Aug 08, 2014 UT: ASAS-SN14cv

Michael Richmond
Aug 08, 2014

On the night of Aug 07/08, 2014, I observed the cataclysmic variable star ASAS-SN14cv. It was roughly steady at roughly V = 14.3, nearly two magnitudes brighter than 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

• The sky was very hazy a sunset, so that the Sun appeared as a sharp-edge red circle. The haze diminished around midnight, but it was never really clear. Note added 8/9/2014: I've learned that smoke from large Canadian forest fires may have been responsible for this phenomenon.
• 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 the gradual decrease as the haze lessened, followed by bumps due to light clouds near the end.

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 a sharp increase near the end as the target dropped lower in the sky.

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 is the outlier around instrumental mag 4.7.

The target, shown in green, shows a slight, gradual decrease in brightness in the second half of the run.

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 8, 2014 UT, 
#    in mediocre 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
Aug08.05584     2456877.55584  2456877.55669  14.371  0.036 
Aug08.05657     2456877.55657  2456877.55742  14.295  0.034 
Aug08.05874     2456877.55874  2456877.55959  14.340  0.035 

• Download ASCII data file 'asas14cv_aug08ut.dat'

Last modified 8/08/2014 by MWR.