Deep-sky astrophotography primer: every clear night backyard observers are turning out images of the cosmos that let our imaginations soar. Here's what it takes to join them.
SAY THE WORD ASTROPHOTOGRAPHY to even casual amateur astronomers, and chances are they'll conjure up scenes of whirling galaxies and colorful nebulae, for it's these iconic deep-sky images that let our imaginations sail across the sea of cosmic wonder. Not long ago the finest deep-sky photographs came from professional telescopes at mountaintop observatories. But digital photography has leveled the playing field, and now many of the most breathtaking images come from the telescopes and cameras of backyard observers. And, as mentioned on page 34, digital photography has also made it easier than ever for anyone to try shooting their own pictures of nebulae, star clusters, and galaxies.
Perhaps you've dabbled with simple astrophotography techniques like those described on page 36 and are looking to move on to the next stage. Or perhaps you want to leap right into deep-sky photography (plenty of people have). So the question is how do you get started?
Many beginning deep-sky photographers ask what telescope they need. But that's jumping the gun. The principal piece of equipment that every advancing astrophotographer needs is a tracking equatorial mount. That's because the biggest hurdle we face is that our targets are constantly moving across the sky. Whether our interests lie in capturing wide-field vistas of the Milky Way or close-up portraits of distant galaxies, we need our cameras and telescopes mounted on a platform that can accurately track the sky's east-to-west motion.
Years ago all tracking mounts were some variant of the equatorial design pioneered by the German instrument maker Joseph Fraunhofer in 1824. These have a shaft, called the polar axis, aligned parallel to Earth's axis of rotation. It turns at the same rate but in the opposite direction of Earth's rotation, thus effectively cancelling the sky's apparent motion for any telescope mounted on it. The mechanical complexities of the equatorial mount (and there are many) are weighed against the simplicity of following celestial objects by turning a single polar shaft at a constant rate. But it's not the only way to track objects.
Thanks to computer technology, many of today's motorized telescope mounts are based on an altazimuth design. They have mechanical advantages over equatorial designs, but they require driving two axes at constantly varying rates (child's play for motors controlled by a computer chip) in order to track the sky's motion. While both equatorial and altazimuth mounts will keep a telescope pointed at an object as it moves across the sky, the altazimuth design has severe limitations for astrophotographers. That's because an altazimuth mount makes the sky appear to rotate around the object it is tracking. There are workarounds for this problem, but the majority of astrophotographers find it far easier to just use an equatorial mount.
Equatorial mounts suitable for astrophotography are priced from a few hundred dollars to well into the five-figure range. In the North American market you'll find quality mounts made by Astro-Physics, Celestron, iOptron, Losmandy, Meade, Mountain Instruments, Orion Telescopes & Binoculars, SkyWatcher, Software Bisque, Takahashi, and Vixen, to name the major players.
Apart from optional features such as computerized Go-To pointing, more money generally gets you an equatorial mount with a higher load capacity and/or greater mechanical precision. Figuring what you need for load capacity is straightforward; you don't want to put a 20-pound telescope on a mount designed to hold only 10 pounds. And even when you are within the weight limit, the stability of a mount can be compromised as you get close to its maximum capacity. There's no penalty for having a mount that's bigger than required.
Figuring what you need for mechanical precision is a bit more complicated. Even the most accurate gears have imperfections that cause small variations in the rotation rate of the mount's polar axis. Called periodic error, this departure from a theoretically perfect drive is typically specified in arcseconds, and it gives the angular amount that a telescope appears to wobble around the point it is tracking. Good mounts today have periodic errors smaller than 20 arcseconds; and those approaching 5 arcseconds or smaller are considered excellent.
How much periodic error you can tolerate depends on the focal length of the lens or telescope you're using, as well as the length of your exposures. In simple terms, the situation is akin to conventional picture shooting with a telephoto lens; your hands shake regardless of the lens on your camera, but the motion is more obvious and likely to blur images when you're shooting with a telephoto and slow shutter speeds. Astronomical telescopes and time exposures are the ultimate in "long" telephotos and slow shutter speeds.
For many years I've used a German equatorial mount that has a rather mediocre 28 arcseconds of periodic error. Working with camera lenses up to about 180-mm focal length, I can shoot exposures many minutes long that show acceptably round stars. Longer telephoto lenses and astronomical telescopes, however, magnify the effects of the mount's periodic error and show star images that appear elongated. To solve this problem I have to guide the mount.
