UT Jul 28, 2024: Photometry of T CrB

Michael Richmond
Jul 27, 2024

On the night of Jul 27/28, 2024, under fair conditions (clear but very hazy), 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, at V = 9.97 this evening. It does seem to be creeping up 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
• guiding with exptime 5 seconds -- worked fine
• focus position on 12-inch:
  • V = 0.794 yielded FWHM = 2.2 at T = 76/24

Notes from the night:

• no clouds, but lots of haze: rising Moon was deep orange
• almost-third-quarter Moon rose just as I was quitting
• camera at T = -10 C
• air temperature dropped from T = 88/31 at start of run to T = 65/18 at the end, around 1:14 AM
• brief problem with the USB connection to the guide camera, forcing me to unplug and replug both cameras. No problems after that, but maybe I should replace the USB cable
• no animals of note

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's smooth, so no significant clouds -- but there was a lot of haze.

The FWHM was pretty steady. I noted turbulence early in the evening again.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Typical increase as the airmass increased.

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, which is okay, In B-band, it was 0.007 -- not very good.

The measurements show relatively steady brightness, with one sizeable brief bump, of amplitude 0.20 mag in B.

I've submitted these measurements to the AAVSO.