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SPM, a submillimeter photometer for PR0NAOS
Infrared Phys. Technol.Vol. 35, No. 213, pp. 217-289, 1994
CopyrIght 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
1350-4495/94 $6.00 + 0.00
SPM, A SUBMILLIMETER PHOTOMETER FOR PRONAOS J. M. LAMARRE’, F. PAJOT’, J. P. TORRE’, G. GUYOT’, J. P. BERNARD’, A. DE LUCA’, M. GIARD~, J. MANGIN~, R. PEYTURAUX’, J. L. P~GET’, G. SERRA’, F. BOULANGER’, X. DESERT’,I. RISTORCELLI~,G. JEGOUDEZ’,H. LAGARD~RE’,J. LEBLANC’, R. PONS~, G. RECOUVREUR’,M. BARTHELEMY’,C. BOURGUIGNON’,J. P. CRUSSAIRE’,G. DAMBIER’, L.
DELILLE’, E.
KORCZAK’, J. P.
LEPELTIER’, B.
LERICHE’, C.
LIZAMBERT’,
J. NARBONNE~,C. PLAISANT’, J. REIGNER’,J. C. RENAULT’ and C. RIOUX’ ‘Institut d’Astrophysique Spatiale, Universite Paris XI, Bltiment 121, F-91405 Orsay Cedex, 2Service d’Aironomie, BP3, F-91371 VerriBres-le-Buisson Cedex, 3Centre d’Etudes Spatiales des Rayonnements, 9 av. du Colonel Roche, BP 4346, F-31055 Toulouse Cedex, “LIRL, Universiti Nancy I, F-54506 Vandoeuvre les Nancy and %stitut d’Astrophysique, 98bis Boulevard Arago, F-75014 Paris, France (Received 14 September 1993)
Abstract-SPM is a multi-channels photometer designed to obtam the best sensitivity at the focus of the 2m balloon-borne PRONAOS telescope. Its scientific goals include the measurement of cosmological background anisotropy, which is important for understanding the origin of the structure in the universe and the observation of cold solid matter in the interstellar medium as an indicator of the physical conditions that prevail in star forming regions. SPM consists of warm optics that ensure beam switching and in-flight calibration, a liquid helium cryostat that contains filters and the )He cooled bolometers, and electronics for housekeeping and data handling. The spectral bands are 18&24Opm, 240-340 pm, 340-540 pm and 540-1200 ym. They are defined by dichroics, which allows simultaneous observations in the four bands. The expected NEAT in flight is significantly less than 1 mK in all the bands. SPM will fly with PRONAOS in May 1994.
I. INTRODUCTION PRONAOS is an acronym for the Projet National d’Astronomie Submillimktrique. On the one hand it acts for research laboratories of the Centre National de la Recherche Scientifique (CNRS), and on the other hand for the Centre National d’Etudes Spatiales (CNES) and industry, in a single effort to construct and operate a large balloon-borne instrument dedicated to submillimetre astronomy. The purpose of this project is to provide astronomers with the opportunity of observations in a wavelength domain (100 pm-1 mm) where the atmosphere is nearly opaque, due mainly to water vapor rotation lines. The two atmospheric transmission windows around 350 and 450 pm are usable only from the best observation sites (‘) during the best weather periods. Even at airplane altitudes, a lot of absorption lines disturb the measurements’*) and even make it impossible or only just possible to observe some species, such as H,O and 0,. Wide band and high sensitivity measurements are then only possible at higher altitudes. The 2 m telescope will give an improved angular resolution compared with the previous submillimeter space-borne or balloon-borne projects.‘3.4) The project has four main sub-systems. The 2 m telescope, the gondola and two focal instruments, SPM (systtme photomktrique multibandes), a multi-channel high sensitivity photometer, and SMH (spectromktre httirodyne), a 380 GHz heterodyne spectrometer.‘s’ Only one of these instruments can be mounted on the Cassegrain focus of the telescope. Each focal instrument has its own scientific goals and its own scientific and technical team. The system is designed to perform 10 flights in about 10 years. The first flight with SPM is scheduled in May 1994. The spectral bands and the fields of view of the four channels of SPM are given in Table 1. The PRONAOS gondola and telescope are described in Section II. The scientific objectives of SPM are listed in Section III. 277
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Section IV is devoted to the detailed report of its design. Section V deals with an analysis of the sources of noise and with calibration results.
II.
THE
PRONAOS
GONDOLA
AND
TELESCOPE
The gondola protects the scientific payload and points it towards the observed astronomical source. It also provides the scientific payload for electrical power and a telemetry-telecommand link with the ground. Its structure comprises mainly aluminium alloys. It is 7 m high and the total weight of PRONAOS is 2.6 tons (Fig. 1). The thermal control of the whole system is achieved by using thermal paints and polyurethane protection. On board energy is provided by about 100 kg of lithium batteries. The gondola electronics are controlled by a DATA computer (Crouzet) and its real time system ASTRES. The pointing system consists of two principal stages. The azimuth stabilization stage orientates the whole gondola and ensures the damping of external perturbations. The fine pointing system orientates the 450 kg scientific payload, i.e. the telescope and its focal instrument, through a Cardan suspension with a nominal standard deviation of 5 arcsec. An inertial platform is used as a short term sensor for the pointing servo-control. Its drifts are measured by a star sensor which works even in day light with stars having a magnitude up to five. The star sensor can be oriented off-axis from the telescope line of sight in order always to have a bright enough star in its field of view. The balance of the pointed mass is achieved by three moving masses that compensate mechanical deformations and helium evaporation in the Dewar. The Cassegrain telescope has a diameter of 2065 mm and a primary-secondary mirror distance of 1718 mm. Its equivalent focal length is 20 m and its total mass 225 kg. The primary mirror is of the segmented type, with six light-weight panels (6 kg/m2). The panels are sandwiches of carbon fiber honeycomb core between two carbon fiber skins. Position sensors at the edge of the segments and position actuators supporting them compensate for the gravity and temperature effects on the supporting structure. This system keeps the long term surface deformations to RMS values less than 12 pm. At the beginning of the project, the specifications were twice as large. The surface quality of the mold made by REOSC and used for the fabrication of the panels, as well as the new fabrication technique of the panels, were the main reasons for this increase in the antenna precision. The whole telescope is embedded in a sunshade that protects the optical system from external sources of heat, such as the sun and the earth. This limits the temperature gradients along a primary mirror diameter to values less than a few kelvin, which is an important characteristic for a beam switching instrument such as SPM. The Toulouse Center of the Centre National d’Etudes Spatiales (CNES) is responsible for the gondola development. The telescope is developed by Matra Espace in Toulouse, under the auspices of the CNES. More information on PRONAOS can be found in Ref (6).
III.
