UT Jun 5, 2025: Photometry of T CrB

Michael Richmond
Jun 05, 2025

On the night of Jun 04/05, 2025, under unusual conditions -- a very thick haze, in part due to smoke from distant forest fires -- 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. Sigh.

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 (342, 1144) to get a decent guide star.
• focus position on 12-inch:
  • V = 0.586 yields FWHM 2.0 pixels at T = 79/26

Notes from the night:

• Very Warm and humid
• Skies were very hazy again. My B-band measurements are again brighter than those of SSOA (by 0.15 tonight). Could the haze be responsible? I couldn't see the Big Dipper much, due to combination of haze and waxing gibbous Moon.
• waxing gibbous Moon pretty bright in southwest most of the night
• camera at T = -9 C (couldn't reach -10 C when starting)
• air temperature was very warm: 79/26 at 9:15 PM, dropping to only 65/18 when I closed at 4:06 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. Small bumps due to minor brief clouds. It was very hazy, and the Moon made the sky much brighter than usual.

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 bumps due to clouds.

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 not good. In B-band, it was 0.009 mag -- yuck.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.