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From: jeff@utastro.UUCP (Jeff Brown the Scumbag)
Newsgroups: net.astro.expert
Subject: Re: photons per stellar magnitude
Message-ID: <71@utastro.UUCP>
Date: Thu, 14-Nov-85 15:06:43 EST
Article-I.D.: utastro.71
Posted: Thu Nov 14 15:06:43 1985
Date-Received: Sun, 24-Nov-85 07:26:36 EST
References: <1863@saber.UUCP>
Distribution: net
Organization: U. Texas, Astronomy, Austin, TX
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> How many photons per second per square meter are recieved from a star of 
> say 10th magnitude on the surface of the earth??

For V magnitude = 0.04 (Vega, that is) and a wavelength of 5556
Angstroms, there are 948 photons per square centimeter per second
per Angstrom (of wavelength) at the top of the Earth's atmosphere.
The absolute calibration is good to a few percent; relative values
(i.e., differences in magnitudes) are more accurate.  Going to
other colors is more complex than scaling a blackbody curve since
Vega (defined to have color index zero) is not a black body, but if
you stay in the visual (longward of the Balmer jump at 3650A and
short of the Paschen jump about 8200) you can probably get away
with it.  To get beyond this you need to do all kinds of horrible
things (like response functions for photometric bands and flux curves
of standard stars) which rapidly go to problems even the experts
would prefer to avoid, and you really have to go to the literature
to find out how to do it right.

To convert your 10th-mag star to photons per whatever you need the
bandpass response function convolved with the spectrum of the star.
The V band is about 900A wide centered at about 5445A.  I will let
you go from there, but if you ignore the color slopes, call the
bandpass function a box 900A wide, and remember that magnitudes are
a logarithmic unit (5 magnitudes <----> a factor of 100) you ought
to get a number that's the right order of magnitude (which is all
a lot of us ask for)....


Jeff Brown the Scumbag
		{allegra,ihnp4}!{noao,ut-sally}!utastro!jeff
		jeff@astro.UTEXAS.EDU
Astronomy Department, U. of Texas, Austin