Astronomy photography: what to know: it used to be tough. But now, modern cameras and telescopes have opened up astrophotography to everyone. Interested? Here's where to start.
That was then, this is now. Modern digital cameras are far more sensitive to dim light than film ever was. Cameras and their adjustments are way more versatile. Telescope mountings can work computerized magic. And once you've obtained a digital image, some click-the-button optimizing in software can bring out what's in the image better than the most skilled darkroom craftsman once could do with smelly chemicals.
So, what are you waiting for? Here's a step-by-step path from some easy start-outs to learning how to get really serious.
(1) Start With What You've Got.
It's a lovely evening and there's a beautiful sight in the west. In the deepening twilight, a thin crescent Moon poses with a bright planet. But maybe the only camera you own right now is a little point-and-shoot. For tricky scenes like this, you're supposed to have an expensive digital SLR, right?
Nope. We're going to start in the shallow end. That little digital camera that you used to bring to picnics and Thanksgiving family dinners (before you switched to your phone camera) can gain a new life capturing beautiful and instructive night-sky vistas. You'll get a feel for what's possible and perhaps become eager for more.
Many small digital cameras have a night mode, a continuous-fire shutter-release, manual exposure control, and manual focusing. These options give you the keys to some great skyscapes. Of course if you already have a more capable DSLR, all the better.
You'll need one other thing immediately: a tripod to hold the camera steady for long exposures. We've seen small, lightweight but adequate tripods at drug stores for $10 to $15.
First: forget the camera's auto-exposure setting. Whether for twilight sky scenes or nighttime constellation portraits, set the exposure time manually. Try exposures ranging from 1 to 30 seconds long, and see what you get. When photographing the sky, it's always good to take many different exposures. That way, at least one of them is likely capture the optimum depth and color. This is called bracketing exposures.
One of the more interesting forms of night-sky photography, and the simplest, is recording star trails. You keep the shutter open as long as possible to reveal the turning of the heavens. Your camera's maximum exposure time may be only 10 or 15 seconds, but with one simple hardware-store tool and computer freeware, you can extend that a lot.
This will be your introduction to two critical, basic astrophotography techniques: long exposures and image stacking.
You'll need a way to hold down the shutter button, so that the camera snaps one image after another without you touching and wiggling it. A small clamp with rubber grips, sold in hardware stores for a few dollars, does the trick.
See if your camera can be set to not display the image after each exposure. That'll avoid wasting battery power and enable you to take lots of images right in a row. And turn off the flash.
Frame your target area--the Big Dipper, say, or Cassiopeia over trees--using the camera's widest zoom angle. Include some foreground scenery to add interest and give the composition an everyday-world sense of scale.
To start your image series, set the camera for its maximum exposure length (or to "night mode" if you can't select a shutter speed). Set the ISO speed to 400, a good start-out compromise that's sensitive to dim light but not too noisy (grainy).
Next, set the lens focus at infinity (">). Select the continuous-shooting function, then attach the clamp to hold down the shutter button. Let it keep shooting for at least 10 minutes. Once you're done, you'll have dozens or even hundreds of modestly long exposures with a few stars visible in each.
Your next step is to download and install one of the free computer programs written for combining (stacking) star-trail frames. Startrails (www.startrails.de/html/software.html) and StarMax (ggrillot.free.fr) for PCs both work well and can accept files directly downloaded from your camera. Spend the time to read about them. These easy-to-use programs will automatically stack all your images into a final composition.
This project is just a start. Experiment! If your light pollution is so bright that it starts fogging the sky in even a 15-second exposure, try stacking three times as many 5-second exposures. Or, use an image-processing program on the final result to push down the brightness levels of the sky background while keeping all the brighter light levels where they are. Which works better? Hint: the earlier along the process chain you do a fix, the better the likely result. But try different things and get a feel for them all. You learn by doing.
Okay, now we're going to go swimming deeper.
Pretty crescent Moons and wide-field constellations are nice enough, but you've pushed your everyday camera as far as it will go. For targets such as galaxies, star clusters, and nebulae, you realize you need more equipment: a bigger, longer lens to collect more light and magnify more, a DSLR camera or even an astronomical CCD camera, and a motorized mount to track the moving sky.
