A 94 GHz differential radiometer for observations of diffuse sky emissionL. Piccirillo Citation: Review of Scientific Instruments 62, 1293 (1991); doi: 10.1063/1.1142531 View online: http://dx.doi.org/10.1063/1.1142531 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/62/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Design and fabrication of a 94 GHz klystron AIP Conf. Proc. 569, 712 (2001); 10.1063/1.1384398 Development of 180 GHz heterodyne radiometer for electron cyclotron emission measurements in JT60U Rev. Sci. Instrum. 66, 413 (1995); 10.1063/1.1146364 A balloon borne 19GHz radiometer Rev. Sci. Instrum. 61, 158 (1990); 10.1063/1.1141867 Simple radiometer for net infrared sky irradiance measurements Rev. Sci. Instrum. 54, 1554 (1983); 10.1063/1.1137296 Absolute Differential Radiometer Rev. Sci. Instrum. 34, 1028 (1963); 10.1063/1.1718649

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A 94 GHz differential radiometer for observations of diffuse sky emission L. Piccirilloa) Istituto Superiore Poste e Telecomunicazioni, Viale Europa 190, 00144 Rome, Italy

(Received 14 September 1990; accepted for publication 10 December 1990)

The design and performance of a portable 94 GHz differential radiometer is discussed. The antenna system is an off-axis paraboloid-hyperboloid combination with a field of view of about 50 arcmin. All mirrors are made of carbon fiber coated with silver. Beam switching is obtained by wobbling sinusoidally the primary (parabolic) mirror. This radiometer was used in Antarctica (Italian Base, Terra Nova Bay, - 74 S, + 164 E) to make measurements to the diffuse sky emission at mm wavelengths. In future, we are also planning to use the antenna, together with a more sensitive detector, to make measurements of the cosmic background radiation anisotropy (CBR).

I. INTRODUCTION

The cosmic background radiation (CBR), emitted by the early universe - lo5 years after the Big Bang,’ domi- nates the sky brightness above the atmosphere, at mm wavelengths. In the Big Bang scenario, during the process of expansion of the universe and its consequent cooling down, the energy density became low enough to permit the formation of the first atoms, while the radiation, decoupled from matter, could propagate freely through space. At the same time, matter started collapsing and forming those structures (e.g., galaxies, clusters, and superclusters of gal- axies) that we see nowadays in the sky. In those regions the temperature was higher than in the noncollapsing ones.* Other physical processes, such as inhomogeneities in the matter lying between the last scattering surface and the observer,3*4 could also be responsible for sky temperature fluctuations. The result is that the CBR would be aniso- tropic and would contain information on the formation of the structures. Anisotropy measurements would produce a map of the spatial distribution of the temperature fluctua- tions of the sky. Up to now, measurements of the CBR anisotropy’ have resulted only in upper limits, which nev- ertheless discriminate between a large number of models of the universe. All the theories on the birth of the universe and on the formation of the structures must take into ac- count upper limits on the CBR anisotropy. The expected value for the CBR anisotropy is less than 10 - 5 in AT/T. This means that the rms temperature fluctuations of the sky would be less than 30 PK.

This low value requires a very sensitive instrument and a good estimate of all spurious signals. An important source of noise is connected with absorption and fluctua- tions of the atmospheric constituents (e.g., water vapor).6 In the sub-mm and mm wavelength region of the spectrum there are several atmospheric windows7 in which the at- mosphere is quite transparent. Some important ones are centered at 800 pm, 1 mm, 2 mm, and 3 mm. We chose to work at 3.2 mm wavelength (94 GHz) where there is a minimum in the atmospheric absorption and in the galactic emission. Although this choice reduces the problem of the

absorption of radiation by the atmosphere, it does not re- solve the problem of sky fluctuations because it is still emis- sive. In order to overcome the latter problem a possible solution would be to go up in the atmosphere by means of a balloon-borne instrument or outside the atmosphere with a satellite, which would prove to be very sensitive but very expensive too. A cheaper alternative could be offered by a suitable cold and dry ground-based location with a very stable and transparent atmosphere. For this reason we chose Antarctica which, thanks to its extremely clean and dry atmosphere, allows, even during the summer season, a percentage of good observing time of about 60%.

