UT Aug 05, 2024: Photometry of T CrB

Michael Richmond
Aug 05, 2024

On the night of Aug 04/05, 2024, under fair conditions, I acquired images of the recurrent nova T CrB. This star undergoes outbursts at long intervals of 80 years or so. Its next outburst is predicted to occur soon, perhaps in 2024, and so I've joined the crowd who are monitoring it.

T CrB is still in quiescence. Sigh. I hope it does something soon.

T CrB

This recurrent nova brightens from by about 8 magnitudes (!), from V = 10 to about V = 2, around every 80 years. Will we see another outburst this summer?

These observations involved:

• Meade 12-inch telescope
• no focal reducer, so working at f/10
• ASI 6200MM CMOS camera
• B-filter 60 sec, V-filter 30 sec
• images binned 4x4
• guiding for the first few hours, then unguided when light clouds arrived
• focus position on 12-inch:
  • V = 0.747 yielded FWHM = 1.8 (!) at T = ??/??

Notes from the night:

• clear but hazy at sunset and for the first few hours of the run. Around JD 527.66, a few bands of light clouds passed through the sky. I continued observing until the source was pretty low -- about 24 degrees above horizons = airmass 2.4
• no Moon
• camera at T = -10 C
• warm and humid all night: air temperature dropped from T = 89/31 at start of run to T = 70/21 at the end, around 1:38 AM.
• the USB connection to guide camera failed right after startup, so I replaced the USB cable with a brand new one. No problems after that.
• loud katydids all night, and coyotes yipping as I left at 1:45 AM

The picture below shows a cropped image of the field of T CrB from Jun 14/15, 2024. The field of view is about 20 arcminutes across.

I've marked the location of several comparison stars, with magnitudes and names taken from the AAVSO's chart.

  star       name                  B          V         
------------------------------------------------------
      A     000-BJS-901         11.190     10.566
      B     000-BBW-805         11.840     11.187
      C     000-BPC-198         13.049     12.336
--------------------------------------------------------------------------

 

When the target is centered, the finder TV shows this field:

Here's the sky background over the course of the run. Note the light clouds arriving late.

The FWHM increased late, especially when I stopped guiding.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. As the object set in the west, the scatter went way up.

Using aperture photometry with a radius of 7 pixels in V filter (binned 4x4, each pixel is 1.036 arcsec, so a radius of 7.3 arcsec), and 7 pixels in B filter (binned 4x4, each pixel is 1.036 arcsec, so a radius of 7.3 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 in V-band was about 0.006 mag, which is not great. In B-band, it was 0.005 mag, due to harder cut on the images included in the solution.

The measurements show that the target is still in quiescence.

I've submitted these measurements to the AAVSO.