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The Space Congress® Proceedings 1981 (18th) The Year of the Shuttle

Apr 1st, 8:00 AM

Optical from Orbiting

P. M. Perry Astronomy Department, Sciences Corporation

William R. Whitman Technical Staff, Astronomy Department, Computer Sciences Corporation

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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 " pollution" £re the significant factors , visible, and ) affecting the quality of obtained conducted by modern ground-based observatories from ground-based optical , degrading is that of atmospheric extinction and seeing. resolution and limiting reach. In addition, Irregularities in the 's atmosphere tend the earth's atmosphere is opaque to to degrade 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 . 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 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 . Another D, where X is the wavelength of the incident orbiting , 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 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 . 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 whose energy and continues to rise. With the orbiting of output peaks at ultraviolet wavelengths. Also, the in the mid 1980s by the of increasing concern to is the , observational astronomy will problem of "," 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 such an instrument offers, there is little The solution to these problems been ob­ 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 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 , the great 16th century , 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 and a 32-inch of 490 miles with a period of about 100 telescope for . 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 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 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&. similarities and differences between these The WEP was operated until February 1973,

2-2 The scheduling of observations and control of a February 1981. Because of the satellite's low earth orbiting observatory like OAO were not excellent stability and the high resolution trivial functions. Observations had to be and wide observing range of the primary scheduled so that they did not interfere with experiment, many observations were made ground contacts. Also the various constraint which cannot be made by any other astronomical regions had to be avoided. Observing was for­ satellite now operating or in the planning bidden within 45 Q of the and within 30° of stage. the anti-sun (to prevent heating of the instru­ ment at the opposite end of the spacecraft) . The OAO series of orbiting observatories has Due to the high radiation levels encountered made many contributions to science. Some of in the South Atlantic Anomaly, observing was the more significant discoveries and obser­ not scheduled when the Spacecraft was in that vations are listed here: area. It was also desirable to avoid observing when the spacecraft was in direct sunlight due OAO-2 to the high level of background illumination. These and other constraints made efficient • 8500 television pictures of stellar scheduling of observations a difficult process fields in ultraviolet light, covering which had to be well planned in advance. 10% of the sky, Pointing of the spacecraft to one minute of • First systematic observations of the arc accuracy was possible with a tracking . Interstellar gas accuracy of about one arc second. The primary was studied by neutral Lyman orientation was provided by six gimbaled star alpha measurements. Properties of trackers with three or more locked onto guide interstellar dust (chemical composition stars. Flywheels inside the spacecraft then and size distribution) were investigated kept the star trackers on their guide stars. by studying the shape and structure of the extinction curve of hot stars, The second of the OAO series to become fully including the prominent 2200& dust ab­ operational, OAO-3 better known as Copernicus, sorption feature. Small carbon grains was launched on August 21, 1972 into a nearly were found to be the main ingredient circular low orbit with a period of' approximately of the interstellar dust, 100 minutes. Very similar to OAO-2 in its operation, Copernicus did include a few im­ • First observations of a in the provements such as improved sun baffles, a vacuum ultraviolet (Tago-Sat:o-Kosaka tracking accuracy to 0.1 arc seconds and a 1969g) revealing among other things gyro inertial reference unit as the primary the extensive Lyman alpha hydrogen attitude sensor to be augmented by four star halo first predicted by Biermann and trackers. (The pointing stability of the Trefftz Z.Astrophys. 59, 1 (1964)), spacecraft during actual operations routinely reached 0.03 arc-second, and for short periods • First observations of a nova in the of time O.Q03 arc-second stability was achieved). vacuum ultraviolet (Nova Serpentis Also carried on Copermirus was an on-board- 1970), computer (OBC) which allowed automatic operation of the spacecraft during intervals between daily • Valuable light curves of variable stars ground contacts. such as pulsating Cepheid variables and eclipsing binary stars, and first ultra­ The main scientific instrument carried by violet observations of cataclysmic vari­ Copernicus, the Princeton Experiment Package ables and faint blue stars, (PEP), consisted of two experiments. One, an X-ray detector, will not be discussed here. • First ultraviolet flux distributions of The second was a 32-inch telescope with of different Bubble types. Cassegrain optics feeding a scanning spectro­ meter. Scanning was done by means of two OAO-3 carriages moving a pair of exit slits and ultraviolet sensitive phototubes. One carriage • Observations giving valuable information scanned the first-order spectrum from 1623A to on the composition and physical properties 3185A and the second order from 711A to 1492A. of both H I clouds and circumstellar H II The scanning was done simultaneously with one regions. For H I regions with strong phototube observing the first order at 0.05A H2 lines the observations indicate mean intervals and the second phototube working at of about 8Q°K, particle 0.025A intervals. The second carriage worked densities between 10 and 1000 cnf"^ and a in a similar way scanning 1550& to 3300& in 0.4$ depletion of heavy elements, steps and 775$ to 1650& in 0.2^ steps. • Observations of wide 0 VI lines indicating Copernicus far outlived its nominal lifetime the presence of a high- (about of one year and was only just shutdown in a million degrees) coronal gas of low

