There's no more seductive image in astronomy than a picture of the night sky filled with stars and colorful glowing clouds of interstellar gas. You know, the kind you see in books and magazines. These images usually come from multimillion-dollar telescopes, not from simple astrophotography setups. But it doesn't take a mountaintop observatory to capture panoramic deep-sky vistas. They're within reach of anyone with modest stargazing equipment and access to dark, rural skies. The technique is called piggybacking photography, and today's high-speed color films and digital cameras have made the process easier than ever before.
In piggyback photography, you use a telescope's mount to track the sky, but the camera shoots through its own lens, not through the telescope.
Night Sky: Craig Michael Utter
Simple Astrophotography: What is Piggybacking?
The richest star fields in the sky are found along the Milky Way, formed by the spiral arms of our home galaxy and well-placed during late summer. The exposure times needed to record the Milky Way typically last a few minutes. For this, you'll need a camera with a B ("bulb") setting on the shutter to permit long exposures and a telescope capable of tracking stars as they move from east to west. But you don't shoot through the telescope itself — it's the mount you want, to serve as a stable, motorized platform. You attach your camera securely to the telescope's tube assembly. It then rides along, similar to piggybacking, as stars are recorded by the camera's lens.
Without motorized tracking, Earth's rotation would cause stars to trail across a camera frame, an effect noticeable with even a 30-second exposure. Star trails can make for great astrophotos, but for rich, star-studded time exposures, accurate tracking is the key.
The beauty of piggybacking is that if the mount is set up correctly and tracks well, all you need to do is lock the camera's shutter open and walk away. You don't need specialized and expensive guiding gear. While the camera and mount do their work unattended, you can sit back, relax, and enjoy the night sky.
The Power of Piggybacking
A "fisheye" lens on a film SLR can capture the sky from horizon to zenith. This shot required a 1-hour exposure on Kodak Ekatchrome E200 Pro slide film.
A camera with a wide-angle lens (18 to 35 mm in focal length) or a standard lens (50 to 55 mm, assuming a 35-mm-frame format) — the same one you use to take daytime landscape or family-vacation snapshots — is ideal for capturing large swaths of the summer Milky Way. These common lenses can reveal the glowing star clouds that traverse the constellation Cygnus and run down to the galaxy's dense central region in Sagittarius and Scorpius, which are prominent in July and August. While to the unaided eye the Milky Way looks like a dim, grayish band, a 5-minute exposure with off-the-shelf equipment will show that it's made up of countless stars laced with delicate wreaths of red nebulosity — the luminous clouds of hydrogen gas that old stars cast off into space and out of which new stars form. With the power of piggybacking photography you can create your own atlas of the Milky Way's bright clouds and dark dust lanes or compile a portfolio of constellation portraits.
Switching to a modest telephoto lens brings a new realm of targets within reach. You don't need very long focal lengths — something in the 85- to 135-mm range is ideal for framing large nebulas, bright star clusters, and the Milky Way star clouds. (Besides, that monster telephoto in your closet may be too heavy to mount securely on the telescope.) The North America Nebula in Cygnus, the Double Cluster in Perseus, the Pleiades star cluster in Taurus, and the Andromeda Galaxy are all suitable telephoto subjects. As a rule of thumb, any object you can see well in binoculars is a good target for a piggy-backed telephoto lens.
Drawbacks of Piggybacking
What piggyback photography can't handle are extreme close-ups of small "deep-sky" objects such as the Ring Nebula in Lyra, the Crab Nebula in Taurus, or the beautiful globular star cluster in Hercules. For these targets you'll need the light-gathering and magnifying power of a telescope (serving as your camera's lens) to obtain good views. That's beyond the scope of this article. Nevertheless, there's still a wide selection of subjects to capture with your simple piggy-back setup.
