UT May 30, 2025: Photometry of T CrB

Michael Richmond
May 30, 2025

On the night of May 29/30, 2025, under mediocre 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.

T CrB was still quiescent, based on our measurements this night.

I again captured some images of T CrB with two SeeStar S50 units, but only incompetently. Need more practice.

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 exposures. Placed T CrB at (388, 752) to get a decent guide star.
• focus position on 12-inch:
  • V = 0.544 yields FWHM 2.1 pixels at T = 63/17

Notes from the night:

• Cirrus and light cloud were scattered over the sky at sunset. It may have cleared a bit in the next few hours, but I suspect the cirrus was present during the entire run.
• young crescent Moon set in the west around 11:30 PM
• camera at T = -10 C
• air temperature was not too cold: 65/18 at dusk, dropping to 57/14 when I closed at 12:41 AM.

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. I see some small bumps due to 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. Note the rises and falls due to passing layers of cloud.

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 so-so. In B-band, it was 0.009 mag, ugh.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.