1. Sunyaev-Zel’dovich
SCIENTIFIC
OBJECTIVES
(SZ) effect and cosmic background
OF SPM
anisotropy
The intracluster gas scatters the (sub)millimeter 3 K cosmic microwave background by inverse Compton effect from its hot electrons (about lo8 Kc’)). In 1980, Sunyaev and Zel’dovich@’ pointed out a way of directly measuring the radial peculiar cluster velocity relative to the 3 K background, Table
1. Bands and field of view of the four channels
Band No. Wavelength (pm) FOV (arcmin)
1 180-240 2
2 240-340 2
3 340-540 2.3
4 _54&1200 3.5
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Fig. I. PRONAOS. 7 m high and 2.6 tons, in flight configuration. The cover of the sunshade is open and a small part of its 2 m segmented mirror can be seen. Focal instrumentation is at the Cassegrain focus. The star sensor is attached under the sunshade. Housekeeping and stabilization equipments are on the lower platform. An 8 m umbrella-like protection is intended to prevent the gondola from being overturned when landmg.
due to the Doppler effect on the scattered photons. For every observed cluster of galaxies, the SZ and Doppler distortions linearly combine but have different spectral distributions and can therefore be separated with observations in several bands in the millimeter and submillimeter range. These measurements would constitute a method totally independent from the so-called 4-D galaxy surveys and would obviously be invaluable for investigating the kinematics of the Universe on the largest observable scale. As shown in Fig. 2, channel 4 of the SPM is an efficient tool for measuring the short wavelength part of the SZ distortion, i.e. its positive part. The atmosphere is opaque and emissive in this spectral band and balloon-borne or space-borne instruments are necessary to complete ground-based observations that can be obtained at larger wavelengths. The other bands of the SPM are also represented in Fig. 2. They will allow the measurement of the possible
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contamination by dust, much brighter at these shorter wavelengths. To check that the measurement of the SZ effect is not due to instrumental artefacts, we plan to observe, during the same integration time, the empty sky, i.e. a region where no potential source has been identified. This measurement will put an upper limit to the anisotropy of the cosmic radiation in this region, at a wavelength and an angular scale (6 arcmin) which differs from other experiments. 2. Interstellar medium (ISM) Submillimeter photometry is the specific tool needed to measure the thermal continuum emission of interstellar dustC9) The four bands of the SPM photometer cover a spectral domain starting at the emission maximum, strongly dependent on the dust temperature (T’ at least). The emission then decreases steeply with the wavelength (A -’ for the AZ,emission-for a Iz -* dust emissivity) but with a weaker dependence on T (proportional). These characteristics enable the dust temperature to be derived (at the wavelength of the emission maximum), as well as the dust emissivity and integrated column density (at long wavelengths). At the distance of the close molecular complexes (100-300 pc, e.g. the Taurus complex), the field of view of the SPM corresponds to 0.1-0.3 pc, typically the size of the molecular condensations detected by the IRAS satellite at 100pm. SPM is expected to detect these with a S/N ratio of 5 in less than 1 s (short wavelength channel) and less than 1 min (long wavelength channel). The diffuse interstellar medium in these regions is also expected to be detected for longer integration times. External galaxies such as M51 can also be studied and resolved with the field of view (a few
-0.5 II-
1
Wavelength
(mm)
Fig. 2. The Sunyaev-Zel’dovich (SZ) effect is a distortion of the 2.7 K cosmic background by Compton interaction with hot electrons m clusters of galaxies. The solid curve at the center of the figure is the SZ effect for a realistic cluster with zero peculiar velocity. The dotted curve represents the component that linearly adds to this effect when the cluster is moving with respect to a cosmological frame with a velocity of 3000 km s- ‘. The four curves at the bottom represent the transmtssion of the four channels of SPM.
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beams per galaxy) and sensitivity of the SPM. Average properties of the interstellar medium at different galactocentric distances can then be deduced from these measurements. Since dust abundance and temperature are indicators of the physical conditions that prevail in star forming regions, such observations may provide unique information on star formation and galactic evolution. 3. Submillimeter
calibration
standards
Special care is taken to calibrate the measurements made by the SPM: pre-flight and inflight calibrations with an internal calibration system will be carried out. They will give accurate flux values for a set of sources in a wavelength range almost out of reach from the ground. This is especially true for the short wavelength channel (18&240 pm) corresponding to the long wavelength channels of ISO. SPM will thus participate in the definition of calibration standards in this new spectral domain.
IV.
INSTRUMENT
DESIGN
1. General description
The main difficulty of infrared and submillimeter instrumentation is the presence of radiation produced by the instrument itself, the telescope, and the atmosphere, which is several orders of magnitude larger than the flux coming from the astronomical source. This flux produces unnecessary photon noise that limits the sensitivity of the instrument. Its mean value must be removed as much as possible by the measurement process. The SPM instrument is designed to make differential measurements between adjacent fields of view in the sky, which is consistent with most of the scientific objectives. This sky chopping allows the suppression of the constant parasitic flux. Periodic nodding of the whole scientific payload (telescope and focal instrument) makes it possible to remove the residual parasitic flux that could be generated by beam switching on non-uniform emissivity and temperature optics. The optics have been designed to minimize the parasitic flux and the electronics to minimize its consequence on the overall sensitivity. The field of view of each channel has been optimized taking account of the size of the diffraction pattern at the focus of the telescope as well as of the predicted amplitude of the pointing error. A beam switching amplitude of 6 arcmin has been chosen for consistency with the fields of view. The observation of the Sunyaev-Zel’dovich effect was the motivation for optimizing the four spectral bands (Table 1). This optimization was made using a physical model for the source, computing for each possible band the photon noise(‘O)and maximizing the S/N ratio. The photon noise is dominant in channels 1 to 3 and detector noise is expected to dominate in channel 4. The SPM is attached to the Cassegrain telescope by three titanium blades that give it a nearly isostatic mechanical positioning. Its main structure is made of two machined pieces of cast magnesium that contain the warm optics and support the different sub-systems: electronic boxes, the calibration system, cryostat, electrical harness, and thermal protection. In addition, the moving mass used to balance the pointed assembly along the X axis (see Fig. 3) is attached to the SPM. The cryostat positioning is achieved with a classical fixation by three points defining respectively 3, 2 and 1 d.f. The whole system weighs 185 kg, approximately equally shared between the main structure, the cryostat, and the electronics. Its largest dimension is 110 cm. The main parts and the general architecture of the instrument are visible in Fig. 3. The cryostat is tilted in order to avoid spilling liquid helium during the different phases of the mission. During the launch and the recovery phases, the telescope is looking at the zenith and the helium exhaust tube makes an angle of 60” with the vertical, which is not a problem due to its eccentric upwards position. Observations begin only after about half of the liquid helium has been naturally pumped during the ascent. The cryostat is then allowed to be oriented in positions where helium would spill if the helium tank were full.
