclipsed. The dotted horizontal line is a nominal value for B-V=0.92 based on literature values for the Full Moon, and the measured values from the composite photo were adjusted up by about 0.6 in B-V to roughly coincide with the 0.92 level. We see that the eclipse was redder than the Full Moon by about 2.5 in B-V.

The jagged outline of the curve, and the evident jumps in brightness in the main photo, seem to indicate that the detector used was not linear – this could be because the images were obtained not only on the linear part of the CMOS devices’ transfer curve.

What are published B-V values from eclipses, studied with professional equipment and attention paid to conservation of linearity? Figure 4 of the paper “Wideband Photoelectric Photometry Of The Jan. 20/21, 2000 Lunar Eclipse”, by Schmude, et al, 1999, in International Amateur-Professional Photoelectric Photometry Communications, Vol. 76, p.75+
(http://adsabs.harvard.edu/abs/1999IAPPP..76…75S) shows a change in B-V of about 2.5 magnitudes during an eclipse – like above.

We obtained other images of the 2015 eclipse and also converted the RGB values to B-V, but found eclipse B-V very different from the above. Attention to obtaining Full Moon images just before and after the eclipse takes place is required, as is some attention for exposure levels, trying always to capture the various stages of the eclipse at mid-sensitivity range of the device used.

Also important is that there are gradients in the shadow of the Earth on the Moon and the geometry of the eclipse and the times of observation – and the location on the lunar surface used – all influence the results obtained.

This is a trial video of all our good V-band images. On the left is the linear image, on the right a somewhat histogram equalized image. Want to embedd this video, but here is a link until further notice.

From the International Space Station (ISS) the Moon can of course be seen, and has been photographed with hand-held cameras. Someone took an image of the Moon with exposure settings chosen to highlight the earthshine. Below, such an image is discussed:

At the bottom the jpeg image is shown. A line has been drawn on the image. The slice of the image is plotted at the top. The R channel in the RGB file was used – G and B are similar but there is a higher sky-level than in R. More details on this image is available at http://eol.jsc.nasa.gov/scripts/sseop/photo.pl?mission=ISS028&roll=E&frame=20073#

On the plot we clearly see the dark sky above the Moon, the entry onto the lunar disk, the extensive dark side, the bright side and the transit of the bright atmospheric layer and then entry onto the dark Earth below.

We note that the bright side is saturated as the intensity reaches 255.

We do not know whether astronauts on the ISS have available high-dynamic range cameras. We think the above image is snapped with a standard 8-bit camera.

Using 16-bit handheld CCD cameras – such as a Hasselblad fitted with a 16-bit digital back – images of the earthshine could be obtained that showed bright side and dark side at the same time without bright side saturation allowing the DS/BS ratio to be calculated. That ratio is proportional to terrestrial albedo.

Such images could be of educational value and could highlight the role that earthshine studies can have in climate change research.

Added later:

Since the atmospheric profile is quite uniform horizontally in the above image, we estimate the mean atmospheric profile and subtract it from the slice across the Moon, thus revealing the profile across the Moon, without atmospheric effects:

The top panel shows, in black, the profile across Moon plus atmosphere; the red graph shows the profile of the atmosphere adjacent to the Moon; the bottom panel shows the difference.

We note that the atmosphere-free profile in the bottom panel above shows almost no ‘halo’ from the bright side – unlike our own images, taken from MLO, through the atmosphere.

That piece of information is quite interesting: As we understand it, our own images have a contribution to a halo from the optics of our telescope along with a variable halo due to the atmosphere scattering. From the ISS a camera, with a lens, was also used and one wonders why there is no sign of this optics-halo? Is the ISS camera optics vastly better than ours? This is not likely, since, probably, any old hand-held camera was used. On the other hand, modern lens-coatings (such as in the Canon ‘Dragon-eye coating’ lenses) have high standards and are designed to suppress lens-scattering and internal multiple reflections.

So, we wonder if the ISS has some special lenses at hand?

We note that the above image is for an extreme ‘almost new Moon’ situation with little flux from the bright side to scatter and interfere – we should see if more ISS images of the Moon, showing earthshine, at phases more like 1/4 are available? If those images also do not have any appreciable halo due to optics we might be on to something.

Note that the image of the Moon is probably taken through a clear spherical dome or window mounted on the side of the ISS – what are the implications of this for observing the Moon through a dome, here on Earth? Is distance between dome surface and camera lens a factor?

