Aug 10, 2014 UT: ASAS-SN14cv

Michael Richmond
Aug 10, 2014

On the night of Aug 09/10, 2014, I observed the cataclysmic variable star ASAS-SN14cv. It was roughly steady at V = 16.6, about one magnitude fainter than its value the previous night. You can read more about ASAS-SN14cv, which was discovered only recently, at

• ASASSN Transients Page

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with clear filter
• 45-second guided exposures

Notes from the night

• Conditions were very good, at last: no haze, little sign of clouds.
• I was able to see all the stars in the Big Dipper clearly, despite the Full Moon to the south.
• Twilight sky flats.
• Very nice guiding, all night long.
• Divided all raw frames by 2 to avoid exceeding the 15-bit limit of XVista software.

Below is a graph showing the sky brightness as a function of time during the observing run. Note just one bump due to clouds.

Below is a graph showing the FWHM as a function of time during the observing run.

ASAS-14cv

Here's a chart of the field of ASAS-SN14cv, which is at

      RA = 17:43:48.58     Dec = +52:03:46.8   (J2000)

Some of the reference stars marked above have magnitudes in the UCAC4. Specifically, star "A" above is

• star UCAC44 711-058151
• has V-band magnitude 11.518

The television camera on the finder scope shows the following when we're pointed at ASAS-14cv. North up, East left, field about 1 degree on a side.

The image adjustment factor graph shows the one bump early, then a smooth curve due to the changing airmass 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. 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. I used the UCAC4 V-band magnitude of star "A" to convert the ensemble instrumental magnitudes to a reported "V"-band magnitude (but remember, it's a clear filter).

Sigma-vs-mag plot: The two brightest stars were both saturated, and so I gave them zero weight in the ensemble calculations. The target blends in with other stars at instrumental magnitude 6.5.

The target, shown in green, shows no significant change in brightness, just a large scatter due to its faintness.

You can see my measurements of the star in the ASCII text file below. The first few lines are shown here:

# Measurements of ASAS_SN14cv made at RIT Obs, Aug 10, 2014 UT, 
#    in good conditions. 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-8E CCD. 
# Exposures 45 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 7.4 arcseconds.  
#    which has been shifted so UCAC4 711-058151 has mag=11.518 
#    which is its V-band mag according to UCAC4.  
# 
# UT_day             JD            HJD        mag    uncert
Aug10.05644     2456879.55644  2456879.55724  16.386  0.098 
Aug10.05715     2456879.55715  2456879.55795  16.614  0.125 
Aug10.05787     2456879.55787  2456879.55867  16.656  0.127 

• Download ASCII data file 'asas14cv_aug10ut.dat'

Last modified 8/09/2014 by MWR.