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2. Warm optics
The warm (i.e. not cooled) optics (Fig. 4) drive the radiation coming from the telescope into the cryostat. Its design provides: a focal plane chopping system, a clean beam from the point of view of geometry and diffraction, and an internal relative calibrator. Chopping is achieved by a wobbling quaternary mirror (M4), optical conjugate of the telescope secondary mirror by M3. M4 is mounted on a galvanometer system manufactured by General Scanning. (‘I) The stability achieved is better than 1 arcsec for the amplitude and 6 arcsec for the average position at the minute time scale. The wobbling mirror is itself imaged by M6 on the entrance cold pupil (PFR) in the cryostat. All mirrors are oversized with respect to the geometrical beam in order to limit the flux due to diffraction from the outside of the mirrors. Special care was taken for M3, located in the focal plane, on which the field modulation takes place. The diffraction computations were made using the reversion theorem of Helmoltz.(‘*) The fraction of the beam outside the mirror varies from 1% (M3) to 2% (M4) in the short wavelength channel and from 3-7% in the long wavelength channel. Baffling with a microwave absorber allows control of the flux coming from these regions. A small mirror (MCI) can be placed inside the beam coming from the secondary mirror. This mirror reflects the flux from the internal calibration source in the beam. 3. Cold optics The image of the telescope secondary mirror is formed by the warm optics on a circular diaphragm PFR cooled at 4 K, which is the actual pupil of the system from the point of view of geometrical optics. The PFR is also used to hold an entrance optical block consisting of a quartz lens L and several filters that reduce the flux at wavelengths than 180 pm to negligible values. This
HOUSEKEEPING
Fig. 3(a)-legend
opposrte
ELECTRoNlCS
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photometer
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FLEXIBLEBIADEMS’Y -
I I
.
CRYOSTAT
Y t>
OPTICAL APERTURE
Fig. 3(b) Fig. 3(a and b). The largest dimension of the SPM is 114 cm. titanium blades. The warm optics are positioned inside the different sub-systems. ABS-X is a motorized moving mass flight. The optical axis of the
It is attached to the telescope by three flexible magnesium main structure that also supports used to balance the pointed assembly during telescope is Ox.
flux had been previously lowered to values acceptable by the cryogenic system by a 100 pm short wavelength blocking filter attached to a 25 K thermal screen. The four spectral bands are separated (Fig. 5) by three dichroics consisting of capacitive mesh filters. (13)Short-wavelength-pass filters made with inductive meshesu4) are situated just in front of the field diaphragms defining the fields of view of the different channels. Off-axis parabolic mirrors form the image of PFR on the entrance of light cones and the image of the field diaphragms at infinity. Toroido-parabolic light concentrators”s.‘6’ achieve an optimal matching of the beam with the bolometers. 4. Cryogenics The 4 channel detection system is fixed on to a 10 in. diameter cold late in a liquid helium cryostat specially manufactured by Infrared Laboratories Inc. (“I The neck of this 62 cm total height Dewar is thrown off-axis of the 13.5 1. helium tank, in order to permit a liquid transfer when tilted at 60-’ from the vertical, its normal standby position on the gondola. The optical signal is received inside the cryostat through a vacuum tight corrugated polyethylene window fixed to its external envelope. Two aluminum gas enthalpy cooled screens at about 25 and 100 K protect the helium tank from thermal radiative and conductive inputs. The coldest one supports a filter cutting all radiation at wavelengths shorter than 100 pm. The cold optics volume is surrounded by an aluminum screen at liquid helium temperature, the inside painted black to avoid optical reflections. This screen is cut away at the optical entrance to support the copper filter holder directly thermally anchored on the cold plate of the cryostat. The cold optical components are screwed on a 3.6 kg auxiliary
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Fig. 4. PRONAOS 2 m telescope and the warm opttcs of the SPM fold the beam onto a cold stop (PFR), which is the actual entrance pupil of the system. Beam switching is achteved by the wobbling M4 mirror, an optical conjugate of the secondary mirror. A small removable mirror reflects the calibration sources in the beam.
cooled copper plate hanging from the cryostat helium plate through 3 large copper 15 cm long columns. Its temperature is controlled by an Allen Bradley resistance thermometer. The total weight of this optical plate is 7.2 kg. Thermally isolated from the cryostat cold plate by very thin wall stainless steel tubes, two large copper holders ( w 300 g), each supporting 2 bolometers and their associated light cones, are
Fig. 5. Scheme of wavelength selectton m the cold opttcs. The elements represented are those attached to the optical plate. Off-axis parabolic mirrors (not represented here) fold the four beams onto the entrance of light collectors parallel to the Z axis. L is a lens. Fl is the blockmg filter at Ic = IgO~m. DII are longwave pass dichroics and F2 to F5 are longwave blocking filters. The fields of view of the different channels are defined by the field diaphragms DCi.
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screwed onto the liquid 3He reservoirs of two miniature hermetic refrigerators developed by Torre and Chanin(‘*’ and cooled down to 0.3 K for more than 48 h when the 4He is pumped down to 1.8 K. The temperatures are controlled by four wire calibrated Ge thermometers. These fridges have been improved to achieve a higher refrigeration power and to allow their operation with the 60” tilted cryostat. Starting with a 1.8 K 4He bath, recycling takes typically about 20 min for the 3He pump desorption and liquefaction followed by about 60 min to obtain a temperature close to 0.3 K. In order to speed up by a factor of more than 15 the cool down time of the detector holders and ‘He reservoirs from room temperature to LN, and then to near cryogenic temperatures, 2 passive automatic gas heat switches,“” parts of the mechanical stainless steel supports, have been installed. Fixed on the cold plate, a small charcoal getter increases the cryostat hold time when filled with LN, . Each bolometer provides a signal to a JFET heated to 130 K. The power dissipation to the 4He bath from the cold electronics is around 1.5 mW. To avoid thermal radiation from the JFETs, they are covered by an optically tight copper cap screwed to the cryostat cold plate. 5. Detectors The most sensitive detectors at these wavelengths are bolometers. To achieve the lowest possible NEP they are cooled down to 0.3 K. Detectors for channels 1, 2, and 4 are commercial silicon bolometers.“” The bolometer of channel 3 is a composite bolometer(20’ with a 30 pm thick diamond substrate and a monolithic doped germanium thermistor (*I)fitted in an integrating sphere in an optimized way following Refs (22) and (23). This bolometer has been calibrated by three different methods including particle detection.‘24’ Their intrinsic NEPs are around lo-l5 W Hz-“* or less at 19.5 Hz. They have been tested with backgrounds much larger than those expected in flight. Then their working temperature and their NEPs obtained in the lab are higher. The currently demonstrated sensitivities to temperature differences are less than 1 mK Hz-“* on all channels with a 300 K background, not far from that induced by photon noise. The field of view of each bolometer is limited by the entrance of light concentrators attached to the detectors. 6. Electronics
The photometer electronics are built around a main computer (UGBS) and six electronic sub-systems for housekeeping (HK), measurement and calibration. The opto-coupled serial links use 16 bit words with a standard protocol, which proved to be very efficient during the development phases, since any of the sub-system could be tested using a single computer interface. The UGBS tasks are: interface with the gondola computer, reception of telecommands, preparation and transmission of the telemetry format, data acquisition from and control of electronic sub-systems, and control of the energy distribution. One of the consequences of the high sensitivity to temperature differences is a tough requirement on the uniformity of the thermal emission of the instrument and on the stability of all the systems having an influence on the signal. A special effort has been made to improve the electronic parts involved in this requirement. A compact and stable read out circuitry for thermometers has been developed especially for this project and is extensively used in the housekeeping electronics. In the same way, a multiplexed 16 bit digital to analog converter electronic card has been developed and included in all the HK sub-systems. The different HK functions are: temperature stabilization of warm optics and the main mechanical structure and position control of the calibration mirror; measurement of cryogenic temperatures, cycling control of helium 3 refrigerators; control of the wobbling M4 mirror, temperature stabilization of its servo-controlled motor and position sensor and sampling of the position signal; thermal control of the outside part of the cryostat, especially of the entrance window to avoid water condensation on the optical path; temperature control of the calibration blackbodies and position control of the associated chopper.
