May 05, 2013 UT: Photometry of SDSSJ1538+5123

Michael Richmond
May 06, 2013

On the night of May 04/05, 2013, I attempted to observe for the first time the cataclysmic variable known as SDSSJ1538+5123. The Hubble Space Telescope will observe this object later this month (May 16-17), so it's important to monitor its brightness.

The setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with clear and R filters
• 30-second to 120-second guided exposures

Notes from the night

• Conditions were good: no clouds at all, good seeing. No moon, so the sky was relatively dark.
• FWHM was good, 2.2 - 2.8 pixels = 4-5 arcsec
• I used twilight sky flats for R, dome flats for clear.

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 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. 

I first took a series of 60-second R-band exposures, but quickly realized that the target was completely invisible. I found a good guide star in the area: if one places star "Q" in the main CCD at position (164, 287), the guide CCD will show a bright star. I was unable to calibrate the guider, because the Dec axis sticks frequently. Nonetheless, the guider did a pretty good job of keeping the telescope fixed in place. The guide star would typically drift very slowly in Dec until, after several minutes, the mount would finally jump in Dec by several arcsec, but that was good enough for my purposes.

I increased the exposure time to 120 seconds and switched to a "clear" filter. These images finally showed the target as a very faint source. I was able to produce a decent signal by stacking a number of these two-minute exposures. The stacked image had decent (for Rochester) quality: FWHM = 2.3 pixels = 4.2 arcsec.

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. Remember, the measurements are all taken with no filter. There are two ways to estimate the magnitude of SDSS 1538.

• no color term This is definitely not correct, but it is easy to do. Using six reference stars, I found

        V  =  (instr. clear mag)  + 1.06  +/-  0.08 mag
     

  from which we can derive

      SDSS 1538  "V"(nocolor) =  18.7
      

• including color term The AAVSO measurements provide (B-V) for the comparison stars, so I can try to correct my instrumental magnitudes for any simple systematic effect as a function of color. Of course, I don't know the color of the target star, but I can make a guess. The SDSS measurements of SDSS 1538 show (g - r) = -0.32, which is quite blue; not a surprise for a CV. Using the transformations from Jester et al. (2005) , I can convert that to an estimate of (B - V) = -0.078 for SDSS 1538. Using six reference stars, I found an empirical transformation:

        V  =  (instr. clear mag)  +  0.761  + 0.3787 * (B-V)
     

  from which we can derive

      SDSS 1538  "V"(incl.color) =  18.4
      

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.
#   No filter, stacked (8 x 120-second) exposure
#   Conditions were good: clear, dark skies.
#   
# Aperture photometry with radius 4 pix = 7.4 arcsec.
#   Reference star photometry from AAVSO chart 11713LT
#   "V" mag is based on unfiltered measurements,
#   compared to V-band reference magnitudes, no color terms.
#   
# UT Date       UT     JD             "V" mag
#----------------------------------------------
 2013 May 05   02:26  2456417.601     18.7 +/- 0.3

Last modified 05/06/2013 by MWR.