12P/Pons-Brooks observing campaign

Comet 12P/Pons-Brooks comes to perihelion this April. It has an orbital period of 71 years and a large nucleus which has frequent outbursts. The BAA comet section is running a programme to measure the brightness of this comet in a small aperture near to the nucleus. This shows the outbursts more clearly than measuring the total magnitude of the comet although that is an important parameter too and it can be measured at the same time using comphot.

A plot of the lightcurve since 2023 November is shown below. You can get the latest version of this lightcurve in full resolution from here (you may need to refresh your browser to get the latest image). For consistency, if you wish to contribute observations to this lightcurve please see the instructions below.


The photometry is performed using unfiltered images, preferably using Astrometrica, which are reduced against Gaia DR2 as the reference catalogue. In Astrometrica set the star catalogue to “Gaia DR2”, colour band to “Gaia Broadband (G)” and the filter to “Clear/None”. On the “Program” tab select an aperture radius which corresponds as close as possible to 9 arcsec radius given your pixel scale. When you measure the comet adjust the contrast so that you can confirm that there are no bright stars within about twice the radius of the aperture (Astrometrica has a strange way of estimating the sky background and stars near to the aperture can bias the measurement). A good example of a clean measurement is shown here:

For observations to be included in the please send me the photometry file (called PhotReport.txt). You can also send me the astrometry file (called ADESReport.PSV in recent versions of Astrometrica) but this isn’t used for the lightcurve although it is a useful check on other parameters. Here is an example of the PhotReport.txt file.

TELESCOPE: 0.28m f/10.0 0.28-m f/10 Schmidt-Cassegrain + CCD
EXPOSURE JD: Mid-exposure, not corrected for light time
JD mag Flt SNR ZeroPt Cat Design.
2460327.34948 12.714 G C 90.40 23.991 Gaia2 12P/Pons-Brooks
2460327.35027 12.737 G C 106.37 23.984 Gaia2 12P/Pons-Brooks
2460327.75453 12.691 G C 130.39 24.440 Gaia2 12P/Pons-Brooks
2460327.75931 12.674 G C 143.54 24.445 Gaia2 12P/Pons-Brooks

Please send the files to my email address and remember to send them as attachments so that they don’t get mangled by email software.

If you do spot an outburst then please alert other users using the Comet Section mailing list. You can subscribe to this here.

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