Contains research objectives and summary of research on five research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400)M.I.T. Sloan Fund for Basic ResearchNational Aeronautics and Space Administration (Contract NAS5-23731)National Aeronautics and Space Administration (Grant NGR 22-009-526

Benford, Richard L.

Downey, Patricia M.

Frerking, Margaret A.

Halpern, Mark

Muehlner, Dirk J.

Weiss, Rainer

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VIII. INFRARED INSTRUMENTATION AND ASTRONOMYAcademic and Research StaffProf. Dirk J. MuehlnerProf. Rainer WeissRichard L. BenfordGraduate StudentsPatricia M. DowneyMargaret A. FrerkingMark Halpern1. INFRARED HETERODYNE DETECTIONJoint Services Electronics Program (Contract DAAB07-76-C-1400)M. I. T. Sloan Fund for Basic ResearchRichard L. Benford, Margaret A. Frerking, Dirk J. MuehlnerThe aim of this project is to develop a compact, continuously tunable, infrared het-erodyne receiver for high-resolution spectroscopy of remote objects. Astrophysicalapplications have provided the principal motivation for the work, but the instrument isalso useful for local measurements, as the observation of ozone in our atmosphere hasdemonstrated. The receiver uses a tunable diode injection laser as the local oscillator.The diodes that we have used have operated in the 9-12 [im range. A HgCdTe photodiode(kindly supplied by David L. Spears of M. I. T. Lincoln Laboratory) is used for the mixer.The complete receiver is assembled in a small liquid helium dewar suitable for boltingto a telescope.The diode laser local oscillators used in previous experiments cannot betemperature-cycled without risk of degrading or altering their characteristics. Thisproblem has now been alleviated, since our present laser (from Laser Analytics, Inc.)operates at liquid nitrogen temperature, which makes it convenient to store the appa-ratus at its operating temperature. We have used this laser in the receiver to observethe absorption spectrum of atmospheric ozone against the Sun. For comparison, wehave also measured the spectrum in the laboratory, by passing the laser radiationthrough an absorption cell to a separate detector, and also by heterodyne detection of ablackbody source through the cell.Figure VIII-1 shows some of the results obtained for ozone. Trace (a) is a labora-tory absorption spectrum of 2% ozone in 48 Torr of oxygen at room temperature in a1-m absorption cell. The laser, operating in a single mode over this range, was tunedby varying its current. Part (b) is a single trace of solar absorption by atmosphericozone. The scan took approximately 10 minutes to complete. The IF bandwidth of100 MHz yields an infrared resolution of 0. 0067 cm - . Part (c) is the average of (b) andPR No. 119(VIII. INFRARED INSTRUMENTATION AND ASTRONOMY)LASER CURRENT (A)1.39I I I I I I I I IWAVE NUMBER (cm - 1)I011.8Fig. VIII-1. Ozone absorption spectra. (a) Spectrum obtained in a laboratoryabsorption cell. (b) Typical atmospheric absorption spectrumobtained against the Sun by using a heterodyne receiver. (c) Aver-age of 6 atmospheric absorption spectra. (d) Spectrum calculatedwith AFCRL atmospheric absorption line parameters.5 similar scans. The atmospheric data were obtained on 1 December 1976, with the Sunat an elevation angle between 230 and 26'. A fit to the laboratory spectrum (a) with theuse of the Air Force Cambridge Research Laboratory compilation of line parameters1is shown in (d). There is only rough agreement between the observed and calculatedspectra. According to S. A. Clough,2 of Air Force Geophysics Laboratory (formerlyAFCRL), the compiled ozone parameters in this frequency region are not very accurate,and so disagreement in detail between calculated and observed spectra is not surprising.The ozone results presented here do not represent the ultimate capability of theheterodyne receiver. The local-oscillator power focused on the mixer was ~40 [W, morethan five times lower than that required for shot-noise-limited detection. The outputnoise seen in Fig. VIII-1 was approximately 10 times larger than the amplifier noise,PR No. 1191.31I.0w0zZI-zIr1011.0I - 7 1 . . ..f i I I 1(VIII. INFRARED INSTRUMENTATION AND ASTRONOMY)primarily because of inadequate shielding against RF interference. Also, approximatelytwo thirds of the available solar power was lost because of inefficient transfer optics.In spite of these less than optimum conditions, we feel that the usefulness of the infra-red heterodyne receiver for high spectral resolution remote sensing has been demon-strated.References1. R. A. McClatchey et al., AFCRL-TR-73-0096, "AFCRL Atmospheric AbsorptionLine Parameters Compilation," Air Force Cambridge Research Laboratories, Bed-ford, Massachusetts, 26 January 1973.2. S. A. Clough, Private communication, 1976.2. DEVELOPMENT OF INTEGRATED SILICON BOLOMETERSNational Aeronautics and Space Administration (Contract NAS5-23731)Patricia M. Downey, Rainer WeissIn this project we have applied the techniques of integrated circuit manufacture tothe ancient art of bolometer construction. An entire bolometer assembly - mountingframe, thermal links, and low heat capacity bolometer element - has been formed outof a single piece of silicon by masking and etching. The central bolometer elementincorporates an ion-implanted resistance thermometer and a thin metallic film radiationabsorber. Broadband detectors with time constants of 1/10 s and radiation noise equiv-alent power (NEP) of 10-5 at 1. 