So very tired…

Getting started with astrophotography

September 8, 2010 at 7:54 pm (Camera Gear, Howto, Photography) (astronomy, astrophotography, Long Exposure, moon, night, photo editing, Photography, star, star trails)

This almost 27 minute exposure was taken on August 5, 2010, at the peak of the Perseid meteor shower. You can see a single meteor, as well as a jet that flew through the frame. Exposure 1609.9s, Aperture f/2.8, Focal Length 11 mm

Astrophotography, defined in simplest terms, is not much more than pointing your camera at the night sky. Of course as with most things it is not really so simple and the upper reaches of astrophotography can involve specialized equipment that costs thousands of dollars, and specialized settings that can be difficult to access. I’m not really going to get into that though, I just want to present some techniques using common equipment that pretty much anyone with a DSLR and a tripod can go out and try for themselves. Thus, I’ll demonstrate three types of shots: the moon, star trails, and milky way shots.

First of all, we will need to assemble some gear. At a bare minimum you should have:

• A DLSR camera, or an advanced point-and-shoot that allows you to dial in shutter speeds manually.
• A decent tripod. Keeping your camera steady is absolutely imperative for star trail and milky way shots.

A few other items that are extremely useful, though not absolutely necessary are:

• An infrared or cable shutter release. If you don’t have one of these then use your camera’s timer function if available.
• A flashlight, or other portable lights for focus assist and general navigation in the darkness.
• Folding chairs…you may want to have a seat while doing a 30 minute star trail exposure.
• Snacks, beverages, suitable clothing and other ‘creature comforts’ to make your night of shooting as comfortable as possible.

A few words on locations

You may be aware of what light pollution is. Suffice it to say, the more electric light polluting the sky, the less stars you are going to see. For moon pictures this is not so important, but for other shots it is imperative to get as far from ‘civilization’ as possible. Depending on where you live in the world, this may be easier said than done. Those in large cities will have to travel farther than those who live in more rural area. I’ve found that if you don’t have time or resources to plan a trip solely for astrophotography you can use camping as a good opportunity, assuming you head out into the bush to do your camping. Now other than getting as far from the city as possible, the location doesn’t really matter that much, just pick a nice quiet spot a little ways off the road. The head and tail lights of passing traffic can influence your exposure, so you may want to keep that in mind. If you are unsure of where to go in your local area, you may want to see if there is a local astronomy club and ask them where they go for their ‘star parties’.

Part 1: Shooting the moon

Exposure: 0.005 sec (1/200), Aperture: f/5.6, Focal Length: 300 mm

Getting a shot of the moon may be the simplest of the three, as you don’t really need to leave home and head out into the wilderness. The moon is very bright, and it stands to reason that if you can see it, you can photograph it regardless of the light pollution present. At the risk of sounding patronizingly obvious, the moon is very, very far away. That means you will want to use your longest telephoto lens. I have the Nikkor 70-300mm VR, and mounted on a D90 with an APS-C sized sensor that gives me an effective focal length of 450 mm which is still not nearly long enough to fill the frame. Unless you can afford a 600mm telephoto at about $10,000 you will have to make a choice of whether to crop the photo, or leave some context into the photo as I have done with my example shot. If you don’t have a telephoto lens, you can still try with your longest focal length, but even with a crop I think you will be disappointed with the results.

Now it is important to keep in mind that although you are likely shooting at night, the moon’s surface is covered in full daylight. That’s why it’s so bright. You need to consider this when selecting your exposure values. As a starting point you can use the ‘sunny-16’ rule which states that in full sun at f/16 your shutter speed should be the reciprocal of your ISO. Thus: if you are using ISO200 your shutter speed should be set at 1/200th of a second. From this starting point you can use a faster shutter speed and larger aperture and vice-versa as you see fit, or disregard the rule altogether as your artistic sensibilities dictate. I am a very strong believer in the mantra “learn the rules, so you know which rules you can break”. Try a few settings and chimp the shots to see what you’re getting. That is one of the foremost benefits of shooting digital: instant results.

If you are not comfortable dialing in your exposures manually you can also use your DSLR’s spot-metering function. Regular matrix or average metering will not give good results, as the darkened sky will cause your camera to ramp up the exposure leaving you with a spot of nuclear burn-out where the moon should be. Select spot-metering, and place your focus/metering point directly in the middle of the moon and take a reading. At this point you can use your exposure/focus lock button and recompose if you wish. This will ensure a shot where the moon is properly exposed and showing good detail, while the rest of the frame will fall where it may, most certainly coming out very dark if not completely black. That’s really not such a bad place for the night sky to be.