2-3 instrument is a 45-cm density, possibly occupying much of The IUE scientific an echelle spectrograph and interstellar space, telescope with four SEC Vidicon television tubes (two of . The telescope is Observations of chromospheres and them in back-up cameras) • so that scattered light coranae of cool stars, carefully baffled entering the tube must suffer at least two black surfaces • Mass loss in hot stars. reflections from diffuse before it can strike a telescope mirror. is capable of obtaining both INTERNATIONAL ULTRAVIOLET EXPLORER (IUE) The instrument high resolution (0.1A) and low resolution ranges which of recommendations made by the NASA (6A) spectra in two wavelength As a result on the dispersion Mission Board on an integrated space vary slightly depending Astronomy interval from about 113OA to astronomy plan for the 1970 f s and from a study but cover the by the Research Unit of the United 3250&. Council concerning Kingdom's Science Research space­ an echelle spectrograph, The stabilization and control of the a 45-cm telescope with reaction on the development of craft is carried out by a set of work began in mid-1970 and pointing. satellite and an associated wheels which control the slews an Explorer-class accuracy is able to operate as a guest observer A one-arc-second pointing system an offset star tracker. An OBC facility. The end result of this work was maintained by all stabilization and control the International Ultraviolet Explorer (IUE) is used for launched in 1978 and placed in a geosynchronous calculations. As its name indicates, IUE is an orbit. in near international facility, the satellite and optical Observations with IUE are performed having been provided by the real-time with the guest observer having the instrumentation data Space Flight Center, (GSFC) the tele­ opportunity to take a quick look at the Goddard the cameras by the Science received from the spacecraft. This gives vision assess Research Council (SRC) and the panels by observer the ability to immediately Space Agency (ESA). The observing the quality of the spectra and to make a the European should the satellite is split between the two decision as to whether the observation time on time, sides of the Atlantic with two-thirds directed be repeated with a different exposure it is then from one control center at GSFC and the remaining etc. If the image is approved from the European control center near processed with the observatory's image third products Madrid. processing software, and the output are quality checked and given to the guest a synchronous orbit for IUE many observer with, in general, same-day or one- By choosing an the problems associated with lower orbiting day turnaround time. In most cases of session satellites are not encountered, and the operation observer can complete his observing -data in both of the satellite from an observing standpoint on IUE and leave will all his becomes very similar to that of a ground-based processed and unprocessed form.. The scientific objectives of the observatory. full year IUE project are given below: IUE has just completed its thit,d of operation (of a 3- to 5-year nominal for" • To obtain high-resolution spectra of lifetime) and is expected to continue three stars of all spectral types in order many more years. During its first to determine more precisely their years of operation more than 19,,000' images physical characteristics, have been, acquired. A few of the highlights of the first three years of observations made « To study gas streams in and around some by IUE are given here. binary systems, • Observations of hot components in a • To observe at low resolution faint stars, variety of binary systems, including galaxies, and quasars, and to interpret companions to stars mot generally these spectra "by reference to high- suspected of being binaries were made, resolution spectra, In particular several Cepheids have 'been, found to "have relatively hot • To observe the spectra of and companions. as those objects become accessible, Observations were made of symbiotic These systems t To make repeated observeations of objects systems* are stars known found to show variable which appear to be red giants but or newly spectra spectra, contain indications in their that hot continua are exciting emission t To define more precisely the modifications lines in circumstellar material. of starlight caused by interstellar dust and gas.

2*4 • Observations of the in M 100 background, increased sensitivity and. large which was discovered in April 1979 re­ aperature the ST should reach objects 100 times vealed many interesting features. Most fainter than those which can be seen by ground interesting of all were the emission based telescopes, and with the abscence of lines which appeared to originate in distortions caused by the atmosphere, time- a rapidly expanding, thin, ionized resolved observations in the range of 1 circumstellar shell located well outside msec to 1 sec will be possible. the exploding envelope. Observations of these lines has given some insight The objectives of the ST program as stated into the situation preceeding the in the Announcement of Opportunity for supernova explosion. Space Telescope are to determine:

