[6.0] Space Astronomy 1960:1989

v1.0.1 / chapter 6 of 10 / 01 aug 08 / greg goebel / public domain

* With the development of space technology in the last half of the 20th century, telescopes could be placed above the Earth's atmosphere to obtain a better look at the skies, particularly in the infrared and high-energy regions of the electromagnetic spectrum. Space astronomy is now well established and has provided many significant discoveries.

[6.1] ORIGINS OF SPACE ASTRONOMY / THE 1960S

* Long before the first space flights, astronomers had dreamed of putting a telescope into space. A telescope in orbit around the Earth, above the Earth's turbulent atmosphere, would be able to obtain crystal-clear images of cosmic objects. After World War II, as astronomers became more interested in observing the regions of the electromagnetic spectrum beyond the range of visible light, they also began to realize that space-based telescopes would be able to map the sky in the infrared and the high-energy regions blocked by the Earth's atmosphere. The big rockets that had been developed during the conflict meant that the technology for putting telescopes into space was within reach.

Initially, space observations were performed by "sounding rockets", which were relatively small space vehicles that could put a payload above the atmosphere for a brief time. In 1946, a rocket-launched spectrometer developed by workers at the US Naval Observatory obtained the first ultraviolet solar spectrogram, revealing absorption features not previously detected in the radiation from any celestial object. This was a step forward, but in those days it wasn't easy to stabilize a rocket so it could perform clear observations of a target.

The development of balloons made of polyethylene plastic sheeting in the postwar period gave astronomers another option for getting above the atmosphere to observe space. Pre-war research balloons had been made of rubberized silk or other cloth, which made them expensive and, more importantly, too heavy to reach really high altitudes. Polyethylene balloons could reach much higher, up to about 32 kilometers (20 miles), on the edge of space where the "seeing" was much better.

Large numbers of balloon flights were made under the US Navy's PROJECT SKYHOOK after the war to study space radiation. Although SKYHOOK balloons were only partly inflated at launch, giving them the appearance of a green onion, the balloon envelope would expand to an ovoid at operational altitude. Sunlight passing through the translucent polyethylene envelope could produce vivid colors, and the balloons could move surprisingly fast in high-altitude winds; SKYHOOK balloons were clearly identified as the culprits in a good fraction of the "unidentified flying object (UFO)" reports of the day.

SKYHOOK led to PROJECT STRATOSCOPE, in which telescopes were lofted to high altitudes by balloons. In the late 1950s, a 30 centimeter optical telescope was carried aloft on the STRATOSCOPE I mission to take very sharp pictures of the Sun. It was followed a few years later by STRATOSCOPE 2, which carried a centimeter telescope to obtain a set of high-resolution photographs of planets and star systems with a resolution close to a tenth of an arc-second.

* Sounding rockets and balloons performed significant observations in astronomy, and remain valuable tools for astronomers in the 21st century, but artificial Earth satellites promised to provide much greater capability; even before the launch of the first Earth satellite, the Soviet "Sputnik 1", in November 1957, astronomers had envisioned placing an optical telescope in space, where there would be no atmosphere to interfere with "seeing".

Typical early space science satellites performed observations of the space environment around the Earth, measuring radiation, magnetic fields, and space plasmas, but although increasingly sophisticated "space physics" satellites continue to be launched, it's hard to generally regard them as a telescopic technology and appropriate for discussing here. The same can be said about multifunction satellites that carried secondary payloads for astronomical research, as well as the planetary probes launched since the early 1960s, even though the probes have returned far more information about the planets than could have ever been obtained by any Earth-based telescopes.

* The first dedicated space astronomical observatories were the US National Aeronautics & Space Administration's (NASA) "Orbiting Solar Observatory (OSO)" spacecraft for scientific studies of the Sun. Nine were launched from 1962 into 1975:

    spacecraft  launch_date  booster     notes
    __________________________________________________________________

    OSO-1       07 mar 62    Delta 
    OSO-2       03 feb 65    Delta C 
    OSO-C       25 aug 65    Delta C     Launch failure.
    OSO-3       08 mar 67    Delta C
    OSO-4       15 oct 67    Delta C
    OSO-5       22 jan 69    Delta C
    OSO-6       09 aug 69    Delta N     Included secondary payload.
    OSO-7       29 sep 71    Delta N     Included secondary payload.
    OSO-8       21 jun 75    Delta 1914
    __________________________________________________________________

