UT Jul 07, 2022: Photometry of U Sco with ASI camera

Michael Richmond
Jul 07, 2022

On the night of Jul 06/07, 2022, under good conditions, I acquired images of the outbursting cataclysmic variable star U Sco.

The night was clear, with just a bit of moonlight adding sky brightness.

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 = 15.5.

The main setup was:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• plate scale 1.052 arcsec/pix
• clear filter, exptime 45 seconds
• images binned 4x4
• no guiding
• camera ran at T = -15 C
• focus
  • foc in clear = 1.020, yielding FWHM 5 at T = 73/23
  • foc in clear = 0.999, yielding FWHM ? at T = 65/18
  At the end of the run, the temperature was 62/16.

Notes from the night:

• clear skies all night
• first quarter Moon in the West, didn't make the sky very bright
• I used twilight sky flats, rather than dome flats.
• both skunks wandered in the back yard and inside the fenced area; deer made a brief appearance
• a satellite swept through the field during my observations, moving directly on top of the target and ruining one measurement. Images of this object (see below) show it varying periodically in brightness during each 45-second exposure. It must be rotating quickly.

  

  

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.

Label	B	V
A	11.255	10.707
B	12.955	12.323

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 clear skies.

The FWHM graph below shows a small increase at the very end of the night, probably due to 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 0.008 mag: pretty nice!

The change in zeropoint shows a gradual increase due to airmass (I deleted a few outlying images).

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 7, 
#    in good 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
Jul07.08024     2459767.58024  2459767.58487  14.965  0.038 
Jul07.08088     2459767.58088  2459767.58551  14.995  0.041 
Jul07.08213     2459767.58213  2459767.58676  14.992  0.041 

• download ASCII text file, UT 2022 Jul 07, U Sco from RIT Obs

Last modified 7/07/2022 by MWR.