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The Astro-2 mission, which will investigate a variety of basic properties of the universe, builds on the experience and scientific data obtained with the same telescopes on the Astro-1 flight in December 1990. This second mission will significantly increase the available library of wide-angle, deep exposure ultraviolet images of the universe, with each UIT exposure capturing a field of view larger than the apparent size of the full Moon. The UIT scientists, with team members and Guest Investigators from Goddard and cooperating institutions, will survey fundamental astrophysical objects such as galaxies, nebulae, and star clusters, in ultraviolet light. This energetic radiation, which is invisible to the human eye, cannot be received on the ground, as it is blocked by the ozone layer and other elements of the Earth’s atmosphere.
As part of their wide ranging science objectives, the UIT scientists will focus on globular star clusters, the oldest star systems in our Milky Way galaxy; such studies help to understand how the galaxy formed and how the physical condition of stars like the Sun change over long periods of time. In another key study, UIT investigators will search for ultraviolet radiation from the recently recognized low surface brightness galaxies that shine so dimly that they have been largely overlooked until recently, although they include the largest known spiral galaxies, they may represent a significant fraction of the visible mass in the universe, and they may even provide an effective tracer of the underlying background of unknown dark matter that pervades the universe. If UIT finds ultraviolet light from these galaxies, the results may help to understand why they seem to be forming very few new stars, yet are unusually blue (which normally signifies a galaxy with a great many newborn stars). And if no ultraviolet light is found, the mystery of the low surface brightness galaxies will be even greater.
The UIT scientists plan to gather new technical data about the surface of the Moon, to add to the knowledge base gathered in the Apollo manned landings and in the recent Clementine mission’s remote sensing of Earth’s nearest neighbor.
Goddard’s UIT will take wide-field photographs of objects in ultraviolet light, recording the images on film for processing back on Earth. The Hopkins Ultraviolet Telescope (HUT), developed at The Johns Hopkins University in Baltimore, Md., conducts spectroscopy in the far ultraviolet portion of the electromagnetic spectrum. The Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), developed at the University of Wisconsin at Madison, measures the intensity and spectrum of ultraviolet light and its degree of polarization.
Astro-2 observations also complement those of ultraviolet instruments on other spacecraft, such as the Hubble Space Telescope, the International Ultraviolet Explorer, and the Extreme Ultraviolet Explorer all currently in operation. For example, UIT photographs a larger area of the sky in a single exposure, so the UIT images are used to locate targets for deeper study with the more powerful Hubble, which concentrates on one small part of the sky at a time. By combining research findings from various instruments, scientists hope to piece together the evolution and history of the universe and learn more about the composition and origin of stars and galaxies.
The Ultraviolet Imaging Telescope makes deep, wide-field photographs of objects in ultraviolet light. Before Astro 1, very few ultraviolet images had been made and mostly during a few brief rocket flights.
The Ultraviolet Imaging Telescope observes a field of view two-thirds of a degree across, an area larger than the full moon. This is considered "wide field" for astronomers. For many galaxies or star clusters, this is large enough to encompass the entire object in a single photo frame. In addition, the UIT utilizes "solar blind" technology, meaning that it is virtually impervious to interference from the bright glare of visible light, which often drowns out the ultraviolet radiation from the same galaxy or star cluster.
Images made in the ultraviolet spectrum clearly show the dynamic events taking place beyond our world. The clutter of objects which produce most of their radiation in visible light disappears. Hot stars leap into prominence, the spiral arms of distant galaxies snap into clearer resolution, and the material hidden between the stars comes into view.
UIT’s wide-field images are ideal for investigating astronomical questions such as the shapes of nearby galaxies as revealed in ultraviolet light, the properties of massive hot stars, the evolution of low-mass stars, and the nature of interstellar dust and gas. UIT galaxy-wide images are sky surveys that can locate bright ultraviolet stars for further more detailed study by the Hubble Space Telescope.