Considering the unusual and adverse environmental conditions in the South Pole, special care was put in the design and realization of the experiment. We thus decided both to make extensive use of titanium and stainless steel for the structures and carbon fiber for the optics to stiffen and lighten the radiometer against the very strong antarc- tic winds and, because of the rapidly changing weather conditions, to make the system easy to transport and dis- assemble. All the components were tested under low tem- perature and humidity conditions before shipment to the site.

II. MEASUREMENT REQUIREMENTS

The rms sky temperature fluctuations can be measured with two distinct techniques:

(a) Absolute: pointing the optical beam alternatively towards the sky and towards a reference load at a known temperature and emissivity;

(b) Differential: pointing the optical beam alterna- tively towards two different directions in the sky.

The detector provides a voltage output proportional to the difference between the strengths of radiation coming from the sky and radiation coming from the reference load [case (a)] or from the two extreme positions of the beam in the sky [case (b)]. When the radiometer is operated in the absolute mode, and neglecting all the spurious signals, the output consists of two components: a dc component, pro- portional to the difference between the sky antenna tem-

*‘Current address: European Space Agency, Space Science Department, Astrophysics Division, ESTEC-Keplerlaan 1, Noordwijk, The Netherlands.

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\ last scattering surface ,’

FIG. 1. Schematic view of the differential measurement showing all the emitting sources between the observer and the CBR last scattering sur- face.

perature T& and the reference antenna temperature ce,; and an ac component, proportional to the time fluctuations of the difference between the sky antenna temperature and the reference antenna temperature:

dc= <7&, - Cer>, ma Gy - CeP

The brackets refer to a In order to exploit

(1)

(2)

time average. the maximum dynamical range of

the receiver, the dc component must be negligible with respect to the ac component. This means that the reference load temperature must be kept very close to the sky equiv- alent temperature. This aim is often difficult to attain tak- ing into account that the sky equivalent temperature could be subject to fluctuations on various time scales ranging from minutes up to days.

The differential radiometer technique (the one adopted here), avoid in principle all these kinds of problems simply using, as reference, another sky position for the beam. In this way, the dc component in Eq. ( 1) goes to zero when the integration time is sufficiently increased. In this way, the receiver is sensitive only to gradients in the sky flux.

When the radiometer is pointed alternatively towards the sky, along direction 1 and 2 (see Fig. 1 ), supposing that a source enters the beam only in position 1, the output signal is given by the relation:

Vo,, = G( q - G) + GTsys, (3)

where

G= 6 + Tk,, + T;rouncit C = T;ource + T:t,r + k, + 2"&m

Tsys = 2 Tmixer/ ( A~71 l/2 .

(4)

(5)

(6)

1294 Rev. Sci. Instrum., Vol. 62, No. 5, May 1991 Diffuse sky emission

G is a calibration coefficient, T,,,,, is the antenna tem- perature of a sky source which is in the beam of the an- tenna, and Tsys is the noise temperature of a Dicke radi- ometer* defined in a frequency band Av, in an integration time equal to r and with a mixer noise temperature Tmixer- Assuming that the atmospheric emission and the spurious radiation coming from the ground are not time- dependent but only responsible for an offset in the detected signal, then

T offset = ( Tit,,, + T;roud - ( %, + T&m,)t (7)

AT,,,= T& - Tibr. (8)

Combining Eqs. (4), (5), (6), (7), and (8), we have:

Vout = G( Tsource + TM) + GAT,,,. (9) It is important to note that, although not explicitly

indicated, all the temperatures given above (with the ex- ception of T,,,) refer to antenna temperatures and not thermodynamic temperatures. Equation (9) shows that, if we want to disentangle the CBR anisotropy, representetby the second term on the right-hand side, we must measure the Tsource and Tof terms with sufficient accuracy. The term ToR can be estimated from the raw data, because the rms computation itself is not dependent on any constant added value (see Fig. 9). The term T,,,,,, is due to all the astrophysical sources which lie between the antenna and the last scattering surface. This term can be estimated only on the basis of a suitable model of the distribution of the sources in the sky.

