A Look Inside the Camera
Of the estimated $70 million Pan-STARRs budget, about one-third will be used to build four of the largest and, in some respects, most advanced digital cameras in the world. At the heart of each Pan-STARRs camera will be a novel digital-imaging chipset developed by the Lincoln Labs at the Massachusetts Institute of Technology. The chipset is specifically designed to correct for the jitter caused by Earth’s turbulent atmosphere and thus improve image quality. These chips, or “orthogonal transfer charge coupled devices” (OTCCDs), adjust the image digitally at the focal plane (the area behind the telescope lens where light is concentrated) of the camera itself, rather than mechanically with a deformable mirror as is common with most telescope adaptive-optics technologies.
The OTCCDs inside each Pan-STARRS camera will be arranged in an 8x8 pattern called an “orthogonal transfer array” (OTA). Each OTCCD is an 8x8 grid of independent imaging cells that are 600x600 pixels across. A full focal plane is equivalent to an array of 4,000 independent imaging devices. During each exposure, the OTAs quickly scan the positions of a few relatively bright stars to get a baseline correction setting from these well-known stellar bodies. The sky image is then adjusted within the charge coupled device itself. Each focal plane is about 16 inches a side. Pan-STARRS project investigators at the IFA have already taken delivery of the first OTAs. Extensive lab tests have shown the devices meet or exceed specifications.
In addition to image quality, the Pan-STARRS design offers many sidebenefits. The small CCDs can read out data much more quickly than large ones. Also, the large number of devices reduces overall project risk. With more than 4000 chips in each camera, even slightly imperfect chips can still be put to good use. And, while bright stars can saturate a single CCD, the thousands of chips that will be in use with Pan-STARRS can be programmed to screen out bright stars and minimize saturation by bright stars across all the CCDs. The technology that powers the Pan-STARRS cameras will allow correction for the effects of wind-shake that would otherwise blur the images. This is similar to the technology, known as optical shift image stabilization, provided in some high-end digital cameras.
The Pan-STARRS project will provide the most comprehensive sky catalog ever compiled
Ancient Hawaiian astronomers studied the sky from Pu‘u Kolekole on the summit of Haleakala, their eyes turned aloft in a quest for a greater understanding of the great celestial canvas. On June 30th, modern-day astronomers gathered on that same summit to mark the start of a project which promises to create the best map to date of that seemingly endless canvas. The gathering marked the dedication of the first prototype telescope for the $70 million Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). Upon its completion in 2010, Pan-STARRS’ linked array, consisting of four 1.8-meter light-gathering apertures, will be able to capture and map a larger percentage of the night sky than any other system on Earth. “It completely dwarfs what people are doing at the moment,” says Nicholas Kaiser, a researcher at UH’s Institute for Astronomy (IFA) who also serves as the principal investigator and chief scientist for the Pan-STARRS project. “Even the prototype camera will be the largest in the world. Pan-STARRS will produce the most extensive catalogs in the history of astronomy.”
Each of the Pan-STARRS telescopes will be equipped with a version of the world’s largest digital camera. Specially designed for the project, the cameras’ sensors will measure more than 1.4 billion pixels across an area of about 40 centimeters per side. The largest digital camera at present, the Canada France Hawaii Telescope’s “MegaCam,” has a sensor area of 340 million pixels, less than one-tenth the amount of pixels in the Pan-STARRS system. With every 30 second exposure, Pan-STARRS will capture about seven square degrees of sky, or about 40 times the area of the full moon as it appears overhead. Pan-STARRS will be able to map the entire sky visible from Hawai‘i once a week, and will track objects as faint as 24th magnitude – roughly equivalent to the brightness of a candle seen from 6,000 miles away. Within its 10-year lifespan, Pan-STARRS will map 90 percent of all “killer asteroids” with dimensions of 1,000 feet or greater. The system will also have the capability to spot and track asteroids as small as a football field.
