[SQM] mag not PER arcsec ?

James Ronback jim_ronback at dccnet.com
Thu Mar 3 06:30:46 UTC 2011


Jan,

There is a typo in your note below. You should replace "arcsec" with
"square arcsec".

The convention used for SQM luminance/ luminosity measure is magnitude
per square arcsec, or magnitude / arcsec², or magnitude / arcsec^2 .

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

See also: http://coolwiki.ipac.caltech.edu/index.php/Units

Cheers
Jim

On 2011-02-27 4:05 AM, Jan Hollan wrote:
> ........
> Luminance units:
>
> 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 
> all.
>
> 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 / /*square*/ arcsec = 42 mag / 2 /*square*/ arcsec
>   -- the nonsense is apparent, isn't it.
>
> The only way is to say something like that 1 /*square*/ arcsec has 21 mag. Then 2 
> /*square*/ arcsec have 20.25 mag, 10 /*square*/ arcsec have 18.5 mag, 100 /*square*/ arcsec have 16 mag. Or, 
> using my programme lum (available online as well, as
>                 http://amper.ped.muni.cz/jenik/astro/lum.php ):
>
> ----------
> jhollan at 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
>                              4.30E-4 cd/m^2
>
> 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)
> ---------
>
> /*Square*/ 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.
>
> best regards,
>   Jenik Hollan
>
>
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