May 07, 2013 UT: Photometry of SDSSJ1538+5123

Michael Richmond
May 07, 2013

On the night of May 06/07, 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
• 60-second guided exposures

Notes from the night

• Conditions were fair: no clouds, but some haze, especially in the west (but I was pointing east and south). No moon, so the sky was relatively dark.
• FWHM was good, 2.2 - 2.5 pixels = 4-5 arcsec
• I used twilight sky flats for V filter
• I re-balanced the telescope, which I _think_ improved the tracking and guiding a bit. See graph below.

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. 

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 60-second exposure time. I discarded just 2 of the 84 images, leaving a set of 82 good pictures.

Stack of (82 x 60 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 small scatter and gradual change: that indicates a lack of clouds and a decreasing airmass. A quick and dirty linear fit yields an extinction coefficient of 0.35 mag/airmass, which is reasonable for the V filter. Two of the three outliers at late times are due to elongated images, due to slight trailing.

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)      - 1.45  +/-  0.03 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 five reference stars, I found an empirical transformation:

        V  =  (instr.V mag)  -  1.37   - 0.098  * (B-V)   +/-  0.02
     

  from which we can derive

      SDSS 1538   V  =  18.5  
      

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 (82 x 60-second) exposure
#   Conditions were fair: dark skies with a light haze.
#   
# 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 07   03:05  2456419.63      18.5 +/- 0.4

Last modified 05/07/2013 by MWR.