27 October
31 October
Orwell Astronomical Society (Ipswich)
Comet C/2023 A3 (Tsuchinshan-ATLAS), 14-31 October 2024
Credit for discovery of comet C/2024 A3 (Tsuchinshan-ATLAS) is apportioned jointly to the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in South Africa which observed the object on 22 February 2023 and to observers at Purple Mountain (Tsuchinshan) Observatory in China, who found it independently in images taken on 09 January 2023.
At discovery, the comet was 7.3 AU (astronomical units) from the Sun, and shining at a dim magnitude 18. It subsequently came to perihelion on 27 September 2024 at a distance of 0.39 AU from the Sun. Its closest approach to Earth was on 12 October 2024 at a distance of 70 million km. Members of OASI observed the comet when it came to prominence in early-evening skies in the days following closest approach to Earth. At its brightest, the comet achieved naked-eye visibility for a few days.
Steve reported two observations of the comet from his back garden in Melton. Both observations were made with a Seestar S50 "smart" telescope, with image processing in PI and Affinity Photo.
27 October
31 October
Martin observed the comet on three nights:
14 October, single image.
16 October, single image.
16 October, stack of 12 images.
18 October, stack of 16 images.
A train of Starlink satellites.
On 16 October, I decided to take advantage of the mainly clear skies to observe and image the comet. I walked the short distance from my home to Bourne Park to gain the benefit of a much improved south-western horizon.
I observed the comet with binoculars from 18:05 UT. The below image was taken at 18:11 UT. The comet appeared to have faded slightly relative to its appearance from Newbourne on 14 October. As the sky darkened, it was nevertheless just about visible to the naked eye. Canon EOS 1200D camera with a Samyang 14 mm f2.8 lens, ISO 800, 4 s exposure. I tweaked the image in Affinity Photo 2 with the Clarity and Denoise filters.
I made a further attempt at observing and imaging the comet on 18 October, also from Bourne Park. The comet had dimmed further, and was only visible with binoculars, no longer with the naked eye. Cirrus cloud and aircraft contrails did not help visibility!
16 October
18 October
The night of 18 October was clear, but I was observing straight over Felixstowe Docks from Felixstowe Ferry Golf Course, so light pollution severely compromised photography.
18 October
Introduction
On occasion, Nick James (Director of the Comet Section of the BAA) and I visit La Palma in the Canary Islands, during the summer, to see the stars, taking with us a load of cameras, tripods, drives and other equipment. We hire a rural house on the western side of the island and have the option of seeing the sky from there, or ascending the Roque de los Muchachos to an altitude of more than 2000 m, close to the observatories. (The observatory complex itself is off-limits at night.)
In 2023, our visit coincided with the passage of C/2023 P1 Comet Nishimura. We also found out that passes are available to use the observatory visitor centre car park at night. We did not see the comet by eye in the twilight, but our digital cameras revealed it. At the date of our visit, comet C/2023 A3 (Tsuchinshan–ATLAS) had been discovered and had the potential to become a prominent object in the sky during October 2024. Although comets are notoriously unreliable (remember Comet C/1973 E1 Kohoutek?), we decided to visit the island to observe it.
By the end of September 2024, reports from the southern hemisphere indicated that the comet was exceptional and might even become visible in daylight on 09 October. We had arranged to be on the island for the nights 11-16 October inclusive. The Moon would be close to full but, of course, there was nothing that could be done about that. We applied in advance for a (free) pass to use the visitor centre car park during our stay.
TUI offer direct flights to La Palma, but only weekly. Daily flights are via Madrid, and we chose this option. On the evening of 10 October there was an aurora alert, but I had to ignore it as all my big cameras were packed and I was due to leave home at 1.30am for Heathrow. My sky cameras captured plenty of activity. Details of the equipment that I brought to La Palma are given below.
11 October
Comet altitude at sunset 10.9°, elongation 11.3°; sets 53 minutes after the Sun. Venus altitude 19.0°. Age of Moon 9.0 days.
We arrived in Santa Cruz de La Palma at 2.30pm. We retrieved our luggage, collected the hire car, visited the shops to buy food, then journeyed directly to the visitor centre – we did not have time to go via the house and unpack first.
Sunset was at approximately 7.45pm and we arrived at the visitor centre some two hours beforehand. We spoke with the security guard and he advised that the forecast was for strong winds in the evening and the next day. A couple of other Brits arrived: Steve Knight and Nicky Fleet from Newbury Astronomical Society. The night-time pass is not widely advertised and we four were the only people holding passes for all six nights.
