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There is a typo in your note below. You should replace "arcsec" with
The convention used for SQM luminance/ luminosity measure is
magnitude per square arcsec, or magnitude / arcsec², or magnitude /
I agree with the rest of your note since magnitudes are on a
logarithmic scale thus they are not additive. As you indicated, you
need to use mcd / m^2 to be able to add or average the luminances <br>
See also: <a class="moz-txt-link-freetext" href="http://coolwiki.ipac.caltech.edu/index.php/Units">http://coolwiki.ipac.caltech.edu/index.php/Units</a> <br>
On 2011-02-27 4:05 AM, Jan Hollan wrote:<br>
it is a bad habit to write mag/arcsec or to say mag per arcsec. It is like
saying db/car. Any metrologist, good physics teacher or a caring
technician would wave a hand and think that such people speaking nonsense
language cannot be taken seriously, as they don't understand science at
If there are hundred cars around, producing noise amounting to 70 db, it
would be absurd to claim that they have "a polluting power" of 0.7 db/car.
"21 mag/arcsec" is no less absurd. The slash is a mathematical operation
and cannot be employed here. If it could, then
21 mag / <i><b><font color="#cc0000">square</font></b></i> arcsec = 42 mag / 2 <i><b><font color="#cc0000">square</font></b></i> arcsec
-- the nonsense is apparent, isn't it.
The only way is to say something like that 1 <i><b><font color="#cc0000">square</font></b></i> arcsec has 21 mag. Then 2
<i><b><font color="#cc0000">square</font></b></i> arcsec have 20.25 mag, 10 <i><b><font color="#cc0000">square</font></b></i> arcsec have 18.5 mag, 100 <i><b><font color="#cc0000">square</font></b></i> arcsec have 16 mag. Or,
using my programme lum (available online as well, as
<a class="moz-txt-link-freetext" href="http://amper.ped.muni.cz/jenik/astro/lum.php">http://amper.ped.muni.cz/jenik/astro/lum.php</a> ):
jhollan@amper:~$ lum s 21
The equivalent of Luminance expressed by a star of faintness of
21.00 mag defocused to "one square second"
means a Luminance of some
and alternative equivalent stars might be faint
21.00 mag when defocused to one "square second",
12.11 mag when defocused to one "square minute",
3.22 mag when defocused to one "square degree".
8.88 mag defocused to a circle of diameter of 5 angular minutes
(on the verge of being a "point" for human nightime vision)
<i><b><font color="#cc0000">Square</font></b></i> arcsec is so tiny that it has no bearing for human perception of the sky.
And it's boring to carry those large numbers in mind, starting somewhere
from 16 in cities. Tenth of a square degree would be a more reasonable
piece of the sky -- actually, it would hint how faint stars may be
well seen. (3.22 + 2.5 = 5.7) mag ones, for the above luminance of
the sky of 0.43 mcd/m2. Numbers from 0 to 7 are easier to remember
and have some idea how bright or dark the sky really is.
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