UT Aug 31, 2024: Photometry of T CrB

Michael Richmond
Aug 30, 2024

On the night of Aug 30/31, 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 star is still in quiescence.

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, except for last half-hour or so
• focus position on 12-inch:
  • V = 0.725 yielded FWHM = 2.3 at T = 82/27

Notes from the night:

• some light cirrus at sunset, with distant clouds on the western horizon. But clear throughout the night, as far as I can tell.
• A breeze picked up throughout the night; It rained around 4 or 5 AM.
• No moon.
• camera at T = -10 C
• air temperature was warm again: T = 93/34 in the dome when I opened. It only dropped to 72/22 at the end, around 12:15 AM, and humidity was pretty high
• no equipment failures, but I somehow stopped the camera from taking pictures for about a half hour by mistake
• bats just after sunset

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 smooth curve indicates clear skies.

The FWHM rose slightly, especially at the end when the target was at high airmass.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. If there were clouds, they were thin.

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, pretty good! In B-band, it was about 0.007 mag.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.