[Aavso-photometry] Measuring aperture

[Lionel Catalan]

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: aavso-photometry-bounces at aavso.org
[mailto:aavso-photometry-bounces at aavso.org] On Behalf Of Barbara G. Harris
Sent: Saturday, January 17, 2009 3:21 PM
To: aavso-photometry at mira.aavso.org
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

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