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For each bolometer, a stabilized cold temperature J-230 JFET preamplifier (PA) is used as an emitter follower with constant drain current and drain source potential to maintain its gain at absolute unity.‘*‘) This provides an impedance conversion necessary for the G = 1000 room temperature external amplifiers fixed on the cryostat external envelope. The analog output signal is then sent to digital lock-in amplifiers (LA) developed to maintain a high stability of signal processing. The LA entrance circuitry includes a band-pass filter, a variable gain amplifier programmable by telecommand, a deglitcher, and a saturation detector. The LA itself uses voltage to frequency converters(*@ synchronized by the phase-shifted reference signal provided by the position sensor of the chopping mirror. The demodulated signal is presented as one 16 bit word per period, the first bit indicating saturation or deglitching. The maximum total gain of the amplifying
chain is 4.5 x 105.
7. Data handling The housekeeping and scientific data are collected by the SPM computer (UGBS) and arranged in a format of 782 bytes sent each second to the telemetry emitter and then down to the ground station. Data from the Global Positioning System and from the telescope pointing system are transmitted from the gondola computer to UGBS and included in the telemetry format. These data are necessary for data reduction. The real time ground-based data handling station is built around a telemetry decommuter that feeds three specialized Personal Computers (PCs). The “technical control” PC records all the SPM data, displays the housekeeping parameters, and is used to prepare and send telecommands to SPM. The “signal quality” PC performs on the scientific signal real-time advanced processing such as digital filtering, statistical and Fourier analysis, and interactive display. It gives a deep insight into the performance of the instrument. The “astronomical” PC performs interactive data reduction and displays the results in a form convenient for a quick evaluation of the scientific return of the flight, which is useful for the flight conduct.
8. Calibrations Calibration is a necessary but laborious exercise for all measurements made in the submillimeter range. The uncertainties on the sidelobes, on the transmission by the optics, the losses by diffraction, the fluctuation in time of the response of the detection system quickly build up errors of lO-50% on the measurements existing today. All of these issues have been carefully addressed for the SPM instrument. Distant sidelobes of the cryostat were measured and found consistent with an Airy pattern energy distribution. Transmission of the optics and diffraction within SPM are taken into account in the calibration process: an extended and modulated blackbody is placed in front of the SPM and completely fills the beam. This blackbody is the primary standard of the system. It is designed for the purpose of submillimeter calibrations and has been used for the Emilie experiment.‘*” The signal of this standard is directly compared to the internal calibrator signal. All calibrations may now refer to the internal calibrator. It consists of two blackbodies (310 and 370 K) seen alternately thanks to a reflection chopper. Optical coupling is made by the removable mirror MCI with a dilution factor of about 3%. The remaining part of the beam comes directly from the sky. This allows calibrations of the photometer, with small changes in the background and therefore in the operating point and the responsivity of the bolometers. The 60 K temperature difference between the two blackbodies has a stability better than 20 mK for a time scale of 1 h and 1 mK for a time scale of 1 min. The internal calibration has been operated under a large number of experimental conditions such as the background flux (with an extended cold-77 K-blackbody in front of the SPM), temperature of the detectors, and presence or not of the atmosphere.
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0.1
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^__.
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7
0.01
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^ _.
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Integration time (s) Fig. 6. Allan variance of the four channels with internal calibration as the optical source. These curves show that even with a strong source, the gain of the detection chain is stable enough to improve the S/N ratio for continuous integration times as long as several hundreds of seconds.
200.0
SIGNAL FILTRE
100.0 0.0 -100.0 -200.0
Fig. 7. Detection of Jupiter from the ground in the presence of strong atmospheric noise and absorption. Dotted lines are the outputs of channels 3 and 4 with effective atmospheric transmissions of lo-* and 5.10m4. The solid line is the result of the optimized difference between 3 and 4, which removes most of the atmospheric noise. Units are 0.5 s and SPM digital output. Jupiter transit in the field of view is visible at ~270 time units with an antenna temperature of 2mK. This type of measurement was used for alignments of the star sensor.
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V.
INSTRUMENTAL
NOISE
ANALYSIS
The ultimate limit on a flux measurement is set by the photon noise of the total incoming radiation on the detectors. Instrumental noise must be added to the photon noise to give the real limit on the measurement. Instrumental noise includes detector noise, thermo-optical noise on the modulated signal and the noise induced by the fluctuations of the pointing system. SPM in-flight sensitivity is expected to be photon noise limited for the short wavelength channels and detector noise limited for the long wavelength channels. Therefore the experimental design must minimize both thermo-optical and pointing related noise. Thermo-optical noise results from thermal fluctuations or inhomogeneity of the parts seen directly or by diffraction, fluctuations of the modulation parameters (amplitude, offset) and fluctuations of the detection electronics. The first term is minimized by a careful thermal control of the structure and mirrors of the warm optics, and by severe control of the diffracted fluxes. The surface of M3 and its environment are a key in this process because the beam scans M3 during modulation. A dedicated modulation scheme (between the on-axis beam and a variable position on M3) gives a direct measure of the spurious fluxes as a function of the modulation amplitude. Other parts such as the edges of the primary mirror may also create a modulated-and varying-signal which adds to the astrophysical signal and cannot be eliminated. Achieving a highly stable modulation system needed intensive tests and work on the driving electronic of the wobbling mirror scanner. Appropriate dynamics for the complete detection system (typically 104), prevents artificial noise generation in the presence of constant parasitic signals. This is demonstrated, as well as the stability of the whole system, by the Allan variance(28~29) of measurements obtained with an internal calibration source (Fig. 6). A high linearity and stability is an efficient tool that makes difficult data reduction possible. The example of the detection of Jupiter from Toulouse (France) with an atmospheric effective transmission of 5.104 and strong atmospheric noise (Fig. 7) is an illustration of the efficiency of good quality multi-channel photometry. Acknowledgements-The development of SPM is funded by CNES and CNRS. We would like to thank Fran9ots Buisson, Jean Audoubert, Bernard Boullet, Jean-Pierre Diris, Andre Laurens, Roger Roulet-Matton, and the teams of CNES who played a decisive part during the integration and tests of SPM with the PRONAOS telescope.
