UT Jun 24, 2025: Photometry of T CrB

Michael Richmond
Jun 24, 2025

On the night of Jun 23/24, 2025, under good 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.

A second nice night - clear, dark, just a bit hazy. T CrB remains at its usual level.

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 with 5-second exposure; T CrB at (340, 1092)
• focus position on 12-inch:
  • V = 0.597 yields FWHM 2.8 pixels at T = 83/28

Notes from the night:

• clear at sunset but hazy. A brief period of light cloud around midnight, then clear.
• VERY small waning crescent Moon barely visible just before dawn, age 28.3 days, 1.7 percent illumination. Probably the oldest Moon I've ever seen.
• camera at T = -5 C because it was so hot
• a VERY warm night: air temperature in the high eighties at sunset, decreasing only to 76/24 at 4:09 AM.
• a pair of turkeys and one small chick were foraging near the entrance to the fenced area when I arrived around 5 PM. They returned an hour or so later, too.

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. Note a very brief, minor set of clouds around midnight.

The FWHM was a bit higher at higher airmass (reached airmass 2.2 at the end of the night)

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Note the change in zero-point due to extinction as the target moves lower in the west.

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 0.007 mag, which is so-so. In B-band, it was also 0.007 mag.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.