May 12, 2013 UT: Photometry of SDSSJ1538+5123

Michael Richmond
May 12, 2013

On the night of May 11/12, 2013, I measured 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 V filter
• 120-second guided exposures

Notes from the night

• Conditions were fair: I observed during a window of relatively clear skies between two long periods of cloud. The air felt damp. No moon, so the sky was relatively dark.
• FWHM was so-so, 2.5 pixels = 4.6 arcsec
• I used twilight sky flats for V filter

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. 

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 the notes from last 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; about two hours of looking at the same field. Note the good performance in RA, and the relatively few jumps in Dec.

I took one series of images: V-band images with 120-second exposure time. I discarded a few of 40 images, leaving a set of 35 good pictures and a total of 70 minutes of exposure time.

Stack of (35 x 120 seconds) with V filter. Note the very marginal detection of the target.

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. Note the jump in zeropoint in the last image, as the clouds returned.

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 five reference stars, I found

        V  =  (instr. V mag)      - 0.83  +/-  0.06 mag
     

  from which we can derive

      SDSS 1538   V  =  18.64 
      

• including color term Because the target is (very likely) bluer than most comparisons stars, this is perhaps a better idea, if one can derive good color terms. 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.

  If we derive a color term from five nearby comparison stars in this night's image, we find

  
        V  =  (instr. V mag)      - 0.717  - 0.113*(B-V)    +/-  0.02 mag
  
      

  Using the color term slope of -0.098 * (B-V) derived on previous nights for this instrument, and discarding the faint star "G" I found

  
        V  =  (instr. V mag)      - 0.750  - 0.098*(B-V)    +/-  0.06 mag
  
      

  from which we can derive

      SDSS 1538   V  =   18.74         (color term from tonight)
  
  
      SDSS 1538   V  =   18.70         (color term from previous nights)
      

I'm going to conclude that V = 18.7 +/- 0.5. The large uncertainty is due to the low signal-to-noise value for SDSS 1538 in these images.

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 (35 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 12   02:06  2456424.59      18.7 +/- 0.4

Last modified 05/12/2013 by MWR.