You might think the next step right away is buying a telescope. But that's jumping the gun. The principal piece of equipment that every advancing astrophotographer needs is a good tracking equatorial mount. That's because the biggest hurdle we face is that our targets are constantly moving across the sky. Whether your interests lie in wide-field vistas of the Milky Way or close-up portraits of distant galaxies, you need to have your camera and telescope 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. This German equatorial mount has a shaft, called the polar axis, that you align parallel to Earth's axis of rotation. It turns at the same rate but in the opposite direction of Earth's rotation, cancelling the sky's apparent motion for the telescope mounted on it. There are also fork-design equatorial mounts and other varieties. The mechanical complexity of the equatorial mount allows the simplicity of following celestial objects by turning a single shaft with a simple motor at a constant rate. But that's not the only way.
Thanks to computer technology, many of today's motorized telescope mounts are the alt-azimuth design. Instead of moving celestial east-west and north-south like an equatorial, they simply move up and down (in altitude) and side to side (in azimuth). An alt-azimuth mount has mechanical advantages over equatorial ones, but it requires driving two axes at constantly changing rates to track the sky's motion. This, however, is child's play for computer-controlled motors.
Although both equatorial and alt-azimuth mounts will keep a telescope pointed at an object as it moves across the sky, the alt-azimuth design has severe limitations for astrophotographers. That's because an alt-azimuth mount makes the sky appear to rotate around the object it is tracking. There are workarounds, but most astrophotographers find it far easier just to use an equatorial mount.
Equatorial mounts good for astrophotography cost from a few hundred dollars well into five figures. You'll find quality mounts made by Astro-Physics, AstroSysteme Austria, Celestron, Explore Scientific, 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 buys you an equatorial mount with a heavier load capacity and/or greater mechanical precision. Advice: take the mount's rated load capacity with a grain of salt. Rather than put an 18-pound telescope assembly (including camera and all else) on a 20-pound-rated mount, you probably want a stronger mount, certainly for photography.
Figuring what you need in terms of 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: the amount that a telescope appears to waver 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. The shorter the focal length and the shorter your exposures, the more slop is allowable. But remember, astronomical telescopes are the ultimate in long "telephoto lenses," and deep-sky photography means the ultimate in slow shutter speeds.
"For many years I've used a German equatorial mount with a rather mediocre 28 arcseconds of periodic error," says Sky & Telescope senior editor Dennis di Cicco. "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."
Guiding used to be done by attaching a small telescope with a crosshair eyepiece (a "guidescope") onto the side of the photographic setup. You would center the crosshairs on a star close to your target, and during the exposure you continually tweaked slow-motion controls on the mount to keep the star perfectly on the crosshairs--for as long as an hour or two, without a moment's break. The process was tedious and mind-numbing, not to mention freezing on cold nights. Fortunately, digital technology has come to the rescue.
Today, most astrophotographers have replaced the guide-scope's eyepiece with a small, specialized digital camera called an autoguider that sends commands to the mount's drive to keep the guide star on virtual crosshairs. This piece of automation has truly brought long-exposure astrophotography into the 21st century. You can now set up a long series of long exposures to run almost unattended all night!
For astrophotography, an equatorial mount has to be aligned: set up with its polar axis parallel to Earth's axis. Most modern equatorial mounts for photography come with aids and instructions to help the process go without too much fuss.
Lenses and Telescopes
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.
A telescope used for astrophotography is nothing more than a big, long camera lens. The two most fundamental aspects of any lens or telescope are its focal length and its aperture. The focal length divided by the aperture is the focal ratio (the f/ number). In the world of conventional photography, lenses are described in terms of focal length and f/number, while in astronomy, telescopes are traditionally described by their aperture and f/number. But we're talking about the same things. For example, a 100-mm-aperture f/4 "telescope" produces the same images as a 400-mm-focal-length f/4 "telephoto lens." They only difference is that the "telescope" comes with an eyepiece holder, while the "camera lens" only comes with a camera adapter.
