[Aavso-photometry] Differential vs. absolute magnitudes

[Michael Koppelman]

You can do differential photometry on the standard system or you can  
do all-sky ("absolute") photometry on the standard system. With  
differential photometry you still use comp star but utilize your  
transformation coefficients to correct your data on to the standard  
system. With all-sky photometry you are literally transforming your  
instrumental magnitude into standard magnitudes. In the latter case  
you use standard stars (like Landolt standards) to find your  
transformation coefficients but you don't use them in the way we  
normally think of comp stars where we are directly comparing against  
them.

All-sky is hard and requires photometric skies but it is very  
educational and, to dorks like us, quite entertaining. It's not for  
the faint of heart, though.

I have never actually done differential transformed photometry but  
that is probably the best place for you to start.

M.


On Mar 8, 2008, at 2:35 PM, 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.