Re: Night Sky Quality Meter

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

> Also according to the catalog its accuracy is good because
> it gives $B!^(B0.10 mag/arcsec2 figure.
> The unit of mag/arcsec2 can be converted to cd/m2, which
> lighting engineer is familiar, by a conversion table
> prepared by astronomers. Dr Dave Crawford has one such conversion
> table, but mag/arcsec2 is good to understand the sky
> brightness and is used more often than cd/m2.

Hello Shigemi, Cliff, Friedel, ...

I am much in favour of using cd/m2. I call it by a well known non-SI
one-word name ``nit'', it eases typography and enables to use the unit
when talking -- in fact, only when using a simple name for the unit, we
can make the most useful quantity, luminance, a common one. In the age of
digital cameras, it's also the easiest-to-measure quantity, see e.g.
many processed images in my working directory (with Czech comments)
http://amper.ped.muni.cz/noc/krnap (explanation of the colour coding
is within http://amper.ped.muni.cz/light/luminance, as well as the
needed SW).

The ``mag/arcsec2'' is no unit at all. There is no conceivable quantity
which could be associated with it. Using such horrible expressions is a
further obstacle to being taken seriously by lighting experts (and
generally by engineers and scientists).

I have a conversion programme online:

              Luminance
        URL: http://amper.ped.muni.cz/jenik/astro/lum.php

Quoting its result for 1 mnt, the proper parlance when using magnitudes (I
use magnitude as a unit, and call the associated quantity ``faintness'')
appears (the software avoids writing ``nit'' to be completely SI-based):

The given Luminance of
                        0.00100 cd/m^2 corresponds to a star of
faintness of some

20.08 mag   defocused to one "square second",
11.19 mag   defocused to one "square minute",
 2.30 mag   defocused to one "square degree".
 7.96 mag   defocused to a circle of diameter of 5 angular minutes
            (on the verge of being a "point" for human nighttime vision)

jenik

PS.
 Working on the evaluation of Giant Mountains National Park night
environment -- its images are in the above-mentioned working directory --
I read almost nothing. I glimpsed the evolution of this topic just by a
chance, and felt obliged to post a remark, as photographed luminances are
the very tool I am using. My single-image accuracy is about 10 per cent
too, or 0.1 mag. The advantage is, I could give scotopic luminances as
well, or even ``circadian rhythm affecting'' illuminances. The only thing
which is needed to get these alternative photometric quantities is to use
a proper linear combination of R, G, B values (or Y, C, G, M ones).