Apr 13, 2015 UT: UX UMa

Michael Richmond
Apr 13, 2015

On the night of Apr 12/13, 2015, I observed the cataclysmic variable star UX UMa. It showed flickering variations during my run, and reached the bottom of an eclipse at the very end.

You can read more about this star at

• Modeling UX Ursae Majoris: An Abundance of Challenges

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• SBIG ST-9E CCD camera with clear filter
• 30-second unguided exposures

Notes from the night

• Conditions were good for the first two hours, but then clouds came and gradually thickened.
• Twilight sky flats.
• No guiding, as there are NO guidestars anywhere. Rats. I had to keep nudging the telescope to maintain UX UMa close to its starting position.
• 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 clouds in the second half.

Below is a graph showing the FWHM as a function of time during the observing run. The telescope gradually became defocused as the temperature dropped, but I didn't notice. Whoops.

UX UMa

Here's a chart of the field of UX UMa which is at

      RA = 13:36:41.0      Dec = +51:54:49     (J2000)

The brown box shows the rough size of the CCD during these measurements.

One of the reference stars marked above has photometry measured by the AAVSO, in their chart 14702AHO. I used this star to convert the instrumental magnitudes to the V-band scale (though of course, they are unfiltered measurements)

     A   =   000-BBT-788  =    UCAC44 710-053537    V = 13.497

The television camera on the finder scope shows the following when we're pointed at UX-UMa. North up, East left, field about 1 degree on a side. The two pieces of tape in the upper-left corner show the location of the center of the CCD field, very roughly. The bright star near the bottom is HD 118557.

I used 30-second exposures during this observing run. The target was not close to being saturated -- typically 2000 - 4000 counts/2, so the exposure time could be doubled without worry.

The image adjustment factor graph shows a relatively smooth curve until the clouds arrived.

Using aperture photometry with a radius of 4 pixels (radius of 5.7 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. I used the AAVSO 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 brightest star is the one labelled "P" in the chart above; it wasn't saturated, but fell so close to the corner of the chip that it fell on and off from frame to frame. The second-brightest star is UX UMa.

The target, shown in green, shows lots of variations, even before the eclipse at the very end of the run.

# Measurements of UX_UMa made at RIT Obs, Apr 13, 2014 UT, 
#    in mediocre conditions, 
#    by Michael Richmond, using 12-inch Meade and SBIG ST-9E 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 5.7 arcseconds.  
#    which has been shifted so UCAC4 710-053537 has mag=13.497 
#    which is its V-band mag according to AAVSO chart 14702AHO. 
# 
# UT_day             JD            HJD        mag    uncert
Apr13.04350     2457125.54350  2457125.54633  13.156  0.011 
Apr13.04387     2457125.54387  2457125.54670  13.145  0.011 
Apr13.04425     2457125.54425  2457125.54708  13.160  0.010 

• Download ASCII data file 'uxuma_apr13ut.dat'

Last modified 4/13/2015 by MWR.