• Observations confirming the existence • the constitution, physical character­ of the galactic corona were made. istics, and dynamics of celestial entities, • Observations were made of the two components of the quasar 0957 4- 561 • the nature of processes which occur suggesting that both components are in the extreme physical consitions indeed images of the same quasar existing in and between astronomical produced by a massive elliptical objects in the foreground acting as a . • the history and evolution of the universe; and, SPACE TELESCOPE • whether the laws of nature are While IUE continues to make observations of universal in the space-time continuum. astronomical objects, personnel at the Marshall Space Flight Center, the Goddard Space Flight To begin this task ST will carry five scientific Center, and astronomers around the world instruments (it can carry as many as five at one await the orbiting of the Space Telescope (ST) time) when it is first placed in orbit. They by the Space Shuttle, currently scheduled for will consist of the Wide-Field Planetary January 1985. The scope of the ST project Camera (WF/PC), The Faint Object Camera (FOG), is beyond that of any other astronomical the Faint Object Spectrograph (FOS), the High satellite or ground-based observatory. It Resolution Spectrograph (HRS) and the High is designed as a long-term program in space Speed Photometer (HSP). astronomy providing astronomers with a capability far surpassing that of any other The WF/PC should be sensitive from about orbiting astronomical observatory or ground 115 OA to 1.2 microns and can perform imaging based observatory. and filter at two focal ratios with fields of view of 2.67 x 2.67 and 1.1 The ST consists of a Support Systems Module, x 1.1, arc minutes. an Assembly, five focal plane Scientific Instruments, and the Scientific The FOC will be a -counting system Instruments Control and Data Handling Subsystem. sensitive from 1200i to 6500& with fields The Fine Guidance Sensors in the Optical of view of 11.0 x 11.0 and 22.0 x 22.0 arc Telescope Assembly will be used for both ST seconds. Also included is a slit Spectro­ pointing and for . The ST will graph mode for extended sources and a be placed in a circular orbit at an altitude coronograph for working near bright stars. of approximately 550 km and at an inclination of about 28° to the , giving a period The FOS will provide low resolution spectra of approximately 95 minutes. The telescope from about 1500A to about 700QA using a itself is an f /24 Ritchie-Chretien system with Digicon detector. a 2.4-meter , and will be the largest telescope ever orbited. It will have a point The HRS will provide moderate and high source spatial resolution of about 0.1 arc resolution spectra from, 1050A to about 3200A seconds with 70 percent of the total energy also using a Digicon detector. enclosed by a circle 0.2 arc seconds in diameter. The wavelength operating range will be from The HSP will provide high-accuracy photometry about 1150A out to about one millimeter; and high time resolution (16 p see) data however, the first group of five instruments will over the wavelength range 1150A to 800Q& not exploit the entire range out to the using four image dissectors, a photomultlplier millimeter end. Pointing and stability of the tube and about 48 filters. ST will be accurate to 0.007 arc seconds* slightly less in the case of objects moving Astrometry can. also be performed with ST "by with respect to the stellar background. With using the Fine Guidance System (FGS) which can the combination of high resolution, low determine, relative positions of stars to an

2-5 accuracy of 0*002 arc seconds. probability will see orbiting astronomical observatories taking a larger and larger share Of the many things which make ST unique of the observational workload. among astronomical satellites, one of the most important is the capability of in-orbit BIBLIOGRAPHY maintenance of the spacecraft and the scientific Instruments, and the ability to change scientific A Brief Description of the International instruments in orbit* These maintenance visits Ultraviolet Explorer, GSFC IUE-701-73-015 are currently planned at about 2-1/2 year in­ 1973 tervale* At approximately five-year intervals the Space Shuttle will retrieve the ST from Announcement of Opportunity for Space orbit and return it to the ground for re- Telescope, NASA March 1977 furblshment* Code, A and Savage, B, Science 177, p. 213, 1972 Astronomers will perform observations with ST from the Space Telescope Science Institute Kuhner, Defense and Space Systems Department (ST Scl) to be located at the John's Hopfclns Battele, Columbus Laboratories, private University in Baltimore, Maryland, Since ST communication will be a" low orbiting spacecraft, observations will be conducted by means of a pre-planned Leckrone,. D., Space Telescope Scientific program* Data will be relayed to ground Instruments, Society of Photo-Optical stations by way of the Tracking and Data Relay Instrumentation Engineers, Vol. 183-Space Satellite System (TDRSS) and then to the ST Optics. Reprint 1979 Scl Institute personnel will process the data. The astronomer will also have an Mar an, S. and Boggess, A.., Ultraviolet opportunity to "custom11 process his own data. Astronomy Enters the Eighties, Today, Vol. 33 No.9 p. 40, 1980 With the difficulties involved in funding projects often finds it necessary O'Dell, C., The Large Space Telescope to defend the worth of one experiment or another. Prograni s Sky and. Telescope, Vol. 44 p. 369, In the of astronomical research projects, 1.972 trying to a value to the knowledge is not an task, Qn.fi approach Orbiting Satellites Project/Science, Vol. 2 is to look at the relative values of certain No. 1, GSFC, January 1981 projects* This is the approach by of Laboratories who. has Space Telescope Mission. Operations Requirements, taken as a of an astronomical OP-01 Vol. Ill, November 21, satellite 1 ! relative worth the number of published observational papers using data from Space Telescope Seien.ce Institute Request fmirtic.iilm.if' satellite. that for Proposals, NASA publication. in 1980 nearly 30 percent of the observational published in. the Agt£QEhyg;|c§l Joygnal Spitzer, L. and Jenkins, E., Ann. Astrcm data," ami ''"in."' particular and Astrophys. Vol. 13, 133-164 nearly one out of of 1m. the Snow, T. , , Vol. 5, No. 2, p. 26 1976 which returned by orbiting to The Scientific Results from the Orbiting (QA0-2), SP-310, collect from ground-based observatories, and in the of ST* of a not the is Watts, R*, An Satellite Copernicus, Sky and Telescope, Vol. 44 the launching of QAO-2 has p/231 concerning procedures for observatories in in. of the observatory to be placed in orbit, ST. in the of however* for and the volumes* archiving* are The mtv two see by St, and In all

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