UT Jul 08, 2024: Photometry of T CrB

Michael Richmond
Jul 08, 2024

On the night of Jul 07/08, 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.

The object is still in its quiescent state, at approximate V = 10.18. The (B-V) color has decreased slightly over the last week or so, and the star has increased in brightness by 0.3 mag or so. Hmmmm.

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 worked well
• focus position on 12-inch:
  • V = 0.784 yielded FWHM = 2.7 at T = 78/25

Notes from the night:

• very clear before midnight. Light clouds arrived after that
• pretty crescent Moon on western horizon at sunset
• camera at T = -10 C, in what will probably become the new norm for a while
• air temperature in the dome dropped from T = 88/31 at start of run to T = 67/19 at the end.
• USB communications between computer and camera failed after moving telescope from park position to flatfield position (scope has to point to very low elevation), and then AGAIN later in the middle of the night. The latter was a big problem, as I lost nearly an hour of data to this not-very-obvious failure. What happens is that the camera starts to take an image, but after just 1 second or so, quits and doesn't read out properly. MaximDL displays what looks like an ordinary image, so the only clue that this is happening is a sudden, very large increase in the rate of images accumulating.

  I will try getting new USB cables and perhaps a new USB hub.

• lots of raccoons, and one skunk who wandered into the fenced area

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 was clear early, but clouds came late; I discarded the final 5 or 10 images. The gap is due to the USB problem mentioned above.

The FWHM was steady in V.

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 0.004 mag, which is very good. It was also 0.004 in B-band.

The measurements show relatively steady brightness. The B-band showed a bit more action, but nothing major.

I've submitted these measurements to the AAVSO.