[Aavso-photometry] photometry variation with measurement aperture

[Walt Cooney]

The additional info is on my website.
http://members.cox.net/waltc/BLACKBERRY_OBSERVATORY.htm

The photometry was done using MIRA.  The sky annulus remained the same for all
measurement apertures.

Thanks!
Walt

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org]On Behalf Of Walt Cooney
Sent: Monday, March 07, 2005 10:33 AM
To: 'arne'; CCD-astronometry-photometry at yahoogroups.com
Cc: aavso-photometry at mira.aavso.org
Subject: RE: [Aavso-photometry] photometry variation with measurement
aperture


Thanks Arne.  I thought I had supplied more details but the attached graph
didn't go through.  They must be turned off for these two lists.  I'll post them
on my web site and send a link in a few minutes.

-Walt

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org]On Behalf Of arne
Sent: Monday, March 07, 2005 10:22 AM
To: CCD-astronometry-photometry at yahoogroups.com
Cc: aavso-photometry at mira.aavso.org
Subject: Re: [Aavso-photometry] photometry variation with measurement
aperture


Walt Cooney wrote:

>Folks,
>
>I could use a little advice.   I am doing some standardized photometry on DK
CVn
>and have found I can move the magnitudes up or down by a few hundredths
>depending on what measurement aperture I use.  Clearly that's bad for
>standardized photometry.  Aperture size appears to be an optimization between
>S/N for seeing the trend behavior vs. producing data with good absolute
>accuracy.
>
>I'd like to understand why it matters.  If you carve off the top of the light
>curve distribution function, I don't see why is should matter whether you carve
>off and measure more or less as long as you carve off and measure the same for
>each star.  It can't (?) be a problem with few pixel statistics because that
>would show up as random noise.
>
>I'd appreciate some thoughts on this.
>
>
>
Walt, give us more details.  Software program?  fwhm in pixels of the
stars?  Size of
the apertures, and magnitude change?  Inner/outer sky annuli radii?
Counts in the
aperture and sky?
  In general, you will get different results as you change the aperture
size, just based
on random statistics.  As the aperture gets smaller, you run into
"rounding" problems,
where there are differences due to including partial pixels in an
attempt to create
a circular aperture.  In addition, for faint stars, as the aperture gets
smaller, the
signal/noise generally increases as you are including fewer sky pixels
in the aperture,
so the measure tends to asymptotically approach the correct answer.
Arne
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