50 Stacked shots. Sigma 35mm 1.4 on a Nikon D810
Introduction
Spotted in February 2023, comet A3 follows the now well established naming convention of it's recorded year, and it's discoverers. This comet hasn’t been seen for 80,000 years; and another 80,000 will have passed when it completes it's orbit!
My son and I went on a hike last night, carrying all the equipment up the side of a hill in the dark. Looking west over Loch Earn, I tracked this comet for about 50 individual pictures, beginning just at the point of darkness. A large and very bright moon was up, hence the sky is fairly bright and blue coloured. I had many individual shots without the huge imposing cloud formation; however I think it frames the image nicely, so I have went with this one to show you. I like the way the comet straddles the side of the cloud down toward the horizon. This being the wider framed shot, I will see about looking at my more magnified view of the comet from my other camera soon. I will add that picture to this article when finished. The other shot is at 200mm on a star tracker mount.
The Technical Side of a Widefield Comet Picture
The equipment I had with me last night was a Star Adventurer Star Tracker, two tripods and two cameras - a Nikon D810 and Z8. This particular shot is with the Nikon D810. I took 50 shots. My settings where ISO 400, f/1.4 and 8 seconds. Don’t bother stopping down for comets; you want a large clear aperture size, and you want to suck in as much light in as short a time as possible. Don’t stop down to sharpen up edge stars or aberrations as you might normally do. (If concerned about that, simply do another shot stopped down for the stars themselves. I would recommend using a star tracker for this, because stacking stopped down is difficult - yet it might work in bright moonlight like this). In addition to the 50 shots taken, I also took x3 of the foreground light from a small caravan site at the southern banks of the loch. This allowed me to use luminosity masking to reduce the blowout of the lights in the other exposures of the foreground. I layered this section up in post production to correct this area as to how my eyes roughly seen it. In processing the aims are to correct this area, and bring out the comet and the background stars. I did not bother taking a final shot of the stars stopped down - mainly because I forgot, so I used a small ‘coma corrector’ brush which I have made in photoshop to tidy up some winged stars in the deep corners.
Processing
I did a very quick vignette removal in lightroom of the 50 pictures and made sure the white balance was 5500K (daylight). I then stacked the 50 sky pictures in Sequator. We can also use Photoshop, however if you go that route it involves manual aligning, which will take you more time. I imported the stacked Tiff into Photoshop and started using curves to dig out the comet signal and tail, and balanced the stars with that. I used a very low amount of Orton applied to the sky only (low amount, lots looks bad!). The reason I do this with most night sky images is due to digital having a lack of Halation. Halation is a photographic effect, normally naturally occuring with film cameras, born when light traverses a film emulsion and reflects back, crafting a soft glow around bright areas of an image. This occurs due to the scattering of light within the film's structure, resulting in a diffusion that bestows a dreamy, ethereal quality. Unfortunately, digital tech doesn’t do this. It’s too sharp. One of my biggest inspirations is the late foremost wide angle astrophotographer, Akira Fujii. His images are iconic and memorable. A prominent astronomer, he is the author of several books and periodicals about the night sky. He featured in Sky and Telescope Magazine for decades and in other prominent astronomy publications. The main-belt asteroid 3872 Akirafujii is named in his honor. He shot almost exclusively on large format colour film, which gives this effect naturally. To get this in digital, we either use filters applied at the time of shooting (which in a way, lack control and can’t be modded later), or we use orton and other effects in post processing, as I have done here. I applied a custom vignette using a curve layer and a large soft brush, painting the edges and corners to bring the eye in centrally and towards the comet on the right of the frame. I applied a bit of noise reduction at a low level with NoiseXTerminator and was done with the sky portion of the image. The foreground needed an exposure blend of x3 pictures in order to better capture the ultra bright area on the Southern banks, as previously mentioned. I blended these together using luminosity masking. Then I masked both shots together, did a final couple of curves and the like and was done. The overall trick to producing good results in astrophotography, or any genre, is small, focused adjustments. Gradual, gradual, gradual, and following masking, some global adjustments to meld the image together. It is very important to consider overall exposure using the histogram at all times, and the brightness of the land vs the night sky. I have written a great deal about how poor night sky images appear when they have massively bright foregrounds and deep, dark skies. Now let’s look at the deep sky picture of the comet that I obtained:
Comet C/2023 A3 Tsuchinshan-ATLAS
The Technical Side of a Deepfield Comet Picture
I obtained this picture in the same location, as the comet appeared in the sky at approximate heading of 245 degrees due west, and approximately 21 degrees inclination. I used a Nikon Z8 camera with their 70-200/2.8S lens on a Star Adventurer star tracker mount. I use the ‘pro pack’ version with wifi for this mount, which allows me to properly balance the payload with the counterweight kit onto the top of the mount. This is essential in order to do any sort of deep sky, or ‘magnified’ close up images of celestial objects such as comets or galaxies / constellations. I took 40 pictures of the comet at 200mm. I used 35; five having some light cloud obstruct the view of the comet during the exposures.
