In the beginning of March, we went for an observation at the good and old Pico dos Dias Observatory (OPD), here in Brazil. I think it was the first time I went there during summer, which is the rainy season around these parts, so it’s not actually a good time for observations. We wanted to assess the B&C 0.60 m telescope in its potential to do followup observations for KELT (Kilodegree Extremely Little Telescope – pretty funny name: a bit of a satire on ESO’s “very large” telescopes, I imagine), which surveys the sky looking for hot Jupiters around bright stars. So yeah, we went there to hunt some exoplanets.
To do that, we used differential photometry, a method that compares how the brightness of different stars in a given field of view vary with time. In our case, we wanted to see how the brightness of the exoplanet-host dipped when a transit occurred. To do that, we need as many photons as possible to fall on our detectors, so we can have measure the dip with a bigger certainty. Results were very interesting, and I will write a more complete blog entry about our observations in the near future. But, while we were there trying to catch the light (or rather, the shadow) of exoplanets, I was, as always, trying my hand on some night sky pictures.
While I left the telescope doing a very long series of exposures, I set out taking my camera and tripod to do some imaging. And during one of my first trials, I was lucky enough to have a really bright meteor cross the sky and go over the 1.60 m telescope, and even more lucky to have my camera exposing at the time. And further lucky to have set a high ISO, so the meteor stood out beautifully in the picture. Unfortunately, since it was one of the first pictures, the camera wasn’t focused very well (in fact, it was pretty terrible), so it didn’t end up as good as it could be. Also, the Moon was 88% illuminated, so we get this effect of “daylight with stars” in long exposures (in all pictures, click to embiggen).
However, things weren’t that bad, because the sky went completely dark after around 4 AM, when the Moon set, so there was this short window of darkness until the twilight, and I was eager to take advantage of it. I haven’t had many chances of taking pictures in really dark places with really dark conditions before, so this was a good opportunity.With the help of some bright stars, I managed to focus the camera and I took these two following pictures. The first one was taken using ISO 3200 and, luckily enough, I got another meteor (a dimmer one, though) and a satellite! The second one was taken with ISO 1600, which has less noise, but it is less sensitive, and so I had to take more exposures, resulting in longer star trails (I don’t have an equatorial tracking tripod). As you can imagine, we have to work with a trade-off between more light and noise/trails.
Here are two other shots taken aiming towards the domes of the observatory, and they are probably my favorite ones. Not only because they look fairly good, but because it was actually very fun to take these shots. The site was so dark, that it was hard to find my way around the observatory until my eyes didn’t get used to the darkness (it took about 10 minutes). Once your pupils are fully open, we can see so many stars in the sky that it is difficult to find the asterisms of the constellations.
I think those were very productive nights. We obtained good results from the observations, I got a chance to take pictures in really dark conditions, and food was good. The food from OPD is never a let down. Word is that, when the weather is bad, astronomers like to spend their time lounging at the 1.60 m telescope pantry. Luckily we didn’t need to do that.
DSLR have become very popular cameras, so much that I managed to get one in 2013, while I was an exchange student in Netherlands (these cameras are too expensive here in Brazil – more than double the price found in North America or Europe). DSLR stands for Digital Single-Lens Reflex, and these cameras have a CCD (Charge-Coupled device) sensor which captures the light and than the information can be translated into images. CCDs have long been used in astronomy and, in fact, there’s quite an amusing history behind it: it is because of astronomy that almost all portable electronic devices and digital cameras today use this technology; if it weren’t for the astronomers interest in making CCDs more efficient and less expensive, Bell Labs (who invented it) would have thrown it in the trash, because they didn’t find a purpose for it. Or so goes the story told among astronomers. In other words, astronomy made it possible for CCDs to become accessible and useful to the public!
Anyway, since DSLR cameras¹ carry in them the same sensor used in astronomy, it is quite possible to do night sky imaging in a similar way that professional astronomers do. And this is what I have been doing since I got my camera: it is a Nikon D3100, a very entry-level device, and I’m even using the stock lenses (18-55 mm). Taking picture of the night sky is a very satisfying hobby, but it can also be time-consuming and a bit frustrating at times, and the learning curve is steep. But you can always start small. In this tutorial, which is the part 1, I’ll guide you from the easiest and less painful way to do night sky imaging. In part 2 (still working on it), I’ll show you the way through the most advanced stage I’ve gotten so far, without a telescope². I’m also thinking about writing a part 3, which would be the processing of images obtained on telescopes, but I still need to acquire more experience on that, so let’s leave that on the bag of ideas.
Since I still consider myself a beginner, suggestions to this guide are always welcome, and I will keep this post updated from time to time. Without further ado, here we go:
Here are the necessary equipment to do DSLR imaging. Keep in mind that the prices I list here are for new, entry-level devices, and are also approximate.
DSLR camera + stock lenses: € 300 or US$ 375.
Class 10 SD card: around € 50. Get the higher class you can! Otherwise you’ll suffer from slow data-writing.
A tripod: starting from € 50.
Your best computer: depending on the number of pixels your camera takes, dealing with various layers of images will take a HUGE toll on your computer’s processing power.