Ill. BEAM MODULATION

The usual beam modulation techniques provide two (or more) beams displaced in the sky symmetrically with respect to the center line of the telescope. The choice of the modulation frequency depends mainly on the receiver noise characteristics. A low frequency produces small me- chanical stresses in the telescope structure and in the mir- rors, resulting in small optical aberrations and negligible microphonic noise. On the other hand, the l/f noise and the atmospheric noise can become important. A good tradeoff is a modulation frequency of the order of 10 Hz or more. Due to our well-balanced wobbling system (see Fig. 2), we were able to operate up to a frequency of about 30 Hz, without noticeable stresses in the mirror or measurable microphonic noise in the receiver.

Beam switching can be achieved by wobbling either the secondary or the primary mirror. In the former case, an important advantage consists in moving small masses re- sulting in a simple mechanical realization. The main dis- advantage comes from the diffraction signal picked up from the edge of the moving mirror. The receiver horn, being directly optically coupled to the secondary wobbling mirror, receives a diffracted signal from the ground (or surrounding noise sources) that is modulated at the same frequency of the beam switching and thus undistinguish- able from the sky signal. Wobbling secondary mirrors are always used in big telescopes where wobbling the primary is impractical.

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FIG. 2. Schematic view of the optics and the modulation system. (1) Attachment to primary mirror; (2) carriage; (3) cradle; (4) preloaded bearings; (5) circular guide rail; (6) eccentric drive.

Wobbling primary mirrors are feasible only in small aperture telescopes. The main advantage comes from the reduced spillover of the ground radiation synchronous with the modulation. The receiver horn does not receive a synchronous diffracted radiation because it is coupled to a fixed mirror. A diffracted modulated radiation is always present due to a double incidence: first on the primary (wobbling) mirror and then on the secondary, but with a negligible contribution.

With reference to Fig. 2, in order to perform the beam- switching, the primary mirror is mounted in a cradle with four bearings and is wobbled by means of a connecting rod attached to an electrically driven wheel. The amplitude of the sinusoidal motion of the mirror may be varied by changing the position of the connecting rod along the di- ameter of wheel. The oscillation frequency of the mirror can be suitably set by choosing the angular velocity of the wheel. We worked at a frequency of about 16 Hz and with a peak-to-peak sky modulation amplitude of about 4”. All masses were well balanced and, as a result, we had a system which (to first order) was virtually free of vibration. The arm connecting the two mirrors, once the elevation is fixed, can be removed in order to minimize transmission of vi- bration between the fixed and the wobbling mirror. How- ever, the level of vibrations was so low that we could not measure any intluence on our measurements. The net weight of the overall antenna together with the detector and electronics was about 70 kg. Owing to a special mount-

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ing, we were able to assemble and disassemble the entire optics in a few seconds. This feature proved very useful during sudden weather changes (quite common in Antarc- tica).

IV. ANTENNA SYSTEM

The antenna is a combination of an off-axis paraboloid with an off-axis hyperboloid (see Fig. 3 and Table I). We chose the off-axis configuration in order to have a high optical efficiency. The optics is designed to give a 50 arc- min beam in the sky. Both mirrors were made using a silver-coated carbon reinforced plastic (CFRP) resulting in very rugged and light optics. All the moving metallic parts, coupled to the wobbling mirror, were made of tita- nium and aluminum in order to minimize the masses. The primary and the secondary mirrors are hooked together by an arm. The primary mirror is mounted on a semicircular guide rail in order to permit the elevation of the antenna to be set. The elevation axis coincides with the optical axis of the receiver horn. In this way, the position of the focus is not changed when the antenna direction changes from the horizon up to the zenith. The optics is such that the direc- tion of the polarization of the horn does not change with the elevation. The antenna can perform an elevation scan without moving the detector: this feature is particularly useful when the detector is placed inside a cryostat for which the tilt angle is limited by cryogen confinement (usually nitrogen and/or helium). The scan of the sky was performed using the drift-scan techniques: the azimuth of the antenna was fixed towards the North-South axis. The rotation of the Earth provided for the transit of the portion of the sky under measurement.

