Each step down the scale is roughly two and a half times brighter. It covered ten of them in under a day.
This comet was supposed to be boring.
220P/McNaught, found by Robert McNaught in 2004, loops the Sun every five and a half years. Small, faint, forgettable. Then in June it exploded, brightening by a factor of thousands in under fourteen hours. Then in August it did it again.
The green glow is real. It's carbon gas, C₂, fluorescing in sunlight at 516 nanometres. The fan above the core is dust being pushed off the nucleus.
Here's the problem. Over three hours the comet drifted across the sky against the stars. Stack for sharp stars and the comet smears into a streak. Stack for a sharp comet and every star becomes a trail.
So you do both, then blend them. Ninety frames, three hours, one comet that refuses to die.
The long version
Robert McNaught discovered this Jupiter-family comet from Siding Spring on 20 May 2004. On a 5.5-year orbit, it's the kind of object that usually passes unremarked.
Then, in a window of under fourteen hours spanning 30–31 May 2026, it brightened by roughly seven magnitudes, with some estimates putting the peak near magnitude 8.2. Perihelion followed on 14 June. A second outburst in August brightened it again by a factor of several hundred, and it was still holding near magnitude 9 at the end of that month. It makes its closest approach to Earth, 1.043 AU, on 12 October 2026.
The coma's green cast is diatomic carbon (C₂) fluorescing at 516 nm, a common signature in an active comet's inner coma. A short dust fan extends from the condensed nucleus.
On the processing: over the 3.6-hour run the comet moved 137 pixels against the star field, measured at 22.8″/h against an ephemeris prediction of 23″/h. A single stack can only keep one of the two sharp. The final image is a composite, one stack registered on the stars and one registered on the comet's own motion, combined so both are sharp. The smeared comet in the star-aligned stack was modelled from the comet's measured velocity and subtracted rather than masked, which leaves the star field completely intact.
Acquisition
| Date | 17 September 2026, 01:23–05:00 EEST |
| Integration | 89 × 120 s at ISO 800, 2 h 58 m total |
| Tracking | Sidereal, dithered every 2 frames, no meridian flip |
| Calibration | 100 bias, 30 flats, matched darks |
Equipment
| Optics | GSO RC8 carbon, 203 mm, with TS 2″ Photoline 0.8× reducer, 1270 mm at f/6.3 |
| Camera | Canon EOS M50, no filter |
| Mount | HEQ5 Pro on PLL C82 pier, EQMOD |
| Guiding | ASI220MM Mini on TS 50 mm, pulse-guided, 4.13″/px, guide FWHM 3.16″ |
| Control | N.I.N.A. 3.2.0.9001, PHD2 2.6.14 |
| Image scale | 0.604″/px, plate-solved at 1269.5 mm |
| Processing | Siril 1.4.4, dual registration (global star alignment and comet/asteroid registration), Winsorized sigma stacking, SPCC against Gaia DR3, background extraction; composite assembled with a motion-blur model of the comet's measured velocity |
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