The sky was reassuringly clear… with one exception. From the direction of the setting Sun, a large bank of cloud was approaching, at higher altitude than the visitor centre. As the Sun set, there was a spectacular show of light and dark cast on the clouds that we could see, by clouds further away that we could not see. But I hadn’t come to La Palma to watch clouds! Unfortunately, I neither saw the comet nor photographed it. Part of the problem was that I knew the position of the comet relative to the setting Sun (directly above), but I could not determine the position of the latter due to the cloud.
Steve had brought a selection of equipment. One item that caught my eye was the Seestar. Steve was able to align it, aim it towards the comet and then photograph it through the clouds. It was not a great shot but it was better than nothing!
12 October
Comet altitude at sunset 15.1°, elongation 15.9°; sets 72 minutes after the Sun. Venus altitude 19.1°. Age of Moon 10.0 days.
We already had the forecast of wind. When we reached the visitor centre, the sky was totally cloudy. Then it started to rain. With no let-up in sight we all called it a day and were back at base just after sunset.
13 October
Comet altitude at sunset 19.5°, elongation 20.5°; sets 93 minutes after the Sun. Venus altitude 19.3°. Age of Moon 11.0 days.
We returned to the visitor centre. It was sunny but there was a lot of high altitude cirrus about. As the Sun was setting sundogs were visible, and the cloud showed no evidence of going away. At one point, only Venus and the Moon were visible. Finally, approximately 45 minutes after sunset, we saw the comet through breaks in the cloud. Several layers of cloud were moving, and we didn't see the entire comet free of cloud. But it was evidently bright!
We could set up our equipment in the same spot each night. Once the comet had set, the sky did clear somewhat. This provided an opportunity to see where Polaris was relative to features of the visitor centre building: on subsequent evenings, this enabled approximate alignment of our equipment before sunset.
14 October
Comet altitude at sunset 23.8°, elongation 25.0°; sets 113 minutes after the Sun. Venus altitude 19.4°. Age of Moon 12.0 days.
This was the day when the Earth passed through the plane of the orbit of the comet. Depending on the geometry of a comet’s orbit it is occasionally possible to see a phenomenon called the anti-tail pointing towards the Sun. It is purely an optical effect, caused by the tail curving when the comet is close to the Sun and appearing, from the perspective of an observer on the Earth, closer to the Sun. The most famous comet to show an anti-tail (often pictured in textbooks) is C/1956 R1 Arend-Roland.
Even though the comet was now visible in the evening sky, the anti-tail might curl sufficiently that, with a clear sky, it might be visible in the morning sky before dawn. We decided to travel to the other side of the island to catch dawn and maybe, just maybe, see the anti-tail in twilight. We checked the webcam at the GTC (10 m Gran Telescopio Canarias) before setting off: it showed a sky that was clear overhead but cloudy at lower altitude. An hour later we stood on the eastern side of the island and, indeed, it was clear overhead but cloudy at lower altitude! We needed a clear sky on the eastern horizon: that wasn’t going to happen, so we returned home.
The evening of 14 October looked to be the best yet! From the visitor centre, the main cloud deck was below us, and a few wispy clouds above us moved quickly. There was no wind. We put into practice the lesson of the previous day regarding initial polar alignment. Unfortunately, after dark, I forgot to check and finalise alignment, but the initial alignment proved adequate! Eventually, we found the comet and followed it down to the (cloudy) horizon. One of my cameras locked up for a period. Once all cameras were running, the main challenge was to not overexpose the background in the twilight sky. Meanwhile, Nick could see the anti-tail in his camera: I did not realise how prominent it would be so changed the framing on one of my cameras to record it better.
Even though the comet was not as bright as on the previous day, we were elated at what we had observed. However, we decided to forego the opportunity to get up at 5.00am on the following morning to visit the eastern side of the island again in another attempt to see the anti-tail in the morning sky.
15 October
Comet altitude at sunset 27.8°, elongation 29.2°; sets 132 minutes after the Sun. Venus altitude 19.5°. Age of Moon 13.0 days.
We found that Mike Harlow had posted the SOHO LASCO C3 image on the BAA images page. This showed the anti-tail crossing the field-of-view of the instrument at a distance of nearly 25° from the comet!
In the afternoon it started to rain. The rain continued as the day wore on. We checked the GTC webcam and it was raining there too. We stayed put.
16 October
Comet altitude at sunset 31.4°, elongation 32.9°; sets 150 minutes after the Sun. Venus altitude 19.7°. Age of Moon 14.0 days.