* The first more or less general-purpose orbiting astronomical satellites were the NASA "Orbiting Astronomical Observatory (OAO)" spacecraft. Four were launched, though only two were successful:

    spacecraft  launch_date  booster        notes
    __________________________________________________________________

    OAO-1       08 APR 66    ATLAS AGENA D  Quickly failed in orbit.
    OAO-2       07 DEC 68    ATLAS CENTAUR  AKA "Stargazer".
    OAO-B       30 NOV 70    ATLAS CENTAUR  Launch failure.
    OAO-3       21 AUG 72    ATLAS CENTAUR  AKA "Copernicus".
    __________________________________________________________________

One of the most significant discoveries of the 1960s in space astronomy was made by a set of spacecraft that weren't actually designed as astronomy satellites. From 1963 into 1970, the US Department of Defense placed a constellation of "Vela" satellites into orbit to monitor nuclear tests. The Velas were in the form of beachball-sized polyhedrons, with radiation detectors at the vertices; the time delay between detection of radiation between the different detectors permitted coarse determination of the direction to the blast.

Although the Velas were military surveillance satellites, they had a secondary mission to detect celestial events that might be mistaken for evidence of a nuclear detonation. They did in fact observe some large celestial X ray events, and more importantly began to pick up what became known as "gamma ray bursts (GRBs)", which were very powerful bursts of gamma rays that happened about once a day and came from no particular place in the sky. Astronomers would find themselves puzzling over GRBs for the rest of the century.

* In the late 1960s, NASA launched the first dedicated radio astronomy satellite, the "Radio Astronomy Explorer (RAE)", more officially known as "Explorer 38". Explorer 38 was launched into Earth orbit by a Thrust Augmented Improved Delta booster on 4 July 1968. It had a launch mass of 190 kilograms, and carried a payload of set of radio receivers to pick up long-wavelength signals. It featured a dipole antenna extending 18.3 meters to each side, and two pairs of antennas mounted top and bottom, with each pair forming a "vee" with an angle of 60 degrees and with each aerial having a length of 229 meters. The long antennas were stowed in the form of reels of copper-beryllium alloy tape, which curled into a cylinder when deployed.

BACK_TO_TOP

[6.2] SPACE ASTRONOMY IN THE 1970S

* The 1960s had introduced dedicated space observatories like the OAO series; the 1970s were noted for the flight of the first generation of high-energy observatories. The instruments used for such observations fell roughly into four categories: "proportional counters", "grazing-incidence telescopes", "spark chambers", and "scintillation detectors".

Proportional counters are somewhat better known as "Geiger-Mueller" or just "Geiger" counters. They are built around gas-filled tubes, with radiation, usually X rays, passing through the tube ionizing the gas and closing an electric circuit to giving an electrical signal that can be read. A heavy-metal "collimating screen" could be fitted to such counters to give them a degree of directionality. Proportional counters had actually been flown during the 1960s on sounding rockets, mostly under the initiative of Riccardo Giacconi (born 1931), an Italian-born physicist residing in the US who became one of the main driving forces behind space high-energy astronomy. Giacconi and his team performed the first space X ray observations on 19 June 1962, with a sounding rocket carrying a proportional counter system. Although the rocket only performed five minutes of observations, that was long enough to spot a bright X-ray source.

As far back as 1959, Giacconi had envisioned a more sophisticated X-ray imaging instrument, the "grazing-incidence" X ray telescope. From the higher end of the ultraviolet spectrum up into the X ray spectrum, a conventional mirror can no longer focus photons, since they go right through or are absorbed. However, if such relatively high-energy photons hit a dense, polished surface at a low angle of incidence, they will be reflected, in much the same way that a rock can be skipped over water. The concept of grazing incidence of X rays was not invented by Giacconi, but he was the first to propose an astronomical telescope based on such principles. In a grazing incidence telescope, the mirrors are in the form of concentric barrels, usually in two stages, focused on a proportional counter system or other X ray detectors.

The spark chamber was developed for ground-based particle physics; it was something of an extension of proportional counter technology and is useful for high energy particle and gamma ray observations. While proportional counters use a single gas-filled tube with a single connection, a spark chamber consists of a gas-filled box, with sets of wires strung across in layers that alternate at right angles, forming a three-dimensional matrix of electrified row and column "lines". When a particle zips through the chamber, it ionizes the gas in its path, causing sparks to jump between row and column lines. Although gamma rays are neutral particles and can't be directly tracked by a spark chamber, high energy gamma rays can convert themselves into electron-antielectron pairs, which can be tracked to provide directional and energetic data on the gamma ray that produced them.

