This image presents Comet C/2021 A1 (Leonard) in a very wide field, which emphasizes not the bright core and coma, but its very long (and faint) tail. The field of view could fit more than 20 lunar disks (10º) across! The brightest comet of 2021, Leonard was nicknamed the “2021 Christmas Comet” by some. Indeed, it displayed a spectacular sight at the end of December, especially for observers to the South of the Equator. It was visible right after sunset before disappearing in the southwestern horizon. From dark locations, it was visible to the unaided eye (I could detect it under direct vision, but it was best seen with averted vision), and there are reports of naked eye visibility of the tail under exceptional sites. This image (and many online pictures), however, emphasizes its faint features: the comet was beautiful, the best of 2021, but its naked eye visibility doesn’t come close to what long exposures are able to capture. 

C/2021 A1 has this name because of its characteristics and discovery. C/ denotes it is non-periodic, 2021 A1 denotes it was the first comet (1) of the first half-month (A) discovered in the year 2021: the first comet identified in 2021, by astronomer Gregory Leonard in images taken from the Mt. Lemmon Observatory in Arizona, exactly one year before its closes solar approach (perihelion). The day of this image is posted, 3 Jan 2022, is Leonard’s perihelion at about 0.6AU (56 million km) from the Sun, and its “one year discovery anniversary”. 

Its nucleus is a mass of rock, ice and dust about 1km across. Its tail of ionized gas, however, extends millions of kilometres into the Solar System. Two distinct components can be seen in a comet’s tail: the “dust tail”, which has a more yellow and often curved appearance, and the “gas tail”, which is usually blue and is more affected by solar wind, thus pointing radially away from the Sun. In the case of C/2021 A1, we can see the brightest coma-nucleus region with a slight green-cyan tint, the dust tail in more white-yellow-ish colour, about 2 degrees across, and a spectacular long gas tail, extending across the image and beyond the field of view. The tail has a twisted appearance, likely the result of activity and flares on the comet’s nucleus by solar winds and magnetic field.

This is my take on this beautiful comet. I was really happy this was the first real opportunity to capture a great comet with my improved equipment and skill. The last comet I was able to capture was Neowise in 2020, but its show was reserved for the Northern Hemisphere. Then I could only make quick, only to record, but not a fully featured image of the comets tail. Before that, in early 2015 I captured Comet C/2014 Q2 (Lovejoy), but my skill at the time and very short integration limited the extraction of all beauty and detail of the comet.

Now, I feel that capturing Leonard was a great challenge and a privilege. Summer is the rainy period in southeastern Brazil: clear nights are rare. This image was captured in the early evening of 26 Dec, the only night where weather allowed me to shoot C/2021 A1. Furthermore, the low altitude of the target poses challenges: we are imaging through thick atmosphere layers, right after sunset, for a very short period of time: I had only about 40 minutes of useful dark time before it was beyond the local horizon. I was very pleased with how this image turned out: processing was a pleasure.

The relative motion of the comet to the starfield is always something to think about when shooting those solar system wanderers. I stacked two sets of data: registered on stars, or on the comet nucleus. However, the motion was relatively limited in the wide field, and I decided to go for the simpler approach of only processing the star-registered version: so, this is a natural result, with no star/comet separate processing or blending. My goal was to really push the faint low-surface-brightness features of the gas tail, and I ended up capturing some very faint clouds of galactic cirrus (IFN), which can be seen in a more brown-ish colour in the background, right in the limits of the singal-to-noise of this short half-hour integration.I really liked the tail's blue color and the overall balance of the image turned out. At the end, I consider this one of my favourite images, and the best comet I have ever photographed!

Constructive criticism, comments and suggestions are more than welcome in the comments section! 

Date and Time: 26 December 2021, 20:00 - 20:30 (local time, UTC-3)
Location: My Observatory, MG, Brazil. Rural Skies (Bortle 3, calculated SQM ~21.6)
Camera: Canon EOS 6D (mod) at ISO 800
Optics: Samyang 135mm f/2 lens, operated at f/2
Mount: Sky-Watcher HEQ5, tracking, guiding
Exposure Detail: 30x60s. Total integration 0.5h. 
Processing software: APP, PI, PS

References:
APOD 25/12/2021, 30/12/2021, 03/01/2022;
Hatfield, M. Views of Comet Leonard from Two Sun-Watching Spacecraft. NASA Image Feature;
Comet. Wikipedia.
If you liked this image, I recommend searching for the incredible work of Daniele Gasparri and Gerald Rhemann.