UT Jun 14, 2026: Photometry of T CrB through clouds

Michael Richmond
Jun 14, 2026

On the night of Jun 13/14, 2026, under poor 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 (but then again, it was also predicted to occur during 2024), and so I've joined the crowd who are monitoring it.

This evening, clouds never really left the skies, though I was able to see the field through them most of the time. The star is still quiescent.

T CrB

This recurrent nova brightens 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 with 5-sec exposures, T CrB at (498, 1252)
• focus position on 12-inch:
  • V = 0.386 yields FWHM 2.2 pixels at T = 77/25

Notes from the night:

• Heavy clouds early, then broken clouds with brief periods of almost-clarity.
• no Moon
• camera at T = -12 C
• temperatures were warm. T = 89/32 when opening, dropping to 69/20 at 4:06 AM
• a number of images at the end of the run showed trails; I think that one of the cables was stretched taut and was pulling against the tracking of the telescope. I loosened some cables and the very last few images looked round again.

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 table X40237AAS. Note that the magnitudes listed for stars "A" and "B" have changed from the ones I listed in last year's notes.

  star       name                  B          V         
------------------------------------------------------
      A     000-BJS-901         11.096     10.554
      B     000-BBW-805         11.779     11.166
      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. The sky is brighter early, but even later there were clouds; I guess there was just little light from the ground to make them appear bright.

The FWHM shows rose toward the end; I'm not sure why.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Lots of clouds, all night long. Bleah.

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 in V, which is poor. It was 0.012 in B -- very poor.

The measurements show that the target is still in quiescent phase.

I've submitted these measurements to the AAVSO.