UT Aug 15, 2024: Photometry of T CrB (and dome maintenance)

Michael Richmond
Aug 15, 2024

On the night of Aug 14/15, 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.

The sky was clear and not too hazy. Measurements indicate that T CrB is still in quiescence. 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 until the airmass grew too high, at which point I stopped guiding
• focus position on 12-inch:
  • V = 0.678 yielded FWHM = 1.9 at T = 75/24

Notes from the night:

• clear, not as hazy as the previous night
• first-quarter Moon low in the south appeared only slightly yellow
• camera at T = -10 C
• air temperature dropped from T = 90/32 at start of run to T = 64/17 at the end, around 12:34 AM
• no equipment failures
• no animals
• earlier in the morning, Joe Wixom and a colleague from FMS came to the Observatory for some maintenance on the dome. They lubricated the dome-slit motor and the wheels on which dome rotates (using Tri-Flow for the latter job). Joe even replaced one of the roller assemblies on which the dome slit rolls up and down. Thanks to both of them!

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 very smooth locus -- a sign of clear skies.

The FWHM was fine and relatively steady.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. No big changes due to clouds.

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 good. In B-band, it was 0.007 mag.

The measurements show that the target is still quiescent.

I've submitted these measurements to the AAVSO.