[Aavso-photometry] Opinion needed

[Wolfgang Renz]

Hi Jim

What software did you use ?

The jumps are in the 0.1..0.2 mag range = ~ 10..20% in flux.
Thats pretty much and should get usually obvious by a visual
inspection if the histogram streching is sufficient and the is-
sue is not spread over too many pixel.

As the two used comps don't jump in different directions (like
in the first obs on the very left of the plot), it should be some-
thing introduced by the measurements of the var itself.

Drifting stars can be the reason for such jumps if the calibration
is not perfect. If one uses moving averages too, one even gets
smooth in- and decreases that look pretty much like true variability.
Did the stars drift a bit on the CCD ?
If you can make a plot of image# or time vs. x and y of the var and
the jumps correlate strongly with a certain range in x and y, you
probably have found the sollution.
Then you should check your bias/dark/flat/light frames for a weak/
hot pixel in the area of the apertures and annuli of the var that did
not calibrate out well. If it would be a dip and not a bump, it could
be also caused by a star (trail) in the flats.

Contaminating star in or close to the star aperture or sky annulus
can also cause such effects. Especially if the FWHM changes
severely between images.
Did you have stars in or close to the var aperture and annulus ?
Did you use fixed apertures or relative apertures that adapt to the
FWHM ?
Does your software allow and did you use some kind of rejection
algorithm in the sky annulus (mean or median/mode/min-max/
sigma/...-reject) ?
If you can make a plot of the image# or time vs. FWHM and the
FWHM values correlate with the jumps, you might have found
the reason too.

It could have been also a cloud that affected just the two comps
but not the var measurements or a clearer spot that affected just
the var but not the comp measurements.
If you can make a plot of the image# or time vs. measured ADUs
of the stars and the sky and one of the ADU values correlates with
the jumps, you might have found the reason too.

Anything else I missed ?

Clear skies
 Wolfgang

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



----- Original Message ----- 
From: "Jim Roe" <jroe at ...>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Cc: "Bruce Gary"
Sent: Wednesday, August 13, 2008 3:06 AM
Subject: [Aavso-photometry] Opinion needed

> I was taking a time series of CoRoT-2 hoping to catch a transit of its 
> planet.  Clouds seemed to have cut off the onset and finish of the 
> transit  ( :-( ) but in between the data seemed fairly smooth - except 
> for the nearly 15 minute brightening shown in the light curve at
> <http://jamesroe.com/CoRoT2-20080812.JPG>
> 
> Remember, the transit was going on during all of this data.
> 
> The target is the data represented by squares and two comp stars (in 
> ensemble) are the diamonds.  The comp curve closest to the target is 
> only 52 arcsecs away from the target, while the other is some 8 arcmin 
> away (presumably negating potential flat field problems with the 
> intermittent clouds?).
> 
> I can't see any obvious changes in the images involved.  Can anyone 
> suggest what might have happened?  Is it worth following up in some
> manner?
> 
> Jim Roe [ROE]