UT Jul 07, 2025: Photometry of T CrB (guide camera failure?)

Michael Richmond
Jul 07, 2025

On the night of Jul 06/07, 2025, under fair 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 (but then again, it was also predicted to occur during 2024), and so I've joined the crowd who are monitoring it.

Another cirrus-y night, lit up by waxing gibbous Moon. T CrB is still quiescent.

The guide camera failed: when I requested a 5-second exposure, the usual 5-second countdown did not appear in MaximDL's windows, and the "exposure" never ended. I could ignore it and use the main camera just fine, and did so. No guiding tonight. But is this the end of the guide camera? Need to check it out.

First racoon sighting of the year!

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, as guide camera wasn't working
• focus position on 12-inch:
  • V = 0.594 yields FWHM 2.2 pixels at T = 85/29

Notes from the night:

• hazy and light cirrus all night, I think
• waxing gibbous Moon in S nearly all night
• camera at T = -5 C
• hot temperatures again: 85/29 at start, 75/24 at end (3:59 AM)
• lots of wildlife: two deer, two raccoons, all after the mulberries
• the guide camera wasn't working properly.
  • When I plugged in the USB cables, the computer "beeped" in the normal manner.
  • MaximDL detected it, showed the usual indications for it
  • BUT when I clicked on "Start" for a 5-second exposure, no count-up ("1", "2", etc.) appeared. The "exposure" never ended.
  • I clicked on "Stop", and could use the other camera just fine.

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 table X40237AAS. Note that the magnitudes listed for stars "A" and "B" have changed from the ones I listed in last year's notes.

  star       name                  B          V         
------------------------------------------------------
      A     000-BJS-901         11.096     10.554
      B     000-BBW-805         11.779     11.166
      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. The sky was bright all night due to cirrus and the Moon, and some light clouds appeared near the end.

The FWHM rose gradually.

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

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, which is okay. In B-band, it was 0.008 mag.

The measurements show that the target is still in quiescent phase.

I've submitted these measurements to the AAVSO.