As long as people have been gazing at the sky, they've been making records of what they saw. Every medium has been used, from smooth faces of rocks to sketchbooks. Cameras and film revolutionized astronomical record-keeping beginning in the late 1800s, and photography remained king of all media for a century.
You may find this hard to believe, but for recording the Sun, Moon, and planets, these venerable tools all pale in comparison to the video cameras found in millions of today's households. No static drawing or photograph can rival the ineffable sense of reality and the emotional impact of a dynamic video recording. Richard Berry, one of the pioneers of electronic imaging for amateur astronomers, summed it up: "Seeing a video recording isn't experiencing the real thing, but it's the next best thing. Although it is impossible to reproduce the effect of a live video in the pages of a magazine, it is utterly wonderful to see the Moon, the planets, or the Sun on your television looking as good as they ever appear in a high-power eyepiece."
Each minute of videotape contains 1,800 discrete images, a number that dwarfs what even the most nimble photographer can achieve by tripping the shutter and winding the film-advance knob of a conventional camera! And just like with today's ever-capable digital cameras, video avoids the need to wait for film to be developed and printed to see your results.
The image sensor in a modern camcorder is a tiny wafer of silicon called a charge-coupled device (or CCD), an electronic chip far more light sensitive than the crystals of silver salts in photographic film. Unlike the mechanical shutter in a conventional camera, however, the electronics in a camcorder usually limit exposures to 1/60 second or less. Consequently, only the brightest astronomical objects make suitable targets: the Moon, the planets, and the Sun (with proper filtration, of course!). On the other hand, a video camera's short exposures can overcome many of the distorting effects of the Earth's turbulent atmosphere. Video recordings can effortlessly capture details that appear during fleeting moments of good seeing.
The videographer has an insuperable advantage over the still photographer using film to capture fine details on the planets. Recording the colorful cloud belts of Jupiter requires an exposure of 2 or 3 seconds on a relatively slow, fine-grained film, while more distant and dimmer Saturn requires exposures of 4 to 8 seconds. But Earth's turbulent atmosphere limits the duration of perfect images to fleeting moments — typically only fractions of a second. That's why professional astronomers are investing small fortunes to install adaptive optics systems on their telescopes that actually compensate for atmospheric seeing. With its ability to record 30 discrete images every second, video can be considered as "the poor man's adaptive optics."
Stills of the Night
"The moving finger writes and, having writ, moves on," lamented Persian poet Omar Khayyám. Both the visual observer and the still photographer must maintain an uninterrupted vigil so as not to irretrievably miss the all-too-rare moments when the air steadies and delicate details flash out in what legendary observer Percival Lowell called "revelation peeps." But a video recording captures an entire observing session, preserving those fleeting lucid moments like insects caught in amber.
Unlike 35-mm single-lens reflex (SLR) cameras, only a handful of the most expensive camcorders are equipped with removable lenses that would allow easy attachment to a telescope (using special adapters). Camcorders with permanent lenses require you to use what's called the afocal system. Here's how it works. The rays of light from a very distant object emerge from a telescope's eyepiece in parallel bundles. You bring this parallel light to focus on the camcorder's CCD by holding the camera's lens up to the telescope eyepiece. It's important to first have the telescope accurately focused, and this can be done by eye as long as you either have 20/20 vision or wear glasses that correct any near-or farsightedness to 20/20.
Sound complicated? Don't worry: most people find that focusing a telescope for afocal work is easy, especially since the real-time video image is visible in the camera’s viewfinder.
The Moon is the biggest and brightest object in the night sky. Odds are that it was the first object you looked at through your telescope, and it's the best subject for learning the ropes of video astronomy. After you've honed your skills videotaping the Moon for a night or two, you'll be ready to try your luck with the planets. The waxing and waning phases of Venus, the seasonal advance and retreat of the polar caps of Mars, the ever-changing features of Jupiter's cloud belts and four bright Galilean moons, and Saturn's magnificent ring system are all within the grasp of your backyard telescope and a camcorder.
