UT Aug 11, 2025: Photometry of T CrB

Michael Richmond
Aug 11, 2025

On the night of Aug 10/11, 2025, under good conditions (the wildfire smoke seems to have abated), 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 measurements are quite nice. I think a slight tweak to the telescope's tracking rate helped.

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 3-second exposures, T CrB at (350, 1116);
• focus position on 12-inch:
  • V = 0.620 yields FWHM 2.3 pixels at T = 83/28

Notes from the night:

• clear skies, as far as I could tell
• waning gibbous Moon in East and South
• camera at T = -6 C
• temperatures were warm to hot: T = 83/28 while focusing, dropping to 73/23 at 1:01 AM
• I set the tracking rate to +001, and the results were good: very few trailed images. Success might be due to the shorter-than-usual 3-second exposure time for the guide camera, too.
• the computer in the dome shut itself down just as I was starting measurements of T CrB. I'm not sure if I accidentally caused it, or if the machine did it by itself. I turned power on again and the machine rebooted just fine.
• no animals

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. Pretty smooth, meaning no clouds, but bright due to Moon.

The FWHM rises just a bit at the end, due to airmass (I guess).

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run.

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 in V, which good. It was the same in B -- also good.

The measurements show that the target is still in quiescent phase, close to the brightness of the "plateau phase" of this system; perhaps accretion has started again.

I've submitted these measurements to the AAVSO.