UT Aug 30, 2024: Photometry of T CrB

Michael Richmond
Aug 30, 2024

On the night of Aug 29/30, 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.

I wasn't able to start observing for more than an hour after sunset, due to clouds. When the skies cleared (mostly), I was able to see that the target was still in quiescence.

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
• (sometimes) guiding with 5-second exposures
• focus position on 12-inch:
  • V = 0.759 yielded FWHM = 3.1 at T = 67/19

Notes from the night:

• clouds for about two hours after sunset. After that, a period of clarity, then clouds returned after midnight, so I gave up on a plan to acquire some color term observations.
• No moon.
• camera at T = -10 C
• air temperature was cool: T = 79/26 in the dome when I opened. It dropped to 65/18 at the end, around 12:32 AM
• no equipment failures

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. Clouds early, but somewhat clear after that.

The FWHM rose slightly.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Not photometric, but the clouds were thin, at least.

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 0.010 mag, ugh. In B-band, it was also about 0.010 mag.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.