UT Sep 02, 2024: Photometry of T CrB

Michael Richmond
Sep 02, 2024

On the night of Sep 01/02, 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.

No outburst yet. Rats.

I also acquired some images of the Landolt standard field PG2213-006 in order to check the color terms I've derived from observations of the field of T CrB itself.

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 (the first 20 images in V-band were also 60 seconds, oops)
• images binned 4x4
• guiding with 5-second exposures, except for a brief cloudy period
• focus position on 12-inch:
  • V = 0.675 yielded FWHM = 2.8 at T = 68/20
  • V = 0.654 yielded FWHM = 2.7 at T = 60/15

Notes from the night:

• very clear after some rain in the early afternoon. One brief period of clouds in the middle of the run, then clear again.
• No moon.
• camera at T = -10 C
• air temperature was much cooler: T = 79/26 in the dome when I opened. It only dropped to 59/15 when I quit at 12:57 AM.
• no equipment failures, but I must remember to reset the "Autosave" parameters at the start of night, from those for PG2213 to those for T CrB

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. Brief clouds interrupt an otherwise very clear night.

The FWHM rose slightly.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Aside from the brief cloudy time, it was clear.

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 0.006 mag. 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.