Conversion from mg/arcsec^2 to cd/m^2

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

> Subject: Conversion from mg/arcsec^2 to cd/m^2
>
> Does anyone have conversion table or converting formula from mg/arcsec^2 to
> cd/m^2 and vise versa.
>
> > read. Even more so with a value like ``one fourth of a millinit'', which
> > is representative for natural moonless clear sky night luminances in
> > zenith.
>
> Judging from Jan's comment I can do it by myself as follows;
>
> cd/m^2         approximate mg/arcsec^2
> 0.00025                       22 (brightness of natural dark sky)
> 0.0016                        20

Shigemi, the formula can be, e.g., found from the code of my programme
fai2bri. The link is given at the bottom of my php answer, the link
being
    http://astro.sci.muni.cz/pub/hollan/programmes/sources/astro/lum.pas
the php text being
    http://amper.ped.muni.cz/jenik/astro/lum.txt
and the working php being
    http://amper.ped.muni.cz/jenik/astro/lum.php

Natural sky luminance in zenith is not equivalent to a star with faintness
of 22 mag defocused to one square second. It's brighter. If you give 22
mag as an imput to my programme, it says

  compute
   22____ [mag__]
   If the unit is one magnitude, it applies to a star of that faintness,
   covering (defocused) a solid angle of:
   [one square_______________] __ [s]

   [ ] save settings    [ ] use my saved settings

   Another option is to use your own command line (ev. [ ] overriding all
   previous data):
   ______________________________________________________________________
   (`?' gives help). compute

The equivalent of Luminance expressed by a star of faintness of
22.00 mag  defocused to "one square second"
      means a Luminance of some

                            1.71E-4 cd/m^2

Vice versa, if you give ``one fourth of a millinit'' as an input somehow
(no way to ask this way the programme, to get a similar speech-based
answer), you get (the last digit is missing in the input box after
computation):


  compute
   0.0002 [cd/m2]
   If the unit is one magnitude, it applies to a star of that faintness,
   covering (defocused) a solid angle of:
   [one square_______________] __ [s]

   [ ] save settings    [ ] use my saved settings

   Another option is to use your own command line (ev. [ ] overriding all
   previous data):
   ______________________________________________________________________
   (`?' gives help). compute

The given Luminance of
                        2.50E-4 cd/m^2 corresponds to a star of
faintness of some

21.59 mag   defocused to one "square second",


Anyway, what's the base of my message:
                   mg/arcsec^2 is a sheer nonsense.
 No less one, than dB/m^2.

If we would like to be taken seriously, we have to express ourselves as if
being not entirely crazy. Unfortunately, it seems we are not even able to
pretend to be sane. What a pity for the night envinonment. We should try
once more.

jenik