30 Nov Astrophotography: An introduction
Living out west has given me a profound appreciation for night sky viewing. Some of the US’s darkest skies are a mere 2 hours away. So you’d think that the opportunities are nearly limitless. The truth is, astrophotography is a tricky beast. Just getting the proverbial stars to align requires some choice weather, the correct moon phase, and depending on what you’re trying to see, the right season. Then add the complexity of the Earth’s rotation, it throws a wrench in classical long exposure methods. By the end of this article I hope to help you learn the fundamentals to at least get started!
THE RIGHT CONDITIONS
A dark sky needs to be dark. Super dark. As you would suspect that means getting away from civilization and out in the middle of nowhere. The Eastern half of the United States is a bit of a challenge as there are city lights just about everywhere. The easiest way to figure out where the dark skies are is this amazing website called DarkSiteFinder. As you can see the western half of the US is basically a dark sky park VS the eastern half. Next we need to get the moon phase right. A full moon will ruin any chance you have of taking night sky pictures. Now you could get lucky and have the moon be low in the sky and blocked by a canyon wall or something but its easiest to just try and go when the moon is new. The moon goes from full to full every 29.5 days, so generally speaking, its going to be a full moon around the same time every month. You can check the moon phase HERE. I put it in my google calendar and just have it repeat every 29 days. Lastly for conditions is weather. No clouds are key, and another reason why West is best. The desert is rarely cloudy!
We’re pushing the limits of camera technology in terms of capturing light. After all thats all a camera does anyway in its basic function. So we need to maximize the ability to capture as many photons as we can. Any modern digital camera body will be pretty adequate but the lens is whats going to define the rest.
A fast wide angle prime lens is what most people start with. Its great for capturing a landscape with a milky way backdrop. As seen on the left. There are quite a few budget lens options out there in this category! Rokinon makes a great 14mm f/2.8 for most lens mounts. Or my dream astro wide lens, the Rokinon 24mm f/1.4. Its wide enough for an epic vista but also fast enough to shoot single exposure milky way images. Generally speaking, you just want to get a lens with a max aperture larger than f/2.8. The wider the focal length the better. More on that later.
A good sturdy tripod is also very important. That cheap $15 Walmart one wont cut it. Get on Amazon and find something with some carbon or metal legs and a beefy ball head mount. Lastly, a remote, which is really only needed if you plan on going further than single exposure night sky images. If you are just doing single exposure shots then just setting a 10 second timer is adequate. The goal being, letting all your equipment settle into its spot before the shutter opens and does the long exposure. Stars are tiny in the frame and any movement will make them look blurry or out of focus.
Another option is to use a sky tracker. The longer you leave your shutter open, the more movement it will capture, and the Earth’s rotation is definitely movement. So if you have a lens with a narrow aperture like f/4 or smaller, then you’ll have to leave the shutter open longer to get the shot. Generally speaking, if you have a 35mm you can only shoot 14 seconds. Theres a rule we’ll get to later on that. You can see that rather quickly you are going to hit your limit on shutter speed with a narrow aperture lens. The work around is to put your camera on a device that moves with the rotation of the Earth. Theres a few cheaper options out there ranging from a few hundred, all the way up to the big money ones in the thousands. It’s also a great option when getting into image stacking or photographing faint deep sky objects like Andromeda, seen right.
CAMERA SETTINGS: SINGLE EXPOSURE
Heres the easy part! Because we are pushing the limits of the camera, we run out of adjustments pretty quickly. For single exposures we have the most limits. Start with shutter speed. The rule is called the 500 rule. 500 / focal length = shutter speed. For example 500 / 35mm = 14 seconds. This is due to the earths rotation. Next up is aperture. As large as you can get it. You have a max aperture of f/1.4? Use it at f/1.4. Which only leaves ISO as an adjustment. Thats why this is the easy part! You only have one adjustment possible for your exposure and its ISO. Go higher for a brighter image or lower for darker. Generally I start at ISO 1600 and adjust from there.
500/ focal length = shutter speed
Aperture set to maximum
ISO adjust for exposure
CAMERA SETTINGS: MULTIPLE EXPOSURE
Heres where things get a little more complicated. First lets address the whole reason were stacking images to begin with. To get rid of noise from using a high ISO. Noise looks an awful lot like stars in the sky, and when you push an image in the post processing to get those juicy looking galaxies, it comes out in force. So to get rid of that, we’re going to stack images. The important thing to remember is to just get the brightest part of what your photographing to be properly exposed. The dark parts will get filled in during the post processing by adding light data from multiple exposures. Take Andromeda for example. The core of the Galaxy is very bright, so focus on that being that part you can see in the image. If you try and expose the outer spiral arms then the center will just get blown out.
Shutter Speed 30 seconds
Aperture set to maximum
ISO adjust for exposure
Now here’s the difference in adjustments. Depending on how well you set your alignment for the sky tracker, you might be able to get a longer exposure before the stars start to trail in your image. Personally I’ve never seen over 45 seconds but I’m also not great at doing polar alignments. So I generally stick to the 30 second rule here. You’ll want to get at least 10 shots to stack, preferably 15-20 if you have time. Now the light frames aren’t the only ones we need to take! We need to get calibration images as well. These are pretty easy thankfully. After you’re done taking the light frames, put the lens cap on and take the exact amount of frames you did for the light with the same settings. This will eliminate temperature noise in the image. Every time you take an image, heat is generated and shows in the form of noise. Next take the same amount of frames again with the lens cap on but with the shutter speed set as fast as it will go, 1/8000th in most cases. This will eliminate sensor readout noise. And job done! On to the post processing.
10-15 light frames – 30 seconds / max aperture / ISO adjust for exposure
10-15 dark frames – same settings you used for light frames
10-15 bias frames – same aperture and ISO as light frames but 1/8000th shutter
10-15 flat frames – same ISO and focus point as light frames (put a white T shirt in front of the camera lens and point at a blue sky the next day)