Page 434 First International Symposium on Space Terahertz Technology
Submillimeter Astronomy in France
P. ENCRENAZ
Laboratoire de Radioastronomie Millimetrique
OBSERVATOIRE DE PARIS - Section de Meudon
92195 MEUDON FRANCE
ABSTRACT:
Submillimeter astronomy has been selected as the top priority forthe next decade by the french space agency (CNES). A balloon bornetelescope of 2m of diameter is built by Matra-Space, using the lightestCFRP materials. The petals of the telescope have been recentlydelivered: the telescope will be diffraction limited at 150 gm, and thepannels will be adjusted during flight. A pointing accuracy of 5" isanticipated. The first flight from Sicily to Spain is scheduled for 1992with a set of bolometers cooled to He3. The first heterodyne receiverwill fly in 1993. It consists of a cooled schottky receiver at 380 GHz toobserve simultaneously H20 and 02. The trilayer Nb/Al203/Nb junctionsmay be used for the first flight.
In collaboration with Cal Tech and Jet Propulsion Laboratory, aproposal for a small submillimeter satellite called SM3 (T. Phillips in theP.1) has been selected for a phase A by CNES. Related technologicaldevelopments are funded in the area of cryo coolers, adjustable pannelsin flight, quantum well devices and local oscillator sources, andmanufacturing of SIS junctions.
Work on different subsystems of the cornerstone FIRST is also inprogress.
First International Symposium on Space Terahertz Technology Page 435
PRONAOS
A French Balloon - Borne Space Project
1) Generalities
PRONAOS is a scientific space project for astrophysical observations in
submillimeter wavelength region with instrumentation borne on a stratospheric
balloon gondola.
The PRONAOS architecture consists of three parts •
- a carry away vehicle : the stabilized gondola
- a common facility : the submillimeter telescope
- a scientific payload : 2 focal plane instruments
A 900 000 m 3 balloon will permit first flights in transmediterranean campaigns from
Sicilia to south Spain (about 20 hours of ceiling at an altitude of 40 km and a latitude of
38° N), in july 1992 and 1993.
2) "Naps" gondoll :
The main characteristics of the vehicle are
- an azimuthal gondola with two stages stabilization delivering a diurnal pointing with
an accuracy RMS in arc seconds on two axes
. absolute 7"
. stability 5"
. drift rate 5" hour)
- the dry mass is 1600 kg for a lauching mass of 2200 kg
- the mass of the pointed set including Telescope, 1 focal-plane instrument and various
equipments is approximatly 500 kg.
- The gondola dimensions are typically : 4m x 4m x 7m (see Fig 111-1.1 and 1.2) and the
on-board energy permits a capacity of 30 kWh by a set of lithium batteries.
First International Symposium on Space Terahertz Technology Page 437
3) Thq telescope
It is a 2m class submillirneter telescope. The main characteristics are
Cassegrain type telescope with a focal length of 20m, with f10,9 primary aperture
and 2.8 m back focus. The pointing (accuracy goal : 5 arc second) is obtained by stellar
sensor.
The 2m of diameter primary mirror has a surface accuracy of 10 g m RMS
including all defects.
The image quality of the telescope will be limited by the diffraction at 300 gm .
Light weight technology is used for the primary mirror realization : carbon fibre
by a replica-technique.
The mirror, segmented in 6 parts semi-active (in flight, aligment of the 6
segments considered as rigid, controlled by an on-board servo-loop), is equiped with 18
actuators and 30 dispacement captors.
Each segment is realized in composite structure (carbon fibre : honeycomb and
skins) on a replica mould : the mass is 3.3 kg, with a coating of 0.15 gm thickness gold
deposit.
4) Instruments,
Two instruments could be used on the focal plane of the telescope
SPM : (Multiband Photometric System)
The spectral resolution v/Av is 1 to 10 on the field : 200 gm to 1.5 mm (in 5 bands).
The photometer employs cooled bolometers at 0.5 K (or 0.1 K).