Scintillation detectors go back to the first decades of the twentieth century, when researchers were trying to probe radioactivity and the atom. Certain materials -- these days, usually plastics -- will emit light or "scintillate" if a high-energy particle passes through them. In the early days, researchers would actually observe experiments through a plate of scintillation material and take notes on when scintillations occurred, but in modern days scintillation detectors consist of a scintillating material capping a photomultiplier tube, which converts the scintillations into electric signals and amplifies them.

* The first serious high-energy astronomy mission was the "Small Astronomy Satellite 1 (SAS-1)", a pioneering orbiting X ray observatory. During the 1960s, Riccardo Giacconi and others had managed to map a few dozen bright X ray sources using sounding rockets; Giacconi, always ambitious, proposed an X ray observatory satellite to NASA in 1963. The spacecraft that emerged weighed only 64 kilograms. It consisted of a small bus, based on that used by the US Navy Transit-O navigation satellites, carrying an X ray payload and surrounded by four solar cell paddles. The X ray payload was a set of two back-to-back proportional counters behind heavy-metal collimating screens; one counter had 5 degree angular resolution, the other had 0.5 degree resolution.

SAS-1 was launched into orbit on 12 December 1970 by a Scout light booster from the Italian San Marco launch platform, which was a modified and somewhat decrepit oil platform off the coast of Kenya. The launch site was chosen to make launch easier, since the rotational velocity of the Earth is greatest at the Equator, making it easier for the satellite to attain orbital velocity; also, the equatorial orbit avoided Earth's radiation belts. After launch, the satellite was named "Uhuru", from the Swahili word for "freedom", since 12 December was Kenya's Independence Day.

At the end of the mission of Uhuru, 27 months after its launch, the number of celestial X ray sources had climbed to about 300. One of its most significant discoveries was that many of the X-ray sources within our galaxy were, in fact, binary stars, providing ammunition for theorists to construct models of X ray star systems. The usual model was of a superdense "neutron star", an object a few kilometers across that had the mass of a full-sized sun, pulling material off a normal stellar neighbor. One of the X ray sources, "Cygnus X-1", seemed to be a normal star around a very massive superdense object, one that was so dense that light couldn't escape from it. Astronomers believed they had discovered the first "black hole".

Two more similar SAS spacecraft were put into orbit by Scout boosters from the San Marco platform, including "SAS-B", launched on 15 November 1972, and "SAS-C", launched on 7 May 1975.

* The final two Orbiting Solar Observatories also carried high-energy astronomy payloads. OSO-7, launched on 29 September 1971, was a larger spacecraft than previous OSOs, with a launch mass of 635 kilograms and only a very general resemblance in configuration to the earlier OSOs. OSO-8, launched on 21 June 1975, was larger still, with a launch mass of 1,066 kilograms, a wheel that looked like a bass drum, and a large rectangular sail. The OSOs performed valuable observations of the Sun in the ultraviolet, X ray, and gamma ray regions of the electromagnetic spectrum.

* Other nations were performing high energy astronomy studies at the time as well. The "Netherlands Astronomy Satellite" or "ANS" in its Dutch acronym was launched on a Scout-D booster from Vandenberg Air Force Base (AFB) in California on 30 August 1974. It carried ultraviolet, soft X ray, and hard X ray instruments, and performed a sky survey during its almost three years of observations. It was also one of the first spacecraft to detect GRBs.

* The British launched a series of six "Ariel" science satellites on US Scout boosters from 1962 into 1979. While the first four were focused on Earth ionosphere studies, the "Ariel V" spacecraft, launched on a Scout B booster from the San Marco platform on 15 October 1974, was a dedicated X-ray observatory with a sophisticated instrument suite. It operated for five years and made a number of major discoveries. It discovered a new class of short-lived transient X-ray "flare" sources.

The "Ariel VI" satellite, launched by a Scout D booster from Wallops Island, Virginia, on 2 June 1979, also performed observations of the X ray and gamma ray sky, but it was badly hobbled by technical problems.

* The European Space Research Organization (ESRO), later and now the European Space Agency (ESA), developed one of the first spacecraft to carry an ultraviolet telescope, the "TD-1" spacecraft, launched by a Delta-N booster on 3 December 1972. The "TD" stood for "Thor Delta", referring to the launch vehicle. TD-1 had a launch weight of 473 kilograms and looked like a rectangular box with solar arrays on each side. It carried ultraviolet, X ray, gamma ray, and particle instruments, including a spark chamber instrument. Over 30,000 UV sources were cataloged.

