May 16, 2013 UT: Photometry of SDSSJ1538+5123

Michael Richmond
May 15, 2013

On the night of May 15/16, 2013, I measured the cataclysmic variable known as SDSSJ1538+5123. The Hubble Space Telescope will observe this object WITHIN 24 HOURS (May 16-17), so it's important to monitor its brightness.

The setup was:

Notes from the night

This is a chart of the field of SDSSJ1538, taken from Aladin and the POSS I E plate. The target is in the square box near the middle of the field.

You can find information on the star, including a photometric sequence of comparison stars nearby, at the AAVSO's entry for SDSS1538 = 000-BDB-429 in the International Variable Star index.

AUID        letter       RA.          Dec.             B              V             B-V    
------------------------------------------------------------------------------------------
000-BJR-224  A  15:38:51.06    51:20:37.2     13.186 (0.077)  12.617 (0.061)  0.569 (0.098) 
000-BJR-225  B  15:38:05.33    51:23:57.9     13.998 (0.006)  13.343 (0.006)  0.655 (0.008) 
000-BJR-226  C  15:38:21.62    51:23:55.9     14.756 (0.008)  14.170 (0.008)  0.586 (0.011) 
000-BJR-227  E  15:38:13.33    51:26:46.1     15.843 (0.010)  15.082 (0.010)  0.761 (0.014) 
000-BJR-228  F  15:38:28.57    51:29:09.1     15.820 (0.004)  15.295 (0.000)  0.525 (0.004) 
000-BJR-229  D  15:37:51.56    51:23:10.8     16.732 (0.017)  15.691 (0.017)  1.041 (0.024) 
000-BJR-230     15:38:37.21    51:23:54.9     16.670 (0.010)  16.161 (0.008)  0.509 (0.013) 
000-BJR-231     15:37:58.24    51:27:07.2     17.283 (0.011)  16.443 (0.008)  0.840 (0.014)
000-BJR-232     15:38:39.75    51:25:36.2     17.897 (0.022)  17.117 (0.011)  0.780 (0.025)
000-BJR-233  G  15:38:24.80    51:25:11       18.194 (0.036)  17.360 (0.008)  0.834 (0.037)
000-BJR-234  H  15:38:21.08    51:25:10.8     18.777 (0.047)  17.864 (0.000)  0.913 (0.047)
000-BJR-235  J  15:37:50.99    51:24:10.5     18.951 (0.025)  18.265 (0.008)  0.686 (0.026) 
------------------------------------------------------------------------------------------
Report this sequence as: 11711AGH in the chart field of your observation report. 
The brightest star, marked "Q", is UCAC4-707-053459, and has magnitude V = 10.77. The target, SDSS1538, is much fainter, with a magnitude in quiescence of roughly V = 18.

Tonight, I planned to acquire images with only the V-band filter. I took twilight sky flats.

I again guided the telescope using a nearby star. As explained in notes from a previous night , the telescope doesn't move smoothly in Dec, so a few exposures suffered from jumps. I discarded those -- they amounted to just a few percent of the total. The graph below shows the motion of stars in the field over the course of the observations. The jump in RA occurred when I paused to re-focus the telescope and then had to reacquire the guide star.

I took one series of images: V-band images with 120-second exposure time. I discarded many images, leaving a set of 34 good pictures and a total of 68 minutes of exposure time.

Stack of (34 x 120 seconds) with V filter.

Using aperture photometry with a radius of 4 pixels (radius of 7.4 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. I ran ensemble photometry on the dataset, to check the quality of the night. The graph below shows the change in zeropoint from image to image. The jump at the end indicates that the clouds were coming back.

There are two ways to estimate the magnitude of SDSS 1538.

I'm going to conclude that V = 18.8 +/- 0.4.

Let me summarize my results:

# Measurements of SDSS 1538+5123 
#   made by Michael Richmond at the RIT Observatory
#   with 12-inch Meade LX200 + SBIG ST-8E CCD.
#   V filter, stacked (34 x 120-second) exposure
#   Conditions were fair.
#   
# Aperture photometry with radius 4 pix = 7.4 arcsec.
#   Reference star photometry from AAVSO chart 11713LT
#   "V" mag is based on V-band measurements,
#   compared to V-band reference magnitudes, (B-V) color terms.
#   
# UT Date      mid-UT  JD              V  mag
#----------------------------------------------
 2013 May 16   05:26  2456428.73      18.8 +/- 0.4


Last modified 05/16/2013 by MWR.