This was our last night on La Palma and we decided, unless the forecast "guaranteed" rain, to attempt one last observation. When we arrived at the visitor centre, the weather wasn’t favourable: we could see neither the observatories nor even the Sun! But the cloud was clearing on the western horizon. There were two layers of cloud above us, one moving swiftly and one slowly. At times we could see Venus and Arcturus, but not the Moon behind us. I deployed two cameras with the 50 mm and 100 mm lenses, but did not set up the Megrez. Fast-moving clouds interrupted our view, but we were able to photograph the comet between clouds. The comet was now definitely fading.
17 October
The journey home, in two stages: Santa Cruz de La Palma to Madrid with Iberia Express, and Madrid to London with BA. We had checked-in on the previous day. I prefer a window seat and, when flying back to London LHR, always try to get a right-hand seat so that I can anjoy the view over the City of London, always amazing, day or night. On this occasion, during check-in with BA I had no opportunity to select a seat, and was allocated a left hand window seat. Nick saw an opportunity! We would leave Madrid after sunset and fly north - would it be possible to see out of the window the comet over the western horizon? Interesting!
So we both carried what we needed as hand luggage:
When the seat belt signs came off we raced to try our luck. We did not see the comet by eye, but we did capture it in some rather poor quality photos. Another first!
What did I do in a previous life to suffer in this one by having an interest in astrophotography?
"Normal" astrophotography in clear skies has one involved in flats, darks, hot pixels, followed by aligning, stacking, binning, colour balance and finally trimming and setting the brightness levels. The target is usually only a degree or so across, with residual background in the image from general light pollution, sometimes with a gradient. This is straightforward to remove from the final image.
On the trip to La Palma to photograph Comet C/2023 A3, there was all of the above plus the following additional issues: the comet was visible against the dusk twilight, wide-field cameras were necessary to capture the full extent of the object, and clouds and strong moonlight were present. The difficulties resulted in a series of images best described as "lacking in contrast".
So how to obtain the best results from a sequence of several hundred photos from sunset to comet-set? Sure, I could select a cloud-free subset and stack them, but how could I make a movie from the entire sequence, and how could I flatten the background and bring out more detail, frame by frame?
The thing that most looks like the background is the background itself. What I needed was a fuzzy version of the background that left the comet untouched. There may be modern software packages that can do this but I am familiar with the capabilities (and limitations) of IRIS. My solution was to make copies of the original image, shift them by different amounts such that the comet did not overlap itself, and stack them. By taking an average or a minimum of the stack, the comet effectively disappears, leaving a smoother version of the background with no stars. Subtracting the smoothed background from the original leaves a background which is much flatter, with a bas-relief effect: edges are more distinct, but big gradients have been removed. This I call a "differential view".
Equipment
By way of equipment, I brought to La Palma three Sony A7S cameras, for use with a Megrez 90 mm aperture refractor (focal length approximately 450 mm), a 100 mm f/2 objective and a 50 mm f/1.8 objective, together with a GoPro camera and a couple of sidereal drives.
Orientation of Images
Below, all landscape views have the horizon at the bottom and portrait views have been rotated so the horizon is to the right. Alignment of frames is on the head of the comet, unless stated otherwise.
13 October
It's not often that a comet is as bright as C/2023 A3. Exposure duration was limited by the sky brightness, caused by both twilight and the Moon.
Below, the first two videos were taken using the 50 mm lens and the second two using the 100 mm lens, providing a higher magnification.
The following video was produced by stacking frames in the standard way.
The following video was produced by differential processing as described above. It provides a better view of the tail, and also makes visible many satellites (most of which I am sure are owned by Elon Musk!)
The following video was produced by stacking frames in the standard way.
The following video was produced by differential processing. Again, this makes more satellites visible, and there is a hint of something to the right of the head of the comet - see below!
By the end of the evening, we were pleased to have seen the comet, given that the two previous nights were clouded out.
14 October
The sky on the evening of 14 October was much improved upon that of the previous evening. This enabled some wonderful images to be captured, even though the comet was already fainter than it had been on the previous evening.
The following three still images and two videos were captured using the 50 mm lens.
The following image is a stack of 10 frames captured with a static camera. The frames were of sufficiently short duration that they could be aligned without blurring. There is a hint of a streak to the right of the head of the comet.
The following image demonstrates the benefit of subtracting the background: the anti-tail of the comet, appearing to the right of the head, is now clearly visible! The image was taken on the day the Earth passed through the plane of the orbit of the comet, and the older material of the tail was therefore invisible until it appeared ahead of the comet.
