[CCD-astrometry-photometry] RE: [Aavso-photometry] photometry variation with measurement aperture

[Walt Cooney]

Thanks Arne,

That fits the data perfectly but I picked another pair of stars in the field and
did the same exercise to test it.  I found the same trend.   One gets brighter
than the other with changing aperture.  I also checked the DSS which has much
tighter star images than I have.  I could not see evidence of a second star.  As
a matter of fact, you calibrated this field some time back and so you may have
some very good images in your archive that you can check.  The star that was
used as the comp is 188.243676 RA and 37.905456 Dec.  DK CVn is at 188.289521
and 37.973055 per your astrometry in your sequence file.

I was thinking this could be indicative of a gradient across my image but it
seems that would be subtracted out with the sky annulus.  Maybe the gradient was
in my flat but I think that would give me an artificially brighter magnitude for
one star vs. another but would not change with measurement aperture.

-Walt

-----Original Message-----
From: arne [mailto:arne at aavso.org]
Sent: Monday, March 07, 2005 12:37 PM
To: CCD-astrometry-photometry at yahoogroups.com
Cc: aavso-photometry at mira.aavso.org
Subject: Re: [CCD-astrometry-photometry] RE: [Aavso-photometry]
photometry variation with measurement aperture



Walt Cooney wrote:

> I did not include some of the other info you asked about.  The maximum
> pixel
> values for the comp star and variable were around 5000-6000.  I have a
> 16 bit
> AP-7p so I was well short of half well depth but not rediculuously
> underexposed
> either.  The net count which I presume to mean measurement aperture
> minus sky
> ranged from 60,000 to 90,000 for a measurement aperture of 6 radii.
> "Background" as MIRA calls the average sky background was around 1500.

Walt further replied that the background was "per pixel."  Since the
peak pixel values
for the comp and variable were 6000-ish, this means that sky becomes
very important once
you are more than a couple of pixels from the star center.  However,
while a mismeasurement
of sky would affect the results for a single image, this should be
random over the entire
image set.  Since each image has a systematic trend (brighter variable
with smaller aperture),
my guess is that the comparison star has a companion.  As you shrink the
measuring
aperture, the comp star companion is removed, making the comp star
fainter and the
variable effectively brighter.
  Note, however, that Walt's light curve does improve as he moves to
smaller apertures,
indicative of less sky background and therefore better signal/noise.
Arne


------------------------ Yahoo! Groups Sponsor --------------------~-->
In low income neighborhoods, 84% do not own computers.
At Network for Good, help bridge the Digital Divide!
http://us.click.yahoo.com/EA3HyD/3MnJAA/79vVAA/1.XolB/TM
--------------------------------------------------------------------~->

------------------------------------------------------------------
Group page: http://groups.yahoo.com/group/CCD-astrometry-photometry/For any
problem or suggestion, please contact the group moderator at:
CCD-astrometry-photometry-owner at yahoogroups.com

lem or suggestion, please contact the group moderator at:
CCD-astrometry-photometry-owner at yahoogroups.com


Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/CCD-astrometry-photometry/

<*> To unsubscribe from this group, send an email to:
    CCD-astrometry-photometry-unsubscribe at yahoogroups.com

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/