Many spectacular objects such as the Andromeda Galaxy, the North America Nebula, and the Pleiades appear relatively large, so they can be captured well with conventional 300- to 500-mm telephoto lenses (or the equivalent telescopes). It's the smaller galaxies, star clusters, planetary nebulae, and other such objects that are best photographed with the greater focal lengths of larger astronomical telescopes.
There is no all-purpose telescope for deep-sky photography. But, says di Cicco, "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 (i.e. not terribly expensive) equatorial mounts. They can also be reasonably portable and easy to set up, meaning they'll fit in a car and you can carry them around without being a weightlifter.
In a perfect world, deep-sky astrophotography would be done with high-performance CCD cameras designed specifically for long astronomical exposures. But these are expensive. Most people start out with a conventional DSLR that's also good for everyday photography, and that's not as much of a tradeoff as it used to be. The performance tradeoff that remains with a DSLR is balanced by its simpler and often more intuitive operation in the field (astronomical CCD cameras require a separate computer). If you already have a DSLR, this is absolutely the way to start out, and you may well stay with it forever.
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 chilling the image sensor with a thermoelectric cooler. Noise shows in images as bright specks and an overall grainy appearance, and it becomes more noticeable in the long exposures needed for dim scenes.
The best, most sensitive astronomical CCD cameras are monochrome (black-and-white). Color photography with these is done by shooting several images through different color filters and combining them with image-processing software.
Straddling the fence between DSLRs and high-performance, monochrome astronomical cameras are entry-level astronomical cameras. Most of these are cooled like their more expensive cousins, making them less noisy than DSLRs, but they often have smaller sensors. Some are based on the same sensors used in DSLRs and thus produce a color image with a single shot. 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, a major component of glowing 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 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). And of course, a DSLR is good for all your other photographic needs.
If you're interested in flat-out performance, then a quality, monochrome, cooled astronomical camera is the clear choice. Prices have come down a lot but still start at around $500 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 pricey for larger filters and computer-controlled wheels. The major North American manufacturers of high-end cameras include Apogee Instruments, Celestron, Finger Lakes Instrumentation, Orion, Quantum Scientific Imaging, and Santa Barbara Instrument Group.
Ansel Adams once said that a photographic negative is like a composer's score, while a print is the musician's performance of it. The modern equivalent of fine printmaking is computer image processing.
There's more to this than fiddling with Adobe Photoshop (or its lower-cost equivalents); that's just the final step. Most first-rate astro images were also processed in their early stages with programs optimized for the special demands of astronomical imaging. All the major astronomical image-processing programs have websites, including Astroart, CCDSoft, CCDStack, DeepSkyStacker, ImagesPlus, MaxIm DL, Nebulosity (Mac computers only), and PixInsight. There are also several up-to-date books that cover image processing in detail; you'll find an excellent selection from introductory 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 back when film ruled the world.
Imaging the Moon and Planets
And now for something completely different.
The challenge that deep-sky objects post for astrophotographers (visual observers too) is that they are dim. Small size is not the problem; many are quite large. The planets pose the opposite challenge. They're brilliant--lit by full sunlight!--but tiny, just arcseconds wide. The details you want to see and record on them are even tinier. Even on the great big Moon, the details you want to record--little craterlets, clefts, mountain crags, everything that gives the landscape texture and realism--are right at the limits of what Earthbound equipment can resolve. The problem is less in collecting dim light and more in seeing sharply through the quivering fuzziness of Earth's unsteady atmosphere.
So, planetary imaging methods are completely different.
One method has blown away every other. It's enabling amateurs to take portraits of the planets that used to be unimaginable at professional observatories. The idea is to use a video camera to take thousands of short exposures for a few minutes and feed them to a computer. Special software looks at them, throws out all but the sharpest ones--those that caught a lucky fraction of a second between atmospheric tremors--and then stacks those so that their noise and errors are reduced and consistent, real features emerge. Standard digital processing can then render the weakest real features plain. The results not only far surpass film photography, but beat by a mile what your eye can see though the same telescope.
And it's cheap. For $100 or less you can now get a specialized astronomical video camera designed for planetary imaging, with the necessary software included. Entry-level planetary video cameras include Orion's StarShoot USB Eyepiece Camera ($70), Orion's better StarShoot Solar System Color Imaging Camera IV ($100), and Celestron's more capable NexImage 5 Solar System Imager ($200).
