Our cameras get most of their workouts during the daytime, when there's plenty of light from the sun to illuminate our subjects. If we shoot at night, it's usually with a flash attached to brighten things up, or with long exposures using man-made light from our streets or houses. That big dark sky above us at night seems to offer little that we can see or photograph... but that's not the case. There's not just a whole other world of things to take pictures of in that dark sky -- there's a whole universe of opportunity. This how-to article will provide the basics to let you start taking dramatic photos of the sky at night.
Astrophotography is the name given to the process of taking pictures of anything not on the Earth, but out in space. Most of us probably think of images from the Hubble Space Telescope when the word is mentioned, but making good astrophotos doesn't require billions of dollars or a ride in the Space Shuttle. This article will show you how to start making photos with equipment almost every photographer has on hand, progress to the next level with some simple equipment you can buy inexpensively or make yourself, and finish with what you can do if you decide to invest in some specialized (and sometimes expensive) equipment.
The first thing to remember when getting into astrophotography is that the earth rotates on its axis once per day. Obvious? Sure, but what it means for astrophotography is that you're never trying to capture a "still" object. The earth's rotation makes the stars, planets, and the moon appear to move across the sky all night long. The moon and planets also have their own motions, which complicate matters. And since most of the things we'd like to take photos of are awfully dim compared to a typical subject in the sunlight, we're going to have to use long exposure times to gather enough light to see them, and they'll be moving (or appearing to) the whole time. There are only two ways around this constant movement: ignore it, or compensate for it.
© Paul LeFevre
Stars appear to circle the sky around Polaris, the North Star. A 60-minute exposure at f/8 taken on Palomar Mountain, California with a Canon 300D DSLR at ISO 400. Click photo for larger image.
Let's start with what we can do by ignoring the earth's rotation. Shooting these kinds of photos only requires a camera capable of long exposures (a "B" or Bulb setting, or exposure settings from 30 seconds to several hours). The obvious example is "star trail" images.
The earth rotates on its axis at about 1/2-degree per minute -- so the stars will appear to rotate that same amount overhead. If we set a camera on a fixed tripod, open the shutter, and let the earth rotate us under the stars, we get an image that shows the stars as "streaks" or trails as they appear to move across the sky. The longer we leave the shutter open, the longer the trails will be in our final image. The image shown here was exposed for about 60 minutes, giving 15-degrees of arc to each star trail. Notice in the image that near the center there appears to be almost no star-trailing, and as you move further out the star trails appear to get longer. That's because the camera was pointed right at the center of the earth's rotational axis: Polaris, or the North Star. Since this is the center of rotation, there's the least amount of apparent movement near this axis. As you move further away from this point, stars will appear to move more and more in a straight line until you reach the "celestial equator" (90-degrees from the North Star in the sky), where the trails are perfectly straight and will appear longer.
Here's how to take star-trail photos:
• Use a sturdy tripod, and secure it in place or weight it down if you can. Any movement of the tripod will show "squiggles" in your star trails.
• Use a fairly wide-angle lens for best results, the 35mm equivalent of 20-50mm focal length is a good place to start.
• Choose a medium-speed film or a digital-camera ISO of 400-800. That's high enough to record even fairly dim stars, but it shouldn't introduce too much grain or noise.
• Set a medium aperture of f/5.6 to f/11. The stars won't change much in brightness no matter which aperture setting you use, but smaller apertures will reduce the brightness of "skyglow" from nearby towns or other light sources.
• Select a dark location away from city lights if possible. Include something interesting in the foreground (such as the trees above) to give scale to the image and to help show the sky's apparent rotation against the earth.
• Make sure you have new or freshly-charged batteries in your camera. Holding the shutter open for long periods drains batteries fast! If your camera has a DC car-power adapter, or a battery pack, use them.
• Use a cable release or remote release, set manual focus, focus on infinity (put a small piece of masking tape on the lens' focus ring to hold it if you can), and open the camera's shutter. Leave it open as long as possible. Longer exposures mean longer star trails, but also pick up more "sky glow.
Star-trail images can be spectacular, but due to the motion of the stars, they can be somewhat "abstract." What if you want to take an image of the stars as we actually see them?
© Paul LeFevre
Shorter exposures with a wide-angle lens can still provide enough light to show the stars in a night sky without "trailing." 30-second exposure at ISO 400 on a Canon 5D DSLR at 14mm and f/8. Click photo for larger image.
