Orwell Astronomical Society (Ipswich)
Comet C/2023 P1 (Nishimura), 15-19 September 2023
C/2023 P1 (Nishimura) was discovered by Japanese amateur Hideo Nishimura on 12 August 2023. It brightened rapidly and, on 08 September 2023, became visible to the naked eye. It was closest to the Earth on 12 September and reached perihelion on 17 September.
By coincidence, I had arranged a trip with Nick James to La Palma in the Canary Islands from 14 to 20 September. We make occasional journeys to the island and our custom when visiting is to hire a rural house in the north-west at a height of about 1000 m above sea level. Often the weather is good enough to see the sky from the house but, on occasion, we venture up the Roque de los Muchachos to a height of 2000 m to enjoy a superior sky. At the top of the mountain, an observatory complex, the Observatorio del Roque de los Muchachos, houses one of the largest telescopes in the world, the 10 m aperture Gran Telescopio Canarias (GTC). To prevent disturbance to the astronomers, tourists are not allowed to stay overnight at the observatory complex.
Recently a visitor centre has been built close to, but just outside, the observatory complex. It is open only during the day. When the visitor centre is closed, the car park of the facility is closed too, so cannot be used at night as an observing location 🙁. When we visited at night in 2022, a guard shooed us away! However, this time we discovered that free permits are available permitting entry to and use of the car park at night. 😊
By the time of our first evening's observing, 15 September, the comet was very close to the Sun, at an altitude of around 2°. Although we were at a height of over 2000 m, we were looking towards the comet through cirrus clouds in the far distance. Observing from the car park, unsurprisingly we could not find the comet; all that was visible was the twilight sky! Our experience highlighted a fundamental problem: how to find a comet against a twilight sky?
Nick fared better than I at finding the comet. He had brought a telescope on a GoTo mount and, although Polaris was not visible against the twilight sky to align the mount properly, was able to use the Sun as a positional marker before sunset then, after sunset, other bright objects as they came into view of the telescope, first Mars then, later, the Moon. Alas, I did not have a GoTo mount, so resorted to pointing my instruments in the general direction of the comet. My primary instrument was a 90 mm Megrez refractor with a Sony A7S on the back; my secondary another Sony A7S with a 100 mm f/2 lens, operating at f/2.8 (a 35 mm f/2.8 telescope!) The latter has a field of view of some 20°, so I was confident that it would capture the comet even if I was somewhat inaccurate in aiming it.
For the four evenings when we tried to observe the comet in the twilight sky, we could not see it with the naked eye, nor with a small pair of binoculars. I had to photograph the comet "blind"; there was no way to verify at the time that my cameras were aligned appropriately, and it was only after each session that a review of the photos revealed a bright smudge against the background sky.
Following the difficulty of photographing the comet, the next challenge was how to process the images to make it visible against the twilight sky. Anyone who has tried astrophotography will likely have encountered difficulties processing images due to light pollution causing a gradient background; twilight too, although it is not light pollution in the usual sense, causes a very pronounced gradient background. I adopted the following processing methodology for the photos shown below. (Other approaches are possible!) For each photo, I made eight copies, translated them according to the table below relative to the original (at translation (0, 0)), and then took the median value of each pixel in the original plus eight translated copies to produce a low frequency (blurred) image. I then subtracted the low frequency image from the original to highlight high frequencies (fine detail) in the latter. The process is similar to unsharp masking, but requires less computation.
| (-x, +y) | (0, +y) | (+x, +y) |
| (-x, 0) | (0, 0) | (+x, 0) |
| (-x, -y) | (0, -y) | (+x, -y) |
In the following view from the Megrez on 15 September, the comet is visible in the standard view, but a pointer certainly helps! In the processed view, it is much more prominent, and even sports a very short tail towards 1 o’clock.
The following short clip from 15 September illustrates how incredibly difficult is was trying to find a bright dot dodging behind clouds!
In the below view from 17 September, a series of frames from the Megrez has been aligned on the comet and added. Not only is the tail more visible, but faint background stars are apparent.
In the video below, also from the Megrez on 17 September, the comet is at higher altitude and in a sky more blue than two days previously, but it remains a challenging object. Background star HIP61103 (magniture 6.0) is visible to the upper right.
The following video, captured on 18 September by the A7S with 100 mm lens, captures the wider view and illustrates the weather conditions. We were situated above the cloud, which was turbulent. At this scale, the comet appears as little more than a dot. Wind shake caused problems with the Megrez, so the images that it captured are not shown.
The following image illustrates what stacking can do. The stack is of images displayed in the above video; the number stacked is limited by the effects of differential refraction across the frames. The faint stars are set in a much darker sky. Alignment on the comet brings out a small tail, but the image scale is tiny.
On 19 September, we enjoyed our first evening free of cloud, but a five-day old Moon was only some 40°s distant from C/2023 P1. Nevertheless, the below view captures 8th magnitude stars close to the comet.
The following video taken with the Megrez shows the sky on the horizon on 19 September; although it was pretty good, we still could not see the comet directly.
A wider view of the horizon, taken with the A7S fitted with 100 mm lens, is similar to that of the day before. The clouds were still very lively.
The following is a stack of frames shown in the above video, aligned on the comet. The smaller aperture (~35 mm) makes visible fainter stars than the Megrez, but they are higher in a darker sky.
While I was unable to see the comet directly, was was pleased to have recorded it only 12° from the Sun.
We were not the only people in the visitor centre car park. A Dutchman, Frank, had brought with him a very impressive telescope, a 760 mm Dobsonian, with 200 mm Schmidt-Cassegrain acting as a finder! No, he didn't transport it by plane, instead taking two days to travel by car and ferry to the island. Late one evening, he invited us to look through it, at M17, M42 and the Horsehead Nebula. Unfortunately, I could not make out the Horsehead – either I was not sufficiently dark-adapted, or my eyes are too vintage! Nevertheless it was a privilege to look through the instrument.
Here I am with the 90 mm Megrez telescope, at the visitor centre, above the cloud-deck. Note the "luxury" counterweight. The objects are heavy (by design!) and, to preserve the airline baggage allowance for other essentials, I left mine at home, intending to use a battery pack in its place. But no: three 500 ml water bottles, total cost less than €1, held in position with the ubiquitous gaffer tape, did the trick admirably!
Nigel Evans