One last thought: I cheated with my example shot, as I didn’t use a tripod. It is hand-held at 1/200th of a second, which is a no-no due to the 450mm EFL. I strongly suggest however, that you do as I say, and not as I do to ensure the sharpest shots possible. This is especially true if you intend to crop the shot.

Part 2: Star trails

Exposure: 1424.6 seconds, Aperture f/2.8, Focal Length: 11 mm

You are probably aware that Earth spins on its axis. This means that if we put a camera in a fixed location relative to the Earth things not on the earth, such as stars, will appear to be moving. When it comes to star-trail photography, this apparent movement can make for quite a dramatic photograph. For this type of shot you absolutely must use a tripod, as you will be making exposures of up to 30 minutes. You should also use an infrared or cable release to ensure the sharpest shots possible. You may want to use your widest lens for this type of shot, however that is of course a judgement call. I like as much sky in frame as possible so I used my Tokina 11-16mm at its widest setting. As for framing, I find that adding something at the bottom or side of the frame relative to the earth adds a lot more interest. I used trees in the example photos, but you can use anything you like, such as a hillside, an old barn, or even a car. As for focus, you will want to set your distance meter to infinity. If you do not have a distance meter on your lens, you can use a flashlight and light up some trees in the distance and use your AF to focus on them, and that should leave the stars in relatively good focus. After you have your focus dialed, change the focus to manual control so the AF doesn’t kick in the next time you hit the shutter release.

As for exposure settings, it is largely personal taste. You need at least 30 seconds to detect any movement in the stars. The longer your exposure, the longer the trails will be. I generally like to expose for 25 to 30 minutes. To get an exposure longer than 30 seconds you will likely have to use ‘Bulb’ mode. On Nikon cameras this requires the ML-L3 wireless remote control, a very useful piece of gear for about $15. For other camera makes, you will have to consult your manual for specifics of enabling bulb mode. As for aperture, that depends on the brightness of the sky. If there is no moonlight present you may want to dial your aperture wide open. If there is a very bright moon you will probably want to stop down a bit or your shot will come out of camera ridiculously bright. Turn your auto-ISO off, and set your ISO to your camera’s lowest possible setting to avoid noise.

Exposure: 1800s (30 minutes), Aperture: f/2.8, Focal Length: 11 mm

You may notice that these example shots have a center point in the sky that the rest of the stars seem to swirl around. That star is Polaris, the star closest to the north magnetic pole. It sits exactly above the axis on which the earth spins, which makes it appear static. If you can locate Polaris then you can use it to effect in your composition. Quite honestly, I have no idea which star has the same effect for our friends in the southern hemisphere. You’ll have to do some research I guess. Also, if you orient your frame 90 degrees or so away from Polaris you can get some neat effects and different lines. This is where experimentation and artistic license come to play. Try all sorts of different things and see what you can get.

Note: rather than doing straight long exposures there is also a method of taking many shorter exposures and stacking them with software. This requires a special cable release and specialized software which I don’t have, so I can’t speak intelligently about it. digital-photography-school.com has an article on the subject you can explore if you like…

Part 3: The Milky Way

This is a simple technique that can afford you some stunning results, however, you do have to ensure there is close to zero light pollution at your chosen location. If you cannot see the Milky Way with your naked eye, you will not be able to photograph it. Basically, you have to set up on a tripod, open your aperture to it’s widest setting and expose for 30 seconds. As I mentioned, any longer than 30 seconds and you will start noticing the movement of the stars which is undesirable in this case. You will also have to crank up your ISO. If you leave it at a low setting you will be left with a decent shot of the brightest stars, but you won’t get the full milky way effect. This will of course introduce noise. That said, a bunch of points of light, ie: the stars will hide the noise well, so it isn’t as annoying as it may be in other types of shots. You may want to just frame open sky for this type of shot. You can see in my example that the noise is most noticeable at the bottom of the frame in the tree line.

Exposure: 30s, Aperture: f/2.8, Focal Length: 11 mm, ISO: 1600

More resources

As I mentioned, these three types of shots just scratch the surface of astrophotography, and are relatively easy to capture with common equipment. If you are already a keen astronomer, you will almost certainly want to look into an adapter so you can attach your camera to a telescope. Jerry Lodriguss has put together a great bit of information and further links on his Introduction to Digital Astrophotography page if you want to explore further. Have fun!

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