REFERENCES 1. P. H. MotTat, R. A. Bohlander, W. R. Macrae and H. A. Gebbie, Nature 269, 676 (1977). 2. J. E. Harries, J. opf. Sot. Am. 67, 880 (1977). 3. J. C. Mather, Proc. SPIE Conf Infrared Spaceborne Remote Sensing, 11-16 July 1993. San Diego, CA, USA and COBE Preprint No. 93-10. 4. M. Fisher er al., 388, 242 (1992). 5. G. Beaudin et al., SPIE Int. Conf. on Infrared and Millimeter Wave Engineerng, 19-21 January 1993, LOS Angeles. Proc. SPIE 1874, 246 (1993). 6. J. M. Lamarre, P. Encrenaz, G. Serra, F. Pajot and G. Bellaiche, J. Opt. 21, 141 (1990). 7. R. A. Sunyaev and Ya.B. Zel’dovich, Comm. Ap. Sp. SC. 4, 173 (1972). 8. R. A. Sunyaev and Ya.B. Zel’dovich, A. Rev. Astron. Asrrophys. 18, 537 (1980). 9. F. Pajot, Boisst, R. Gispert, J. M. Lamarre, J. L. Puget and G. Serra, Astron. Asfrophys. 157, 393 (1986). 10. J. M. Lamarre, Appl. Opt. 25, 870 (1986). 11. General Scanning Inc., 500 Arsenal Street, Watertown, MA 03372, USA. 12. M. Born and E. Wolf, Principles of Optics, 6th edn, p. 381. Pergamon Press, Oxford (1984). 13. J. M. Lamarre, N. Coron, R. Courtin, G. Dambier and M. Charra, Int. J. Injiared Millimeter Waves 2, 273 (1981). 14. J. M. Lamarre and B. Leriche, To be published. 15. V. K. Baranov and G. K. Mel’nikov, Sov. J. Opt. Technol. 33, 408 (1966). 16. D. A. Harper, R. H. Hildebrand, R. Stiening and R. Winston, Appl. 0p1. 15, 53 (1976). 17. Infrared Laboratories Inc. 1808 East 17th street, Tucson, AZ 85719, USA. 18. J. P. Torre and G. Chanin, Rev. scienf. Instrum. 26, 318 (1985). 19. J. P. Torre, To be published. 20. J. Leblanc, G. Dambier, N. Coron and J. P. Moalic, French Patent No. 75 36104; 26 NOV. 1975. 21. J. Leblanc, G. Dambier, N. Coron and J. P. Moalic, US Patent No. 4116063; 26 Sept. 1978. German Patent NO. 2653865; 8 June 1977.
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J. Leblanc, G. Dambier and N. Coron, CNRS French Patent No. 8508945, 13 June 1985. J. M. Lamarre, Internal LPSP Note; 19 April 1977. N. Coron, Internal IAS, Note; 26 April 1992. D. P. Stokesberry, Proc. IEEE S4, 66 (1966). S. Cova and A. Longoni, Rev. scienf. Instrum. SO, 296 (1979). F. Pajot, R. Gispert, J. M. Lamarre, R. Peytureaux, J. L. Puget, G. Serra, N. Coron, G. Dambier, J. Leblanc, J. P. Moalic, J. C. Renault and R. Vitry, Asrron. Asrrophys. 154, 55 (1986). 28. D. W. Allan, Proc. IEEE 54, 221 (1966). 29. P. Lesage and C. Audoin, Radio Science 14, 521 (1979).
CopyrIght 0 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved
Pergamon
1350-4495/94 $6.00 + 0.00
SPM, A SUBMILLIMETER PHOTOMETER FOR PRONAOS J. M. LAMARRE’, F. PAJOT’, J. P. TORRE’, G. GUYOT’, J. P. BERNARD’, A. DE LUCA’, M. GIARD~, J. MANGIN~, R. PEYTURAUX’, J. L. P~GET’, G. SERRA’, F. BOULANGER’, X. DESERT’,I. RISTORCELLI~,G. JEGOUDEZ’,H. LAGARD~RE’,J. LEBLANC’, R. PONS~, G. RECOUVREUR’,M. BARTHELEMY’,C. BOURGUIGNON’,J. P. CRUSSAIRE’,G. DAMBIER’, L.
DELILLE’, E.
KORCZAK’, J. P.
LEPELTIER’, B.
LERICHE’, C.
LIZAMBERT’,
J. NARBONNE~,C. PLAISANT’, J. REIGNER’,J. C. RENAULT’ and C. RIOUX’ ‘Institut d’Astrophysique Spatiale, Universite Paris XI, Bltiment 121, F-91405 Orsay Cedex, 2Service d’Aironomie, BP3, F-91371 VerriBres-le-Buisson Cedex, 3Centre d’Etudes Spatiales des Rayonnements, 9 av. du Colonel Roche, BP 4346, F-31055 Toulouse Cedex, “LIRL, Universiti Nancy I, F-54506 Vandoeuvre les Nancy and %stitut d’Astrophysique, 98bis Boulevard Arago, F-75014 Paris, France (Received 14 September 1993)
Abstract-SPM is a multi-channels photometer designed to obtam the best sensitivity at the focus of the 2m balloon-borne PRONAOS telescope. Its scientific goals include the measurement of cosmological background anisotropy, which is important for understanding the origin of the structure in the universe and the observation of cold solid matter in the interstellar medium as an indicator of the physical conditions that prevail in star forming regions. SPM consists of warm optics that ensure beam switching and in-flight calibration, a liquid helium cryostat that contains filters and the )He cooled bolometers, and electronics for housekeeping and data handling. The spectral bands are 18&24Opm, 240-340 pm, 340-540 pm and 540-1200 ym. They are defined by dichroics, which allows simultaneous observations in the four bands. The expected NEAT in flight is significantly less than 1 mK in all the bands. SPM will fly with PRONAOS in May 1994.
