UT Aug 04, 2024: Photometry of T CrB

Michael Richmond
Aug 04, 2024

On the night of Aug 03/04, 2024, under fair (hazy) 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, though it continues to creep up very very slowly in brightness.

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
• no guiding, due to expected clouds
• focus position on 12-inch:
  • V = 0.789 yielded FWHM = 1.7 (!) at T = 78/25

Notes from the night:

• broken clouds at sunset mostly disappeared, and the sky was mostly clear during the run. Very hazy, and two brief periods of clouds as well. I did continue to altitudes less than 30 degrees.
• no Moon
• camera at T = -10 C
• warm and very humid all night: air temperature dropped from T = 85/29 at start of run to T = 71/21 at the end, around 1:15 AM. Sheets of paper in the dome felt pretty damp by then.
• no USB issues, hooray
• no animals

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. Note the two brief periods of clouds.

The FWHM was pretty steady, until the end of the run 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. Not too bad.

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, which is not good. In B-band, it was also 0.007 mag.

The measurements show that the target is still in quiescence.

I've submitted these measurements to the AAVSO.