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.
