WRITTEN BY LORIS FERRINI AND SILVIA POLITO
Welcome back to our second article on how to capture the Cosmos!
In our previous article, we talked about how to photograph our natural satellite, the Moon. This time, we’re pushing ourselves a bit further: we’ll try to photograph the Milky Way! Unlike the Moon or the planets of our Solar System, it is a bit more complicated to observe the Milky Way with the naked eye. You probably have noticed that the Milky Way is not visible from our cities or the surrounding countryside. Indeed, to observe it with the naked eye, we will need extremely dark skies, far away from the light pollution of cities, which would disturb our observations.
Places where nature is almost unspoiled are to be preferred, like national parks or deserts. There are a bunch of places on Earth that are known for being the best locations to see the Milky Way: Chile, Iceland, and Peru are just a few of them. However, we don’t need to push ourselves at the World’s ends to see the Milky Way: no matter where you’re reading this from, there will be at least a handful of dark corners nearby which can be good for beginners, from where you can see the Milky Way with your naked eye; you might need to drive few hours to reach it, but it will be worth it, I promise!
But no more talk, let’s see how we can capture this majestic celestial object that hosts our Solar System.
To photograph the Milky Way, we use a basic astrophotography technique: we take many long-exposure shots and then sum them together to increase the signal-to-noise ratio.
In astrophotography, the concepts of summing and merging are not synonyms: summing means assembling all the photos, one on top of the other, making sure every single point, every single star is perfectly overlapped to bring out more signal than photo noise; on the other hand, merging means putting one photo after another, to compose the trail of stars in the sky (to create the so-called star trail). For this kind of photo, we can use wide-angle lenses, or at least focal lengths between 15mm and 35mm and bright (focal ratios less than 4).
Another important thing to keep in mind is that at night, all the automatisms of the camera don’t work, so we have to precisely set our device: the camera (SLR or mirrorless) should be set to manual mode (M program), as should the focus (MF on the lens); then we need to focus on a distant light source (a bright star, a planet, a distant streetlight...); this must be as point-like as possible; the aperture must be open, not to the most open value that the lens allows, but closed of one stop (full-aperture lenses will not perfectly correct the framed field and we will have elongated stars as a result, especially at the edges of the frame. By closing the aperture of one stop we solve this problem); high ISO sensitivity, using the "best-ISO" value, which is the ISO value at which the sensor has the best response with respect to the light signal it receives (to find it, search on the internet for the value of your sensor) and, finally, saving the files in RAW (even better if R+L so you have the JPG of each photo and you can decide which ones to keep).
Now it’s time to set the shutter speed. To find the correct time we will have to do some testing to get the point-like stars (generally, we could stay from 10'' to 20'' depending on the focal length). Since we will be taking long shots, remember to keep a pause between each shot (about 5'' to 10''). It’s important to not physically press the shutter release button, as this will create vibrations; a delayed shutter release in the camera, a remote shutter release, or a timer remote are a better choice, so you can schedule the whole session.
If only the sky is present in the photo, once the series of sky shots are done, the session will be over; however, if a foreground is also present, in addition to the series of sky shots, we will have to focus on the foreground as well. We will take a properly exposed shot for the ground to be used as the foreground of the sum of the sky shots: lowering the ISO, closing the aperture, and lengthening the time. A useful tip is to take the foreground shot during the so-called blue hours (after sunset or before sunrise) so that we still have enough light to best portray the foreground.
In processing, the program will hold the stars still by summing the whole sky, and thus we will get the sum master, to which we will overlay the foreground from the photo we took for the ground. Programs that can be used for summing and processing are Sequator, DSS, Siril (free), or PixInsight (not free, but it is the best program for astrophotography processing).
As long as the camera is stationary on the tripod, we will have time limitations because after a certain amount of time, the sky has moved (or rather, we have moved). To solve this problem, some mounts called astro trackers move contrary to the Earth's rotation to compensate for it. Thus, the sky and camera move in unison, and in this way, much longer shots (even up to 2') can be taken.
Now that we actually start chasing the sky, we can call it true astrophotography.
There are two types of astro trackers: spring-loaded or battery-operated. The former are very light, practical, and do not need any external power supply, but their sky tracking lasts only one hour, and being so light they have a limited carrying capacity; the latter, on the other hand, are heavier, and need an external power supply (4 AA batteries or via power bank), but can track all night long and have a greater carrying capacity.
Since you can take much longer shots than before, you can close the lens aperture by another stop, so that you have an even more leveled and corrected field. Once the whole session of sky and ground shots has ended, we have to deal with the corrective shots, namely: dark, bias, and flat. Darks are shots taken with the same settings as the sky shots (same ISO, times, and aperture), but with the cap in front of the lens. They are corrective shots used to eliminate thermal noise due to the long duration of the photos. The most important thing is that darks are taken at the same temperature so that their correction is the best possible. Bias shots are also shots taken with the cap in front of the lens, but at the highest shutter speed that our camera body allows; they are used to eliminate electrical noise due to the sensor turning on to initiate the shot. Finally, flats are used to correct the vignetting effect in sky shots and all the spots of dirt or dust on any optical surface; these must be taken framing an evenly lit area: they must be properly exposed (histogram between 1/3 and 1/2).
ABOUT THE AUTHOR
Hi, I'm Loris, I mainly deal with astrophotography and daytime landscapes. My passion for astronomy began as a child when I was given my first telescope, then it remained with me and has grown to this day. I have been practicing night photography for 8 years now and I have tried to create a set of instruments that allow me to do every type of astrophotography: from the wide field of the Milky Way, to the deep sky of nebulae, long focal length for distant galaxies and high resolution images of Moon and planets.
Silvia: Hi! I'm Silvia Polito, a linguist and financial translator working in English, French, and Italian. When I'm not working, you'll often find me gazing up at the stars, completely fascinated by astronomy and the universe. In fact, I'm about to enroll at University (again), this time for an astronomy course. I also work with the UAI, which stands for the 'Italian Amateur Astronomers Union'. I'm currently living in Forlì, Italy.
DO YOU WANT TO BE FEATURED IN OUR BLOG?
If you are passionate about photography and enjoy sharing your expertise with the community, our blog is for you. Go to https://www.theartistgallery.art/write-with-us read the rules, and submit your work.
Being featured in our blog will bring you several advantages, we will share your work not just in the blog but also in our social media and newsletter. You can talk about a photographic project you are working on, the story behind your best shot, your technical skills or just share tips with the community.
Comments