* The ESA also developed the first dedicated gamma-ray mission, designated "COS-B", which was launched into Earth orbit from Vandenberg AFB by a NASA Delta 2914 booster on 9 August 1975. It was in the shape of a drum covered with solar cells and had a launch weight of 278 kilograms.

COS-B was built around a spark chamber, which was electrically armed by a separate instrument on the bottom of the chamber composed of a set of scintillation counters. The entire assembly was surrounded by a scintillating screen that was sensitive to particles but not gamma rays, allowing confounding events to be detected and rejected. COS-B also carried a proportional counter system to measure X rays in hopes of linking gamma-ray emission to X ray sources. Although COS-B was only designed for two years of operation, it was still operating when the mission was finally terminated in 1982.

* The Japanese Institute of Space & Astronomical Sciences (ISAS) launched the first Japanese space observatory, the "CORSA-B" X ray satellite, on 21 February 1979. It was launched from the ISAS space center at Kagoshima on an ISAS M-3C-4 booster, and renamed "Hakucho (Swan)", since among its prime targets were X ray sources in the constellation Cygnus, the Swan. Hakucho performed valuable observations of variable and transient X ray sources into 1985.

* Several manned space flights conducted in the 1970s also performed astronomical research. The NASA Skylab space station, launched on the last flight of a Saturn V heavy-lift booster on 14 May 1973, was visited by three crews through the rest of 1973, each launched on an Apollo capsule carried by a smaller Saturn 1B booster. The Skylab crews performed a wide range of astronomical observations, particularly in the UV and X ray regions of the sky. Most of the instruments were carried on a specialized module, the "Apollo Telescope Mount (ATM)", but one of the X ray instruments was actually flown on the Saturn IB upper stage that put the second Skylab crew into orbit. Skylab was also the first spacecraft to carry a grazing-incidence telescope, in the form of a relatively small camera system for solar extreme UV / soft X ray studies.

The Soviets conducted manned space astronomy missions as well. The first, the "Soyuz 13" space capsule, carrying the two cosmonauts Pytor Klimuk and Valentin Lebedev, was launched by a Soyuz booster from the Baikonur Cosmodrome in Kazakhstan on 18 December 1973 on an eight-day mission. Soyuz 13 was a unique mission, the only time a manned space capsule as such was flown as an astronomical observatory, with the "Orion" UV telescope array in place of the docking adapter normally fitted to the Soyuz capsule. The capsule also performed Earth observations and carried a biological studies payload.

The very last Apollo flight, the rendezvous between Apollo 18 and Soyuz 19 in July 1975 as part of the American-Soviet "Apollo-Soyuz" test project, also carried an astronomical payload, an "Extreme Ultraviolet (EUV)" telescope. It was a very modest experiment, intended mainly to see if there was any point to conducting observations in the EUV range; there was some thinking that the tenuous haze of interstellar hydrogen would block EUV emissions, but the Apollo-Soyuz instrument found four sources. As it later turned out, the density of interstellar hydrogen in our region of space is relatively thin. A supernova in the distant past cleared out the interstellar gas over a radius of hundreds of light years, forming a "local bubble" in which EUV observations were possible.

Astronomical studies were also performed as part of the experiments on the Soviet "Salyut (Salute)" space stations, placed into orbit beginning in 1971 with the "Salyut 1" civilian space station, a mission that would end in tragedy. Four successful Salyut stations were launched in the 1970s, including two military Salyuts (3 & 5) that did not emphasize scientific research, and two civilian Salyuts (4 & 6) that did. Salyuts 4 & 6 carried a suite of astronomical instruments, and Salyut 6 was even temporarily fitted with the small "KRT-10" radio telescope for long-baseline observations in collaboration with ground-based radio telescopes, but this exercise was not a big success.

* In fact, although useful astronomical observations were performed by manned spacecraft, the real stars of the show were unmanned satellites. The final Orbiting Astronomical Observatory, OAO-3 or "Copernicus", launched on 21 August 1972, was partly a pathfinder for such missions. Copernicus was much more sophisticated than the previous OAOs, with a launch weight of about 2.2 tonnes. Copernicus featured an 81 centimeter telescope and a high-resolution spectroscopic system; the pointing system employed by Copernicus could keep the spacecraft on target for several minutes with a maximum deviation of about 0.02 arc-second. It was a highly successful mission, remaining in service into 1981, almost ten years,

The first of three "High Energy Astrophysical Observatories (HEAO)" was launched from Cape Canaveral on 12 August 1977; it was not a radical departure from earlier X ray spacecraft in terms of technology, with a payload of survey instruments operating through the X ray and into the low gamma ray regions of the electromagnetic spectrum, but it was much bigger and more sophisticated, with a launch mass of 2,720 kilograms, requiring use of a big Atlas Centaur booster. HEAO-1 conducted sky surveys into 1979.

