UT Sep 20, 2024: Photometry of T CrB

Michael Richmond
Sep 20, 2024

On the night of Sep 19/20, 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 quiescent, but it showed a nice little wavy variation over the last hour which is pretty.

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 until late in the run
• focus position on 12-inch:
  • V = 0.609 yields FWHM 2.3 pixels at T = 75/24

Notes from the night:

• The sky was clear all night -- no sign of clouds
• nearly full Moon rose over the house soon after I started my run; it was on the opposite side of the sky, but still increased the sky background
• camera at T = -10 C
• air temperature was pretty warm: temperature was 75/24 at dusk, 62/17 at 10:36 PM when I stopped
• no woodland creatures

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. The very smooth locus indicates very clear skies.

The FWHM rose just a bit over the run.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Again, no evidence for 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. In B-band, it was also 0.007 mag.

The measurements show that the target is still quiescent. The little wave in the last hour, with an amplitude of nearly 0.10 mag, is a bit unusual; it's nice to see _something_ happening.

I've submitted these measurements to the AAVSO.