Traditionally, guiding was done by placing a small telescope (a guidescope) with a crosshair eyepiece alongside the photographic setup. You would center the crosshairs on your target or a star close to it, and during the exposure you used slow-motion controls on the mount to keep the star perfectly on the crosshair. The process was tedious and mind numbing. Fortunately, digital technology has come to the rescue.
Today, most astrophotographers have replaced the guidescope's eyepiece with a small digital camera that issues commands to the mount's drive to keep the guide star on virtual crosshairs. There are several variations on this theme of autoguiding. For example, some of the astronomical CCD cameras made by Santa Barbara Instrument Group (SBIG, for short) have a pair of digital sensors--one that records the image, and another, smaller one that monitors a guide star.
For astrophotography, an equatorial mount has to be set up with its polar axis parallel to Earth's axis. The process is called polar alignment, and there are a variety of ways to do it accurately, all of which are too involved to cover in detail here. Many books on astrophotography describe the process, but there's also good material suited to beginning astrophotographers in the third edition of The Backyard Astronomer's Guide by Terence Dickinson and Alan Dyer (2008, Firefly Books). As a bonus, this book's astrophotography section is one of the best and most up to date to appear in a general compendium on amateur astronomy. I highly recommend it.
Once you have a tracking equatorial mount with slow-motion controls set up and polar aligned, the astrophotography world is your oyster. With a suitable camera and lens attached to it, you can tackle just about every astrophotography project imaginable.
While they may seem like very different things, a telescope used for astrophotography is merely acting as a lens for your camera. Indeed, it's a somewhat fuzzy line that separates some small telescopes from traditional camera lenses.
The two most fundamental aspects of a lens or telescope are its focal length and aperture, which together determine its focal ratio (called f/number or f/stop by photographers). Mathematically, the f/ratio is simply the focal length divided by the aperture. In the world of conventional photography, lenses are described in terms of focal length and f/ratio, while in astronomy, telescopes are classed by their aperture and f/ratio. For example, I have a Pentax 4-inch (100-mm) aperture f/4 telescope designed mainly for astrophotography. It has a focal length of 400 mm. It produces the same images as the company's 400mm f/4 telephoto lens made for medium-format film cameras.
Since the focal length of the lens or telescope determines the field of view recorded by a given camera, this is usually the parameter of most interest to astrophotographers. Because many spectacular objects such as the Andromeda Galaxy, North America Nebula, and Pleiades star cluster are relatively large, they can be captured with conventional 300- to 500-mm telephoto lenses. It's the smaller star clusters, planetary nebulae, and galaxies that are best photographed with the greater focal lengths available with telescopes.
Conventional wisdom from the days of film holds that telescopes for deep-sky astrophotography should be about f/5 or faster. This remains good advice in the digital age, but because digital chips are more sensitive than film, somewhat longer f/ratios are also viable now.
There is no all-purpose telescope for deep-sky photography, but if I had to pick one that can do a lot, I'd choose something with about 6 to 8 inches of aperture and a focal ratio of f/4 to f/8. In addition to being reasonably priced, scopes this size work well with many mid-range (think modestly priced) equatorial mounts. They can also be highly portable and easy to set up.
In a perfect world, deep-sky astrophotography would be done with high-performance CCD cameras that are designed specifically for long astronomical exposures. And while it's true that most of today's elite astrophotographers use such cameras, most beginners start out with conventional DSLRs. The tradeoff in performance that comes with a DSLR is balanced by its simpler and often more intuitive operation in the field (astronomical cameras require a separate computer). Then too, performance is relative. Even today's run-of-the-mill DSLRs produce images that exceed the best deep-sky photographs from the film age.
Internally, the principal difference between an astronomical CCD camera and a DSLR is that astronomical cameras are optimized to reduce "noise" in long exposures. This is usually done by reducing the temperature of the image sensor with a thermoelectric cooler. Noise manifests itself as bright specks and an overall grainy appearance in images, and it becomes more noticeable in the longer exposures needed for dim subjects.
For purely technical reasons having to do with the color-filter array incorporated in the sensors of cameras that produce "one-shot" color pictures, the most sensitive cameras are monochrome--in other words, they shoot only black-and-white images. To create a color image with these cameras, astrophotographers shoot several images through different color filters and combine them with image-processing software.
Straddling the fence between DSLRs and high-performance, monochrome astronomical cameras are entry-level astronomical cameras. Most of them are cooled like their more expensive cousins, making them less noisy than DSLRs, but they often have smaller sensors. Some of them are based on the same sensors used in DSLRs and thus produce a color image with a single exposure. But it's more than just cooling that sets them apart from DSLRs. These entry-level astronomical cameras have been modified to make them far more sensitive to the deep-red wavelength of hydrogen-alpha light, which is a major component of emission nebulae. Like their high-performance counterparts, they require a separate computer, and their overall operation is much the same. The biggest difference is that models with color sensors don't need filters and multiple exposures to create color images.
