- FireWire Camera Removes Bottlenecks in Astroimaging
Anne L. Fischer
Obtaining high-resolution photographs of planets and other interstellar objects is the quest of many amateur and professional astronomers. With advances in digital photography and telescopes, astrophotographers can take higher-quality images of objects at greater distances. To obtain high-quality planetary pictures, astronomers take videos and then piece together the best frames to obtain the clearest image. The challenge is to take enough frames with as little noise as possible.
Astronomer Daniel Llewellyn obtains high-resolution images with a FireWire camera, shown here at the top of the imaging trane with a cable leading to the computer that stores the images.
In the past, planetary imaging systems used webcams, which are low-resolution — usually, 640 × 480 at around 10 fps. They connect to a computer through a USB port, which, even at its highest speed (USB 2.0), uploads at 50 MB/s. Daniel Llewellyn, an astronomer and president of Telescope Atlanta, a retail store in Georgia, used a webcam in the past for planetary imaging but found it noisy and the frame rate hampered by USB 1.0. He did obtain good images, but he saw vast improvement with the next level of FireWire (IEEE 1394b) data transfer — better electronics, with lower noise and higher frame rates.
Great moments of clarity
When he is imaging the planets, Llewellyn films a series of short intervals — anywhere from 2 to 5 min. During each interval there is distortion in the image, which is caused by turbulence, moisture and vapor in the atmosphere. One example is the twinkling of stars, which is actually the Earth’s atmosphere causing light to defract. When photographing through that “busy” atmosphere, the astronomer is limited not only in the amount of magnification that can be used but also by the atmospheric distortion, which causes the object to quiver. The planet is quivering because of the scintillation in the atmosphere, which can be seen when looking down a straight road in the desert at high noon. Magnification is limited because the object appears to be moving. This is overcome by taking several video streams and breaking them up into frames. The result of filming a high number of frames (30 to 60 per second) is a handful that show “great moments of clarity,” according to Llewellyn.
When any of Jupiter’s Galilean satellites pass in front of the sun, they cast a shadow on Jupiter. In this photo, 10 shadows are seen.
In his experience, a video stream of 3 to 5 min can result in as many as 8000 frames, of which only 600 might be used. When Llewellyn used an 8-bit webcam with USB 1.0, he was limited by the data transfer from the camera to the computer. When he used the webcam at 25 fps, the USB 1.0 interface could not handle the data stream and would compress the images, producing artifacts. And because the webcam had minimal electronics, the images were noisy or grainy. Recently, in the March 2008 issue of Sky & Telescope, Llewellyn had an image of Mars published — a major achievement in the amateur astronomer community. The image is far superior to others he has taken because he now uses a professional video camera.
He’s found that the 12-bit camera provides much better resolution than the 8-bit, with higher bit depth. “In planetary imaging, you’re scraping and clawing for every advantage you can get,” Llewellyn said. He uses a Flea2 camera from Point Grey Research of Vancouver, British Columbia, Canada, placed at the focal plane in a C-14 from Celestron of Torrance, Calif., on a Paramount ME German equatorial mount from Software Bisque of Golden, Colo.
Saturn, the sixth planet from the sun and second largest (after Jupiter), can present a challenge for astropho?tographers, especially when low on the ecliptic plane, the path that defines the orbit of the Earth around the sun.
The native focal length is 3910 mm, which is near 4 m; during planetary imaging, the focal length increases to 10,166 mm, or just over 10 m. With a 2.6 Barlow lens that is used for magnification of the image, he can capture 1024 × 768 raw (unprocessed) 12-bit images at 45 fps with a FireWire 800 interface.
Although the 1024 × 768 is capable of just 30 fps, he pushes this to 45 by using the region-of-interest feature on the camera. This cuts off the top and bottom of the chip, making it smaller — though not as small as the 640 × 480 — and increases the frame rate.
To acquire a high-quality image such as his published Mars photo, he makes several series of at least 2 min each, using the MCMP/MJPG compression from Lead Technologies of Charlotte, N.C., to the live video stream. This is done to help manage the large color data stream to the computer and to ensure that there are no dropped frames or added artifacts. The capture file results in 2000 to 6000 frames, which are processed and stacked using Regi-Stax, a free image-processing software from RegiStax in the Netherlands. Final image adjustments are done in Photoshop from Adobe Systems Inc. of San Jose, Calif. The high-speed interface of FireWire 800 is a great advantage for color cameras because of the large amount of data that is streaming, he said. It increases the number of frames taken in a second, which increases the chances of getting “good frames and beating the seeing,” he noted. “If I could get 60 frames per second color, I’d do even better.”
This image of the 17/P Holmes — taken Oct. 27, 2007 — is the brightest recorded outburst of a comet.
Many astronomers use black-and-white chips and then add color, but Llewellyn opted for a Bayer color array camera. The color can reduce sensitivity but, by coupling the camera with his telescope setup, he can get the same results as those from using a black-and-white chip and an RGB filter in front of the camera. The drawback to the black-and-white route, he noted, is processing time, because you must take four sets of images and then stack them.
When the goal is a perfect image of Mars, confidence in his equipment is paramount to Llewellyn, because the Earth passes Mars in its orbit only every 25 months. “When Mars is around, I have limited time to take really good photos. Other planets pose different challenges, such as Saturn and Jupiter. Jupiter is low on the planetary ecliptic, so you have up to 10 times more image-degrading atmosphere you have to image through. After next year, it will start its journey back and will be higher in the sky. Saturn is starting to move downward for the Northern Hemisphere and will present the same challenge in the years to come. With the new higher-frame-rate, high-resolution camera, many previous challenges have been overcome.”
Contact: Daniel Llewellyn, Telescope Atlanta; e-mail:email@example.com.
- digital photography
- A form of photography in which an electronic camera converts an image to an electronic signal that is stored in digital format on magnetic media or film.
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