I shot the comet as mentioned, at 200mm with the following settings:
Aperture of f/2.8
Exposure time of 20s
ISO of 500
I will address these in order. The widest aperture is always where we should use the lens for comet imaging. As previously noted in this article, forget worrying about lens aberrations and the stars in the midframes corners, at least for now. The most important point to hold in your mind is that we need to collect light and fast. We do this by having the largest aperture. This concept is called clear aperture and you can read more about this online if you want to understand it in a deeper way as it relates to astrophotography. This brings me nicely to the exposure time. The other way we gather light, is clearly via exposure length; in fact these are the only two variables that influence the final collected light. (ISO is a digital boost for the most part, and does not change the physical light collected during at exposure). Lastly, the ISO of 500 was selected because it is sufficiently low enough as to protect the star cores from blowing out to white during the individual sub exposures, and because it, along with the aperture and exposure time, created a histogram with the data bump being approximately 1/3 from the left wall. We have to be really careful here. I am often slightly under this, to protect the stars blowing out to white, which doesn’t tend to look great in the final picture. Notice that I have picked up the individual star colours in the final shot above? Let me be clear here, that the Star Adventurer mount is capable of easily doing one minute tracked sub exposures at 200mm. In fact, I’ve managed two minutes with accurate stars. I’ve even pushed to 3 minutes before…The reason I didn’t need to do this, or couldn’t, is because it was a moonlit night, and that comes with a lot of additional light. That night, longer sub exposures than 20 seconds would have given me general overexposure.
This is a very good result for only approximately 11 minutes of total data. The more data we get, the better the image can be, technically speaking. You should always aim for this, however sometimes cloud, rain or life stops us from going any further. This image could only really be improved from a technical standpoint by increasing the number of sub exposures that I can stack together in post processing and / or by moving up to a telescope with a larger effective aperture. (Remember that aperture is a ratio of focal length. There are f/11 scopes with greater aperture than my 200/2.8 lens which has approximately 71mm of aperture). We can surmise this via the basic equation:
Focal Length / lens aperture ratio = effective aperture for light collection
So imagine I used an 800mm telescope with an f4 aperture. I would have 200mm of aperture to gather light. Significantly more aperture, yes, however with that comes the demand for significantly more equipment, accuracy and alignment. And of course…expense and weight! Bearing in mind that my son and I hoofed all this stuff up the side of a very steep hill, you will see why I am not enamoured by that thought! There aren’t enough clear nights in Scotland in my opinion for me personally to take this side of what I do any further. I do absolutely enjoy every minute of maxing out what I have. There are of course limits of what a 200mm lens can resolve, with regards to the physical size or the ‘angular view’ of the object in the sky. The smallest of objects need more aperture and focal length, that’s just the way it is. I find it extremely satisfying to max out or ‘use up’ every possible trick in the book to create the best picture from the equipment I own, in a carriable sized package.
Processing
Comet processing is very technical. Comets are one of the hardest astrophotography target types of process effectively; the process of trial and error can literally have you tearing your hair out. Eventually I may get around to making a tutorial however to surmise:
The 35 images where stacked in Deep Sky Stacker on the stars only, producing a resulting linear 32 bit Tiff file
Then I aligned (called registering) each file on the comet because the background stars and the comet move at different rates to one another
Further to this, those resulting 35 intermediate registered files had StarXTerminator ran on each one to remove the stars, leaving only the sharply aligned comet in each one
I then took those resulting 35 files and stacked those together
So far, I had two stacked tiffs, one where the comet was sharp and in focus, and another where the background stars where
I went further and split the blurry comet out of the background star stack by using StarXTerminator then subtracting one from the other
As a result, I now had x2 workable files, one of the stacked and aligned comet, one of the stacked and aligned stars, with the comet removed
I then processed using most of the normal workflow available here
The Dust Tail of the Comet and very good star quality
Ta-dah! And that’s it. I highly recommend trying much easier astro targets than comets initially, because comets move differently against the background stars, they are much more complex to process. I appreciate that people will want a shot of the comet in the sky right now. That’s fine - I encourage you to go ahead, and try processing in a simplistic manner intially to get a resulting image, hopefully the information in this article can help. There can be great satisfaction to be had in this process. Comets come when they come. You can even keep the resulting data for when you are able to process them better than your initial try, after learning / improving your skills over time. As you can see with my crop into the ion tail, the stars are very good with this lens, considering it is a zoom, I use it because of it’s aperture and I am very pleased with what it can do. More later, maybe…
Steve