The other two HEAOs were of similar size and were also launched by Atlas Centaur boosters. It was HEAO-2 that was the most innovative of the two. HEAO-2, which was launched on 13 November 1978 and renamed "Einstein" once it was in operation, was the first spacecraft to carry a grazing-incidence X ray telescope. This instrument had an outer diameter of 58 centimeters and consisted of a set of four nested metal "barrels" with parabolic surfaces, feeding X rays into a second set of four nested barrels with hyperbolic surfaces, which in turn fed four instruments that were mounted on a turntable at the focus of the telescope. Two instruments were specifically designed for producing X-ray images -- one with a large field of view of 1 degree but a low resolution of 1 arc-minute, the second with a smaller field of view of 20 arc-minutes but a high resolution of 4 seconds -- and the other two obtained X-ray spectra.

The Einstein mission was remarkably successful. It revealed that most stars generate far more energy in the X-ray region than our Sun; it discovered many hundreds of X-ray binary systems, with enough accuracy to allow the corresponding optical objects to be located. Hundreds more extragalactic X-ray sources, mostly active galaxies, were found. Some remarkable images of supernova remnants were obtained; and observations of quasars showed them to have X-ray cores that fluctuated in brightness so quickly that they could not have been more than a few light-hours across.

In 1980, Einstein's attitude-control system went crazy, and a considerable amount of thruster fuel had to be expended to regain control of the spacecraft. The thrusters ran out of fuel a few months later, and the satellite became useless, after surveying only 5% of the sky.

HEAO-3 was more similar to HEAO-1 -- carrying a suite of survey instruments but no grazing-incidence telescope -- except for the fact that HEAO-3 was built to perform surveys in the "hard" X ray and gamma ray regions of the spectrum. It was launched on 20 September 1979 and operated into the spring of 1981.

* Earlier experiments in ultraviolet space astronomy also led to a larger UV observatory spacecraft, the Anglo-American "International Ultraviolet Explorer (IUE)" mission. It was launched by a Delta 2914 booster on 26 January 1978, and had a launch mass of 671 kilograms. It was built around a 45 centimeter ultraviolet telescope, which could feed two spectrographs, one in the longwave region of the UV spectrum, the other in the shortwave region.

IUE was an astoundingly successful mission, remaining in service for 18 years, until 1996. The IUE kept a mascot, the Energizer Bunny, a toy mechanical bunny that banged on a drum as a sales gimmick for Energizer batteries; like the Energizer Bunny slogan, IUE just kept on "going and going and going". It obtained ultraviolet spectra of planets, moons, and comets, on stars in all stages of their evolution, X-ray sources, active galaxies and quasars, and coronas and gaseous bridges associated with galaxies.

* On the other side of the spectrum, in 1974 NASA began operation of the "Kuiper Astronomical Observatory (KAO)", which was a four-jet Lockheed C-141 cargolift aircraft modified to carry a 91 centimeter infrared telescope to high altitude, above most of the atmospheric water vapor that blocks out celestial infrared radiation. It was the predecessor to infrared space observatories and made a large number of important observations in over two decades of operation.

A second and final RAE radio astronomy satellite was also flown in the 1970s. "Explorer 49" was of similar configuration to Explorer 38. It was launched by a Delta 1913 booster on 15 June 1973, being placed into an orbit around the Earth's Moon to reduce radio interference from terrestrial sources. The RAEs performed observations of signals below about 10 MHz; it appears that their antenna array was directional, but it is difficult to determine specifics. In any case, they were the last dedicated radio astronomy satellites for decades. Two more RAEs were planned, but they were not funded.

BACK_TO_TOP

[6.3] SPACE ASTRONOMY IN THE 1980S

* The 1980s saw the launch of the first major space infrared observatory, the NASA-Dutch "InfraRed Astronomy Satellite (IRAS)", which was put into orbit by a Delta 3910 booster launched from Cape Canaveral on 26 January 1983. IRAS carried a 60 centimeter infrared telescope, fitted into a Dewar flask chilled by liquid helium to increase its sensitivity. (It is difficult to spot thermal sources using an instrument that is warmer than the sources.) IRAS only operated until its helium ran out, but in that time it performed the first infrared sky survey, mapping 250,000 sources.

