[SQM] mag not PER arcsec ?
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...)
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