[Aavso-photometry] Differential vs. absolute magnitudes

[Jeff Hopkins]

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
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