Jul 11, 2010 UT: Photometry of cataclysmic variable SDSS 162520.3+120309

Michael Richmond
Jul 11, 2010

On the night of July 10/11, 2010, I observed the cataclysmic variable star SDSS 162520.3+120309 which is currently in outburst.

The setup was:

• Celestron 14-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-9E CCD camera with no filter
• 30-second unguided exposures

Notes from the night

• The conditions were poor: thin clouds passed overhead early, and dew formed on the telescope's front corrector plate later in the run.
• The FWHM was around 3.0 pixels = 3.1 arcseconds (but much larger during a brief period when heater was turned on too high). I used a radius of 4 pixels = 4.2 arcsec for photometry
• There is a short gap due to clouds early in the dataset, and a larger gap due to dew forming on the corrector plate later. I think that the proper setting for the dew heater is somewhere between 10 and 20 percent of full power.

This is a chart of the field based on one of my images last night.

The chart has several of the brighter stars in the field labelled with letters, just to keep me straight as I perform the reductions. Star P is in the Tycho-2 catalog, but it's so bright that it is saturated in my measurements. The star labelled "C" is in the USNO B1.0 catalog:

         name                   RA             Dec           B         R
-----------------------------------------------------------------------------
 USNOB1.0 1019-0299359      16:25:25.739    +11:59:21.18   14.27     13.24
                                                           14.46     13.30
-----------------------------------------------------------------------------

Because the conditions were poor, I am not commenting on the other variable stars in the field.

I measured the instrumental magnitude of each star with aperture photometry, using a radius of 4 pixels = 4.2 arcseconds and sky defined by an annulus around each star. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

One output of the ensemble solution is the value of the zero-point of each frame relative to the others. In the graph below, I plot this zero-point as a function of time. Note the brief rise due to clouds, and the overall gradual rise due to dew; the sharp drop occurs when I cleared the corrector plate of dew.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.

The floor of this diagram corresponds to a scatter of about 0.015 mag, which is rather mediocre. The target is the second-brightest star, at differential magnitude 0.1. its scatter of 0.106 mag (about the same as scatter on Jul 08 UT) is due to the changes in its light.

Light curves for selected stars (SDSS162520 and stars A - D, as well as X) in the field are shown below. The target is shown by light green crosses.

Here's a closeup of the variation in SDSS162520 and two comparison stars.

I've made a table of the measurements themselves, with three different flavors of time. The differential magnitudes from the ensemble solution have been shifted so that star "C" in my chart, USNOB1.0 1019-0299359, has value 13.8; that should be close to its V-band magnitude.

Here's the start of the table.

# Measurements of SDSS162520+120309 made at RIT Obs, Jul 11, 2010 UT, 
#    in poor conditions 
#    by Michael Richmond, using 14-inch Celestron and SBIG ST-9E CCD. 
# Exposures 30 seconds long, no filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 4.2 arcseconds. 
#    which has been shifted so USNOB1.0 1019-0299359 has mag=13.8 
#    as a very rough estimate of its V-band value. 
# 
# UT_day             JD            HJD        mag    uncert
Jul11.07882     2455388.57882  2455388.58220  14.136  0.015 
Jul11.07919     2455388.57919  2455388.58257  14.130  0.015 
Jul11.07957     2455388.57957  2455388.58295  14.136  0.015 

• download UT 2010 Jul 11 SDSS162520 photometry (plain ASCII)

Last modified 7/11/2010 by MWR.