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Optical Astronomy from Orbiting Observatories The Space Congress® Proceedings 1981 (18th) The Year of the Shuttle Apr 1st, 8:00 AM Optical Astronomy from Orbiting Observatories P. M. Perry Astronomy Department, Computer Sciences Corporation William R. Whitman Technical Staff, Astronomy Department, Computer Sciences Corporation Follow this and additional works at: https://commons.erau.edu/space-congress-proceedings Scholarly Commons Citation Perry, P. M. and Whitman, William R., "Optical Astronomy from Orbiting Observatories" (1981). The Space Congress® Proceedings. 5. https://commons.erau.edu/space-congress-proceedings/proceedings-1981-18th/session-2/5 This Event is brought to you for free and open access by the Conferences at Scholarly Commons. It has been accepted for inclusion in The Space Congress® Proceedings by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. OPTICAL Dr. P. M. Perry, Manager Mr. William R. Whitman, Astronomy Department of Technical Staff Computer Sciences Corporation Astronomy Department Computer Sciences Corporation ABSTRACT Atmospheric extinction, seeing, and "light pollution" £re the most significant factors ultraviolet, visible, and infrared wavelengths) affecting the quality of observations obtained conducted by modern ground-based observatories from ground-based optical telescopes, degrading is that of atmospheric extinction and seeing. resolution and limiting reach. In addition, Irregularities in the earth's atmosphere tend the earth's atmosphere is opaque to radiation to degrade star images to much below the per­ shorter than 0.3 microns preventing the ultra­ formance potential of good instruments. Even violet from being observed in detail from the under the best observing conditions with the ground. The solution to these problems has been best ground-based instruments 5 a resolution of to move astronomical telescopes into earth orbit. 0.5 arc seconds is the best that can be ob­ Initially these orbiting observatories carried tained and then for only a short period of instruments sensitive to ultraviolet and higher time. Modern optical telescopes are theo­ energy radiation since it was otherwise unob- retically capable of doing nearly an order servable. The success of the first series of of magnitude better. For optical surfaces these orbiting observatories, the Orbiting whose irregularities are a small fraction of Astronomical Observatories (OAO), established the wavelength of light, the theoretical these satellites as one of a new generation of resolution in seconds of arc is given by 25A/ tools for exploring the universe. Another D, where X is the wavelength of the incident orbiting observatory, the International Ultra­ light in microns and D is the diameter of the violet Explorer (IUE), is unique among the objective in centimeters. For a telescope current orbiting observatories in that it is in such as the Kitt Peak 4-meter, a theoretical a geosynchronous orbit and provides a guest resolution of about 0.03 arc seconds is pos­ observer facility serving the international sible in green light. In addition to limiting community. IUE has had a significant impact the resolution of ground-based telescopes^the on observational astronomy. Nearly 10 percent atmosphere absorbs most of the radiation of all observational papers published JLi the shorter than about 0.3 microns preventing Astrophysical Journal in 1980 reported or used studies of astronomical objects at those observations made by IUE. The figure for all wavelengths. This is of particular importance astronomical satellites is about 3 times higher to the study of young hot stars whose energy and continues to rise. With the orbiting of output peaks at ultraviolet wavelengths. Also, the Space Telescope in the mid 1980s by the of increasing concern to astronomers is the Space Shuttle, observational astronomy will problem of "light pollution," airglow enter a new era. For the first time, as­ scattered light from cities, which increases tronomers will have access to a large (2.4 the brightness of the background sky meter) high-resolution telescope unhindered it more difficult to observe faint objects. by the earth f s atmosphere. With the potential ob­ such an instrument offers, there is little The solution to these problems been doubt that the near future will see a large vious for a long time but it not until fraction of observational astronomy performed the creation of the National Aeronautics from orbiting observatories, and Space Administration in late 1958 that the opportunity for conducting astro­ INTRODUCTION nomical observations from space arose. It shortly after its creation that By far the most significant factor affecting invited interested astronomers in to discuss the quality of astronomical observations, at possibility of orbiting an optical wavelengths (here defined to include satellite. It from these 2-1 that the Orbiting Astronomical Observatories orbiting observatories, their scientific (OAO) were conceived. Two of the four satellites instruments,and modes of operation. We will in the OAO series, OAO-2 and