Film Versus Digital
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The film camera of choice is the traditional 35-mm SLR (single-lens reflex) model for simple astrophotography. If you've purchased such a camera in the last few years, chances are it uses battery power to keep the shutter open. Even a couple of 15-minute exposures in the cold night air can quickly drain the battery, shutting down the camera. In the past, astrophotographers opted for low-cost, no-frills SLRs with mechanical shutters that worked without batteries (for sky shooting you don't need the autoexposure and autofocus features). Classic mechanical models such as the Pentax K1000, Canon F-1, Nikon F2, Minolta SRT-101, and Olympus OM-1 are still available at used-camera stores and through online auctions such as eBay.
On the other hand, digicams are taking over just about every sector of celestial imaging. For piggybacking simple astrophotography, the best choices are today's digital SLRs. These "prosumer" cameras offer 6 to 11 million pixels (megapixels) of resolution and inter-changeable lenses, usually the same lenses that fit on older film SLRs. (Most compact point-and-shoot digicams, the kind with nonremovable lenses, generate too much electronic noise during exposures lasting more than a minute.) While they depend on batteries, digital SLRs can deliver one to two hours of long-exposure shooting before you need to recharge the battery.
Film die-hards scoff at digital SLRs — until they see the results appear on the camera's built-in LCD viewing screen as soon as the exposure is completed. Then they sell off their aging film gear. I know I did! Digicams are wonderful for on-the-spot checking of the framing, focusing, exposure time, and tracking. A lot can go wrong during even a 5-minute exposure. Having instant feedback allows instant fixes, thereby improving your best-to-blooper ratio. Admittedly, digital SLRs are still pricey (starting at $700 to $1,000), but their cost has been coming down steadily.
Getting Lined Up
For a good initial polar alignment, point the tube parallel to an equatorial mount's polar axis, look through the finderscope, and fine tune the altitude and azimuth to center Polaris.
Night Sky: Craig Michael Utter
Trailed stars are the number one flaw in piggybacked shots. This is caused by poor alignment of the telescope mount with the north celestial pole, the point in the sky near Polaris, the North Star, about which stars appear to rotate as Earth spins on its axis. To track stars accurately, you must use an equatorial-type mount and align it so that its polar axis aims as close to the celestial pole as possible.
While computerized "Go To" telescopes can also follow the stars, they do so by constantly moving two axes. Piggybacking a camera onto one of those scopes can result in badly trailed stars, since the field will rotate during the exposure. Optional wedges or tripods for popular Go To models, such as those from Meade and Celestron, allow them to be tipped over and polar aligned, which is essential for piggyback shooting.
How accurate does this polar alignment have to be for simple astrophotography? It depends on the focal length of the lens you're using. Simply aiming along the mount's polar axis by eye on Polaris, nearly 1° from the north celestial pole, might be good enough. Wide-angle lenses, which cover relatively large areas of the sky, can be forgiving of errors in polar alignment. Long telephoto lenses, on the other hand, magnify everything, including any tracking errors. So they demand greater accuracy when aligning the telescope's polar axis, to within a fraction of a degree of the true pole. Some equatorial mounts have polar-axis finderscopes built in, and these make accurate alignment much easier than with fork-mounted telescopes.
Even with precise polar alignment, however, stars can still appear to trail. The culprit here is the telescope's motor-drive mechanism, which might not be tracking smoothly or simply is not turning at the correct speed. In that case, the solution is to manually "guide" the telescope with the aid of a high-power eyepiece fitted with illuminated crosshairs. The trick is to select a moderately bright star to guide on and keep that star perfectly centered on the crosshairs throughout the exposure. Dual-axis drives (with a motor and push-button speed control on each telescope axis) greatly ease the guiding process and keep your shaky hands off the mount.
What Exposure Should You Use?
On a digital camera at f/4 and ISO 400, a 1-minute exposure (left) reveals the Orion Nebula, but 3- and 6-minute exposures record fainter details.
Piggybacking shots of the Milky Way demands dark, moonless skies. To record the most stars and nebulosity, the best exposure is usually the longest one you can take before skyglow from city lights starts to wash out the details. There's more to setting the exposure, however, than just opening the camera shutter. The lens's aperture (opening) also controls the amount of light reaching the film or digicam sensor. Opening the lens to its widest aperture (that is, the smallest f/number, typically f/2) lets in more light and keeps the exposure time to a minimum, while still picking up stars as faint and as numerous as your sky will allow. Opening the lens by one f/stop, from f/4 to f/2.8 or from f/2.8 to f/2, cuts your exposure time in half.
