UT Jul 26, 2024: Photometry of T CrB

Michael Richmond
Jul 26, 2024

On the night of Jul 25/26, 2024, 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, perhaps in 2024, and so I've joined the crowd who are monitoring it.

T CrB is still in quiescence, at V = 10.05 this evening.

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 exptime 5 seconds -- worked well for a time, then problems (see below)
• focus position on 12-inch:
  • V = 0.813 yielded FWHM = 2.2 at T = 72/22

Notes from the night:

• no clouds at first, but then a thin layer of haze or cirrus arrived.
• Waning gibbous Moon lit up the sky after midnight
• camera at T = -10 C
• air temperature went from T = 86/30 at start of run to T = 62/17 at the end, around 1:30 AM
• MaximDL froze several times, forcing me to kill the process, unplug/replug the cameras, and restart. I think the guide camera was the culprit, and I was forced to give up on guiding after 11:46 PM. I am guessing that the USB hub may be the root of the problem, so I'll try buying a new one.
• coyotes yipped in the distance to the west around 10 PM, and the skunks made a visit, based on odor

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 chart.

  star       name                  B          V         
------------------------------------------------------
      A     000-BJS-901         11.190     10.566
      B     000-BBW-805         11.840     11.187
      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. It looks smooth, so the clouds weren't bad until the end.

The FWHM rose a bit at the end.

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 bump early due to haze/cirrus, and more clouds/airmass near the end.

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, which is okay, In B-band, it was 0.006.

The measurements show relatively steady brightness, with a couple of tiny, brief bumps in B-band.

I've submitted these measurements to the AAVSO.