Telescopes In Space (Astronomy In Space)

We have already emphasized, in the introduction to this chapter, the great value of launching telescopes into space. This section concentrates on the infrared, optical and ultraviolet wavebands. It is unlikely that radio telescopes will be placed in orbit in the near future, owing to the enormous size necessary for reasonable angular resolution, but several designs have been proposed which make full use of the weightless environment. Spacecraft carrying radio detectors (other than those necessary for spacecraft performance) may also serve as links in very-long-baseline interferometers, or can be designed to explore very long wavelengths.

Early attempts to overcome the atmospheric limit to optical spatial resolution included balloon-borne telescopes. The STRATOSCOPE missions carried a 90-cm telescope and associated television camera to a height of 20km. The resolution was degraded to about I 0.5 arc second, from the theoretical diffraction limit of 0.1 arc second owing to the convection of residual atmosphere in the telescope tube. Several flights were failures. The Orbiting Astronomical Observatories were initially conceived in the 1950s and several successful launches have been made. OAO-2 launched in 1968 carried equipment to survey the ultraviolet sky from about 100 to 300 nm. This follows the general pattern for opening new wave¬bands: surveys followed by more detailed studies.

OAO-3, renamed COPERNICUS after launch in August 1972, is a good example of precision instrumentation actually in action. The major experiment consisted of a ultraviolet telescope spectrometer of 80cm clear aperture. The spectrometer entrance slit is at the Cassegrain focus and the spectrometer itself and a fine guidance assembly, (generating signals to maintain spacecraft pointing directions), are situated in front of the primary mirror, partially obscuring it. Two movable carriages containing photo¬tubes can be programmed to scan the spectrum from about 71 to 328 nm with a resolution of at best about 0.5nm. Such a system is ideal for probing the interstellar medium along the line of sight to hot stars, and since it operates in the ultraviolet it can, for example, detect interstellar molecular hydrogen absorption lines. Commands to, and data from, the spacecraft are transmitted when it passes over one of the satellite ground stations, such as the one maintained at Quito in Ecuador. Gas jets and gyroscopes can swing the space¬craft around to any selected target. Pointing on that target can then be maintained to better than, a tenth of an arc second if the fine guidance system in the telescope is locked onto a bright star. A small package of X-ray telescopes and detectors is also included on Copernicus. Many X-ray sources are not bright enough in the optical or ultraviolet wavebands to operate the fine guidance system. In that case, a bright nearby star would be observed and then the instrument rotated in the required direction to align precisely the X-ray telescopes on the required object. Spacecraft technology is at present capable of producing inertial platforms that point with milliarcsecond accuracy or less, which is much less than the image size on even the most ambitious space telescopes yet proposed.

Infrared observations can be made from the Earth’s surface through a number of poor windows at scattered wavelengths. Balloon and aircraft-borne telescopes have extended the range of results, but telescopes have been limited so far to about 2m diameter, and the residual atmosphere has still severely hampered observations. The rewards of infrared astronomy are enormous; the investigation of star formation, molecular lines, galactic nuclei, the planets and so on. A major difficulty in developing satellite instrumentation is the necessity to cool at least the detectors and possibly the telescope itself. An active refrigeration scheme would use a lot of power, and a more passive approach using liquid helium would limit satellite operations to a relatively short lifetime. Nevertheless, survey-type satellites have been proposed, and a large infrared telescope has been proposed as an instrument for some Space Shuttle flights.

The SPACE SHUTTLE is a reusable transportation system designed to carry scientific and application payloads routinely to and from low Earth orbit during and after the early 1980s (figure 23.6). The shuttle can put free-flying spacecraft into orbit, possibly using an additional module called the TUG, to inject these into higher orbits. It can also retrieve and refurbish other satellites already in orbit, and it can take a laboratory, such as the proposed European SPACELAB into low Earth orbit for a period of a week to a month. Spacelab need not entirely fill the space within the shuttle, which would then be available for other instrumentation, perhaps astronomical. A 3-m diameter infrared telescope with liquid helium-cooled detectors could be carried on such a mission, yielding vast quantities of important data from even one week’s operation.

Perhaps the most significant astronomical instrument so far proposed for a shuttle launch is the US SPACE TELESCOPE (ST). This is intended to contain a 2.4m Cassegrain telescope injected into orbit from the Space Shuttle in about 1985. Its design lifetime is at least 15 years; regular maintenance and per¬haps retrieval back to Earth for brief periods by the crew of the Shuttle make this realizable. The angular resolution of such a telescope would be better than 0.1 arc second. This is in the optical band, although it is designed to operate from 91.2nm to about 1mm, thereby spanning ultraviolet, optical and infrared wave¬bands. The telescope could be pointed at any desired target with an accuracy of 0.03 arc second and stabilized on that target to 0.005 arc second. The implications for resolving planets round other stars, binary star systems, astrometry and extending all the fundamental reference scales are enormous. The focal plane will probably contain several detectors on a turret arranged so that anyone can be commanded into the observing position. Suggested instrumentation includes cameras, photometers and spectrographs, to cover the different spectral ranges.

The Space Telescope will be able to detect optically objects over 100 times fainter than those detectable on Earth, that is, objects down to magnitude 29. A space telescope similar to that described here would have the capability of detecting extended objects of low surface brightness. This would have a significant impact on our knowledge of the outer parts of galaxies, as well as revealing nebulae and clouds located within our own galaxy. The giant radio emitting lobes on either side of most radio galaxies may in fact emit more energy in optical wavebands than in the radio. This has been detected by careful ground-based studies, being but a few per cent of the night-sky background. Optical emission at this level would become a relatively easy target for a space telescope.

The use of an occulting disc, similar to that in a coronagraph, may reveal planetary systems in orbit about neighbouring stars. Scattering in the Earth’s atmosphere renders such a task completely fruitless from the ground. Studies of our own outer planets would allow Uranus, for example, to be resolved to about the same degree as the Pioneer spacecraft photographs of Jupiter. Complete spectra, from the infrared through to the ultraviolet, of a wide range of stars and galaxies will be invaluable for stellar evolution and cosmology.

Other telescope systems can be devised such as very wide-field (60° or so) cameras, deep-sky telescopes and so on that can perform complimentary tasks to a telescope similar to that outlined above. The instrumentation would be similar to that used and designed for ground bused telescopes, excepting that modifications may need to be made to compensate for the space environment and a long unattended life

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