A two-element telescope of high collecting efficiency for sub-millimetre astronomy

Abstract

The special problems of the sub-millimetre region of the spectrum for observational astronomy are outlined and followed by a detailed description of the properties of the hot electron InSb bolometer. Optimization of the size and shape of the detector element for minimum noise equivalent power leads to a well-defined set of constraints on the geometry of the focal plane distribution from a reflecting telescope when such a detector is used for astronomical applications. It is then shown how, with a knowledge only of this focal plane distribution, a shaped two-element reflector, optically well matched to the detector, can be evolved. The resulting telescope which has a virtually conical secondary and quasi-paraboloidal primary, is not only more efficient at a defined wavelength than its classical paraboloid-hyperboloid counterpart of equivalent focal ratio, but maintains its enhanced efficiency throughout the half-decade range covered by the sub-millimetre portion of the spectrum.