Comet Tsuchinshan-ATLAS’s show

In the end, the expected show took place, and it turned out to be a really nice one, although unfavorable weather patterns over all of Central Europe, and in particular in Northern Italy, made observation very hard if not almost impossible.

Anyway, first things first. After the 27 September perihelion at a distance of 0.39 AU, which the comet survived untouched, and after treating southern hemisphere observers to a very nice show, comet C/2023 A3 Tsuchinshan-ATLAS reached solar conjunction again on 9 October. After that, it was predicted to come back into view just after sunset, with better observing conditions for the northern hemisphere. The closest distance to Earth (0.47 AU) took place on 12 October, after which it became higher above the horizon over the following days, while growing fainter because of the increasing distance.

In the days around solar conjunction (8-10 October), the comet reached its peak brightness of  magnitude -4 thanks to the “forward scattering” effect (light scattered by dust expelled by the comet and “back-illuminated” by sunlight). This effect was confirmed in the images from the SOHO spacecraft, whose field of view is about 16-17 degrees wide and is permanently centered on the Sun, making it possible to study phenomena related to solar activity such as CMEs. Comet C/2023 A3 was imaged by the SOHO spacecraft for a few days, during which the spike in brightness was confirmed.

Now, back to my observation plans. The best comet visibility timeframe was expected to run from 12 to 20 October. Unfortunately, the weather forecast for Northern Italy was abysmal: overcast with heavy rain for the whole week, and no chance to find a spot with some decent weather within a 500-plus km radius.

Therefore, I made a last-minute decision and on Sunday 13 I hopped on a flight from Milan to Sicily (Catania), where the weather was expected to be much better, especially in the first days of the visibility window. Thanks to the advice of my friend and first-class astrophotographer Giovanni Benintende, I used Cesarò (ME, Italy) as my base, with an observing location he recommended to me located at about 1400 m elevation in the Nebrodi Park and blessed with a wide and unobstructed view to the west, which provided ideal observing conditions right after sunset.

Here’s the equipment I took with me to image and observe the comet:

• ZWO AM5 Mount, ASI 2600MM CMOS camera with Optolong LRGB filters, Sigma 150-600 f/5-6.3 telephoto lens;
• Skywatcher Star Adventurer tracking mount, Canon 90D DSLR, Samyang 135 mm f/2 lens;
• Fixed tripod, Baader-modified Canon 6D DSLR with a 50-mm f/1.8 lens for time-lapses;
• 7x50 binoculars for visual observation.

Here’s a couple pictures of my gear in the field:

I managed to observe the comet on four evenings, from 13 to 16 October. The table below contains the detail of the circumstances for my observing location (CEST timezone):

Unfortunately, a waxing Moon in the sky hindered observation quite a bit.

On 13 October, just after landing at Catania airport, I directly rushed to the observing spot. This was the best, cloudless night. The comet was clearly visible although very low above the horizon: its tail extended beyond the 7x50 binocs’ FOV, so it was at least 10 degrees in length. In all frames, however short, the comet sported a jaw-dropping anti-tail, which was invisible to the naked eye and through the binocs. The phenomenon of the anti-tail, which is quite rare, is caused by dust particles released by the comet along its orbit: when the Earth crosses the comet’s orbital plane, this thin dust layer becomes visible in all its glory.

On the second evening, 14 October, the comet was higher above the horizon, but unfortunately my imaging session was partly spoiled by thin cirrus clouds.

On the third and fourth night, 15 and 16 October, the comet further increased its altitude but my imaging sessions were even more jeopardized by thin clouds and, from the morning of the 15, even by a thick haze, probably caused by dust from the Sahara brought in by strong southerly winds, which was also an omen of the bad weather pattern that kicked in in the following days and put an end to my observing campaign.

Had I traveled further east, for example to Greece, I would have pulled off even better results, but of course, long-term weather forecasts are still unreliable, so Sicily was the best destination based on the forecast I had at the time of my decision. Therefore, considering the prospects for Northern Italy (check out the satellite image below to see what I mean), I think my decision was really spot on! 🙂

The unfavorable weather pattern over Northern Italy went on for a full week, yielding to a good weather spell with clear days and mild temperatures only in the last days of the month. Meanwhile, the comet had grown noticeably fainter (down to about mag. 6) but also higher above the horizon and was still an interesting and feature-rich object at longer focal lengths. So, on 28, 29 and 30 October I took advantage of the favorable weather to image the comet from Aosta Valley (Italy) through my GSO 10” RC reduced to f/5.5 (1370 mm FL). These three imaging nights produced two luminance-only B/W images and one LRGB color image, all of which look quite nice overall.

Below is the complete “photobook” of the comet with details about data and gear used: some images were processed by my long-time astrobuddy Lorenzo Comolli. The gallery also includes:

• A wide-field photo shot with a camera phone (Google Pixel 8 Pro) on a fixed tripod. Though falling short of anything achievable with a specialized camera, I think it still decent and probably unthinkable of just a few years ago;
• An image of mine processed by Andrea Aletti (Osservatorio del Campo dei Fiori, Varese, Italy) using a specific algorithm to bring out the details in the tail’s gas jets.

Enjoy! 🙂