May 08, 2013 UT: Photometry of SDSSJ1538+5123

Michael Richmond
May 08, 2013

On the night of May 07/08, 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 not great: thin cirrus always present, occasionally thicker. It caused approx 0.4 mag of extinction on average. No moon, so the sky was relatively dark.
• FWHM was good, 2.2 - 2.3 pixels = 4 arcsec
• I used twilight sky flats for V filter
• I increased exposure times 60 -> 120 seconds. Looks like this was a good idea.

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 just 3 of the 56 images, leaving a set of 53 good pictures and a total of 106 minutes of exposure time.

Stack of (53 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. Note the large scatter: a clear sign of cirrus.

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. V mag)      - 1.11  +/-  0.07 mag
     

  from which we can derive

      SDSS 1538   V  =  18.4  
      

• 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.

  Using the color term slope of -0.098 * (B-V) derived on previous nights for this instrument, I found

  
        V  =  (instr. V mag)      - 1.032  - 0.098*(B-V)    +/-  0.06 mag
                                     (if we include all 6 stars)
  
  
        V  =  (instr. V mag)      - 0.992  - 0.098*(B-V)    +/-  0.02 mag
                                     (if we discard faint G, H)
  
      

  from which we can derive

      SDSS 1538   V  =   18.50         (including all 6 stars)
  
  
      SDSS 1538   V  =   18.54         (discarding faint G, H)
      

I'm going to conclude that V = 18.5 +/- 0.4. 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 (53 x 120-second) exposure
#   Conditions were fair-to-poor: light cirrus came and went.
#   
# 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 08   02:24  2456420.60      18.5 +/- 0.4

Last modified 05/07/2013 by MWR.