UT Jul 15, 2026: Photometry of T CrB through wildfire smoke

Michael Richmond
Jul 15, 2026

On the night of Jul 14/15 2026, under unusual conditions -- wildfire smoke from Canada arrived mid-run -- 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.

The star is still quiescent. Given the conditions tonight, that's about all I can say; the photometry was significantly biased by strange extinction.

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
• guiding with 5-sec exposures, T CrB at (532, 1238)
• focus position on 12-inch:
  • V = 0.402 yields FWHM 2.5 pixels at T = 82/28

Notes from the night:

• smoke from Canadian wildfires arrived around 1 AM, I would guess, and added quite a bit of non-standard extinction. As T CrB set in the west, it appeared to be brighter than its neighbors, on average, the difference growing with airmass. About all we can say tonight is that the star didn't go into outburst; I wouldn't trust the measurements much beyond that.
• no Moon
• camera at T = -6 C (not the usual -15 C, due to the high temperatures)
• temperatures VERY warm, to an uncomfortable extent. (I became somewhat dehydrated). T = 82/28 when focusing, 78/25 when I quit at 2:49 AM.
• ran Seestar 0911 with 120-second exposures (10-sec subs). No problems.
• used dark frames of 30 and 60 seconds to reduce T CrB images
• bats flittered over the pad and woods to the west at dusk

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. Note the increased brightness at late times, due to smoke.

The FWHM was measured as much worse than usual -- I blame the smoke.

The graph below shows changes in the photometric zeropoint of an ensemble solution of the instrumental magnitudes over the course of the run. Note the steeper-than-usual slope in the second half of the run, due to smoke.

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 in V; It was 0.004 in B.

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

I've submitted these results to the AAVSO but only the V-band; the B-band were more strongly affected by smoke. .