A ray-tracing simulation of the optical performance was carried out. The main aberration revealed was coma produced when the primary mirror is tilted at the maxi- mum angle of 1.16”. We calculated a loss of 0.1 dB due to the mentioned aberration. The geometrical plus diffrac- tional point spread function (PSF) of the entire optical system is shown in Fig. 4. In Fig. 5 the calculated on-axis beam profile is reported.

The conical horn (Hughes mod. 45826xH), consid- ered as a transmitter, illuminates 97% of the hyperbolic mirror adding an additional loss equal to about 0.1 dB. In Figs. 6 and 7, the measured E- and H-plane antenna pat- tern of the conical horn at 92 GHz frequency test is re- ported. The entire antenna system is surrounded by four ground shields made of reflecting aluminum panels. The shields, which are placed at an inclination of 45”, are used in order to reduce the spurious radiation coming from the ground. The inclination of 45” permit the optics to receive only radiation coming from the sky, whose equivalent tem- perature (about 30 K) is ten times lower than the equiv- alent temperature of the ground (about 300 K).

V. RECEIVER

A brief description of the receiver is given. The spa- tially modulated radiation coming from the conical horn reaches the Schottky diode balanced mixer (Hughes model

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EARING /ENCODER

LEVATION ARM

IXED STRUCTURE

FIG. 3. Section view of the optics. The 45” ground screens are not shown. AI1 the moving parts are made of CFRP and titanium,

m CFRP ‘/A? Al ;$$$ STAINLESS

47416H). The local oscillator consists of a phase-locked Gunn diode (Hughes model 47746H) operating at 94.1 GHz. The IF output is sent, via a pstrip bandpass filter with 400 MHz bandwidth and 1.5 GHz central frequency, to a first IF amplifier (Avantek AWT-2034). A second bandpass filter and a second IF amplifier is followed by a variable attenuator that keeps the power level in the linear region of the detector diode response (OMNI Spectra model 2086-6000-00). The video output from the detector diode is sent, together with the reference signal coming from the wobbling mirror, to a l’ock-in amplifier that pro- duces a voltage output proportional to the difference be- tween the radiation coming from the two extreme positions of the beam in the sky.

TABLE I. Instrument characteristics.

Flux collector Primary mirror: off-axis paraboloid 48 cm diameter Secondary mirror: off-axis hyperboloid 22.5 cm diameter EFL: 851.65 cm f/number: 0.95 Field of view (FOV): 50 arcmin Modulation amplitude (A@: 3.86” AR: 0.1 cm2 sr

Detector Superheterodyne mixer DSB Mixer noise figure (NF): 6.3 dB IF amplifier gain: 50.3 dB IF amplifier noise figure: 2 dB IF amplifier bandwidth: 0.5-2 GHz IF bandpass filter: 1.3-1.7 GHz Local oscillator center frequency: 94.1 GHz Local oscillator output power: 22 mW

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TI

VI. CALIBRATION

Calibration was performed by measuring the coeffi- cient G of Eq. (9) (see Fig. 8). We recorded the output voltage from the radiometer for a known change in an- tenna temperature. The calibration targets were two mi- crowave absorbers (eccosorb) , one at ambient temperature and one at liquid nitrogen temperature. We used a reflect- ing chopper made of polished aluminum blades driven by

P up- FIG. 4. Geometrical plus diffractiona point spread function (PSF) of the optics when the primary mirror is tilted at the maximum angle (I, 16”). No substantial aberrations are evident.

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OdB

IOdB

15dB

2OdB

25 dB

I f 1 I 1 I 1 I I 1

40” 32’ 24’ 16” 8” 0’ 6” 16” 24” 32’ 40 0 1 degrees

FIG. 7. H-plane radiation pattern of the receiving horn.

