May 11, 2016 UT: Photometry of the cataclysmic variable ES Dra

Michael Richmond
May 11, 2016

On the night of May 10/11, 2016, I acquired a set of observations of the cataclysmic variable star ES Dra. The variable star showed an average magnitude of about 15.0, faint enough that my measurements barely show its flickering light curve.

The main setup was:

ES Dra

This nova-like variable is currently in "standstill", and so a target of observations by the Center for Backyard Astrophysics.

The object is located at

  RA = 15:25:31.86   Dec = +62:00:59

A chart of the field is shown below. The size of the chart is about 20-by-20 arcmin.

Entries for some of the marked comparison stars from the UCAC4 follow:

  star        UCAC4              B          V
   A       761-044722         11.723      11.227

   B       761-044711         14.037      13.143



I used 30-second exposures with no filter.

Conditions were good, though the waxing gibbous Moon was bright. Notice how the sky background decreases long after sunset, as the Moon was going down in the west.

The lack of guider was less important on this night, since I could use a simple mechanism to nudge the telescope a bit when it drifted away from its starting location.

As before, I removed any measurements which were made more than 600 pixels (791 arcsec = 13.2 arcmin) from the center of the field.

Using aperture photometry with a radius of 5 pixels (radius of 6.7 arcsec), I measured the instrumental magnitudes of a number of reference stars and the target. Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame.

Sigma-vs-mag plots show that the floor was about 0.003 mag. ES Dra is the outlier at diff mag = 4.2.

Image adjustment factor shows evidence for little cloudiness -- the night was pretty clear.

Here are light curves of the variable and the field stars:

A closeup shows the variations in ES Dra more clearly.

I used the UCAC value for the V-band magnitude of star "A" = UCAC4 761-044722 to shift the ensemble magnitudes to the standard V-band scale -- but remember that these are UNFILTERED measurements.

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of ES_Dra made at RIT Obs, May 11, 2016 UT, 
#    in good conditions (but bright moonlight) 
#    by Michael Richmond, using 12-inch Meade and ATIK 11000 CCD. 
# Exposures 30 seconds long, no filter. 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#    and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 6.6 arcseconds.  
#    which has been shifted so UCAC4 761-044722 has mag=11.227 
#    which is its V-band mag according to UCAC4.  
# UT_day             JD            HJD        mag    uncert
May11.09656     2457519.59656  2457519.59758  15.059  0.036 
May11.09704     2457519.59704  2457519.59806  15.030  0.035 
May11.09751     2457519.59751  2457519.59853  15.095  0.037 

Last modified 4/20/2016 by MWR.