[Aavso-photometry] photometry variation with measurement aperture

[Pertti Pääkkönen]

Hi,

I was very impressed on your asteroid pages! We have also started asteroid 
observing practically one year ago, see: 
http://cc.joensuu.fi/seulaset/ccd/Asteroids/ 

We mainly concentrate astrometry on NEO asteroids and this year we started 
co-operation with University of Helsinki in order to do asteroid time series 
photometry. The photometric observations are directly submitted to SAPC 
(Standard Asteroid Photometric Catalogue), see 
http://www.astro.helsinki.fi/~jtpesone/harras.html

Your new discoveries seem to end at year 2000. What might be the reason? Robot 
telescopes automatically gazing to mag 21 and reporting monthly 100 
discoveries? We have also been trying to discover our first asteroid but no 
success so far...

Talking about your photometry variation with aperture size. Please check your 
measurement precision either from 1/SNR (SNR should contain uncertainties 
from both C and V stars as well as from sky background etc.) or simply the 
standard deviation of your check star (of approx. the same mag than your 
target). If the variation caused by aperture size is of the same order than 
your uncertainty estimate, then you don't have to worry at all. In fact, 
there SHOULD be variation order of the uncertainty estimate, otherwise your 
estimate is wrong. 

Latest note:

According to your latest e-mail I estimated your worst-case uncertainty: 
(sqrt(2) comes from assumption that the C and V stars are of the same 
brightness, gain=3, r is the aperture, and read-out noise R=5)

dmag = sqrt(2)*1/SNR
SNR = (sqrt(gain)*60000/sqrt(60000+pi*r.^2*(1500+R^2)))

For r<10 we get dmag < 0.01 which is much lower than variation in your image. 
Better to wait for Arne's comments as well.

Clear skies (we are currently clouded),

Pertti (PPK)



On Monday 07 March 2005 18:59, Walt Cooney wrote:
> 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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-- 
Pertti Pääkkönen, PhD, Laboratory engineer
University of Joensuu, Department of Physics
Tel: +358 13 251 3238
Fax: +358 13 251 3290