UT Aug 25, 2025: Photometry of T CrB

Michael Richmond
Aug 25, 2025

On the night of Aug 22/23, 2025, under excellent 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.

Tonight's observations might have the lowest noise of any I've collected this year. No outburst, but one can see short, small flickers.

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
F
• guiding with 3-second exposures, T CrB at (338, 1096);
• focus position on 12-inch:
  • V = 0.581 yields FWHM 2.4 pixels at T = 72/22
• custom tracking rate +001

Notes from the night:

• completely clear skies toward T CrB, though I did see clouds low on the eastern horizon early
• no Moon
• camera at T = -10 C
• temperatures were comfortable: T = 72/22 while focusing, dropping to 64/17 at 12:13 AM
• a group of grad students came out to the observatory to hang out on the concrete pad and look at the sky, just for fun. Excellent!

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 smooth curve means no clouds.

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. Very nice -- no sign of clouds, very few trailed images.

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 in V, very nice; it was 0.004 in B, which is the best I can recall.

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

I've submitted these measurements to the AAVSO.