The following image was captured with a static camera. Because twilight was saturated, particularly at the start of the video, I have not aligned the frames on the comet.
In the following image, the frames have been centred, so the horizon appears to approach from the right. The anti-tail is much more obvious.
How many satellites are visible in any one frame? Many are visible in a typical standard view, but there are even more in a differential view. Here the differential view is formed by subtracting nearby views from the frame: almost everything that does not move should disappear. I count 24 satellites.
The following five still images and two videos were captured using the 100 mm lens.
Initially, I did not realise how prominent the anti-tail would be, so I set the camera to capture the tail best, by positioning the field so that the head was towards the edge of the frame, as shown in the following image.
The following image was taken after I adjusted the position of the camera so that the head of the comet was at the centre of the frame.
A good 5° of anti-tail is visible in the following image. How much more would have been visible when the comet was at higher altitude?
In the following image, the sub-frames have been aligned on the stars prior to stacking. This shows a second comet, 13P/Olbers, in the field-of-view. It is considerably fainter than C/2023 A3!
The following two videos were formed from the frames used to make the above images. Both have a rather awkward jump when I realigned the field of view of the camera to centre on the head of the comet. During twilight, I adjusted the exposure manually so that the sky was not burned out and, in processing the video, I have evened out the jumps in exposure.
The following four still images and three videos were captured using the Megrez refractor.
In the following image, the head is rather burnt out. There is the appearance of a groove in the tail.
In processing the above images, my aim was to isolate the comet and remove the background. By way of contrast, to produce the following two images, I made a blurred (smoothed) image of the comet and subtracted it from the original, making details within the comet more easily visible. The following image shows that there is indeed a groove in the tail due to a continuation of the anti-tail.
The following image is a blend of a differential view and a scaled-down normal view. The groove is a reduction in brightness of about 5%. I believe that it is the shadow cast by the anti-tail on the tail. Cometary tails viewed face-on are optically very thin: stars are easily visible through them. However, edge–on they are optically thick, and can restrict or block the passage of light.
The following video follows the comet from initial capture until it sets at sea. It does not reveal a groove-like structure in the tail.
The following differential view reveals the groove clearly.
The following video is a combination of a differential view and a faint normal view.
The following two videos and three images were captured with a GoPro camera!
Owners of GoPro cameras are expected to capture videos while participating in extreme sports such as skydiving, surfing, mountain-biking, or tobogganing, or maybe to mount the camera inside a rally car, or to record an interview of a famous personality. Product advertising does not mention Astronomy and my expectations of what the camera, equipped with a tiny aperture of approximately 1 mm, could achieve, were not great. However, it captured some impressive shots, even when the comet was close to the horizon!
The following video shows the onset of darkness. Venus was the first object to appear in the heavens, followed by Arcturus and the comet. It also shows the general conditions that prevailed during the night.
The following cropped version show things more clearly.
The GoPro was mounted in a fixed position, so image stacking required frames to be aligned first on the central part of the image. The anti-tail is visible in the following image.
The following two images show the anti-tail more clearly (and also many stars), even though the comet is close to the horizon.
16 October
Our last night on the island. Conditions were challenging to the extent that, without the enticement of a bright comet, I would ordinarily not have attempted observations.
The following two videos were captured with the 100 mm lens.
The following video shows just how much cloud was present.
The following differential view, although it can't see through clouds, does show that the anti-tail is still visible.
The following two videos were captured with the 50 mm lens.
The following video records a moment of great disappointment! Just when I thought that the clouds were about to clear enabling the comet to be seen, another bank formed in front of my eyes!
The most striking feature in the following video is not the comet, but the appearance of what look like crepuscular rays. However, the Sun was 18° below the horizon at the start of the video. I do not think the effect is a processing artefact.
17 October
By luck, we had left-side window seats on the northbound flight home from Madrid to London, providing an opportunity to see the comet in the west from the air. Below is a single frame taken inside the aircraft.
I tried taking various hand-held shots - literally, shooting in the dark! The results were not brilliant, but were unusual, and of course we were far above the level of the clouds. The following image contains two of the better results.
Aftermath
So how long was the anti-tail? The following clip from ESA shows the view from the SOHO LASCO C3 camera. When it was taken, the comet was almost 25° away!
Out of six nights spent on the island, we saw the comet on three, and benefitted from really good skies on one. There are a few things I would do differently next time, but I doubt there will be a next time to capture a comet with its anti-tail, or to record a comet from a plane!