[Aavso-photometry] Measuring aperture

[Wolfgang Renz]

Hi

Its of course the easiest way to do photometry with a fixed aperture
and annuli. Mut it is not a must. If one changes them, one must cali-
brate it by deriving a "growth curve".
See e.g.:
1990PASP..102..932S - On the growth-curve method for calibrating
stellar photometry with CCDs - Stetson, Peter B.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990PASP..102..932S

Assuming that the FWHM of the stars in an image is constant is
common, but its usually not 100% true. Especially not if the chip
and the FOV are large. It depends on the actual properties of the
optical system. Even RCs (which should not show coma) have
noticably growing star diameters to the edges if the FOV is large
enough.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Lionel Catalan"
To: "'Barbara G. Harris'"
Cc: <aavso-photometry at mira.aavso.org>
Sent: Saturday, January 17, 2009 10:05 PM
Subject: Re: [Aavso-photometry] Measuring aperture

> Hi Barbara,
> 
> You should use the same aperture, gap, and annulus size for all
> your stars (target, comps, and check). This is because the size
> of the aperture controls the percent of the star light that is actually
> measured. > Using the same aperture ensures that the same
> percentage is used for all stars. This percentage is independent
> of the star magnitude because all stars have the same full width
> at half maximum in a given image. 
> 
> Lionel Catalan, CTE


----- Original Message ----- 
From: "Barbara G. Harris"
To: <aavso-photometry at mira.aavso.org>
Sent: Saturday, January 17, 2009 9:20 PM
Subject: [Aavso-photometry] Measuring aperture

> I have been doing CCD photometry for a few years but I have been
> getting back to basics and digging into how my program (mostly
> Maxim, but also use AIP4Win) calculates magnitudes.  I have a
> question that is probably pretty basic but I would like to have it
> confirmed:  When measuring the comp, check, and variable do you
> have to use the same size aperture and annulus for each star in the
> field or can you adjust the diameters because of field crowding?
> I have made sure to always use the same size and I assume that is
> why it is recommended to use stars of similar magnitudes.
> 
> Barbara