Capturing the solar system: recording impressive portraits of the Moon and planets is surprisingly simple.
IT'S OFTEN SAID that we live in a golden age of amateur astronomy. Large, high-quality telescopes are readily available, and for some purposes, these instruments rival the capabilities of professional observatories. Indeed, it's not uncommon to see amateur photographs of the Moon and planets that are surpassed only by interplanetary space probes and the Hubble Space Telescope. The biggest surprise, however, is how easy and inexpensive it is to take these amazing images. If you can see the planets from your location, you can shoot them.
The best tool to record the planets through a telescope is the computer webcam. Shortly after these tiny cameras were popularized for video conferencing in the late 1990s, amateur astronomers saw their potential to record digital videos of the planets. These pioneering astrophotographers hoped that by capturing thousands of frames in rapid succession to a computer, they would be able to select the moments when the planets were least affected by atmospheric turbulence. Boy, were they right! Soon webcam imagers were routinely producing planetary images better than the finest ground-based photos from the pre-digital age.
Today, specialized planetary webcams are available for as little as $100, opening up the possibility that you can join the ranks of these elite imagers.
Which scope is best?
The best planetary images are captured through telescopes with at least 10 inches of aperture, but smaller instruments can take great shots, too. Whether you own a refractor, reflector, or compound telescope, there's plenty of detail within your reach. (If you're not familiar with these terms, read "What to Know Before You Buy" on page 22.)
Refractors are simple and rugged, but they're perhaps the least suitable design for planetary imaging. Achromats, the most inexpensive models, produce a bluish-purple halo of unfocused light around bright subjects such as the Moon and planets. This effect can be minimized with the use of a specialized minus-violet filter that will block most of the unfocused light, resulting in sharper images with less of a noticeable halo.
Apochromats are free of the halo issue, but big ones are extremely expensive. Also, refractors tend to be quite long and are not very portable in apertures of 8 inches and more. So if you're considering serious planetary astrophotography, your best choices are reflectors and compound telescopes for the same reason that professional observatories use them--they gather lots of light, yet they're short, easy to mount, and relatively cheap to make.
Magnification and Moving Targets
Due to the small angular size of the planets, high magnifications are necessary to record significant detail on them. Even Jupiter, the largest planet, never appears bigger than 50 arc-seconds--the size of a soccer ball 3A of a mile away. Consider the purchase of a good Barlow lens or tele-extender to increase your telescope's magnification.
Another way to amplify your image is to shoot with an eyepiece in place, a technique known as eyepiece projection photography. While this technique can take advantage of eyepieces you may already own, you'll need additional adapters to connect your camera close enough to the eyepiece to come to focus.
Regardless of how you choose to increase the image scale, you'll find that a good tracking mount of some form is necessary to keep your target on your camera's tiny detector. Fortunately, because you'll be recording many video frames per second, you don't need the same kind of accuracy that's essential for multiminute deep-sky exposures (see page 48). Most of the popular planetary image-processing programs automatically align your frames, so as long as you can keep your target on the chip, you can even get away with strong wind gusts wobbling your scope!
A telescope on an equatorial mount is preferable. Altazimuth mounts, which are aligned with the ground rather than Earth's rotational axis, introduce rotation in your movie clip, which may lead to problems when stacking the resulting video. A mount with electronic correction controls for both axes is highly desirable, because you'll want to adjust the position of your target on the camera while you're shooting. And an electronic focuser is highly desirable; just touching your telescope at high magnifications will induce vibrations, making it very difficult to see when it's truly in focus.
Your Digital Eyepiece
Choosing your camera can be just as difficult as choosing your scope. Many options are available today, depending on your level of interest. The original webcam that started the whole planetary revolution, the Philips ToUcam, has long since been discontinued, but it can sometimes be found on the used market for as little as $50. Nowadays, the most inexpensive camera to start shooting the solar system is Meade's Lunar and Planetary Imager (LPI), for $99. The LPI comes with everything you'll need to get started, including its own software, though it doesn't record as many frames per second as some other cameras. However, its control software Autostar Suite automatically registers and stacks your image during the recording, rejecting blurry frames, making the LPI one of the easiest routes to get involved with solar system imaging.
Another low-cost option is the Orion StarShoot Solar System Color Imager III ($189.95). This camera features a larger detector than the LPI, and also includes a custom version of the popular astro imaging software MaxIm DL Essentials.
More advanced cameras capable of capturing 60 frames per second and faster are offered by companies such as The Imaging Source (astronomycameras.com), and Lumenera (lumenera. com), and can cost as much as several thousand dollars each.
There's also another option that you perhaps already own. Many point-and-shoot digital cameras have a video mode capability. While the movie format your little camera records might not be compatible with some planetary image-processing software, it can be converted to a usable format with free software such as VirtualDub (virtualdub.org). Using a point-and-shoot camera will require an adapter to hold the camera close to the eyepiece.
Regardless of which camera you choose, the fundamental approach to planetary imaging is the same: capture many frames in a short period of time, and then stack the best frames of the video using the software of your choice. Cameras such as those mentioned earlier from Meade and Orion come with their own proprietary 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), and AviStack (avistack.de). Mac users should check out Keith's Image Stacker (keithwiley .com/software/keithsImageStacker. shtml). All of these programs are free or very inexpensive.
Getting the best out of your equipment
So you're ready to start shooting our solar system neighbors. Here are a few tips to get the most out of your setup.
(1) First you need to make sure that your telescope has had time to cool to match the outdoor temperature. A warm or cold telescope coming out of the house will produce thermal currents, which ruin the steadiness of your view. Allow about an hour before shooting if you take your telescope outside from a temperature-controlled environment.
(2) If you're using a reflector or compound telescope such as a Schmidt-Cassegrain, check your collimation before shooting. (See "No-Tears Collimation" on the included CD.) Even slightly misaligned optics will significantly degrade your results. This can't be stressed enough. Telescopes with movable mirrors often slip out of collimation, so taking a few minutes to check will pay dividends in your images.
(3) Avoid shooting directly over rooftops or asphalt parking lots early in the evening. Rooftops build up heat in the daylight, and then slowly radiate that heat back into the sky. The views you get over a hot rooftop will be similar to looking at something directly behind a fire. This may be unavoidable in a large city.
(4) Try not to overexpose the movies recorded with your webcam--there shouldn't be any white, blown-out regions in your video, because any sharpening by your post-processing software will increase that area. There's often subtle detail in the bright areas of craters on the Moon, or the polar caps of Mars, that can be revealed if care is taken to avoid overexposure. Most of the webcam-capture software that came with your camera includes a tool to monitor the histogram of your video.
(5) Finally, practice whenever you can. Just like visual observing, you learn more about your equipment's capabilities the more frequently you use them. And because planetary imaging can often take only a half-hour or so, you can take advantage of those partly cloudy nights.
Perhaps the best aspect of planetary imaging is that it's impervious to light pollution. The Moon and major planets are bright enough to be seen from even the most light-polluted cities on Earth. So as long as you have a clear shot, you have a pretty good chance of getting a respectable planetary portrait.
Sky & Telescope imaging editor Sean Walker has been capturing the planets with webcams for more than a decade.
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|Date:||Jan 1, 2010|
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