Infrared Heterodyne Detection of the Moon, Planets and Stars at 10 µm

Abstract

THE possibility of applying heterodyne detection techniques in astronomy is now becoming well known^1-3. Earlier observations made in this way in the visible region of the spectrum^4,5 have indicated that its importance for astronomy lies primarily in the infrared. Preliminary measurements with a 10 µm system for solar heterodyne interferometry were recently published by Gay and Journet⁶. In August and September 1973 we carried out heterodyne detection measurements of planets, stars and the Moon at wavelengths around 10.5 µm. The observations were made at the Coudé focus of the 1.5-m spectrographic telescope of the European Southern Observatory at La Silla in Chile. Figure 1 shows a block diagram of the experimental equipment. The local oscillator was a linearly polarised single line, single mode CO₂ laser (Sylvania, model P941) tunable to different laser transitions around 10.5 µm. Tuning is achieved by changing the temperature of the cooling water and thereby varying the cavity length of the laser tube. Two different types of detectors were available as photomixing elements: a liquid nitrogen-cooled HgCdTe photodiode (Société Anonyme de Télécommunications, quality B₁) and a liquid helium-cooled Ge : Cu photoconductor (Santa Barbara Research Center, type HS). The detector signals were amplified by radiofrequency amplifiers and subsequently rectified by a quadratic detector. For amplifying the diode signals we used two CA1003 E + M amplifiers with a pass band between 10 and 300 MHz; the photoconductor signals were amplified by two AWL-1200 M Avantek amplifiers having a pass band between 0.1 and 1,200 MHz. A high-pass filter was used to produce a flat frequency response over the largest possible bandwidth. This resulted in an effective bandwidth of about 250 MHz for the photodiode system and of 1,000 MHz for the photoconductor system. The application of a chopper allowed synchronous detection. A rotating mirror (MC in Fig. 1), which chopped the source against the sky background, was used when observing planets or stars, whereas a plastic chopper (PC) was used in the calibration measurements and in the observations of the Moon to chop against room temperature (291 K). The sensitivities of both heterodyne detection systems were measured with the aid of a calibrated blackbody source. Heterodyne detection with a photodiode of an extended blackbody of absolute temperature T should have an associated signal-to-noise ratio (SNR)