As mentioned before, the stars appear to move at about 1/2-degree per minute across the sky. If we use a wide-angle lens (which has a wide field of view), you can leave the shutter open for a short while before that movement becomes visible in the image. The stars will still be "trails" in your picture, but the trails will be so small they're indistinguishable from single points. In the image above, I used a 14mm lens (with a field of view of 104-degrees horizontally and 81-degrees vertically), and exposed for 30 seconds. In that time, the stars moved less than 1/4-degree, taking up less than 1/400th of the horizontal field of view. In the full-sized image you can see that they're little trails, but just barely. This kind of image shows the sky as we appear to see it with our eyes. Only the brightest stars will show up clearly this way, but as you can see from the image above you can clearly make out the constellation Orion, the Pleiades star cluster, and other constellations and stars.
Here are some tips for shooting non-trailed fixed-position star images:
• Use a very wide-angle lens -- the wider the better. At 28mm (35mm film equivalent) focal length, you can expose for about 20 seconds without significant trailing. At 50mm, you can only expose for about 10 seconds, which is only long enough to record the very brightest stars. At 14mm, you can shoot for 30 to 40 seconds, which should show more lower magnitude stars.
• As with star-trail images, include something interesting in the foreground! The image above was shot on a night when the nearly-full moon was just rising in the east, and the camera was aimed west, away from the moon. The moon illuminated the foreground without washing out the sky.
• The same setup rules apply as with star-trail images; use a sturdy tripod, a remote release, ISO 400-800, and f/5.6 to f/8.
So far we've only looked at what we can do with a fixed tripod, with our camera still while the sky rotates above us. We're limited in what we can shoot that way. Most of the really interesting things in the sky are quite dim, so we need much longer exposure times in order to see them in our images... but longer exposure times mean the stars leave trails in our images.
However, we can get longer exposure times without trails by mounting our camera to something that compensates for the Earth's rotation. If we could have some kind of rig that rotated around one axis (like the Earth does), align our rig's axis of rotation with the Earth's axis, and then rotate in the opposite direction the same speed as the earth rotates (about ½-degree per minute), then our camera would stay pointed at the same spot in the sky as the Earth rotated, and we could do longer exposures without having the stars trail.
Hmm, that sounds pretty complicated, though. How can we make such a rig? Actually it's very simple, and all you need are a couple of boards, a hinge, and a couple of bolts... it's called a "Barn Door" mount.
A Barn Door is simply two pieces of wood mounted around a simple hinge. At the end of the boards opposite the hinge, you put a screw-bolt through the bottom board in a threaded nut, so that it pushes the top board when you turn it. Using a simple formula, you can calculate how many turns per minute you need to give the screw to match the earth's speed of rotation. Line the hinge (the Barn Door's axis of rotation) up with the North Star (the Earth's axis of rotation), and you can very closely counteract the movement of the stars across the sky for fairly long lengths of time. With a Barn Door mount, you can use longer focal lengths (up to around 135mm) and exposure times of up to about 15 minutes and not see any star trailing -- all for about $10-$20 in materials.
Designs for Barn Doors vary from the ridiculously simple and inexpensive to deluxe versions with battery-operated drive motors (to save you from having to turn the screw manually). A simple single-arm version will track accurately for 10-15 minutes, while only-slightly-more-complex double-arm versions can track accurately for an hour or more! The Internet is rife with design possibilities; here are a few of the many that are worth looking at (I built the second one in the list below, and used it for the Milky Way photo above):
Anthony Galvan III's Simple Barn Door Mount
Dave Trott's Motorized Double-Arm Drive
The next step up in "tracking" mounts from a Barn Door is a German Equatorial Mount. A GEM (sometimes acronyms are very fitting) in its simplest form is a tripod head that has two axes that rotate 90-degrees from each other. When you align the main axis with the Earth's celestial pole, it can be driven to counteract the Earth's rotation. The other rotational axis lets you position the camera or telescope anywhere in the sky while still maintaining the first axis' alignment with the Earth. Commercial equatorial mounts vary greatly in size, quality, and price; a decent-quality mount with a drive motor (to keep you from having to turn it by hand) that will support a camera and short lens can be purchased for under $200, while a higher-quality mount that will support long lenses or telescopes (and often includes a computerized control that will automatically point it to specified celestial objects) can cost between $1,000 and $10,000.