Because entry-level astronomical cameras are often priced competitively with higher-end DSLRs, the deciding factors when purchasing a camera primarily for astrophotography are usually whether you're interested in the best performance (astronomical cameras win in this category) or you want to avoid using a computer in the field (DSLRs win here). Orion Telescopes & Binoculars has recently introduced several entry-level astronomical cameras that are setting new standards for price and performance. You'll also find advertisements and product reviews for many cameras in Sky & Telescope.
If you're interested in flat-out performance, then a quality, monochrome,
cooled astronomical camera is the clear choice. Prices have come down significantly in the last few years, but still start at around $2,000 and head into the stratosphere. Color filters and a manually operated filter wheel start around $500 for small filters (suitable for cameras with small sensors) and also can get rather pricy for larger filters and computer-controlled wheels. The major North American manufacturers of high-end cameras include Apogee Instruments, Finger Lakes Instrumentation, Quantum Scientific Imaging, and Santa Barbara Instrument Group.
Ansel Adams once said that a photographic negative is comparable to a composer's score and the print to a performance. The naturalistic appearance of Adams's famous landscapes was achieved by hours of careful manipulation in the darkroom. Here, too, technology has come to the photographer's aid, with image-processing software replacing enlarging lenses, masks, and smelly chemicals.
The final touches on most of the deep-sky images found in magazines and on the web were done with conventional image-processing programs such as Adobe Photoshop. But in early stages, most were also processed with programs optimized for the special demands of astronomical imaging. Whole books exist on this subject, and even the basics are too detailed to cover here. All of the major astronomical image-processing programs have websites, including Astroart, CCDSoft, CCDStack, DeepSkyStacker, ImagesPlus, MaxIm DL, Nebulosity (Mac computers only), and PixInsight. Any Internet search engine will locate the websites if you type in the software name. There are also several up-to-date books that cover image processing in detail. You'll find an excellent selection ranging from introductory level to advanced published by Willmann-Bell (www.willbell.com).
Armed with a good equatorial mount, a small telescope, and a DSLR camera, today's beginning digital astrophotographers can soon be turning out deep-sky images to rival some of the best ever made when film ruled the world. It's a great way to get started in a hobby that can last a lifetime.
* The most important piece of equipment that astrophotographers need to move into the exciting field of deep-sky imaging is a tracking equatorial mount that allows their cameras and telescopes to follow the sky's apparent motion during long exposures. The German equatorial style of mount, shown here carrying a DSLR camera and telephoto lens, is especially popular with astrophotographers.
* Many of today's telescopes have motor-driven altazimuth mounts that keep them pointed at celestial objects as they move from east to west across the sky. The iOptron MiniTower seen here is a popular altazimuth mount that can carry a wide range of instruments. While altazimuth mounts are good for "snapshots" of the Sun, Moon, and planets, as explained in the accompanying text, they are not suitable for long-exposure astrophotography.
* You can spend a king's ransom on deep-sky astrophotography equipment, but a far more modest investment will allow you to produce excellent images. For example, the shot of the Orion Nebula on the opposite page was captured with this setup. Chinese clones of the venerable Vixen Great Polaris German equatorial mount pictured here are available for around $400, and the Astro-Tech 6-inch f/9 Ritchey-Chretien astrograph retails for $795.
* It's a fuzzy line that separates telephoto lenses and some small telescopes. The author's 4-inch f/4 Pentax refractor is specially designed for astrophotography, and it's seen here with a high-performance Apogee CCD camera. But the refractor's optics are the same as those in the 400-mm f/4 Pentax telephoto lens made for medium-format film cameras. The telescope next to the lens is a guidescope fitted with an autoguiding camera.
SkyWatch senior editor Dennis di Cicco began shooting pictures of the night sky in the early 1960s as a teenager. His reviews of astronomical equipment, especially astrophotography gear, frequently appear in Sky & Telescope.
|Printer friendly Cite/link Email Feedback|
|Author:||di Cicco, Dennis|
|Date:||Jan 1, 2010|
|Previous Article:||Capturing the solar system: recording impressive portraits of the Moon and planets is surprisingly simple.|
|Next Article:||Finding your way: here's how to use our monthly sky charts to identify the evening stars and constellations.|