Notes:
1) Extinction not accounted for – this may cause the ‘slope to the left’ of the DS seen above.
2) The image was taken in August 2011 using a Nikon 3DS camera. That is a 14-bit camera taking .NEF frames. We have this frame from NASA now.

An example of what our fit residuals look like. Note the structure around the BS (right, on the lunar disc). Also the straight-edge structure on the BS sky … do we have internal reflections going on here?

Structure on the DS is in the tenths of counts range.

Here is the same image, but now in % of the local value … don’t scream! We have +/- 50% errors on the BS – but less elsewhere …

Here is a movie of what 100 images in a stack, on a VERY CLEAR NIGHT, looks like if you look at frame-to-frame changes, and histogram equalize the images:

//www.youtube.com/embed/g2uyeH5X96I
If you monitor 4 patches of 20×20 pixels each (one on the dark side of the Moon, one on the dark side sky, one on the bright side of the Moon and one on the bright side sky), you get these mean series:

There is some food for thought here. Consider the DS patch – it varies by tenths of counts (this is mean over a 20×20 patch) – the mean of that patch is typically several counts – so we are looking at several percent to ten percent changes with time here. Of course, we do not fit individual images, but only the stack average.

Notice how BS sky patch is dropping in intensity in this sequence – this is consistent with BS patch getting brighter – less light is being scattered to sky here.

If the sequence of images is scaled to the total flux of one of the images in the sequence we get a small drop in BS patch variability – it goes from 0.22% to 0.12%. SO either the shutter is variable and causing this, or the total amount of light in the 1×1 degree frame is variable, for other resons: scattering or extinction may remove light from the beam entering the telescope. But certainly, the light in the small 20×20 BS patch varies on its own even in flux-normalized images, so non-shutter variability is present.

We have the unique B and V images from JD2455945.17xxx in which the ‘halo’ cancels almost perfectly, allowing us to see the colour of the DS. At the time of observation we can ask what the Earth was like. Here is the image generated from the Earth Viewer at Forumilab ( http://www.fourmilab.ch/cgi-bin/Earth ):

Above is the model image of Earth seeen from the Moon at the time /Jan 18 2012, about 1600 UTC) of our observations. Below is imagery from the GOES West satellite. There are also GOES images from 12 UTC and 18 UTC, the above one is from 15UTC and is the closest in time.

Evidently the Moon and the GOES satellite were not in line as the aspect is slightly different but we get the idea: Southern Ocean and Antarctica was white with cloud and ice, North and South America were partially cloudy, and the sunglint was on ocean off the coast of Peru, approximately.

Here is a simple model image, as if seen from the Moon at the relevant time. The image is generated from geometry and the ‘NCEP reanalysis cloud product’. You notice some similaririties in cloud cover between the satellite image above and this model image. The model only does the perspective projection of pixels onto a sphere – there is no modelling of limb brightening or anything like that.

The images does allow a simple analysis of the effects of various extreme values for land albedo, and used for two assumed wavelength bands could be used to set limits on what we expect the colour of the earthshine to be at this particular time. More to follow!

We help illustrate the theme of ‘simplicity’ for Aberdeen:

http://www.simplyaberdeen.com/simply_being.php#.UbiukM7CVic

Here are some images of the telescope assembled – perhaps it is possible to use the images to understand which plug goes where? Sharper jpegs are in /home/pth/SCIENCEPROJECTS/EARTHSHINE/JPEGS/ – they are the ‘imgXXX.jpg’ files.

I’ve also added images from the construction in Lund – those images are in same place as above, but are called imagexxx.jpg (xxx from 332 to 347).

This link shows more images from the telescope on MLO.

Here is an image of the cabling at the back of the CCD, during MLO.

Presentation at Swinburne (Melbourne) in November 2012

This earthshine presentation was given at DMI, on May 3 2013.

During processing a strange signal was found in a frame combined from 100 images. It turns out that in one image of this stack we have what appears to be a meteor or a satellite flash – or something:

The image is from Jan 17 2012: (UTC 2012-01-17T13:28:48). The exposure time was 0.009 seconds! The trail is about 3/4 of the lunar diameter in length – i.e. about 22 arc minutes. The orientation is such that it is travelling almost Due North (or South!).