* The 1980s saw the introduction of a new class high-energy telescope, the "coded mask" telescope. The grazing incidence telescope was a great step forward in high energy astronomy, but it had an inherent limitation: as the energy of the X rays increases (or, equivalently, their wavelength decreases), the grazing incidence angle became smaller, and the telescope becomes impractically long and heavy, sort of a modern equivalent of the aerial telescopes of Huygens' day.

The only practical way to obtain an image from high-energy X rays is through a "pinhole camera" scheme. Pinhole cameras consist of a box with a small hole on one side and a translucent screen on the other; an image passes through the pinhole and is displayed, upside down and reversed, on the screen. It's a primitive tool, in particular limited by the fact that the flux of light through the pinhole is very small, giving a faint image.

The magnification of a pinhole camera is proportional to the distance between the pinhole and the screen, with the image becoming fainter as it is magnified. The clarity of the image is a function of the pinhole size, with a small pinhole yielding a crisp but faint image, and a large pinhole yielding a fuzzy but bright image.

A pinhole camera could be built for X rays using a heavy metal plate with a pinhole in its center, but that would give a faint image. To give a brighter image, multiple pinholes could be drilled through the plate, but that would result in a different image for each pinhole, with no two images registering on top of each other. However, the pinholes can be very carefully arranged or "coded" to permit the overlapping images to be sorted out using computer power to perform a mathematical operation known as "correlation", providing a single bright image. An X ray pinhole camera with multiple coded pinholes is called a "coded mask imager".

The coding scheme is based on a mathematical concept known as a "cyclic different set". To understand a cyclic difference set, suppose the integers from 1 to 15 are arranged as follows:

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ... 

   1 2 3 - 5 6 - - 9  - 11  -  -  -  -

   1 2 - - - - - - -  -  -  -  -  -  -
   - 2 3 - - - - - -  -  -  -  -  -  -
   - - - - 5 6 - - -  -  -  -  -  -  -

   1 - 3 - - - - - -  -  -  -  -  -  -
   - - 3 - 5 - - - -  -  -  -  -  -  -
   - - - - - - - - 9  - 11  -  -  -  -

   - 2 - - 5 - - - -  -  -  -  -  -  -
   - - 3 - - 6 - - -  -  -  -  -  -  -
   - - - - - 6 - - 9  -  -  -  -  -  -

   1 - - - 5 - - - -  -  -  -  -  -  -
   - 2 - - - 6 - - -  -  -  -  -  -  -
   - - - - 5 - - - 9  -  -  -  -  -  -

   1 - - - - 6 - - -  -  -  -  -  -  -
   - - - - - 6 - - -  - 11  -  -  -  -
   1 - - - - - - - -  - 11  -  -  -  - 

This pattern holds for all values up to 15, at which point the pattern simply starts over again. This is a "base 15" cyclic difference set. A cyclic difference set could be generated for other bases -- for example, selecting a cyclic difference set from 1 through 27 would give a "base 27" cyclic difference set -- but the basic pattern is the same no matter what the set. So how are such cyclic differences found? There's actually no neat way to do it, they simply have to be found by an educated trial-and-error approach, something along the lines of the techniques used to find prime numbers. There are several different families of cyclic difference sets that have different patterns.

From the point of view of designing an X-ray coded mask, the virtue of a cyclic difference set is that the pattern of intervals remains distinct even if all the values are incremented or "shifted" by a certain amount. If we have the base-15 cyclic difference set:

   1 2 3 - 5 6 - - 9  - 11  -  -  -  -

   - - - X X X - X X - - X  -  X  - 

To apply cyclic-difference sets of base N to construction of a coded X-ray mask, first visualize a regular pattern of N equally-spaced holes in a grid, and then number them from 1 to N, moving diagonally down through the grid from the upper left corner and wrapping back up to the top when the bottom is reached. For example, given a base-15 set, we get three rows of five, numbered as follows:

   [ 1 ] [   ] [   ]  [   ]  [   ]
   [   ] [   ] [   ]  [   ]  [   ]
   [   ] [   ] [   ]  [   ]  [   ]

   [ 1 ] [   ] [   ]  [   ]  [   ]
   [   ] [ 2 ] [   ]  [   ]  [   ]
   [   ] [   ] [   ]  [   ]  [   ]