I. INTRODUCTION PRONAOS is an acronym for the Projet National d’Astronomie Submillimktrique. On the one hand it acts for research laboratories of the Centre National de la Recherche Scientifique (CNRS), and on the other hand for the Centre National d’Etudes Spatiales (CNES) and industry, in a single effort to construct and operate a large balloon-borne instrument dedicated to submillimetre astronomy. The purpose of this project is to provide astronomers with the opportunity of observations in a wavelength domain (100 pm-1 mm) where the atmosphere is nearly opaque, due mainly to water vapor rotation lines. The two atmospheric transmission windows around 350 and 450 pm are usable only from the best observation sites (‘) during the best weather periods. Even at airplane altitudes, a lot of absorption lines disturb the measurements’*) and even make it impossible or only just possible to observe some species, such as H,O and 0,. Wide band and high sensitivity measurements are then only possible at higher altitudes. The 2 m telescope will give an improved angular resolution compared with the previous submillimeter space-borne or balloon-borne projects.‘3.4) The project has four main sub-systems. The 2 m telescope, the gondola and two focal instruments, SPM (systtme photomktrique multibandes), a multi-channel high sensitivity photometer, and SMH (spectromktre httirodyne), a 380 GHz heterodyne spectrometer.‘s’ Only one of these instruments can be mounted on the Cassegrain focus of the telescope. Each focal instrument has its own scientific goals and its own scientific and technical team. The system is designed to perform 10 flights in about 10 years. The first flight with SPM is scheduled in May 1994. The spectral bands and the fields of view of the four channels of SPM are given in Table 1. The PRONAOS gondola and telescope are described in Section II. The scientific objectives of SPM are listed in Section III. 277
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Section IV is devoted to the detailed report of its design. Section V deals with an analysis of the sources of noise and with calibration results.
II.
THE
PRONAOS
GONDOLA
AND
TELESCOPE
The gondola protects the scientific payload and points it towards the observed astronomical source. It also provides the scientific payload for electrical power and a telemetry-telecommand link with the ground. Its structure comprises mainly aluminium alloys. It is 7 m high and the total weight of PRONAOS is 2.6 tons (Fig. 1). The thermal control of the whole system is achieved by using thermal paints and polyurethane protection. On board energy is provided by about 100 kg of lithium batteries. The gondola electronics are controlled by a DATA computer (Crouzet) and its real time system ASTRES. The pointing system consists of two principal stages. The azimuth stabilization stage orientates the whole gondola and ensures the damping of external perturbations. The fine pointing system orientates the 450 kg scientific payload, i.e. the telescope and its focal instrument, through a Cardan suspension with a nominal standard deviation of 5 arcsec. An inertial platform is used as a short term sensor for the pointing servo-control. Its drifts are measured by a star sensor which works even in day light with stars having a magnitude up to five. The star sensor can be oriented off-axis from the telescope line of sight in order always to have a bright enough star in its field of view. The balance of the pointed mass is achieved by three moving masses that compensate mechanical deformations and helium evaporation in the Dewar. The Cassegrain telescope has a diameter of 2065 mm and a primary-secondary mirror distance of 1718 mm. Its equivalent focal length is 20 m and its total mass 225 kg. The primary mirror is of the segmented type, with six light-weight panels (6 kg/m2). The panels are sandwiches of carbon fiber honeycomb core between two carbon fiber skins. Position sensors at the edge of the segments and position actuators supporting them compensate for the gravity and temperature effects on the supporting structure. This system keeps the long term surface deformations to RMS values less than 12 pm. At the beginning of the project, the specifications were twice as large. The surface quality of the mold made by REOSC and used for the fabrication of the panels, as well as the new fabrication technique of the panels, were the main reasons for this increase in the antenna precision. The whole telescope is embedded in a sunshade that protects the optical system from external sources of heat, such as the sun and the earth. This limits the temperature gradients along a primary mirror diameter to values less than a few kelvin, which is an important characteristic for a beam switching instrument such as SPM. The Toulouse Center of the Centre National d’Etudes Spatiales (CNES) is responsible for the gondola development. The telescope is developed by Matra Espace in Toulouse, under the auspices of the CNES. More information on PRONAOS can be found in Ref (6).
III.
1. Sunyaev-Zel’dovich
SCIENTIFIC
OBJECTIVES
(SZ) effect and cosmic background
OF SPM
anisotropy
The intracluster gas scatters the (sub)millimeter 3 K cosmic microwave background by inverse Compton effect from its hot electrons (about lo8 Kc’)). In 1980, Sunyaev and Zel’dovich@’ pointed out a way of directly measuring the radial peculiar cluster velocity relative to the 3 K background, Table
1. Bands and field of view of the four channels
Band No. Wavelength (pm) FOV (arcmin)
1 180-240 2
2 240-340 2
3 340-540 2.3
4 _54&1200 3.5
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Fig. I. PRONAOS. 7 m high and 2.6 tons, in flight configuration. The cover of the sunshade is open and a small part of its 2 m segmented mirror can be seen. Focal instrumentation is at the Cassegrain focus. The star sensor is attached under the sunshade. Housekeeping and stabilization equipments are on the lower platform. An 8 m umbrella-like protection is intended to prevent the gondola from being overturned when landmg.
due to the Doppler effect on the scattered photons. For every observed cluster of galaxies, the SZ and Doppler distortions linearly combine but have different spectral distributions and can therefore be separated with observations in several bands in the millimeter and submillimeter range. These measurements would constitute a method totally independent from the so-called 4-D galaxy surveys and would obviously be invaluable for investigating the kinematics of the Universe on the largest observable scale. As shown in Fig. 2, channel 4 of the SPM is an efficient tool for measuring the short wavelength part of the SZ distortion, i.e. its positive part. The atmosphere is opaque and emissive in this spectral band and balloon-borne or space-borne instruments are necessary to complete ground-based observations that can be obtained at larger wavelengths. The other bands of the SPM are also represented in Fig. 2. They will allow the measurement of the possible
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contamination by dust, much brighter at these shorter wavelengths. To check that the measurement of the SZ effect is not due to instrumental artefacts, we plan to observe, during the same integration time, the empty sky, i.e. a region where no potential source has been identified. This measurement will put an upper limit to the anisotropy of the cosmic radiation in this region, at a wavelength and an angular scale (6 arcmin) which differs from other experiments. 2. Interstellar medium (ISM) Submillimeter photometry is the specific tool needed to measure the thermal continuum emission of interstellar dustC9) The four bands of the SPM photometer cover a spectral domain starting at the emission maximum, strongly dependent on the dust temperature (T’ at least). The emission then decreases steeply with the wavelength (A -’ for the AZ,emission-for a Iz -* dust emissivity) but with a weaker dependence on T (proportional). These characteristics enable the dust temperature to be derived (at the wavelength of the emission maximum), as well as the dust emissivity and integrated column density (at long wavelengths). At the distance of the close molecular complexes (100-300 pc, e.g. the Taurus complex), the field of view of the SPM corresponds to 0.1-0.3 pc, typically the size of the molecular condensations detected by the IRAS satellite at 100pm. SPM is expected to detect these with a S/N ratio of 5 in less than 1 s (short wavelength channel) and less than 1 min (long wavelength channel). The diffuse interstellar medium in these regions is also expected to be detected for longer integration times. External galaxies such as M51 can also be studied and resolved with the field of view (a few
-0.5 II-
1
Wavelength
(mm)
Fig. 2. The Sunyaev-Zel’dovich (SZ) effect is a distortion of the 2.7 K cosmic background by Compton interaction with hot electrons m clusters of galaxies. The solid curve at the center of the figure is the SZ effect for a realistic cluster with zero peculiar velocity. The dotted curve represents the component that linearly adds to this effect when the cluster is moving with respect to a cosmological frame with a velocity of 3000 km s- ‘. The four curves at the bottom represent the transmtssion of the four channels of SPM.
