Jun 20, 2014 UT: ASAS-SN14cl

Michael Richmond
Jun 20, 2014

On the night of Jun 19/20, 2014, I observed the cataclysmic variable star ASAS-SN14cl. You can read more about this star, which was discovered only recently, at

• ASASSN Transients Page
• Astronomer's Telegram 6233
• Astronomer's Telegram 6235

This was my first observing session in about two months, so there were a number of minor items that I had to fix.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-8E CCD camera with clear filter
• 30-second guided exposures of ASAS-SN14cl

Notes from the night

• No moon until I was just leaving, skies were dark.
• No clouds early, but some light cirrus for the final half hour or so.
• Twilight sky flats.
• The camera had some moisture inside, so ice crystals ruined the area around the bottom of the images. It didn't really affect the measurements, as far as I could tell. It was warm enough (upper fifties Fahrenheit) that the camera would only go down to -10 Celsius.

Here's a chart of the field of ASAS-SN14cl, which is at

      RA = 21:54:57.62     Dec = +26:41:16   (J2000)

Some of the reference stars marked above have magnitudes the UCAC4 AAVSO chart 13493SS. Specifically, star "B" above is

• star "123" in the AAVSO chart
• star UCAC44 584-123918
• has V-band magnitude 12.335

I took 30-second guided images through clear filter. There's a good guide star available if the target is placed on the chip at (353, 143). After discarding the bad images, I was left with 164 images.

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 AAVSO 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).

The target is the second-brightest star below; its scatter is just a bit larger than that of stars of similar brightness.

The target had a "V"-band magnitude of roughly 12.0, and it showed just a little bit of gradual, slow variation over the course of the 2.4 hour observations:

Grab the text file below for all the RIT measurements of ASAS-SN14cl. The header of the file is shown below.

# Measurements of ASAS_SN14cl made at RIT Obs, Jun 20, 2014 UT, 
#    in good conditions, 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-8E 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 7.4 arcseconds.  
#    which has been shifted so UCAC4 584-123918 has mag=12.335 
#    which is its V-band mag according to AAVSO chart 13493SS.  
# 
# UT_day             JD            HJD        mag    uncert
Jun20.14007     2456828.64007  2456828.64140  12.105  0.007 
Jun20.14065     2456828.64065  2456828.64198  12.104  0.007 
Jun20.14124     2456828.64124  2456828.64257  12.085  0.006 

• ASCII text file with measurements

Last modified 6/20/2014 by MWR.