A Thermal Infrared Heterodyne Receiver with Applications to Astronomy

A method for detection of thermal infrared radiation from astronomical sources is discussed from an instrumentation point of view. This method utilizes heterodyne detection, whereby light collected by a telescope is spatially mixed with single-mode radiation from an infrared laser local oscillator onto a HgCdTe photodiode. The operating wavelength range of the receiver is adjustable from approximately 9 to 12 μm, with an intermediate frequency response of the photodiode and associated electronics spanning from 200 to 2800 MHz, or 2.6 GHz in each sideband. The instrumentation which will be discussed is currently in use by the UC Berkeley Infrared Spatial Interferometer (ISI) at the Mount Wilson Observatory. Each of the three ISI elements consists of a 1.65m telescope with complete heterodyne receiver, and is capable of operating as the front end to a variety of different instruments. The sensitivity of the receiver to measuring weak stellar sources will be discussed and contrasted with homodyne receiver performance. Actual spectroscopic and interferometric measurements of astronomical sources will be mentioned.