UT Aug 27, 2024: Photometry of T CrB

Michael Richmond
Aug 27, 2024

On the night of Aug 26/27, 2024, under very 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.

Tonight was clear and dark -- very good conditions. No outburst -- sigh.

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
• focus position on 12-inch:
  • V = 0.740 yielded FWHM = 2.4 at T = 79/26

Notes from the night:

• clear all night. Nice!
• third-quarter Moon rose around midnight. No problem.
• camera at T = -10 C
• air temperature was high in the day, due to sun: T = 91/33 in the dome when I opened. It dropped to 67/19 at the end, around 12:16 AM
• no equipment failures, though I did accidentally shift the telescope away from the target field for 10 minutes or so in the middle of the run.
• some noisy insects bumped into things in the dark dome, making perceptible "thumps" and "whaps".
• since the Fall Semester has just started, I had to wait for the students in the intro astro lab class to clear out before I could work in the house.

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. A smooth curve indicates clear skies.

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. Few outliers -- good.

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; perhaps no smaller due to the large airmass at end of observing run. In B-band, it was 0.007 mag. Still quite nice.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.