Re: exact answer on shielding and skyglow

[Jan Hollan <jhollan@amper....muni.cz>]

Dear Keepers of the Night,

I'm rather busy with too many things, but I've succeeded to get some
images which are relevant to my old posting, see e.g. a copy in

             exact answer on shielding and skyglow
        URL: http://amper.ped.muni.cz/jenik/letters/public/msg00130.html

 and the next one, containing a part of the Fabio's reply (his full reply
should be in the magnitude6 archive).

The weather was not cold enough, so the snow at major streets was gone due
to lots of salt applied there. Still, some half of the illuminated ground
was covered by fresh snow from the evening shower.

The zenith sky luminance at our observatory was just 5 millinits, what is
just about what is expected for a terrain without snow at the estimated
extinction of 0.50 mag. (I have to compute the extinction from the images
themselves, this is an estimate from the next morning and direct
sunshine as measured with a luxmeter).

The increase of sky luminance due to the snow was unperceptible. I expect
to identify some minor increase later, with more data and more processing.
But the message is clear already:

Sky luminance is little affected in Brno by the light dispersed from the
ground. The dominant reason for skyglow is direct light from luminaires,
even some 1 km from the city centre.

As this is one of our main controversies with the lighting industry (they
mostly claim the opposite), it's really important to prove that sky
luminance almost nowhere increases several times due to snow, as it would,
if the lit terrain would be dominant (it would be dominant in a completely
FS-luminaires lit city).

So,
                    please, TRY to take images

which could be processed to demonstrate the influence of snow. Even pairs
of non-raw images made with the same settings can be used for the purpose.
Some guess of zenith extinction should be of course made, at least like:
very clear skies, distant mountains visible etc.  (man-made skyglow in
millinits = 1 mcd/m2, close to light sources is roughly proportional to
the amount of zenith extinction in decimagnitudes;  accidentally, at our
observatory in Brno, they are the almost the same in zenith).


(I've put the zip with all data and software, apart from a stable
dcraw152j, to
  http://amper.ped.muni.cz/light/luminance/snow/
 for those who would like to see the results or help me to get an accurate
extinction value for zenith.)


wishing you some more snow before the outbreak of spring
 (if you live in northern latitudes...),

jenik