[SQM] Lunar eclipse, a calibration opportunity

Jan Hollan jhollan at amper.ped.muni.cz
Mon Feb 18 19:31:03 UTC 2008


I like the idea of taking measurements by SQM during the eclipse night.
All the Earth hemisphere will see the Moon with almost the same original
luminance (as would be observed above the atmosphere), changing a lot in
time and across the Moon, creating a unique gray wedge during the eclipse
time. Well, not really gray, rather a coloured one, but that may be still
better. A good calibration standard...

(almost: the opposition surge affects it a bit, so those observers being
closer to the line ``Sun -- some spot on the Moon'' see a bit higher
luminance of that spot, but this is but a minor effect)

However, to infer any knowledge from the measurements, some procedures
might help:

Whenever the Moon shines directly onto the sensor, it will affect it in a
rather unclear manner -- SQM having a complicated pattern of sensitivity
across the space angle. So I'd recommend taking always an additional
measurement in a lunar shadow.  To get reproducible results, the baffle
casting a shadow onto the sensor should be always in the same distance, to
obscure a piece of the sky of the same size, with the Moon in the middle.

Direct readings toward the Moon would be interesting as well, again with a
baffle and without it (the difference being due just to the direct
moonlight).

What about extinction of direct moonlight? The larger the extinction, the
brighter the moonlit sky (differing results from various sites would say
something about air transparency). Direct moonlight, measured by SQM
(outside eclipse mostly) would be a way how to get good estimates of
zenith extinction. I've attempted to do it in 2006, see the graph
  http://amper/light/luminance/lun_eclipse/2006/bri.png

Then the direct moonlight values obtained by various
SQMs might be compared (providing a sort of their calibration after years
of use).

Plain SQM without added optics is good for such purposes. A lens helps, of
course, to measure zenith sky only. For calibration, see my posts within
radiometry letter archive, e.g.
 Linkname: a glass-ball collimator for SQM
      URL: http://amper.ped.muni.cz/jenik/letters/radiometry/msg00021.html
 (I'm going to add the current long letter to the same archive...)

Apart from measuring with SQM (or even with a luxmeter which detects units
of centilux), digital imaging in raw formats (as raw as possible, with all
camera processing switched off) would be interesting for camera
calibration. I don't long to process images by many authors, but if
somebody is really interested, I might help.

Some further text on calibrating by Moon is within the above-mentioned
directory,
  http://amper/light/luminance/lun_eclipse/2006
 within the description at the bottom of a thumbnail overview,
  http://amper/light/luminance/lun_eclipse/2006/overview.htm

(an older photometry, with a bit differing luminance scale -- kcd/m2 were
deep blue and not light blue, as I use now -- is
  http://amper.ped.muni.cz/light/luminance/lun_eclipse/photom.htm )

----------

My ``today's'' forecast of clear-sky lunar horizontal illuminance for two
sites is within
  http://amper.ped.muni.cz/light/luminance/lun_eclipse/

The partly eclipsed moon brightnesses are not very accurate, but better
than nothing at all. Fully eclipsed Moon is perhaps twenty times fainter
than shown in the graphs, so the well should be deeper by 0.3 in log10
scale (I've assumed crudely that the ratios of full sunlight to refracted
light of the Sun behind the Earth is 1e4, whereas it's rather 5e6).  And,
finally, the non-eclipsed moon should be up to 0.06 brighter in log10
scale as the opposition surge is still not included in my programme; this
would be an inconspicuous difference within the graph, however.

----------

Last, nothing to do with the eclipse now:

I've written a yearly report for the IDA, it's within the
   http://amper.ped.muni.cz/light/ida/
 directory, e.g., as
   http://amper.ped.muni.cz/light/ida/2007.en.html
 or
   http://amper.ped.muni.cz/light/ida/2007.en.pdf
 -- there are many links from it, including some on radiometry.

A second novelty concerns photometry, my raw2lum has a new, alternative
output for reporting luminances of the scene... showing how many
magnitudes correspond to a square second. An updated programme is within
  http://amper.ped.muni.cz/light/luminance/
 -- even current binaries for linux and win32 are there. At last I find
magnitudes convenient, as the numbers don't take too much space in the
images (there is an option to skip the grid lines etc. as well, see an
a view from our observatory toward the centre of Brno, with Sirius below
the centre and Orion's knees up and right from it,
   http://amper.ped.muni.cz/light/luminance/sites/brno-vyskov/img/69gf.jpg
).

cheers,
 jenik






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