Combined with typical amateur telescopes, even the most affordable camcorders are capable of capturing dramatic lunar sequences. Examples include the spires of shadows cast by mountains towering above plains of frozen lava and the rims of craters flashing into view as they catch the first rays of the morning Sun.
Set up your telescope on an evening when the Moon is between the crescent and gibbous phases, so that deep shadows throw lunar topography into bold relief near the Moon's day-night terminator. Select a low-power eyepiece (the one with the largest number printed on it), point your telescope at the Moon, and bring the image into focus.
Now manually set the focus of your camcorder's lens to the infinity position. If your camcorder has an autofocus feature, turn it off; otherwise it may continue to hunt for focus while you stand there exasperated. Carefully bring your camcorder up to the eyepiece of the telescope, taking pains to center its lens directly over the eyepiece while keeping it parallel to the emerging beam of light. To avoid jarring the telescope, don't let the camcorder touch the eyepiece. A small gap between the camcorder's lens and the eyepiece will not affect the image.
If the image in the camcorder's viewfinder appears slightly out of focus, tweak the telescope's focusing knob until the lunar landscape snaps into crisp view. Depending on your telescope's eyepiece and the camcorder's lens, the viewfinder may show a diffuse, dark circle surrounding the brightly illuminated lunar image. Since you'll want the image to fill the frame, adjust the zoom setting of the camcorder's lens until this effect — called vignetting — disappears. The zoom lens also provides a very convenient way to increase magnification without changing eyepieces.
If your camcorder is equipped with a digital zoom, don't use it. This dubious "feature" actually creates a lower-resolution picture because the camera uses fewer picture elements (pixels) of the CCD to fill the field. Used in conjunction with an eyepiece of moderately long focal length (15 to 25 mm), the optical zoom of most camcorders will provide plenty of magnification for afocal imaging.
The more you enlarge the image, the dimmer it will become, so experiment with the camcorder's zoom and exposure controls until you achieve an acceptable compromise between magnification and image brightness. Technically speaking, dim images have a poor (or low) signal-to-noise ratio that gives them an objectionably grainy appearance — a look appropriately called "snow." The definition and sharpness of your video recordings will not be determined solely by the optical quality of your telescope and the steadiness of the atmosphere. Older VHS or 8-mm camcorders record only 240 to 250 lines of horizontal video resolution. The S-VHS and Hi8 formats represent a marked improvement of 400 to 420 lines of resolution, while today’s popular digital video (DV) approaches 500 lines. High-definition (HD) cameras have 1,080 lines.
Another way to get a view of the heavens onto video is to use special electronic eyepieces in which the lens you would normally look through has been replaced with a silcon chip. Meade Instrument’s Electronic Eyepiece ($70) features 320-by-240-pixel, black-and-white video that is fed through a cable directly to the video input of a TV or video recorder. The camera is powered by an internal 9-volt battery and is available from Meade retailers.
Barska makes a similar eyepiece that captures a 640-by-480-pixel color image and sends the signal through a cable that plugs into your computer via a USB connection, which also powers the eyepiece. The Digi-Eyepiece ($65) comes with Windows software to display, edit, and save the movies.
Used in conjunction with a TV, these products are great for showing groups of people the Moon and planets. But with limited ways to change exposure settings manually, they aren’t as capable as other video recorders.
You’ll Need Support
After your first videotaping session, fatigued muscles will probably make you wish for another way to keep a camcorder at the eyepiece. Try mounting the camcorder on an ordinary camera tripod. With small equatorially mounted refractors and compound telescopes like Schmidt-Cassegrains and Maksutov-Cassegrains, the movement of the eyepiece as the telescope tracks the sky's motion will be modest relative to a stationary tripod-mounted camcorder.
Supporting the camcorder on a separate tripod is more difficult with larger refractors and Newtonian reflectors because their eyepieces move more as these telescopes pivot. You'll be forced to reposition the camcorder and its tripod frequently, so it's better to attach the camcorder to the telescope and let it go along for the ride. Brackets for this purpose can be purchased from astronomy retailers or fashioned from wood, plastic, or aluminum.
