[Aavso-photometry] Aavso-photometry Digest, Vol 52, Issue 8

[Brad Walter]

 I think there is confusion between differential vs. absolute photometry and
absolute magnitude.

Absolute magnitude (often shorthanded as M or Mag) is the magnitude of a
star at 10 parsecs. 

Absolute photometry is a photometric magnitude converted to a standard
system. Differential magnitude is simply the magnitude difference between
measured values of a comparison star and a target star.
(instrumentalTarget-instrumentalComp), normally with both stars in the same
FOV of your CCD so that the images of the two stars are at the same airmass.
If you have the magnitude of the comp star determined in a standard system
(transformed), you can convert your differential magnitude into an absolute
magnitude by adding the transformed magnitude of the comp star to the
differential magnitude. This gives you a magnitude determined by absolute
photometry. However, if you only do the process in this simplest form, you
will have inaccuracy in your measurement. First you have introduced
inaccuracy due to any color difference between the stars. To eliminate that
error, you should correct your measurements for second order extinction.
First order extinction is taken care of because your comp(s) and target are
in the same FOV (and for amateurs this almost always assures they are << 1
degree apart) so they have the same airmass. The second inaccuracy is that
the differential magnitude hasn't been transformed. To improve accuracy you
would apply the transformation coefficients you established for your
telescope-filter-camera combination, after extinction correction, to your
comp and target magnitudes. Then calculate Target - comp and then add the
"standardized" magnitude you have for the comp in the filter you are using
(e.g. V). Adding back the "standardized" magnitude of the comp star adds in
the Zero Point. If you don't have a standardized magnitude for your comp
star then you have to measure the zero point for your evening by measuring
several standard stars (say 10) near the meridian. Correct those
measurements for extinction and transform them. Then average the differences
you get between your transformed values for the standard stars and their
standard values to give you your zero points. Then Target-Comp + Comp + zero
point (where the Target and Comp magnitudes have been transformed) gives you
a magnitude for your target star using absolute photometry. It does not give
you the absolute magnitude of the star. 

>
> ------------------------------
> 
> Message: 3
> Date: Sat, 8 Mar 2008 14:42:18 -0700
> From: Jeff Hopkins <phxjeff at hposoft.com>
> Subject: Re: [Aavso-photometry] Differential vs. absolute magnitudes
> To: gianlucaros at gmail.com, aavso-photometry at mira.aavso.org
> Message-ID: <a05210603c3f8b576cc02@[65.39.86.82]>
> Content-Type: text/plain; charset="us-ascii" ; format="flowed"
> 
> Hi Gianluca,
> 
> You cannot increase your precision by adding a B filter. You 
> might be able to increase your accuracy, however.
> 
> Things seemed a  lot clearer back in the single channel 
> photometry days. Most people did all-sky photometry for 
> determining their transformation coefficients and for special 
> projects and differential photometry for most other projects. 
> Everyone transformed their data and accounted for extinction.
> 
> A word of caution, absolute magnitudes are the magnitudes of 
> stars at a standard distance from the star (10 parsecs or 
> 32.6 light years). 
> Naturally you cannot determine that directly using 
> photometry. I think what you are referring to is the star's 
> published or standard magnitude/extraterrestrial magnitude as 
> seen from outside the Earth's atmosphere. Star magnitudes are 
> usually expressed that way.
> 
> With CCDs many programs let you enter a comparison star's 
> magnitude and then the difference between it an the program 
> star can produce a normalized magnitude for the program star. 
> With practice you can increase the precision of that 
> magnitude, but not the accuracy. To increase the accuracy you 
> must use a standard filter and transform the data. If the 
> stars are far apart you must also allow for extinction. The 
> zero points will drop out as they are the same for both the 
> comparison star and program star.
> 
> Good luck.
> 
> Jeff
> 
> At 13:35 -0700 03/08/2008, gianlucaros at gmail.com wrote:
> >I have been doing some differential CCD photometry with a V filter.
> >Since I would like to increase precision I am thinking of buying a B 
> >filter and to use transformation coefficients. Reading the 
> AAVSO manual 
> >I haven't a clear picture on how to trasform differential magnitudes 
> >into absolute one. Once the data have been transformed using 
> >transformation coefficients, shall I calculate zero point to get the 
> >absolute magnitudes? What is the procedure? Are absolute magnitudes 
> >only possible with all sky photometry or all sky photometry is 
> >compulsory only if the comparisons stars are more than a 
> field apart? I 
> >am getting a bit confused.
> >
> >Gianluca (RGN)
> 
> --
> Jeff Hopkins
> HPO SOFT
> Counting Photons
> http://www.hposoft.com/Astro/astro.html
> Hopkins Phoenix Observatory
> 7812 West Clayton Drive
> Phoenix, Arizona 85033-2439 U.S.A.
> (623)849-5889
> (623) 247-1190 (Fax)
> www.hposoft.com
> 
> 
>