OAO-3 (Copernicus), report some of the significant discoveries by became fully operational. The first of these, each, or in the case of ST, the expected OAO-2, was placed in a low circular orbit in capabilities. December 1968 and carried two experiments de­ signed to make ultraviolet observations of ORBITING ASTRONOMICAL OBSERVATORIES (OAO) stars, nebulae and star fields. The second, OAO-3, which was named Copernicus in honor of The first of the successful OAO spacecraft, the great 16th century astronomer, was launched OAO-2, was launched on December 7, 1968, and in April of 1972 also in a low circular orbit. placed into a circular orbit at an altitude It carried an X-ray experiment and a 32-inch of 490 miles with a period of about 100 telescope for ultraviolet astronomy. The latter minutes. On board were two telescope systems was, at the time, the largest astronomical tele­ for measuring star brightness in the ultra­ scope to be placed in orbit. violet and for obtaining low-resolution spectra. The configuration of the spacecraft was to Both of the OAO spacecraft were low-orbiting have the two experiments located at opposite observatories whose access was basically ends of the satellite. At one end was the limited to the principal investigators who Smithsonian Astronomical Observatory's (SAG) had proposed the experiments carried on board. Celescope experiment, and at the other the In contrast, the International Ultraviolet University of Wisconsin's Wisconsin Experiment Explorer (IUE), as its name indicates, is an Package (WEP). international observatory. It is open to any­ one in the world whose program of observation The Celescope experiment was designed to is accepted by the Observatory. In a synchronous produce television pictures of 2-degree-square orbit above the Atlantic Ocean, IUE is in constant areas of the sky in ultraviolet light, the contact with one of its two ground stations, principal objective being to measure the either at the Goddard Space Flight Center (GSFC) ultraviolet magnitudes of very many stars in in Greenbelt, Maryland, or the European station a statistically significant fraction of the at Villafranca del Castillo, Spain (VILSPA). sky. The instrument itself consisted of four IUE is operated in much the same way as a 12.5-inch Schwarzschild telescopes using ground-based observatory like the Kitt Peak specially developed Westinghouse Uvicon ultra­ National Observatory. Guest Observers whose violet sensitive television tubes, and an proposals have been accepted arrive at one of electronics package to control the operation the two ground stations at their designated of the Uvicons and to encode the data for times and conduct observations in real-time. transmission. Two types of Uvicons sensitive The scientific instrument carried on board IUE to different wavelength ranges were used. One is a 45-cm telescope with two spectrographs for type was sensitive to the range 1050-3200 X. obtaining spectra of a variety of astronomical and the other from 1050-2000 A. The field of objects at ultraviolet wavelengths. Now in its each Uvicon was optically split into two areas fourth year of operation, IUE is expected to of different sensitivity by mounting two dif­ continue providing astronomers with ultraviolet ferent semicircular filters in the focal plane spectra for the remainder of its nominal 3-to 5- of the photocathode. The photoelectrons years lifetime and in all probability for years emitted by the photocathode were imaged on a afterward. target where the image was integrated and stored as an electrical-charge pattern for In the mid 1980s, IUE will be joined by the readout at the desired time. Space Telescope (ST), the largest astronomical instrument to be placed in orbit yet. Carried The sensitivity of the Uvicons during orbital into orbit by the Space Shuttle, the SI is a operations decreased with time, but the pre- high-resolution 2,4-meter telescope which is launch goal of one year of operation was designed to generate a long-term program in still exceeded by 4 months. Celescope was space astronomy providing astronomers with a turned off in April 1970. capability not achievable by any current or foreseeable ground-based telescope. The ST The second experiment carried by OAO-2, the shares some of the characteristics of the OAQs. WEP, consisted of five telescopes (a 16-inch and IUE. It will be a low-orbiting spacecraft, and four 8-inch) employing photoelectric so operation of the scientific instruments will filter photometers over the spectral region be by means of a pre-planned program, as it from about 120oS to 4QQQ&, and two objective- was for OAO. After ST's first few months of grating scanning spectrometers which view operation, it will be available, as a Guest areas 2-1/2 degrees long (in the direction of Observer facility serving the international ^ dispersion) and 8 minutes of arc wide. The community, as is IUE, spectrometers provided resolution of about 10X in the wavelength range 1100& to 2000& In this paper we present a description of the and about 20& in the range 2000i to 4000&.
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