Equatorial mounts can allow you to use longer focal lengths (using either camera lenses or telescopes) to "zoom in" on galaxies, star clusters, and nebulae. The image at left shows two telescopes mounted together on a mid-range (in both price and capacity) GEM, with the camera mounted on a 540mm FL telescope. Why two telescopes? Because once you start using focal lengths longer than 250mm or so, you must track the sky VERY accurately. Even a tracking error as little as a tenth of a degree can ruin an image, making stars look like little oblong streaks rather than nice, sharp points of light. Since even rather expensive GEM mounts are driven by mechanical gears, imperfect machining causes those gears to be slightly inaccurate, and produce tracking errors. To get a good image, these tracking errors have to be corrected. This is called "guiding."
© Paul LeFevre
Imaging and guiding telescopes mounted on a German Equatorial Mount, ready for taking pictures. Click photo for larger image.
Guiding an exposure involves looking through a separate telescope from the telescope or lens being used to take the picture. A telescope eyepiece with crosshairs embedded in it is used -- the photographer centers a star on the crosshairs, starts an exposure, and then carefully watches the star through the guidescope. When mechanical errors begin to move the star off the center of the crosshairs, the mount's drive controls can be used to put it back dead center. Keeping the guide star on the crosshairs during the exposure will ensure that tracking errors are minimized, and long exposures (up to several hours) can be made very accurately. Obviously, this can get quite tedious -- you must keep your eye glued to the guidescope for the entire exposure, and delaying too long in making corrections will ruin an image. Technology has offered a solution to manual guiding tedium -- a CCD guide camera. Commercial guide cameras are available, but even inexpensive digital Web cams can be easily modified to act as guide cameras. Attached to a computer, the camera takes an initial exposure, and notes the pixel position of a guide star (specified by the user). Then the shutter is opened on the imaging camera, and the guiding sequence started through the computer. It takes a new image every few seconds, and again notes the pixel position of the guide star. If it's moved from its original location, the computer sends signals to the equatorial mount to move it back to where it was, just as you would do manually. This "auto-guiding" can easily be more accurate than manual guiding, and doesn't require the photographer to sit at the guidescope for hours on end. The links section at the end of this article has resources for making your own auto-guider from an inexpensive Web cam.
With a good-quality GEM mount, even fairly short focal lengths can provide stunning "close-ups" of many celestial objects. The image at right is of the Andromeda Galaxy, taken with a 540mm telescope. While Andromeda (also known as M31) is one of the brightest galaxies we can see from earth, it's still very dim by photography standards. To make this image, the total exposure time was 12 hours!
© Paul LeFevre
The Andromeda Galaxy -- 12 hours total exposure time on a 540mm telescope. Click photo for larger image.
Once you start using longer focal lengths, a whole universe of celestial objects become potential targets. But those longer focal lengths, while magnifying those smaller celestial objects, also magnify tracking errors -- making it more and more difficult to get good images of them. Taking "deep-sky" multiple-hour exposures of tiny, dim objects at long focal lengths will require investment in a high-quality GEM mount (which is expensive!), sophisticated tracking methods and equipment, and lots of practice. The details of this kind of work are beyond the scope of this beginning article. If you start with star trails or perhaps build a Barn Door mount, you can make wonderful night-sky images without investing a lot of money in equipment. But beware -- astrophotography can be addictive! Once you start getting beautiful night-sky pictures, it's quite likely you'll want to move up to higher levels, which means a significant investment of time and money. The results are very satisfying and beautiful -- but when your spouse starts asking why you need to spend $5,000 on some fancy tripod, don't say I didn't warn you!
There's one night-sky target we haven't covered yet. I saved it until last because it's one of the easiest to take pictures of... our Moon. While the moon appears to move across the sky at nearly the same rate as the stars, it's very bright -- about the same brightness as a daylight scene on Earth (since the light we see from the Moon is just reflected sunlight, that makes sense!). Since it's so bright, it doesn't require the long exposure times that are needed for stars, galaxies, etc. But the Moon can be a great target to photograph.
The issue with taking pictures of the Moon is really focal length of the lens you're going to use. On 35mm film or full-frame digital cameras, it takes a focal length of about 2000mm to fill the frame with the Moon. APS-C DSLRs need about 1200mm to do the same. While you can get satisfactory Moon pictures with shorter focal lengths, the closer you can come to those frame-filling ideals, the better.
