Orwell Astronomical Society (Ipswich)
Comet 3I/ATLAS, 21-26 July 2025
The ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Rio Hurtado, Chile, first reported an observation of comet 3I/ATLAS on 01 July 2025. (Subsequently, "pre-discovery" observations were found extending back to 14 June.) The object is the third known interstellar visitor to the Solar System (following 1I/Oumuamua and 2I/Borisov) and is a little different to its predecessors. The first two were predominantly rocky or asteroidal, whereas 3I/ATLAS is more gassy or comet-like. As it draws nearer to the Sun it is expected to develop a tail, but it is unclear whether it the tail will be bright enough to appear in a modest telescope. The object is expected to attain peak magnitude around the end of November 2025 (fortuitously, this coincides with the best time to view the comet from the UK), and predictions of its peak magnitude have ranged from an initial estimate of 11.5 to an estimate in late summer of 14.5.
When the discovery was announced, I was disappointed to realise that, from my home observatory, the comet would appear only some 20° above the southern horizon, playing hide-and-seek behind a tree. But I was off to La Palma in the Canaries, where it would be some 23° higher in a very, very clear, dark sky!
I packed my black-and-white ZWO ASI6200 camera on a 90 mm Megrez refractor on a ZWO AM3 mount. Together with Nick James, Director of the BAA Comet section, I set off early on 21 July to fly to Madrid, then on to Santa Cruz de La Palma, arriving about 6pm at our rural house.
After such a long journey, we decided to observe from the house. Unusually, it was "blowing a gale"; such weather would normally banish thoughts of staying outside to observe or photograph anything. But we persevered and set up in the gap between the house and an outbuilding. Although I was in the lee of the house, gusts often curled round the corner and, before I could get everything focussed, the wind swept away my rather flimsy Bahtinov mask, so I had to focus by eye. (Later, I searched the area and found the mask some 10 meters away.)
The comet was at best only a pixel in diameter and, after the difficulty focussing, I was pleasantly surprised with the resultant image (below): it shows a lot of stars! Although I recorded 60 minutes worth of frames, because of the difficulty focussing, I did not consider it worthwhile combining them into a video.
Although night-time skies over La Palma are dark, the comet transited the meridian only some two hours after sunset. Therefore the window of opportunity to see it in a truly dark sky was narrow. I always waited until it had transited before starting to record it.
We had reserved spaces, and two powered platforms, at the visitor centre car park of the Roque de los Muchachos Observatory. (The car park is at an altitude of 2112 m. Reserving space is free by contacting roque@sodepal.es.) When we arrived it was just as windy as on the previous night at the house. The forecast was for 12 m/s (25 mph) winds from the north-east. The platforms are totally exposed, with no windbreak of any kind. But we soldiered on, hoping that the weight and rigidity of the kit would be enough to keep it stable. The star images displayed differing degrees of roundness, but we did record the comet again.
The weather forecast was no better than on the previous night. I decided not to take the Megrez to the car park, and instead took wide angle views. Meanwhile Nick set up his kit behind the car, acting as a windbreak, with a very, very long extension lead providing power.
The weather forecast for the top of the mountain was still no better, so we decided to stay at the house and take our chances there. It was clear but still windy, and the house offered a wider selection of windbreaks!
No sooner had we set up, aligned everything and started imaging, than clouds started to appear. The house was at about 1050 m altitude in the north-west of the island and the clouds were approaching at that level from the north-east. At times we were inside the clouds yet, overhead, we could still see the Milky Way in Cygnus. We appeared to be just at the edge of the "cloud-shadow" of the island: our view of the northern sky was totally obliterated but our southern view (towards the comet) fared much better. When the fine mist that lurks inside clouds turned into more traditional rain, we stopped observing. In the video below, the rather uneven star trails illustrate the changeable conditions.
The weather forecast for the top of the mountain had improved, but was not perfect. Anyway, we went up to the car park once more. There, we met two Spaniards, from La Palma but working in Tenerife. Together with their telescopes, they had brought tables and chairs. Luxury! Of course, a forecast is just a prediction. In the event, there was hardly a breath of air. Finally we were able to see the comet, correctly focussed, and without dancing stars (and in a dark sky, of course).
In the following image, the comet is nearly "run over" by a magnitude 14.8 star. The inset at top right excludes some frames to show the comet more distinctly.
Currently, the comet is at the limit of detection, appearing as little more than a faint dot and requiring a total exposure time in excess of 60 seconds. A simulated five minute exposure in ASTAP (an astrometric stacking programme) indicates the object is at magnitude 17.6. Because the comet is so faint, stacking images on the stars showed only an indistinct trail. In the following image, the left hand pane is centred on the comet. In the centre and right panes all the frames are aligned on the comet, summed in groups of 5 or 10, then the resultant images realigned on the elongated stars to show the motion of the comet amongst the latter.
We again ascended the mountain, photographed the two day old Moon, then went back home. Tomorrow was to be another long day of travelling!
The comet will move into conjunction with the Sun and will be better placed for observers at northern latitudes in November and December 2025.
Nigel Evans, Paul Whiting, FRAS