FIG. 5. Calculated beam pattern of the antenna. Full line: plane of offset; dashed line: perpendicular plane.

an electric motor. We assumed zero emissivity for the chopper blades and unit emissivity for the microwave ab- sorbers. We took into account the refractive index of the liquid nitrogen surface (r = 1.2) producing an emissivity slightly less than unity. Special care was taken in order to be sure that the receiver, during the calibration, was work- ing in the linear region. This was done adjusting the IF power level, on the IF amplifiers chain and on the diode

Ode

5dfl

lOd9

2OdE

25d8

40” 320 24’ 16” 8” 0” 8” 16” 24“ 32” 40”

FIG. 6. E-plane radiation pattern of the receiving horn.

1297 Rev. Sci. Instrum., Vol. 62, No. 5, May 1991

detector, by means of calibrated variable attenuators (see Sec. V). The calibration was performed before, during and after every set of measurements. The measured change of the parameter G was less than 5%.

VII. DISCUSSION

The instrument was used during the 1989/1990 Italian Antarctic Expedition and was devoted to a sky survey searching for dust emission at mm wavelengths. The data will be discussed in detail elsewhere;’ however a brief de- scription is given here. The instrument was placed in the vicinity of the OASI observatory and collected in total about 50 h of observations. We detected emission from both the large and small magellanic cloud which are two irregular galaxies very close to our own galaxy. The flux intensity at 3.2 mm, coming from these objects, seems to be emitted by a very cold dust component (T- 15 K) coex- isting with the warm dust (T-30 K) detected by the IRAS satellite in the far infrared. Our measurements are

k ‘.’ ., --

k :’

- -

reflecting chopper

I I L

microwave absorber I , * liquid nitrogen bath

FIG. 8. Calibration assembly. The microwave absorbers are made of ec- cosorb. The chopper blades are made of polished aluminum.

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3.00

dTJ K)

REAL TIME SIGNAL FROM THE LARGE MAGELLANIC CLOUD

C 6=-68.7 26 JAN 1990)

0-0~S:6~jG;1r~Lct*~ Ii 1’1 R.A. 6.00 7.00

FIG. 9. Scan, in right ascension, of the large magellanic cloud obtained with the differential radiometer. The straight line at 900 mK antenna temperature represents the (spurious) constant offset introduced by the side lobes of the antenna.

also in good agreement with other measurements of the same objects but at different wavelengths ( 1 and 2 mm).” We also detected signals coming from our galactic plane and from other sky sources. In Fig. 9 a scan through the large magellanic cloud (LMC) is shown. A clear signal, coming from LMC, is detected well above the receiver noise.

Having demonstrated good performance of the optics and of the modulating system, we are planning to execute,

with this instrument, CBR anisotropy measurements dur- ing the 1991 Antarctic expedition. In order to reach the required sensitivity, however, we will change the Schottky mixer detector to a 3He bolometer system or to an SIS superheterodyne mixer.

ACKNOWLEDGMENTS

I would like to acknowledge G. Dall’Oglio for his con- siderable guidance during all phases of this project. Partic- ular thanks are extended to all the “antarctic” colleagues: P. G. Calisse, A. Iacoangeli, L. Martinis, L. Rossi for their help in the experimental setup; all the coheagues of the Istituto Superiore P. T. for their help in the early phase of the project; Urban Frisk, who provided many useful com- ments on the paper; Giorgio Bagnasco for a critical revi- sion of the manuscript; Willie Fischer, who provided the drawings of the optics.

Special thanks go to L. Pizzo for the considerable help she gave me before, during, and after the antarctica expe- dition. This work was supported by the Istituto Superiore P. T. and by the ENEA Progetto Antartide.

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173 (1972). ‘R. B. Partridge, III ESO/CERN Symposium, Astronomy, Cosmology and Fundamental Physics, Conf. Proc., Vol. 155, Bologna (1989).

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‘H. J. Liebe, NTIA Rep. 83-187 (1983). *J. D. Kraus, Radio Astronomy (Cygnus-Quasar, Powell, OH, 1986). ‘L. Piccirillo (unpublished).

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