UT Aug 13, 2024: Photometry of T CrB

Michael Richmond
Aug 13, 2024

On the night of Aug 12/13, 2024, under mediocre 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.

Tonight's weather was so-so. I made measurements during some clear periods, and it seems T CrB may be very slightly fading now.

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 at first, then none while clouds came; restarted guiding at the end, when clear again
• focus position on 12-inch:
  • V = 0.689 yielded FWHM = 2.0 at T = 68/20

Notes from the night:

• clear at sunset, a bit breezy and coolish. Comfortable. Clouds appeared after about an hour, and remained for the next 2.5 hours. Clear-ish skies again for a short time at high airmass.
• first-quarter Moon low in the south at sunset; no big effect, I think
• camera at T = -10 C
• cooler -- temperature dropped from T = 79/26 at start of run to T = 66/17 at the end, around 12:56 AM
• no equipment failures
• coyotes yipping in the west around 12:30 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. Clear, then long period of clouds, then clear again late.

The FWHM was decent.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. I discarded a number of cloudy images in the middle of the run.

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 0.005 mag, which is so-so. In B-band, it was 0.004 mag.

The measurements show that the target is still quiescent, maybe fading very slightly.

I've submitted these measurements to the AAVSO.