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beams per galaxy) and sensitivity of the SPM. Average properties of the interstellar medium at different galactocentric distances can then be deduced from these measurements. Since dust abundance and temperature are indicators of the physical conditions that prevail in star forming regions, such observations may provide unique information on star formation and galactic evolution. 3. Submillimeter
calibration
standards
Special care is taken to calibrate the measurements made by the SPM: pre-flight and inflight calibrations with an internal calibration system will be carried out. They will give accurate flux values for a set of sources in a wavelength range almost out of reach from the ground. This is especially true for the short wavelength channel (18&240 pm) corresponding to the long wavelength channels of ISO. SPM will thus participate in the definition of calibration standards in this new spectral domain.
IV.
INSTRUMENT
DESIGN
1. General description
The main difficulty of infrared and submillimeter instrumentation is the presence of radiation produced by the instrument itself, the telescope, and the atmosphere, which is several orders of magnitude larger than the flux coming from the astronomical source. This flux produces unnecessary photon noise that limits the sensitivity of the instrument. Its mean value must be removed as much as possible by the measurement process. The SPM instrument is designed to make differential measurements between adjacent fields of view in the sky, which is consistent with most of the scientific objectives. This sky chopping allows the suppression of the constant parasitic flux. Periodic nodding of the whole scientific payload (telescope and focal instrument) makes it possible to remove the residual parasitic flux that could be generated by beam switching on non-uniform emissivity and temperature optics. The optics have been designed to minimize the parasitic flux and the electronics to minimize its consequence on the overall sensitivity. The field of view of each channel has been optimized taking account of the size of the diffraction pattern at the focus of the telescope as well as of the predicted amplitude of the pointing error. A beam switching amplitude of 6 arcmin has been chosen for consistency with the fields of view. The observation of the Sunyaev-Zel’dovich effect was the motivation for optimizing the four spectral bands (Table 1). This optimization was made using a physical model for the source, computing for each possible band the photon noise(‘O)and maximizing the S/N ratio. The photon noise is dominant in channels 1 to 3 and detector noise is expected to dominate in channel 4. The SPM is attached to the Cassegrain telescope by three titanium blades that give it a nearly isostatic mechanical positioning. Its main structure is made of two machined pieces of cast magnesium that contain the warm optics and support the different sub-systems: electronic boxes, the calibration system, cryostat, electrical harness, and thermal protection. In addition, the moving mass used to balance the pointed assembly along the X axis (see Fig. 3) is attached to the SPM. The cryostat positioning is achieved with a classical fixation by three points defining respectively 3, 2 and 1 d.f. The whole system weighs 185 kg, approximately equally shared between the main structure, the cryostat, and the electronics. Its largest dimension is 110 cm. The main parts and the general architecture of the instrument are visible in Fig. 3. The cryostat is tilted in order to avoid spilling liquid helium during the different phases of the mission. During the launch and the recovery phases, the telescope is looking at the zenith and the helium exhaust tube makes an angle of 60” with the vertical, which is not a problem due to its eccentric upwards position. Observations begin only after about half of the liquid helium has been naturally pumped during the ascent. The cryostat is then allowed to be oriented in positions where helium would spill if the helium tank were full.
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2. Warm optics
The warm (i.e. not cooled) optics (Fig. 4) drive the radiation coming from the telescope into the cryostat. Its design provides: a focal plane chopping system, a clean beam from the point of view of geometry and diffraction, and an internal relative calibrator. Chopping is achieved by a wobbling quaternary mirror (M4), optical conjugate of the telescope secondary mirror by M3. M4 is mounted on a galvanometer system manufactured by General Scanning. (‘I) The stability achieved is better than 1 arcsec for the amplitude and 6 arcsec for the average position at the minute time scale. The wobbling mirror is itself imaged by M6 on the entrance cold pupil (PFR) in the cryostat. All mirrors are oversized with respect to the geometrical beam in order to limit the flux due to diffraction from the outside of the mirrors. Special care was taken for M3, located in the focal plane, on which the field modulation takes place. The diffraction computations were made using the reversion theorem of Helmoltz.(‘*) The fraction of the beam outside the mirror varies from 1% (M3) to 2% (M4) in the short wavelength channel and from 3-7% in the long wavelength channel. Baffling with a microwave absorber allows control of the flux coming from these regions. A small mirror (MCI) can be placed inside the beam coming from the secondary mirror. This mirror reflects the flux from the internal calibration source in the beam. 3. Cold optics The image of the telescope secondary mirror is formed by the warm optics on a circular diaphragm PFR cooled at 4 K, which is the actual pupil of the system from the point of view of geometrical optics. The PFR is also used to hold an entrance optical block consisting of a quartz lens L and several filters that reduce the flux at wavelengths than 180 pm to negligible values. This
HOUSEKEEPING
Fig. 3(a)-legend
opposrte
ELECTRoNlCS
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photometer
283
for PRONAOS
FLEXIBLEBIADEMS’Y -
I I
.
CRYOSTAT
Y t>
OPTICAL APERTURE
Fig. 3(b) Fig. 3(a and b). The largest dimension of the SPM is 114 cm. titanium blades. The warm optics are positioned inside the different sub-systems. ABS-X is a motorized moving mass flight. The optical axis of the
It is attached to the telescope by three flexible magnesium main structure that also supports used to balance the pointed assembly during telescope is Ox.
flux had been previously lowered to values acceptable by the cryogenic system by a 100 pm short wavelength blocking filter attached to a 25 K thermal screen. The four spectral bands are separated (Fig. 5) by three dichroics consisting of capacitive mesh filters. (13)Short-wavelength-pass filters made with inductive meshesu4) are situated just in front of the field diaphragms defining the fields of view of the different channels. Off-axis parabolic mirrors form the image of PFR on the entrance of light cones and the image of the field diaphragms at infinity. Toroido-parabolic light concentrators”s.‘6’ achieve an optimal matching of the beam with the bolometers. 4. Cryogenics The 4 channel detection system is fixed on to a 10 in. diameter cold late in a liquid helium cryostat specially manufactured by Infrared Laboratories Inc. (“I The neck of this 62 cm total height Dewar is thrown off-axis of the 13.5 1. helium tank, in order to permit a liquid transfer when tilted at 60-’ from the vertical, its normal standby position on the gondola. The optical signal is received inside the cryostat through a vacuum tight corrugated polyethylene window fixed to its external envelope. Two aluminum gas enthalpy cooled screens at about 25 and 100 K protect the helium tank from thermal radiative and conductive inputs. The coldest one supports a filter cutting all radiation at wavelengths shorter than 100 pm. The cold optics volume is surrounded by an aluminum screen at liquid helium temperature, the inside painted black to avoid optical reflections. This screen is cut away at the optical entrance to support the copper filter holder directly thermally anchored on the cold plate of the cryostat. The cold optical components are screwed on a 3.6 kg auxiliary
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Fig. 4. PRONAOS 2 m telescope and the warm opttcs of the SPM fold the beam onto a cold stop (PFR), which is the actual entrance pupil of the system. Beam switching is achteved by the wobbling M4 mirror, an optical conjugate of the secondary mirror. A small removable mirror reflects the calibration sources in the beam.
