Hopkins, Space Telescope Scientists Design Advanced Hubble Camera
The team developing HACE is made up of scientists and engineers from Johns Hopkins, the Space Telescope Science Institute (STScI) in Baltimore, the University of Arizona, the University of California, Santa Cruz, and the University of Leiden in the Netherlands.
Dr. Ford and many other members of the science team also were involved in the design of COSTAR, a device installed in December 1993 that corrected the blurred vision for three of the space telescope's instruments, impaired by a flaw in the primary mirror.
HACE will improve HST's vision even more, enabling astronomers to probe deeper into mysteries surrounding the age and "missing mass" of the universe. And HACE will be equipped with a "solar blind channel" that blocks out all but certain ultraviolet light, enabling scientists to make detailed atmospheric studies of other planets in the solar system, said Paul Feldman, a Johns Hopkins professor of physics and astronomy.
The camera will be built by Ball Aerospace Corp., in Boulder, Colo., which also built COSTAR. HACE is scheduled to be installed by astronauts during a servicing mission in November 1999.
Astronomers plan to use HACE to peer to the very edge of the universe, using the camera to survey nearly a square degree of the sky -- a region about the size of four full moons. Within that region, scientists hope to see literally all of the galaxies that emit optical light.
"That's a very exciting possibility, the idea that we may be able to photograph an area of the sky and in those directions see virtually all of the galaxies in the universe," said Dr. Ford, a professor in the Department of Physics and Astronomy who has a dual appointment at STScI.
The design was one of three submitted to NASA. It competed against proposals from the Jet Propulsion Laboratory and the Goddard Space Flight Center. NASA announced the selection in December.
"I think, for the first time, with this instrument we will be able to fully exploit the capabilities of the space telescope," said Jim Crocker, HACE systems engineer, who conceived COSTAR's design while director of STScI's advanced programs office. He is now on a one-year leave of absence at the European Southern Observatory in Germany.
Because HST is above the image-distorting atmosphere, it is ideal for precisely measuring the distances of galaxies. By carefully measuring the way that light from a distant galaxy fluctuates, astronomers can calculate precisely how far away it is. The current camera, the Wide Field and Planetary Camera 2 (WFPC-2), isn't efficient enough to make the measurements for a large sample of distant galaxies -- it takes too much observing time to collect enough light. A large sample is needed in order to map the large-scale distribution of matter. HACE will be seven times as efficient as WFPC-2, making those measurements possible, said Marc Postman, an astrophysicist at STScI.
After learning how far away a distant galaxy is, scientists can determine the amount of matter that exists in a large region in space. Then they can calculate how much of that matter is "dark matter." Astronomers think that at least 90 percent of the mass in the universe has not yet been detected, and various theories have been proposed for the nature of this "missing" matter.
HST researchers plan to map the large-scale motion of galaxies, analyzing how the galaxies are affected by the attraction of other galaxies and matter in a region spanning 100 megaparsecs. A megaparsec is 3.26 million light years. A detailed study of the motions of galaxies may provide a direct measure of the amount of matter in that region of space. If scientists know precisely how fast a galaxy is moving, they can figure out how much of the velocity is due to the expansion rate of the universe. By subtracting that percentage from the galaxy's total velocity, scientists can learn what percentage of the galaxy's velocity is caused by its attraction to other objects. Then, astronomers will be able to calculate how much mass exists in the region, leading to calculations of the total mass in the universe and refined estimates of how much dark matter exists, Dr. Postman said.
The camera will enable astronomers to analyze the "gravitational lensing" affects of large clusters of galaxies in the region. The images of objects behind the clusters are distorted by the gravity of the clusters. By analyzing the shapes of those distorted images, scientists can learn the actual mass of the galaxy clusters producing the lensing affect. Then, by comparing the differences in gravitational lensing among galaxy clusters of varying ages and distances, astronomers plan to learn how dark matter has evolved over time.
"I have no worries that the advanced camera will make some rather surprising discoveries," Dr. Postman said.
HACE will not replace WFPC-2, which was installed with COSTAR in 1993. HACE is an axial instrument, meaning it is located perpendicular to the focal plane. It will replace one of HST's four axial instruments, depending on which one is least needed by the time of the servicing mission, Crocker said. WFPC-2 is a radial instrument, the only such instrument on HST, and is located parallel to the focal plane.
HACE will be eight times as sensitive as WFPC-2, using a deep red filter needed to image galaxies in the early universe, and its field of view will be twice as large. It will feature a novel design that minimizes the number of reflecting surfaces, so that fewer photons are lost. Other advanced features include highly reflective coatings and a larger and more sensitive imaging chip, made up of imaging sensors called charge-coupled devices. The CCDs will form an array containing 4,096-by-4,096 pixels, compared with the current camera's 1,600-by-1,600 pixels.
It all translates into a camera with superb efficiency. Whereas WFPC-2 might capture 6.6 percent of the deep red photons entering the telescope, at best, HACE will capture up to 53 percent of the photons, making it eight times more sensitive than the WFPC-2.
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