High-end astro videocams, capable among other things of capturing 60 frames per second and faster, are offered by various companies for prices reaching up to $1,000 or more.
You do need a fairly large telescope, the larger the better. A telescope's resolution--sharpness of vision--is fundamentally limited by its aperture (the diameter of its main lens or mirror). There's no getting around the aperture limit, no matter how well a telescope is designed and made. A 6-inch scope is considered the bare minimum for planetary imaging. A 10or 12-inch is much better.
At least the mount doesn't need a top-quality drive with accurate tracking. An equatorial mount is preferable (tracking alt-azimuth mounts have that image-rotation problem), but you hardly need good tracking for fraction-of-a-second exposures! It just has to be good enough to keep the planet somewhere on the tiny imaging chip for several minutes. If the planet wanders around the frame during that time? Who cares? The stacking software will align ("register") all the images correctly.
Magnification and Moving Targets
Because planets are small, you need high magnification. Even Jupiter never appears bigger than 50 arcseconds wide, the size of a soccer ball 3A mile (1.2 km) away. You're trying to image the printing on the soccer ball. Accordingly, planetary imagers generally use a Barlow lens or tele-extender to increase the telescope's focal length and magnification.
Another way to amplify your image is to shoot with an eyepiece in place, a technique known as eyepiece projection photography. Although this can take advantage of eyepieces you already own, you'll need additional adapters to connect your camera close enough to the eyepiece to come to focus.
There's an option you may already own. Most consumer digital cameras have a video mode. With a DSLR, you can acquire an adapter to fit it into your eyepiece holder, assuming you can move the focuser in far enough to reach focus. If not, or if you have a little point-and-shoot, you'll need to use the eyepiece-projection method. The movie-file formats of some consumer cameras aren't compatible with some planetary image-processing software. If so, no problem. Convert formats with free software such as VirtualDub (virtualdub.org).
An electric focuser for the telescope is highly desirable. Touching the telescope's focusing knob at high magnification wiggles the scope, making it hard to see when it's truly in focus. This matters a lot, because the feedback loop between hand, telescope, and eye is much slower when there's an imaging system in the loop.
Astro video cameras such as those mentioned from Celestron and Orion come with their own software for camera control and processing. Other options include the PC programs RegiStax (www.astronomie.be/registax), K3CCDTools (pk3.org/Astro/index.htm?k3ccdtools.htm), andAviStack (avistack.de). Mac users should check out Keith's Image Stacker (keithwiley.com/software/keithsImageStacker.shtml) or AutoStakkert! (autostakkert.com). All are cheap or free.
In planetary photography, there is no substitute for practice and working patiently up the learning curve, which can be long if you want fine results. But a few important tips are easy:
* As with visual observing, give your telescope time to cool to the outdoor temperature. It may take an hour or more for a telescope stored indoors to become free of image-blurring thermal air currents.
* If you're using a reflector or a compound telescope, check its collimation (optical alignment) before shooting, and touch it up if needed. Even slightly misaligned optics will seriously degrade your results on the tiny, finicky planets.
* Avoid shooting over roofs or warm asphalt parking lots. They give off plumes of image-muddying thermal currents.
* Don't overexpose your videos--there should be no white, blown-out regions, because you lose whatever information is there. Any sharpening by your post-processing software will increase that area. There's often subtle detail in the brightest areas of the Moon, or the polar caps of Mars, that can be revealed if you take care to avoid overexposure. Most webcam-capture software includes a tool to monitor the brightness levels of your video.
* Practice, practice, practice. Like everywhere else in astronomy, patience is an even bigger virtue than it is in the rest of life. Just like visual observing, you learn more about your equipment's capabilities the more time you spend using it with care and attention.
Perhaps the best thing about planetary imaging is that it's impervious to light pollution! The Moon and major planets blaze right through the skies of the most light-polluted cities in the world.
With practice, growing skill, and equipment upgrades when you're ready for them, your images of the planets can become truly world-class.
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|Title Annotation:||Shoot the Sky|
|Date:||Jan 1, 2013|
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