UT Aug 01, 2024: Photometry of T CrB through clouds

Michael Richmond
Aug 01, 2024

On the night of Jul 31/Aug 01, 2024, under poor 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 weather was bad, but it seems T CrB is still at its quiescent value.

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 clouds
• focus position on 12-inch:
  • V = 0.789 yielded FWHM = 2.3 at T = 81/27

Notes from the night:

• lots of clouds, with short intervals of 10-20 minutes when I could see stars in clear areas. No data at all for about an hour, but I did manage to make a few measurements after 1 AM, when the field was low in the sky.
• no Moon
• camera at T = -7 C, due to very high temperatures in the dome
• very warm and humid all night: air temperature dropped from T = 93/34 at start of run to T = 75/24 at the end, around 1:30 AM. A very light rain sprinkled from the clouds at one point!
• guide camera lost USB connection once, at start of run. Had to disconnect, un-plug, re-plug, start MaximDL.
• a bat flittered above the dome at dusk, and katydids were very loud this evening
• around 1 AM, a driver was pulled over by three policecars on the road across from the observatory. Their flashing lights were very bright.

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. Clouds, clouds, clouds. I stopped collecting data for a while during the middle of the cloudiest period.

The FWHM was pretty steady -- the drop is due to clouds.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Ugh.

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 bad. In B-band, it was 0.016 -- the worst I've measured on this field.

The measurements show only that the target is still in quiescence, _perhaps_ continuing its very slow, gradual, rise.

I've submitted these measurements to the AAVSO.