SMH : (Heterodyne Spectrometer)
With a high spectral resolution (v/Av., 106)
The submillimeter heterodyne radiometer will operate in the first generation at
380 GHz with a next goal of operating to 550 GHz. Noise temperatures as low as 500 K at the
lower frequency and 1000 K at the high end of the frequency range are expected. The
heterodyne radiometer mixes the input signal with a local oscillator (LO) signal at
approximately the same frequency to generate an output at the difference or
intermediate frequency (IF) which is then amplified and processed.
An ambient thermal load and a cold load will be used for absolute calibrations of
the radiometer. The mixer subsystem consists of a cooled Schottky diode, or three element
arrray of superconducting - insulating - superconducting tunnel junctions (SIS)
Page 438 First International Symposium on Space Terahertz Technology
operating at 3.5 K. The IF subassembly includes the cooled low-noise preamplifiers
which are followed by warm amplifiers. The LO subsystem contains the injection optics,
a set of frequency multipliers, Gunn oscillator to drive the multipliers , and a phase
locking system for the Gunn oscillator.
Acousto-optic spectrometers (AOS) will be used for the spectral analysis of the
heterodyne radiometers. The instrument block diagram is shown in Fig. 111-2. Following
are descriptions of the heterodyne radiometers and the AOS.
5) SMH main characteristics and description, •
The first interstellar molecules in galactic sources studied with SMH are H20 and
02 at 380.2 and 368.5 GHz respectively.
Atmospheric attenuation for different sites, shown on Fig. 111-3, indicates the
necessity to observe from space.
Using the double side band operation on the submillimeter heterodyne receiver,
both lines will be observed simultaneously by a optimized choice of the intermediate
frequency of the receiver (see picture below)
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first a cooled Schottky diode followed by an HEMT 6 GHz amplifier will be used. In this
case a over-all 500 K noise temperature will permit to detect a line in half of hour. For
the second flight, an SIS mixer will be used to increase the sensitivity..
Local oscillators
Optimum performance of a Schottky or an SIS mixer is achieved with an absorbed
LO power of about 300 p.W in the first case or less 10 jiW in the second case. To provide
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Page 440 First International Symposium on Space Terahertz Technology
even this level of power over the 300 GHz band is a challenge ; however, an approach
based on frequency multiplication of phase-locked solid-state millimeter-wave sources is
adequate and should be sufficiently reliable for space applications.
IF amplifiers
The IF frequency will be set at about 6 GHz with a bandwidth of 500 MHz . To take
advantage of the low noise performance of the mixers devices, the first IF stage, with 30
dB gain, will be cooled to about 40 K and located close to the mixers. HEMT devices are
being developped in the frequency range with noise temperatures less than 20 K and
low power dissipation.
Spectrum analysis
Acousto-optic spectrometers (AOS) will be used to analyze the spectra from the
heterodyne radiometers. In the AOS, the IF signal modulates a transducer which
generates a beam of acoustic waves in a crystalline material. These acoustic waves
diffract a separately injected laser beam. The deflected laser light is then detected by a
linear array of photodiodes to integrate the signal power in each frequency
channel.The signals from each diode are read out to a computer for the processing of the
spectral data.
The spectrum resolution of 500 kHz with 1000 pixels is obtained by 2 acousto-
optical spectrometers (Fig 111-4). The AOS bandwith is 180 MHz with a PbMo04 bragg cell.
To obtain high spectrum resolution, all local oscillators are stabilized by a phase
and frequency loop on a quartz reference. (stability of 10 4 is required).
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First International Symposium on Space Terahertz TechnologyPage 44/
Data acquisition and controls An in-board computer can control the receiver
(house keeping) and compact the data to send them to the ground station. The
transmission flow from the instrument is about 150 kbyte/s.
Cryogenics
The constraints and requirements for the cryogenic system 3.5 K operating
temperature with a 40 hours lifetime. The baseline design is a liquid Helium Dewar with a
40 K stage for the IF cooled amplifier.
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Page 442 First International Symposium on Space Terahertz Technology
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3 +
64)
+(C
+ D
+ E
1 +
E2
+ E
3 +
E4
+ E
5)1 1
53.
1
ACTU
ATO
R A
LON
E/.
0.0
5 m
icro
n