   [ 1 ] [   ] [   ]  [   ]  [   ]
   [   ] [ 2 ] [   ]  [   ]  [   ]
   [   ] [   ] [ 3 ]  [   ]  [   ]

   [ 1 ] [   ] [   ]  [ 4 ]  [   ]
   [   ] [ 2 ] [   ]  [   ]  [   ]
   [   ] [   ] [ 3 ]  [   ]  [   ]

   [ 1 ] [   ] [   ]  [ 4 ]  [   ]
   [   ] [ 2 ] [   ]  [   ]  [ 5 ]
   [   ] [   ] [ 3 ]  [   ]  [   ]

   [ 1 ] [   ] [   ]  [ 4 ]  [   ]
   [   ] [ 2 ] [   ]  [   ]  [ 5 ]
   [ 6 ] [   ] [ 3 ]  [   ]  [   ]

   ...

   [ 1 ] [ 7 ] [ 13]  [ 4 ]  [ 10]
   [ 11] [ 2 ] [ 8 ]  [ 14]  [ 5 ]
   [ 6 ] [ 12] [ 3 ]  [ 9 ]  [ 15]

   [ 1 ]   -     -      -      -  
   [ 11] [ 2 ]   -      -    [ 5 ]
   [ 6 ]   -   [ 3 ]  [ 9 ]    -  

In practice, a coded mask pattern is generated by a computer program. The blank coded mask plate -- which may be made of tungsten, or other materials with a top layer of gold -- is covered with a photosensitive material and then the pattern is exposed on it. The photosensitive material is cured, and then washed away from where the holes are supposed to be. The holes are bored through using chemical etching. To finish the telescope, the coded mask is mounted over the top of a proportional counter array. Masks can be made with tens of thousands of holes. Of course, performing a correlation with such a large array requires some serious computing horsepower.

Incidentally, although coded mask telescopes can reach short wavelengths that grazing incidence telescopes can't, in their own domain grazing incidence telescopes can achieve a better focus and greater image brightness. The two technologies are complementary.

The first flight of a coded mask telescope was in 1976, when a test article built by researchers from the University of Birmingham in the UK flew one on a sounding rocket. The first serious use of a coded mask telescope was during the NASA / ESA "Spacelab 2" mission, launched on the NASA space shuttle Challenger on 29 July 1985. The ESA's Spacelab was a modular system of manned or unmanned science payload modules carried in the shuttle cargo bay; on this flight, the Spacelab payload suite included two coded mask telescopes.

Both were about 3 meters long, but one was a high resolution instrument, while the other was a low resolution instrument. The high resolution instrument had a mask with small holes, each about 2.5 millimeters in diameter, giving it a resolution of three minutes of arc, about a tenth the apparent diameter of the Moon as seen from Earth. The low resolution telescope had a mask with holes about four times as big, giving it poorer resolution; it was used to image extended, diffuse regions of X-ray emission. Image data was relayed to the NASA Johnson Space Flight Center in Houston, Texas, for data crunching, with crude "quick and dirty" images returned to the shuttle to guide operations of the telescopes.

The Soviets had launched their very last space station, Mir, on 20 February 1986 using a Proton booster. It was a "modular" space station, with a hub with six ports allowing new station components to be "plugged in". One of the components was the "Kvant (Quantum)" astronomy module, which was launched on 31 March 1987 on another Proton, carrying UV and X ray telescopes, including a coded mask telescope developed by British and Dutch researchers. It was smaller than the Spacelab coded mask telescopes, but it was launched in time to observe the later stages of Supernova 1987.

The Soviets followed up the Kvant instrument with a satellite named "Granat". launched on 1 December 1989 on a Proton booster. It was built as a collaboration between the USSR and several European countries, carried an sophisticated suite of high energy astronomy instruments, including a large coded mask telescope, providing close-up observations, wide-area surveys, and capture of transient cosmic high-energy events.

* Another landmark space astronomy mission of the 1980s was the ESA "Hipparcos (High Position Parallax Collecting Spacecraft)" stellar astrometry spacecraft, which was put into space by an Ariane 4 booster from the ESA launch center at Korou on 8 August 1989. Hipparcos was to measure the positions of the stars in the sky to up to 1 milliarcsecond, an accuracy 100 times greater than ever achieved before. As a member of the Hipparcos science team put it: "You put a golf ball on the Empire State Building and view it from Europe -- that's the kind of measurement accuracy we're talking about."