What is it? Well – it is clearly between us and the Moon! If it is a meteor its height would be something like 50-100 km. The speed would then be 35 – 70 km/s. A satellite in low earth orbit has speed 8 km/s. Since the image is taken from Hawaii at UTC 13:28 it is near midnight on Hawaii – i.e. the Sun is behind the Earth and unlikely to be illuminating a LEO satellite.

An airplane flies at 800 km/hr at altitude 10 km, so the distance covered in 0.009 s is 2 m which would subtend an angle of 1 arc minute. This is no airplane – or it is much closer, in which case we should see details of the plane.

As far as I can tell there is no pronounced peak in meteorite activity in January.

I have made four colour maps now in B-V on four different nights.

The nights are arranged like this:

JD2455938 JD2455945
JD2456015 JD2456034

A uniform scale is used for all four images, from B-V = 0.0 to 1.4. The BS light is coming out in all four images around B-V=1.0, and the ES at around B-V = 0.6-0.8,
depending on whether one is looking at highlands or lowlands (lowlands are redder).

The cause of the black dips along the BS rim is the problem that the B images are slightly larger than the V images – so even with good registration of the center of the moon (to the closet pixel), there are issues in producing these colour maps. The falloff in the halo light cannot be the same power law in V and B, because the halo changes colour — but this is still to be checked.

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.

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.

We have been using the Clementine mission lunar albedo map for our work. There is an older digital map, by Wildey (The Moon, vol, 16, 1977), which we obtained access to thanks to Tom Stone of the USGS.

Here is a graphical comparison of the two:

Here they are again, now interpolated to the same 2.5×2.5 degree grid so that they can be compared numerically, along with their relative difference:
Upper left: Wildey map. Upper right: Clementine map. Lower left: the relative difference = (W-C)/C. There are some interpolation artefacts on the rhs of both maps as well as in some of the edges, brought about when the full-globe result from Clementine (orbiting lunar satellite) and one-side-only result from Wildey (ground-based telescope observations) are accessed.

The map of the relative differences reveals an overall albedo offset with Wildey being darker than Clementine. In the Mare areas differences of 40-50% are found whil ethe brighter highlands have differences in the 5-20% range. The differences in albedo is therefore not a constant but depends on albedo.

We see no obvious longitude or latitude dependence in the difference. This may be relevant to whether we are interpreting the Clementine map correctly in terms of angle-dependence in the conversion from observations to the map. Or the same convention was used in the Wildey map, produced 30 years earlier!

[added later: more here]

Movie shows 9 images of the moon centered on a particular crater on the bright side.

Greyscale is “minmax” + “log” in DS9.

The halo light looks very stable around the bright side edge. Naively at least, it appears as if the determinations of alpha (power law fall off of scattered light at large angle) should be very stable.

In this time sequence one can see the terminator changing slightly — certain features brighten, others dim – – especially crater walls and mountains.

Movie is here:

http://www.astro.utu.fi/~cflynn/Vmovie.mpeg

play it with e.g. mplayer Vmovie.mpeg -loop 0
for a continuous loop.

Click.

Torben took these pictures at MLO:

Click here.

Our telescope is in the Groundwinds building:

The Earthshine observatory on Mauna Loa. Housed in the previous Groundwinds dome (left), our telescope (right), and Torbjörn and Henriette who installed the telescope and the system with Dave Taylor.

Here is the resulting DMI news on the net: http://www.dmi.dk/dmi/dmis_teleskop_fanger_asteroiden_yu55

Some photos of the MLO site taken by Henriette:
http://www.dropbox.com/gallery/27975715/1/Earthshine?h=425dae

The reflection of the Sun travels across the Earth as seen from the Moon. Since a majority of the earthshine originates near the sunglint it is of interest to map the position of the sunglint.

Below is a map of the footprints of the sunglint for all observations we have so far, from Hawaii. Since almost all the observations are morning observations the sunglint is East of Hawaii and has travelled across South America, Mexico and even North America (the last one may be an error and relate to observations of the Moon from Lund).

A few evening observations have been performed and they correspond to sunglint positions in the Indian Ocean, South-East Asia and Australia.

Close parallel tracks are separated by one day.

On the map below I show all the sunglint positions for the year 2011 – at half-hourly intervals. Times are selected for the Moon being up and the Sun being down. Blue points have Moon’s illuminated fraction less than 25% – i.e. acessible with the CoAdd technique.

The assymetry between the two ‘patches’ is not yet understood by me.

http://vimeo.com/27064549

Moon observations during 2½ hours. Performed with Hans Gleisner.