Imagine turning your home computer into the equivalent of a professional telescope which can display millions of stars and galaxies located anywhere in the sky. Astronomers as well as educators will soon be able to have the sky at their fingertips thanks to an ambitious effort now being funded by NASA and being carried out by the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
Astronomers at STScI are about to begin converting a vast survey of the heavens into a digital format which can be stored on CD-ROMs (compact disk read-only memory) which are commonly used on computer workstations. It will take approximately two years to complete the compression and transfer of data to the CD media. When this sky survey conversion bas been completed, astronomers will have a powerful astronomical resource for doing a wide range of research.
The compressed digitized sky survey project is being led by Dr. Michael Shara and co- investigators Drs. Barry Lasker, Marc Postman, Brian McLean, Rick White, Vicki Laidler, and Doris Daou. This two-year conversion effort is being funded by the Astrophysics Division of NASA's Office of Space Science and Applications (OSSA).
"The sky survey CD will be one of the most important astronomical research tools ever created," says Project Scientist Dr. Marc Postman. "It will afford astronomers rapid access to images of the sky in a format which is readily digested by modern computers. One of the very exciting scientific projects which will be greatly aided by this database is the mapping of the three-dimensional distribution of galaxies 900 million light-years away."
The digitized sky survey is a result of an intensive eight-year effort by STScI astronomers to prepare the Guide Star Catalog (GS C) which provides the coordinates of target stars which are used by NASA's Hubble Space Telescope for acquiring and locking onto celestial targets. To construct the catalog and to support planning for HST observations, astronomers spent five years scanning 2100 photographic plates of the sky (including the 1500 deepest ones being used for the present CD project), and converting them into a huge computer data base.
The original sky survey photographs were taken with wide-angle Schmidt telescopes at Caltech's Palomar Observatory in California, and the United Kingdom Schmidt Telescope at Siding Spring in New South Wales, Australia. Each 14-inch square photographic plate in these surveys covers an area of sky about the size of the bowl of the Big Dipper, and reveals stars and galaxies down to 21st magnitude, a million times fainter than the limit of the naked eye.
As the GSC was being constructed, STScI astronomers realized that the "raw" (i.e. not computer processed for catalog construction) digital plate scans, containing images of one hundred million stars and nearly ten million galaxies, also offered a unique and valuable astronomical resource. Because the sky images are in digital form they can be readily manipulated through computer programming for a variety of research applications.
The problem is that the digitized scans represent a huge quantity of data. Each of the 1500 plate scans is 14,000 picture elements on a side (each picture element or pixel subtends an angle of 1.7 arc seconds - the apparent size of a dime when viewed at a distance of 1.3 miles). This adds up to 600 billion bytes of data in all. The raw scans were initially stored on 4,500 magnetic tape and then converted to 400 double-sided, 12-inch diameter optical disks.
To make the digitized sky survey more accessible to researchers, STScI astronomers identified and extensively tested an algorithm which can compress the data by a ratio of 10:1 without significantly (less than one percent) degrading the accuracy of stellar positions and brightnesses.
This is possible because the algorithm preferentially compresses the dark, background sky regions, thus preserving detail in stars and galaxies where there is a much higher signal-to-noise ratio. "The process is essentially like using an adaptive filter, which blocks the sky background glow but lets starlight through," says Dr. Brian McLean.
The STScI team plans to produce one CD-ROM per week, which would contain the equivalent of 15 survey plates, roughly equivalent to the sky area covered by the winter constellation Orion the Hunter. Even at that ambitious conversion rate, it will take two years to compress the whole sky.
Once this compression is completed, images from the 1500 digitized plates will be distributed on 100 CD-ROMS. At an estimated price of under $2,000, the compressed sky survey will be affordable to libraries and astronomy departments around the world - and will take up only two linear feet of bookshelf space.
Astronomers will have innumerable uses for such a computerized, "on-line" survey of the heavens. They essentially could accomplish many of the tasks that now are done with a one-meter aperture telescope. Some examples:
Galaxy Counts: Researchers can inventory the millions of uncataloged galaxies that are in the sky survey. These galaxies can be used to probe the large-scale structure of the universe. The new galaxy catalog could also be used to find clusters of galaxies, aggregates of many hundreds of galaxies formed billions of years ago.
Supernova and Variable Star Searches: By comparing recent sky images with those in the digitized sky survey, astronomers will be able to discover all types of new variable stars as well as supernovas in other galaxies.
Identifying Optical Counterparts: Earth-orbiting spacecraft have performed initial surveys of the universe in X- rays, gamma-ray and infrared wavelengths. Astronomers will be able to look for optical counterparts to these sources of invisible radiation.
Multi-object Spectroscopy: Astronomers can use the scans to fabricate precise "aperture plates" which are drilled and outfitted with fiber optics to take spectra simultaneously of many stars and galaxies in a telescope's field of view.
Finder Charts: Astronomers need accurate finder charts to help them find celestial objects, even when a target's coordinates are well known. Published charts aren't always available.
For small research projects and classroom instruction, STScI also plans to compress the plate scans by a ratio of 100:1, squeezing the entire survey down to about 10 CD- ROMs. Though the resolution would not be adequate for many research applications, this would make the survey affordable to amateur astronomers and educators, at an estimated cost of about $100. Stars fainter than 19th magnitude would not appear in this super-compressed version, but positional accuracy would be adequate for preparing simple finder charts and developing classroom exercises.
Dr. Gunter Riegler, Chief of OSSA's Science Operations Branch, Dr. Gary Neugebauer, at the California Institute of Technology, and Dr. Malcom Longair, at the Royal Observatory Edinburgh, contributed to this project.