Aug 20, 2014 UT: ASAS-SN14cv

Michael Richmond
Aug 21, 2014

On the night of Aug 19/20, 2014, I observed the cataclysmic variable stars ASAS-SN14cv. It was roughly steady at V = 16.9, showing variations with a small amplitude.

You can read more about this star at

The main setup was:

Notes from the night

Below is a graph showing the sky brightness as a function of time during the observing run. Note the bump, then sharp rise, near the end: clouds started to move in.

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


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, I use these stars to convert the instrumental magnitudes to the V-band scale (though of course, they are unfiltered measurements)

     A          UCAC44 711-058151    V = 11.518 
     B          UCAC44 711-058147    V = 11.899 

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.

I used 120 second exposures because the target has faded to around mag 16. With these long exposures, star "A" is slightly saturated, so I switched to using star "B" for photometric calibration.

The image adjustment factor graph shows a smooth curve for the first half, then a bump and rise in sync with the "sky" values: a sign of clouds.

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 "B" to convert the ensemble instrumental magnitudes to a reported "V"-band magnitude (but remember, it's a clear filter).

Sigma-vs-mag plot: The brightest 3 stars saturated (or nearly so), so I gave them zero weight in the ensemble calculations. The target has a slightly higher scatter than other stars at instrumental magnitude 6.4.

The target, shown in green, shows a pretty clear variation.

# Measurements of ASAS_SN14cv made at RIT Obs, Aug 20, 2014 UT, 
#    in good conditions. 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-8E CCD. 
# Exposures 120 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 711-058147 has mag=11.899 
#    which is its V-band mag according to UCAC4.  
# UT_day             JD            HJD        mag    uncert
Aug20.06638     2456889.56638  2456889.56696  16.900  0.085 
Aug20.06796     2456889.56796  2456889.56854  16.970  0.084 
Aug20.06956     2456889.56956  2456889.57014  16.940  0.083 

Last modified 8/22/2014 by MWR.