UT Sep 11, 2025: Photometry of T CrB (and swapping computers)

Michael Richmond
Sep 11, 2025

On the night of Sep 10/11, 2025, under 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 (but then again, it was also predicted to occur during 2024), and so I've joined the crowd who are monitoring it.

Tonight's run was short, due to equipment issues. The star remains quiescent.

For the first time tonight, I used a new computer in the dome to control the camera. I wasn't able to get the autoguider working, so all the images were un-guided.

T CrB

This recurrent nova brightens 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
• focus position on 12-inch:
  • V = 0.333 yields FWHM 2.3 pixels at T = 64/18
• custom tracking rate +001

Notes from the night:

• clear during observations
• bright waning gibbous Moon was in the East in second half of run
• camera at T = -10 C
• temperatures were comfortable: T = 67/19 while setting up, dropping to 57/14 at 11:27 PM
• I installed the new Windows 11 computer in the dome. Electric focuser is now on COM6 (it was on COM5). Unable to get Orion StarShoot camera to work with MaximDL, so no autoguiding.
• Midway through the run, the connection with the main camera failed, so each exposure aborted after 1 or 2 seconds. I quit and restarted MaximDL, which fixed the problem.
• If focusing software complains about errors, check the black phone-jack-cord running from focuser control unit to motor, as well as the cable from computer to focuser control unit.

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. Some small bumps -- maybe clouds, maybe dome slit issues.

The FWHM varied a lot, probably due to trailing.

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 0.008 mag in V, which is not great. it was 0.008 in B as well.

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

I've submitted these measurements to the AAVSO.