UT Sep 20, 2025: Photometry of T CrB and V527 And

Michael Richmond
Sep 19, 2025

On the night of Sep 19/20, 2025, under good conditions, RIT graduate student Scott Adler and 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.

We tried using a new autoguider camera, but ran into technical issues. We'll try again another night, so no guiding tonight.

The star remains quiescent.

I also acquired images of the eclipsing binary star V527 And for a capstone project.

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 = 0.381 yields FWHM 2.0 pixels at T = 61/16
  • V = 0.369 yields FWHM 3.0 pixels at T = 53/12
  • V = 0.338 yields FWHM 2.8 pixels at T = 44/6
  That yields a change in focus of 0.0052 or 0.0043 units per degree C, with the sense that colder temperatures correspond to smaller focus positions.
• custom tracking rate +001

Notes from the night:

• clear, or nearly clear, during the entire run
• no Moon
• camera at T = -10 C
• temperatures were cold: T = 61/16 while focusing, dropping to 44/6 at 5:09 AM
• No guiding, which caused a number of images (the longer B-filter images especially) to trail.
• This was the first night after the dome door repair, which meant that the telescope had been handled rather roughly during its last parking, and while parked. When I executed the "park scope" command, the telescope didn't go to the normal pointing-up position, but slewed far north, starting to push the camera into the base of the fork. I stopped it via the emergency stop feature, but I'll have to re-set the orientation the next time we open.

  (Note added after the following night -- done! All is well now).

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. Very smooth -- no evidence for clouds.

The FWHM was pretty steady, but had a large scatter due to the lack of guiding.

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 in V, which is pretty good. it was 0.009 in B, not so good.

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

I've submitted these measurements to the AAVSO.

Images of eclipsing binary star V527 And

One of my capstone students is studying the eclipsing binary star V527 And. I acquired some images of it after T CrB reached too large an airmass. The finder telescope shows this on the TV screen when V527 And is in the main telescope's field of view. The two stars at bottom right are good markers.

I acquired 201 pairs of B (60 sec) and V (30 sec) images, all unguided.

                       Start                    End
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   EDT    2025 Sep 20  23:02        Sep 21     05:06

   UT     2025-09-20 03:02:19       2025-09-20 09:06:56

   JD       2460938.62661             2460938.87981
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