5K appear to be feasible. The techniques that areemployed should produce high yield rates and lead to the manufacture of detector arrays.This project is being carried out in collaboration with the Microelectronics Group atM. I. T. Lincoln Laboratory.3. MEASUREMENT OF THE LARGE ANGULAR SCALE ANISOTROPYOF THE PRIMEVAL COSMIC BACKGROUND RADIATIONNational Aeronautics and Space Administration (Grant NGR 22-009-526)Dirk J. Muehlner, Rainer Weiss, Richard L. BenfordThe goal of this experiment is to search for intrinsic anisotropies of the cosmicbackgound radiation, which is adduced to be a remnant of the primordial cosmicexplosion. The high degree of isotropy of this radiation is a deep mystery and measure-ment of the angular distribution on both large and small angular scales constitutes oneof the fundamental problems in cosmological observation. At some level the earliestvestige of galactic formation must have left its imprint on the primeval explosion,PR No. 119(VIII. INFRARED INSTRUMENTATION AND ASTRONOMY)and the explosion was not necessarily spherically symmetric. Observations of the back-ground can also be turned around: Under the assumption of isotropy in some referenceframe, a measure of a global anisotropy in a frame tied to the Earth will give a measureof the velocity of the Earth relative to the reference frame that was at average rest withthe matter that last scattered the background radiation. The upper limits on this veloc-ity, established by isotropy experiments of the background, are in conflict with earliersystematic anisotropies measured in the distribution of galactic red shifts.In this project, which has been in progress for three years, we are conducting aballoon-borne survey of the sky, using a dual-channel differential radiometer with a 170-l -lbeamwidth. The two channels have bandwidths extending from 3 cm - 1 to 10 cm-1 andfrom 10 cm to 25 cm. The lower frequency channel is chosen to include the bulk ofthe energy in the cosmic background radiation, while the higher frequency channel moni-tors inhomogeneities in the atmosphere and is sensitive to local astronomical sourcessuch as interstellar dust clouds. With existing detectors, the survey sensitivity isapproximately 0. 1°K/Hz /2 in the low-frequency channel. In a typical 8-hour flight it-4is possible to set fractional temperature anisotropy limits of 3 X 10-4 on global-scaleanisotropies such as those caused by the velocity of the Earth relative to the universalco-moving frame. The results of 4 balloon flights, covering approximately one thirdof the celestial sphere, show no anisotropies larger than 1/1000 and, with a 90% confi-dence limit, indicate that the velocity of the Earth is less than 350 km/s relative to theco-moving frame. These limits are not set by the ultimate survey sensitivity but ratherby systematic correlations in the data that may well be due to galactic dust clouds orintrinsic anisotropies in the background. Improved sensitivity in the survey, as wellas greater sky coverage, are needed to unscramble the correlations.4. SKY SURVEY AT MILLIMETER AND SUBMILLIMETERWAVE LENG THSNational Aeronautics and Space Administration (Grant NGR 22-009-526)Dirk J. Muehlner, Rainer Weiss, Richard L. BenfordThis project is closely related to the anisotropy measurements described in Sec-tion VIII-3. It is a balloon-borne survey of the sky to search for discrete sources withthe spectral bands used in the anisotropy experiment but with a smaller beamwidth,approximately 1'. A spin-scanned telescope that has a sensitivity of ~104 flux units perangular resolution element is used in the experiment. With improved detectors suchas those described in Section VIII-2, we hope to increase the sensitivity to 100 flux units.Aside from mapping the sky in this unknown spectral region, our goal is to measure thefrequency dependence of the emissivity of typical dust clouds in our Galaxy. We havesurveyed the galactic plane from longitude -50 to +500. The emissivity of the dust in thePR No. 119(VIII. INFRARED INSTRUMENTATION AND ASTRONOMY)galactic center has been found to vary faster than linearly with frequency. Limits onthe millimeter and submillimeter brightnesses of several sources observed at 100 Imhave also been set.5. COSMIC BACKGROUND EXPLORER SATELLITE (COBE)National Aeronautics and Space Administration (Contract NAS5-23731)Mark Halpern, Rainer WeissWe are engaged in a one-year study with scientists from several other laboratoriesto investigate the feasibility of using a single integrated satellite to carry out the fol-lowing measurements of the cosmic background radiation:a. A precision measurement of the spectrum of the background radiation between3 cm-1 and 20 cm-1 by using a cryogenic Fourier transform spectrometer. The designgoal is to make an absolute determination of the spectrum with a signal-to-noise ratioof 1000/1 at the blackbody peak. The instrument is designed to measure the deviationsfrom a Planckian spectrum.b. An absolute measurement of the diffuse background in 6 broadband channelsbetween 300 pm and 10 im. The background radiation in this spectral region is virtuallyunknown, particularly in the 100 - 300 ym band.c. A multiband measurement of the anisotropy of the cosmic backgound radiation,with the use of microwave differential radiometers at 20, 30, 55, and 90 GHz. Theangular scale will be of the order of 70 with a fractional temperature sensitivity of0. 2 millidegrees per resolution element extending over the entire sky in a one-yearflight.This project, if approved by NASA for continued support, should lead to the mostauthoritative measurement of the background radiation. Collaborating with us are JohnMather and Michael G. Hauser, of Goddard Space Flight Center, David T. Wilkinson,of Princeton University, Samuel Gulkis, of the Jet Propulsion Laboratory, CaliforniaInstitute of Technology, and George F. Smoot III, of the University of California atBerkeley.PR No. 119

Infrared Instrumentation and Astronomy

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Infrared Instrumentation and Astronomy

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