Space telescopes and components

Abstract

view Abstract Citations (10) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Space telescopes and components Spitzer, Lyman, Jr. Abstract A brief analysis is given of some of the major problems involved themselves must be used for fine orientation, either with television in the design of a large satellite telescope for stellar observations. cameras and ground control, or by precise measures of angles from A 1000-kg structure is assumed, 2 m on a side, in orbit at an a few bright stars. Once a program star or guide star has been altitude of either 800 km or 36 000 km, with orbital periods of 101 acquired, automatic guidance with an accuracy better than a min and 24 hr, respectively. second of arc would appear entirely possible. Stabilization during guiding requires countering the perturbing Since storage batteries and electronic components are best kept torques due to air drag, solar radiation pressure, the tidal force of at about , while photoelectric detectors should be cooled to the earth, and magnetic forces of ferromagnetic materials. The for low dark current, a two-chamber satellite seems desir- first three torques can be reduced by appropriate symmetry, and able. The satellite rotates about the optical axis automatically to the last by use of iron with low permanent magnetization. A torque keep the warm chamber facing the sun. Solar batteries producing of 100 dyne cm should be adequate for stabilization. This torque a mean power of about 100 w are placed on this side. The other can readily be produced by applying a rotating magnetic field to side comes into equilibrium with the infrared radiation from the an "inertial sphere," consisting of an aluminum shell, positioned earth and with the heat leakage from the warm chamber; a tem- by a rapidly oscillating magnetic field. At 800 km the eddy cur- perature between - 65 and - can be achieved. To maintain rents in a sphere 40 cm in diameter, rotating in the earth's mag- the focus and resolution of the optical system one may use fused netic field, will transmit a torque as great as 100 dyne cm directly silica mirrors, with a relative thermal expansion of less than 10-6 to the earth, and thus the sphere can continue to absorb a torque over this temperature range. For positioning the various optical of this magnitude indefinitely. At an altitude of 36000 km, the elements, thermally compensated structures, using dissimilar angular momentum accumulated by the sphere must be dumped metals, may be used. An arrangement of concentric tubes promises in some way ; jets of gas, and controlled magnetic and radiative to yield the necessary thermal homogeneity required. Alterna- torques provide possible means. To set the telescope, rotation of tively, the mirrors and all supports could all be made of alu- the entire satellite by as much as 180 in 6 min can be achieved minum, and conduction through sufficiently massive supports either by an inertial sphere or by three separate wheels, one for relied upon for thermal homogeneity. each axis. As an example of these concepts, a preliminary design is pre- To sense the direction in which the telescope points, for con- sented for a spectroscopic instrument, with a 24-in. mirror and a trol of setting, a number of techniques are possible. Coarse orien- number of phototubes for scanning a stellar spectrum with high tation can be obtained by simple measures of the light from the resolution. The over-all reliability is investigated, on the basis of sun and the infrared radiation from the earth; measurement of minimum failure rates for standard components. A mean life of the direction of the earth's magnetic field and use of gyroscopes about a year is computed before accumulating failures render the present alternate possibilities. If observations can be limited to instrument unusable. The design includes considerable redundance stars of the fifth magnitude or brighter, the automatic guidance of separate systems. If each electronic component could be made system can accept all light within a circle ` degree in radius, and separately redundant, this mean life would be increased by an the coarse orientation may suffice for initial acquisition, since the order of magnitude. Because of failures not envisaged in the angle of the sun from the optical axis can easily be measured with analysis a number of separate launchings would presumably be considerable precision. For acquisition of fainter stars, the stars required to yield a fully operating instrument. Publication: The Astronomical Journal Pub Date: June 1960 DOI: 10.1086/108242 Bibcode: 1960AJ.....65..242S full text sources ADS |