Another option for shortening exposure time is to use a "fast" (sensitive) film or to switch your digicam to a high ISO setting. A film rating or digicam setting of ISO 400 will require only half the exposure time of ISO 200. If you're shooting with slide film, you can request that your photo lab "push" the film when it's developed — for example, to ISO 800 if the film is ISO 400. This will increase the picture's contrast, but at the cost of a slight increase in grain (coarseness).
A computer can work wonders on astrophotos. The raw image at left shows the low contrast typical of mediocre sky conditions. At right, image processing has boosted the contrast and corrected the colors.
Short exposure times may record less detail, but they'll minimize the streaking of stars due to poor or imperfect tracking and avoid aircraft flying through your field of view. So is a high ISO and a wide-open lens the best combination? Not necessarily. Wide-open apertures can reveal optical flaws in the lens, producing bloated and distorted star images, especially at the corners of the frame. Stopping down the lens (decreasing its opening) by one full f/stop to f/2.8 or f/4 reduces lens flaws and sharpens star images, at the expense of longer exposures.
Faster ISO speeds also have their drawback. They introduce grain with film or electronic noise with digicams. Fuji's Provia 400F offers remarkably fine grain with fast speed and is my choice for all-around piggybacking film shoots. With digital cameras, try settings of ISO 400 or 800. Their electronic noise decreases if the ambient air temperature is chilly, such as on winter nights. It gets worse after the camera has been used to take several images and has warmed up, especially during summer. If that happens, turn off the camera for a few minutes to let it cool before resuming shooting.
Finally, film loses sensitivity over long exposures — a 20-minute exposure does not record twice as much light as a 10-minute exposure. By comparison, digicams maintain their full recording ability throughout an exposure. That's why a digital SLR can pick up in 3 minutes what it might take a film camera 9 to 12 minutes to record. That can make all the difference between images that are trailed and ones that aren't.
Secrets of Success
Don't be afraid to experiment with these simple astrophotography tips. Try a variety of exposure settings (a technique known as "bracketing"), and keep a record to help you determine the best combination of film or ISO rating, exposure time, and f/stop for your particular site.
Unlike shooting through a telescope, focusing a camera lens is usually easy — just turn the focus ring to the infinity setting. But many autofocus lenses can actually focus past infinity. Determining the lens's sharpest focus may be a trial-and-error process, involving shooting several frames at different focus settings. The instant feedback of digital cameras makes it easy to find the best focus, and it's worth taking the time to do so. I've found that even a slight shift of focus, by no more than the width of an index mark, can make the difference between stars appearing as pinpoints or as fuzzy blobs.
Everything went wrong with this astrophoto. The motor drive stalled and trailed the stars; the sky background was overexposed; the lens frosted over; and high clouds rolled in, making the bright stars hazy.
A lens aimed skyward for extended periods will likely have dew or frost form on its front surface. At home I use a small hair dryer to warm the lens and telescope. At remote sites, a 12-volt heater coil wrapped around the lens barrel (be careful it doesn't turn the focus or f/stop ring) provides enough constant warmth to ward off dew.
An open shutter seems to attract aircraft! Always have a dark cloth or opaque card handy to quickly cover the camera lens until the intruder flies away.
On warm summer nights you might find yourself shooting through a flickering swarm of fireflies. On cold nights batteries can lose power, shutting down your camera or telescope drive. Then there are the plain dumb mistakes, like tripping on the tripod leg, shooting the whole night with the lens stopped down to f/16, or shining a flashlight onto a lens to check for dew, only to realize in your late-night daze that the shutter is still open!
Don't get discouraged — getting it all right is part of the satisfaction of taking souvenir images of the night sky. Just beware: astrophotography can be addictive. But it can also be immensely rewarding.