During Astro-1, UIT obtained many striking and scientifically important images, including views of clusters of young, massive stars. In observations of globular clusters of very old stars, UIT found unusual objects, unusually hot and bright stars in little known and short-lived conditions. Galaxy findings including data on the smaller "irregular galaxies" that experience sudden bursts of star formation. Astro-2 will expand this pioneering work on imaging the ultraviolet sky.
UIT is a powerful combination of telescope, image intensifier and camera. Unlike data from the other Astro instruments, which will be electronically transmitted to the ground, UIT images will be recorded on very sensitive astronomical film. The film will be processed at GSFC after Endeavour returns to Earth. Then, the digital data will be scanned into a computer to be analyzed at GSFC, cooperating institutions, and elsewhere in the academic community.
Light is reflected from a 15-inch (38-centimeter) primary mirror, at the middle of the telescope tube, to a secondary mirror near the front. The secondary mirror is linked to an image motion compensation system, which adjusts it slightly as necessary to offset any motion or jitter in the pointing system. This is critical since any motion would blur the resulting photographs.
Reflected from the secondary mirror, the light passes through filter wheels containing six filters each. These different filters allow specific wavelength bands of the ultraviolet spectrum to be selected. By comparing two images of the same area with different filters, the UIT team can measure the temperatures and brightness of thousands of stars in the field of view.
The light then enters one of the telescope’s two image intensifier/film transfer units. The image intensifiers amplify and convert the ultraviolet light into a visible image that can be recorded on astronomical film. Each unit contains 1,000 film frames.
A 30-minute exposure can record a blue star of 25th magnitude, at least 40 million times fainter than the faintest visible light star which could be seen by the naked eye on a clear, dark night. Developed after the mission, each frame of film is scanned and digitized to form an array of 2,048 x 2,048 picture elements, called pixels, for computer analysis. This analysis produces quantitative information about the objects whose images appear on the film.
The ultraviolet telescope assembly rests on a Spacelab pallet in Endeavour’s cargo bay. The Shuttle and Spacelab systems provide power, pointing and communications links for the observatory.
The flight is scheduled for at least 16 days’ duration, which will be the longest Shuttle mission yet. After launch, the plan calls for a roughly 20-hour checkout period, though fine-tuning the observatory could take somewhat longer. Observations will begin immediately after checkout is complete and continue throughout the mission, with only brief interruptions for activities such as waste-water dumps and Shuttle tests.
The scientific information gathered by UIT in Astro-1 has led to more than 30 scientific articles, summarizing findings on subjects ranging from dust grains in space, the material that eventually collects into planets like the Earth, to supernova remnants, the shattered debris of exploding stars. That same debris is the ultimate source of most of the chemical elements on Earth, or in the human body.
Many Astro-2 observations will build on discoveries from Astro-1, while others will seek to answer additional questions about the ultraviolet universe.
On Astro-1, UIT images identified rings of massive star formation in several galaxies, and roughly half of the instrument’s science program on Astro-2 is devoted to studies of star-forming galaxies. A unique UIT contribution is the identification of thousands of individual hot stars in other galaxies; this serves as a target list for investigators with the Hubble Space Telescope.
UIT also photographed globular clusters, where there are often so many stars grouped together in the central region that it is impossible to distinguish individual stars. The ultraviolet images picked out hot stars in late stages of evolution, where hydrogen has been depleted from the cores and energy is provided by burning helium. By comparing photographs taken in different wavelengths, scientists were able to measure the temperature as well as brightness of the individual stars. Observing more globular clusters is a high priority for the imaging telescope on Astro-2. Astronomers will compare the observations to theoretical predications, to help fill in gaps in their knowledge about these late evolutionary stages.
Galaxies have a variety of shapes and sizes: gigantic spirals like the Milky Way, round and egg-shaped ellipticals and irregular shapes with no preferred form. Studying the appearance of galaxies in the ultraviolet is a key to the study of galaxy evolution in the early universe. Before Astro-1, there were only a handful of ultraviolet pictures of nearby galaxies available. UIT images from that mission revealed that the shapes of galaxies seen in ultraviolet wavelengths are strikingly different from their familiar forms in visible light. One UIT goal for Astro-2 is the construction of an ultraviolet atlas of spiral galaxies.