The shift in position of the stars over a six-month period could be used to obtain much better estimates of the distances of relatively nearby stars. These estimates could be used in turn to providing calibration for other, longer distance "yardsticks", most particularly the "Cepheid variable" stars, whose brightness is proportional to their period of variation, allowing them to be used as distance indicators.

Hipparcos suffered a serious malfunction during launch. Due to a failure of the spacecraft's rocket motor, instead of being injected into its "geostationary" orbit at an altitude of 36,000 kilometers (22,400 miles), the spacecraft remained in its highly elliptical "transfer" orbit, rising up to geostationary distance and then falling back low to Earth again. Trying to perform observations from such an orbit seemed difficult at best, and worse, the transfer orbit took Hipparcos through the Earth's radiation belts, slowly frying it.

However, after an initial period of despair, the science team managed to make the measurements, and the spacecraft remained in operation until 1993, returning a terabyte of data. Since all stars have some proper motion of their own, the data had to go through considerable crunching by computer. The initial catalogues of star positions based on Hipparcos data were published in 1997, but there were discrepancies with ground-based observations. It turned out that temperature cycling of the spacecraft had caused it to twist slightly, skewing some of the results. After ten more years of work, a revised catalogue was released in 2007. The refined data gave astronomers new insights, for example in showing that the Cepheid variable yardstick was off by about 10%; objects whose distances had been calculated using the Cepheids turned out to be about %10 farther away than had been previously estimated.

* Hipparcos was quickly followed into space by another groundbreaking space astronomy mission, the NASA "Cosmic Background Explorer (COBE)", launched from Vandenberg by a Delta booster on 18 November 1989. The satellite carried instruments to map the "cosmic microwave background (CMB)", which is believed to be the afterglow of the explosive "Big Bang" in which the Universe was born, and the "cosmic infrared background (CIB)", which gives an indication of the distribution of matter in the Universe.

The CMB appears as a faint emission from all over the sky, matching the thermal emission of a body at a temperature of 2.7 degrees Kelvin. A map of the subtle variations in the CMB would give major clues to the state of the early Universe. To create this map, COBE carried a suite of three instruments a set of set of "differential microwave radiometers (DMRs)", an infrared spectrophotometer, and an infrared photometer. The spectrophotometer and photometer were mounted in a liquid helium-filled Dewar to provide cooling for sensitivity, with enough liquid helium to provide cooling for a year. COBE spun while in orbit, scanning the sky, with bright sources like stars and galaxies subtracted from the data using computer power.

* Other space astronomy missions were conducted by Japan, the Soviet Union, and the ESA in the 1980s. Japan's ISAS launched a series of three "Astro" high-energy astronomy satellites during the 1980s, including "Astro-A" or "Hinotori (Phoenix)"; "Astro-B" or "Tenma", after the Japanese name for the constellation Pegasus; and "Astro-C" or "Ginga (Galaxy)". All were launched by M-3S boosters from the ISAS launch center on Kagoshima:

• Astro-A / Hinotori was launched on 21 February 1981. Hinotori had a launch weight of 188 kilograms, resembled an octagonal box with four solar arrays around the bottom, and performed imaging and spectroscopy of solar flares during a solar maximum.

• Astro-B / Tenma was launched on 20 February 1983. Its configuration was similar to that of Hinotori, but Tenma was focused on deep space observations, using a set of proportional counters for X ray observations, a wide-field transient event detector, and a gamma ray burst detector.

• Astro-C / Ginga was launched on 5 February 1987. It was much like Tenma, with a similar but enhanced instrument suite.

The USSR's "Astron-1" was the first major Soviet space astronomy satellite. It was launched by a Proton heavylift booster from the Baikonur Cosmodrome in Kazakhstan on 23 March 1983. It was a large satellite with a launch mass of 3,250 kilograms, using a spacecraft bus based on that developed for the Venera series of planetary probes. Its major instrument was an 80 centimeter UV telescope, built with French assistance. It also carried an X ray spectrometer. The mission was very successful, operating into 1989 despite the fact that it was only designed to last a year.

The ESA "Exosat" X ray observatory was launched from Vandenberg AFB on 16 May 1983. It had a launch mass of 510 kilograms and looked like a box with a single solar sail on top. Its instrument suite included two grazing-incidence X ray telescopes and two proportional counter systems. It performed observations into 1986. However, development of the spacecraft had been so protracted that the results of the mission were no more than satisfactory.

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