Once you devise a way to attach your camcorder to your telescope, one additional challenge will remain. Despite remarkable advances in miniaturization, a camcorder's videotape mechanism, viewfinder, and battery can weigh several pounds, so you'll need to counterbalance that extra weight carefully. Many amateurs have found a handy solution at their local sporting-goods store in the form of jogger's ankle weights, which can be attached to a telescope using strips of Velcro.
Supporting your camcorder on a tripod or coupling it directly to your telescope will also let you use the remote controls that come with many camcorders. These accessories are a real boon because they let you adjust the camcorder's zoom lens and exposure settings without touching the camera and jiggling the highly magnified image.
To make prints of scenes from your videotapes, you might imagine that simply employing the camcorder's freeze-frame mode while playing back the video on your TV will show a nice image, and perhaps you can take a photograph of your TV screen. Unfortunately, if you're using an analog camera (VHS or Hi8 formats) this will only teach you some harsh lessons about the effects of electronic noise and the physiology of human vision.
When a videotape is played back, 30 discrete images (usually called frames) are displayed every second. Your eye and brain integrate them as a seamless view. At a rate of 30 frames per second, phenomenon called flicker fusion causes the sequence to appear continuous. Your eye-brain combination fills in between the frames and also averages the noise (snow) of individual frames, creating the perception of a vivid moving picture. But in the freeze-frame mode of analog video, the display will invariably suffer from a poor signal-to-noise ratio and have a grainy salt-and-pepper look reminiscent of an overenlarged photograph. To add insult to injury, successive frames will almost invariably exhibit a phenomenon called image excursion — small, erratic displacements caused by atmospheric turbulence. When examined frame by frame, a videotape of a planet that appears stationary and well-defined in the "play" mode will reveal contortions reminiscent of an amoeba under a microscope.
Using a DV-format camcorder avoids some of these problems, because digitally captured video images don't suffer from the "blurry" effects of analog recording. Nevertheless, you’re still at the mercy of our unstable atmosphere. To smooth out the noise and incrementally displaced images, it's necessary to combine dozens or even hundreds of the sharpest frames,a process called "stacking." This was a very laborious, time-consuming task before the advent of software capable of automatically detecting the best frames and precisely superimposing them to create a composite image.
A very popular program for this is RegiStax, the brainchild of Dutch amateur astronomer Cor Berrevoets. This free software for Windows is accompanied by an excellent tutorial. Macintosh users can perform similar processing with Keith's Image Stacker, shareware by Keith Wiley. And another powerful program is AstroStack ($39) by Robert Stekelenburg, which will work on a variety of computer platforms.
Of course, to use these programs, you must first get the video onto your computer. There are several ways to accomplish this, thanks largely to the prevalence of software to edit home videos and burn DVDs. Many video cameras now come equipped with ports and cables that will send video output to a computer's USB or FireWire input. The movie-making software will then capture the video and save it in a file format for video editing, usually AVI or QuickTime.
If you have an older analog camera, you can install a television card in your computer. These cards have TV tuners that allow you to watch broadcasts on your computer monitor (and sometimes record them to disk). Many also feature inputs to connect VCRs or other video sources, allowing analog signals to be converted to digital video. While intended for people to turn their home videos into DVDs, you can use it to import your astro video and run it through image-stacking software.
The result of stacking will be a single picture that you can save to your computer. Then make a copy on your printer or at a local photo-service center to show your family and friends just how good a planetary imager you are!
A valuable book that delves into the digital details of astrovideography is the recently updated Video Astronomy by Steve Massey, Thomas A. Dobbins, and Eric J. Douglass ($24.95 from Sky Publishing), which further illustrates the techniques in this article and much more.
Camcorder-to-telescope adapter brackets are available from:
11969 Livonia Lane
Redding, CA 96003
Orion Telescope & Binoculars
P.O. Box 1815
Santa Cruz, CA 95061
800-447-1001 or 831-763-7000
As with any realm of consumer electronics, the equipment, tools, and techniques change quickly. Join the VideoAstro e-mail discussion group to remain up to date with the latest trends in video imaging. The site offers valuable infor-mation on video equipment and techniques as well as an excellent gallery of amateur video images.