The following items are optional, but strongly recommended:
First of all, get to know your camera, read the manual, become its very best friend. And then get out at night with it. I say this because: 1) you’ll be messing with it in the dark, so it is a must to know the controls very well; 2) night sky imaging cannot be done with a pre-built shooting mode: it has to be completely manual, there’s no auto-pilot! So the best you know how to adjust the shooting settings, the best results you’ll get. Also, it’s helpful to know the nomenclature and the jargons of photography.
If you know your camera well, it’s time to start imaging. The settings on the camera will basically depend on two factors: light-pollution level and phase of the Moon. If you live a very light-polluted place or if the Moon is close to the full phase, you might want to gather less light, otherwise you’ll end up with a picture that is too red (sign of light pollution) or one that looks like daytime (caused by Moon illumination – but some people like pictures that show this feature, maybe because it seems to be a bit surreal – it’s up to you in the end). There are three ways to control how much light the camera gathers:
ISO: controls the sensitivity of the CCD to light
Focal ratio (or F number): controls the aperture of the camera’s “pupil”
Exposure time: controls how much time the shutter of the camera stays open after you release it
For all of these, the higher, the more light you gather, but you’ll also get more noise if the ISO is high.
Focus at infinity
The auto-focus function that most DSLR have need a minimum level of contrast, and because the dark sky is not the queen of contrast, you’ll have to do it yourself. We need to find the focus at infinity. And there is a simple trick to do that. Find a very bright, very far source of light (maybe the tip of a radio antenna in the mountains, they usually have lights; or the light coming from the window of a very far house); then zoom it to the maximum and put it into live view (image on the LED screen), because we’re gonna use digital zoom – if your camera doesn’t have live view, well, just try to do the best you can in the eyepiece. Turn up that digital zoom at the light source, and focus the lenses manually until the source is as small as possible: that should be the focus at infinity. Don’t touch the focus knob after this, otherwise you’ll need to re-focus it. Also, do not mistakenly change it to auto-focus, otherwise the camera will try to do it when you press the shutter release and lose the focus at infinity.
Before shooting, keep in mind that when the camera shutter opens, the smallest rumble will affect the picture, so in an ideal situation, you should not touch the camera, because our hands make it shake. That’s why I recommend using a remote controller. If you don’t have one, the best procedure is to set your camera to the delayed shooting mode. In this mode, after you press the shutter release button, the camera waits a few seconds before actually taking the picture. By doing that, you give some time for the camera to stabilize and stop shaking after you touch it.
For first time shooters, I’d say: pick a region of the sky that is close to the milky way, set your ISO to 800, maximum F number, and an exposure time of 10 seconds (needless to say, use a tripod) and see how things go. If the picture is too bright or too dark, tweak the exposure time and the ISO. Remember that you’ll get a bit of star trails because of the rotation of the Earth (unless you have a tracking tripod, but these babies are expensive as heck).
On the other hand, if you have really dark skies and no prospects of clouds, you can try your hands at getting some nice star trails. I do not have experience on that (I’ll update the guide when I do), but I would imagine that you have to set a very long exposure time (something like more than 30 minutes), set a lower ISO and/or smaller aperture (so it doesn’t get a crazy amount of sky glow in this long period of time), point the camera to a region near the pole of the sky and let it rip.
Post-processing this images shouldn’t be a big deal if there isn’t much light pollution (believe me, it is a pain in the butt trying to get rid from the reddish glow of the sky – but we’ll see how to do that in part 2). It consists of turning down the sky glow and enhancing contrast or colors: to do that, you change the curves or the levels of the picture. I use the open-source software GIMP to edit images, and the options to change curves and levels are inside the menu “Colors”. I’m not going to tell you how to change these, you basically just tweak them until you’re satisfied with it, there is no secret. The following pictures were taken with this basic imaging technique, and had only minor post-processing.
And this is it, basically. That’s how I started taking pictures of the night sky. I think the most difficult part of it is getting acquainted with the camera, especially if you’re new into photography. The options are overwhelming, and to this day I am unsure about some of the settings of my camera, such as how the white balance affects the final images and if I should just leave it to auto to take night sky photos. I need to study those. I suggest you to watch some videos about photography on YouTube (I particularly like this channel), or check some other blogs by professional photographers, there are many things to be learned on the internet.
Clear skies, and see you on part 2 of the guide!
Things to add to this guide in the future: shooting lightnings through a thunderstorm, shooting star trails, shooting meteors.
¹ Why use DSLR cameras? Can’t we use a smartphone or one of those “plug and play” easy to use compact cameras? Short answer: you can try, but the results will probably be underwhelming. Long answer: the problem with these cameras is that you don’t have much control over their settings, such as the sensibility of the CCD or the aperture; some of them allow only short exposures, and others don’t even let you control the focus; night sky imaging demands very precise, user-defined settings; so, no.
² It is possible to do DSLR imaging with the camera attached to the prime focus of a telescope, but in principle the imaging process shouldn’t be very different, except that you wouldn’t use lenses. The biggest challenge with that is to set up a telescope and a mount.