Proper exposure values for the Moon are easy to figure out -- just use the "sunny 16" rule. At f/16, use a shutter speed that is the reciprocal of your film or sensor's ISO value; at ISO 200, you'd use 1/200th sec. at f/16. Adjust the shutter speed up or down according to how many f-stops above or below f/16 your lens is operating at. Use a sturdy tripod, use your camera's mirror lock-up function (if it has one), and use a remote or cable release. Focus at infinity, and shoot away. It's best NOT to use your camera's auto-exposure modes; most camera exposure meters see all that black surrounding the Moon, and add exposure time to compensate, which over-exposes the Moon itself.
If your goal is to shoot frame-filling photos of the Moon showing lots of detail, the best time to shoot isn't when it's full: at full Moon, the Sun is lighting the Moon face-on. Just as with your on-camera flash, straight-on lighting is flat, boring, and doesn't provide any shadow details. Try taking Moon pictures at first or third quarter (when the Moon appears half-lit, as above). At these times, the Sun provides side-lighting, showing relief on lunar surfaces and providing interesting shadows. Try not to shoot Moon pictures when it's low on the horizon. At those positions you're shooting through the thickest part of Earth's atmosphere, and the turbulent, moving atmosphere makes it difficult to see fine details on the lunar surface. Wait until the Moon is high in the sky, where the atmosphere is thinnest, even if that means getting up at 2AM to shoot!
© Paul LeFevre
Moonrise over Tokyo, with Mount Fuji at left. Shot with a Canon 5D DSLR at ISO 50, 0.6 second at f/22 with a 135mm lens. Click photo for larger image.
Another way to shoot pictures of the Moon is to include it in landscape shots for dramatic impact. You can use much shorter focal lengths than for the frame-filling detail shots, but stay on the longer side (100mm or above) or the Moon will look very small in the image. The same "sunny 16" exposure guide applies, with a caveat: when the Moon is near the horizon, the thick layer of atmosphere you're looking through to see it will dim its light, sometimes dramatically. When it's near the horizon, start with about two stops more exposure than when it's high in the sky, and bracket your exposures if you can. For the moonrise image above I used about three stops more than the "sunny 16" guideline. A full Moon will rise in the east about the same time the Sun is setting in the west; a crescent Moon will rise just before sunrise in the east or set just after sunset in the west. Balancing the near-daylight exposure for the Moon with twilight can be tricky, making sunrise and sunset the best times to shoot. Just be careful not to over-expose the Moon or you'll wind up with a white blob that shows no detail.
I hope this How-To has piqued your interest in the possibilities of astrophotography. Don't put that camera away when it gets dark, get outside and take pictures! A whole universe of wonderful images awaits you.
Resources and Links
To do astrophotography well, you need to get to know the night sky -- when and where the Sun and Moon will rise and set, which constellations are in the sky at different times of the year, and which celestial objects are visible. Here are some internet resources to help get you up to speed:
An ephemeris gives the positions of the Sun, Moon, and planets. The NASA/JPL site lets you enter your position on earth to get specific information for your location, at any time in the present, future, or past.
Jim Kaler's Constellation Maps
Provides maps of the constellations and their brightest stars, with different maps for the different seasons of the year.
Sky and Telescope's Interactive Star Charts
An on-line star chart customizable for your specific location and time.
The sky is full of interesting objects to photograph besides stars -- galaxies, nebulae, clusters, and more. French astronomer Charles Messier catalogued 110 of the brightest of these objects in the late 1700's, and his Messier List is a good starting point to find targets of interest. The Andromeda Galaxy above, for example, is also known as Messier 31 (M31). You can get a list of all 110 objects, along with information about where to find them in the sky, at: SEDS Messier Catalog.
If you start going for long exposures at long focal lengths on an equatorial mount, you're going to need to guide your shots -- as mentioned in the text, a simple Web cam can be easily modified to be used as an "autoguider." Ash's Astro Pages has several articles and tips for using Web cams for guiding and astro-imaging.
If you're using a DSLR to do your astrophotography, Michael Covington offers a comprehensive Web page with lots of tips to help you get started: Michael Covington's DSLR Astrophotography.
Paul LeFevre is a photographer, writer, and astronomer living near San Diego, California. He can be reached at firstname.lastname@example.org.