The raw survey power of Pan-STARRS (measured by total light collecting area, amount of sky surveyed, and quality of images) will be unmatched by any telescope on Earth. “Some [telescopes] can detect objects nearly as faint as Pan-STARRS but do not have as wide a field. Some have a wide field but cannot see faint objects. We are building the only instrument that combines both wide field and high sensitivity,” says Kaiser. All told, Pan-STARRS will be about 100 to 1000 times more powerful than the best existing wide-sky survey telescopes, such as the Near-Earth Asteroid Tracking telescope, also on Haleakala. Each night, Pan-STARRS’ telescopes will produce several terabytes of data. To handle these massive flows of data, the project’s researchers are creating new software techniques to extract only the valuable information from the images. Supercomputers or powerful computing grids will be required to process the images in a timely fashion.
The power of Pan-STARRS illustrates the remarkable impact the Digital Age has had on astronomy. When CCDs, or “charge coupled devices” began to replace photographic plates in astronomical instruments in the 1970’s, the chips were tiny, covering only 100 pixels (short for picture element, a pixel is the smallest viewable element of a digital image). As advanced imaging technology based on silicon detecting media developed, the power of telescopes using CCDs followed roughly the same curve as Moore’s Law, with power doubling and tripling for each successive generation of telescopes. “It’s astonishing how rapidly detector and computer technology has advanced,” says John L. Tonry, an IFA researcher who is leading the Pan-STARRS camera design subsystem group.
The four cameras are being built in workshops at the IFA. The organization has become a key provider of instrument know-how to the world astronomical community. Since the late 1960s, the Institute has developed scientific detectors for its own two telescopes and for the NASA Infrared Telescope Facility. The Institute has also built and sold instruments for CFHT, the Gemini Observatory, the Japanese Subaru telescope and the US Air Force’s telescope on Maui. “We have built instruments for many of the premier astronomical installations on Earth. So we have all this know-how,” said Rolf Kudritzki, director of the IFA and the UH detector-development group.
While asteroid spotting will be a key activity of Pan-STARRS, the unprecedented volume of images collected by the telescope could spark new findings across a variety of specialties in astronomy. Cosmologists will have the capability to study entire populations of stars and galaxies across the universe as a whole, potentially providing them with clues about how the universe developed and how it continues to evolve. The myriad billions of newly discovered galaxies that Pan-STARRS will map could be used to study dark matter – an entity completely unknown, except for the fact that it exerts gravitational force on visible matter. “Pan-STARRS will map out dark matter by observing the effect it has on images of distant galaxies,” explains Kaiser.
Even more mysterious is so-called “dark energy,” a purported form of mass that may permeate space and time with repulsive force and is often used to explain the acceleration of the expanding universe. Together, dark matter and dark energy are believed to comprise 95 percent of all matter in the universe. By providing a big picture of the universe and allowing astronomers to apply multiple techniques for dark energy observations – measurement of the expansion-history of the universe, evolution of the clustering of dark matter, and evolution of the clustering of galaxies – Pan-STARRS could provide key clues as to how “dark energy” functions.
Some of the most significant discoveries of Pan-STARRS may be within our own solar system. Pan-STARRS is likely to find more asteroids, comets and objects past the orbit of Neptune in the outer reaches of the solar system than all previous mapping projects combined. In fact, some astronomers believe that Pan-STARRS will do such a good job that it will leave little to find for future near-Earth sky mapping systems in terms of asteroid surveys. Says Al Harris, a senior research scientist and asteroid expert for the Space Science Institute, “I believe this transition will be reached approximately with this next generation of surveys, such as Pan-STARRS. These telescopes will find most near-Earth objects larger than about a quarter-kilometer in diameter, maybe a bit smaller. What remains after that is not worth the cost of finding.”
Laura K. Kinoshita is the Public Information Officer at W.M. Keck Observatory and a JPL Solar System Ambassador.
Pan-STARRS Prototype Illustration: Hawai‘i Institute for Astronomy
Massive Terrestrial Strike Illustration: Don Davis, NASA
Near Earth Object Diagram: NASA Jet Propulsion Laboratory
Gaspra Asteroid Mosaic Photo: NASA