UT Jul 23, 2025: Photometry of T CrB

Michael Richmond
Jul 23, 2025

On the night of Jul 22/23, 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.

The sky was dark and clear tonight. T CrB 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-second exposures, T CrB at (326, 1100)
• focus position on 12-inch:
  • V = 0.512 yields FWHM 2.0 pixels at T = 69/21
  • V = 0.490 yields FWHM 1.8 pixels at T = 58/14
  This change in focus with temperature corresponds to delta-foc = 0.03*(delta-T), where temperature is in C.

Notes from the night:

• sky showed just a few high cirrus at sunset, and little haze for a change. I could see faintish stars and even glimpsed the Milky Way. Light clouds arrived late and brought the run to an end.
• no Moon
• camera at T = -10 C
• temperatures rather cool: 72/22 during darks, 58/14 at 2:35 AM
• two raccoons in the mulberry tree
• when I first turned on and connected the camera, the USB connections were flaky. I had to unplug/replug the camera and restart MaximDL. Then the guide camera complained "could not initialize guider output", which is another symptom of bad USB connnections. The guide camera's output runs over the Keyspan USA-19H device. The original setting was for COM6. Afer some unplugging and re-plugging, changing the COM value several times, I put it back at COM6 and the guider no longer complained.

  Do I need a new USB hub?

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 the smooth curve and low values -- a good night.

The FWHM was pretty steady.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. No evidence for clouds until the end. The outliers are due to slight trailing.

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 pretty good. In B-band, it was 0.007 mag.

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

I've submitted these measurements to the AAVSO.