UT Jul 08, 2022: Photometry of U Sco with ASI camera

Michael Richmond
Jul 08, 2022

On the night of Jul 07/08, 2022, under fair conditions, I acquired images of the outbursting cataclysmic variable star U Sco.

Several times during the night, light broken clouds moved across the sky, and the waxing gibbous Moon wasn't too far away. As a result, the measurements of U Sco have a pretty large scatter tonight: about 0.06 mag.


U Sco

U Sco is a cataclysmic variable which only rarely is caught in an outburst. This one was first noticed at UT 2022 June 6.720 by Masayuki Moriyama. The star reached a peak brightness of about V = 8, but faded quite a bit by this evening; it is currently about V = 15.2.

The main setup was:

Notes from the night:

The object is located at



  RA = 16:22:30.78  Dec = -17:52:42.8   (J2000)

A chart of the field is shown below. The size of the chart is about 42 x 30 arcminutes.

I've marked the location of several comparison stars as well, which appear on the AAVSO chart of comparison stars.

I'll use star "A" to shift my instrumental magnitudes to the V-band scale.

I took a photo of the finder TV's screen when pointing to this target; this could be a useful reference for the future:

The sky value shows several episodes of clouds.

The FWHM graph below shows an increase near the end of the night; could be due to temperature change, or increased airmass.

Using aperture photometry with a radius of 7 pixels in clear filter (binned 4x4, each pixel is 1.052 arcsec, so a 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.

Sigma-vs-mag plots show that the floor was about 0.010 mag after discarding a dozen particularly bad images.

The change in zeropoint shows several episodes of clouds.

The measurements show a small decline in brightness.

You can download my measurements below. A copy of the header of the file is shown to explain the format.

# Measurements of U_Sco made at RIT Obs, UT 2022 Jul 8, 
#    in fair conditions, 
#    by Michael Richmond, 
#    using Meade 12-inch LX200 and ASI 6200MM. 
# Exposures 45 seconds long, clear 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 pix = 7.4 arcseconds.  
#    which has been shifted so AAVSO 000-BBX-431 has mag=10.707 
#    which is its V-band magnitude according to AAVSO.  
# 
# UT_day             JD            HJD        mag    uncert
Jul08.08200     2459768.58200  2459768.58657  15.316  0.057 
Jul08.08263     2459768.58263  2459768.58720  15.237  0.061 
Jul08.08388     2459768.58388  2459768.58845  15.170  0.057 


Last modified 7/08/2022 by MWR.