[SQM] mag not PER arcsec ?

Jan Hollan jhollan at amper.ped.muni.cz
Thu Mar 3 09:53:24 UTC 2011

> 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 / arcsec?, or magnitude / arcsec^2 .

Jim, I've realized that soon after sending it..., texts should be not 
written in a hurry.

But as you've noticed, the unit of (space) angle was not important. Such a 
logarithmic quantity cannot be per _anything_. Has to be OF something 
(like of a star, of a cluster of stars, of a piece of the sky). Per is a 
mathematical operation, which is not applicable here. 2.5 mag corresponds 
to 10 dB (that was another typo, I wrote db erroneously).

The language within my old programme lum is the correct one, including the 
units of a space angle.

SQM reports how faint is 1 sq." - how many magnitudes it has, if magnitude 
is a unit.

(The other use, magnitude as a quantity without any unit, is 
difficult to maintain, nobody was able to do it all the time, see ev. my 
old directory with a single long text, 
http://astro.sci.muni.cz/hollan/a_papers/english  -- but if it is used 
this way, then SQM reports magnitude _of_ 1"^2)

(After sending the letter, I've realised that writing arcsec instead 
of sq. arcsec is not entirely nonsensical, just it is not clear if it 
should be rather a strip over the sky 1" wide or a slice which is 1" wide 
just in the centre, going to 0 at the horizon...)


More information about the sqm mailing list