cooled copper plate hanging from the cryostat helium plate through 3 large copper 15 cm long columns. Its temperature is controlled by an Allen Bradley resistance thermometer. The total weight of this optical plate is 7.2 kg. Thermally isolated from the cryostat cold plate by very thin wall stainless steel tubes, two large copper holders ( w 300 g), each supporting 2 bolometers and their associated light cones, are
Fig. 5. Scheme of wavelength selectton m the cold opttcs. The elements represented are those attached to the optical plate. Off-axis parabolic mirrors (not represented here) fold the four beams onto the entrance of light collectors parallel to the Z axis. L is a lens. Fl is the blockmg filter at Ic = IgO~m. DII are longwave pass dichroics and F2 to F5 are longwave blocking filters. The fields of view of the different channels are defined by the field diaphragms DCi.
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screwed onto the liquid 3He reservoirs of two miniature hermetic refrigerators developed by Torre and Chanin(‘*’ and cooled down to 0.3 K for more than 48 h when the 4He is pumped down to 1.8 K. The temperatures are controlled by four wire calibrated Ge thermometers. These fridges have been improved to achieve a higher refrigeration power and to allow their operation with the 60” tilted cryostat. Starting with a 1.8 K 4He bath, recycling takes typically about 20 min for the 3He pump desorption and liquefaction followed by about 60 min to obtain a temperature close to 0.3 K. In order to speed up by a factor of more than 15 the cool down time of the detector holders and ‘He reservoirs from room temperature to LN, and then to near cryogenic temperatures, 2 passive automatic gas heat switches,“” parts of the mechanical stainless steel supports, have been installed. Fixed on the cold plate, a small charcoal getter increases the cryostat hold time when filled with LN, . Each bolometer provides a signal to a JFET heated to 130 K. The power dissipation to the 4He bath from the cold electronics is around 1.5 mW. To avoid thermal radiation from the JFETs, they are covered by an optically tight copper cap screwed to the cryostat cold plate. 5. Detectors The most sensitive detectors at these wavelengths are bolometers. To achieve the lowest possible NEP they are cooled down to 0.3 K. Detectors for channels 1, 2, and 4 are commercial silicon bolometers.“” The bolometer of channel 3 is a composite bolometer(20’ with a 30 pm thick diamond substrate and a monolithic doped germanium thermistor (*I)fitted in an integrating sphere in an optimized way following Refs (22) and (23). This bolometer has been calibrated by three different methods including particle detection.‘24’ Their intrinsic NEPs are around lo-l5 W Hz-“* or less at 19.5 Hz. They have been tested with backgrounds much larger than those expected in flight. Then their working temperature and their NEPs obtained in the lab are higher. The currently demonstrated sensitivities to temperature differences are less than 1 mK Hz-“* on all channels with a 300 K background, not far from that induced by photon noise. The field of view of each bolometer is limited by the entrance of light concentrators attached to the detectors. 6. Electronics
The photometer electronics are built around a main computer (UGBS) and six electronic sub-systems for housekeeping (HK), measurement and calibration. The opto-coupled serial links use 16 bit words with a standard protocol, which proved to be very efficient during the development phases, since any of the sub-system could be tested using a single computer interface. The UGBS tasks are: interface with the gondola computer, reception of telecommands, preparation and transmission of the telemetry format, data acquisition from and control of electronic sub-systems, and control of the energy distribution. One of the consequences of the high sensitivity to temperature differences is a tough requirement on the uniformity of the thermal emission of the instrument and on the stability of all the systems having an influence on the signal. A special effort has been made to improve the electronic parts involved in this requirement. A compact and stable read out circuitry for thermometers has been developed especially for this project and is extensively used in the housekeeping electronics. In the same way, a multiplexed 16 bit digital to analog converter electronic card has been developed and included in all the HK sub-systems. The different HK functions are: temperature stabilization of warm optics and the main mechanical structure and position control of the calibration mirror; measurement of cryogenic temperatures, cycling control of helium 3 refrigerators; control of the wobbling M4 mirror, temperature stabilization of its servo-controlled motor and position sensor and sampling of the position signal; thermal control of the outside part of the cryostat, especially of the entrance window to avoid water condensation on the optical path; temperature control of the calibration blackbodies and position control of the associated chopper.
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For each bolometer, a stabilized cold temperature J-230 JFET preamplifier (PA) is used as an emitter follower with constant drain current and drain source potential to maintain its gain at absolute unity.‘*‘) This provides an impedance conversion necessary for the G = 1000 room temperature external amplifiers fixed on the cryostat external envelope. The analog output signal is then sent to digital lock-in amplifiers (LA) developed to maintain a high stability of signal processing. The LA entrance circuitry includes a band-pass filter, a variable gain amplifier programmable by telecommand, a deglitcher, and a saturation detector. The LA itself uses voltage to frequency converters(*@ synchronized by the phase-shifted reference signal provided by the position sensor of the chopping mirror. The demodulated signal is presented as one 16 bit word per period, the first bit indicating saturation or deglitching. The maximum total gain of the amplifying
chain is 4.5 x 105.
7. Data handling The housekeeping and scientific data are collected by the SPM computer (UGBS) and arranged in a format of 782 bytes sent each second to the telemetry emitter and then down to the ground station. Data from the Global Positioning System and from the telescope pointing system are transmitted from the gondola computer to UGBS and included in the telemetry format. These data are necessary for data reduction. The real time ground-based data handling station is built around a telemetry decommuter that feeds three specialized Personal Computers (PCs). The “technical control” PC records all the SPM data, displays the housekeeping parameters, and is used to prepare and send telecommands to SPM. The “signal quality” PC performs on the scientific signal real-time advanced processing such as digital filtering, statistical and Fourier analysis, and interactive display. It gives a deep insight into the performance of the instrument. The “astronomical” PC performs interactive data reduction and displays the results in a form convenient for a quick evaluation of the scientific return of the flight, which is useful for the flight conduct.
8. Calibrations Calibration is a necessary but laborious exercise for all measurements made in the submillimeter range. The uncertainties on the sidelobes, on the transmission by the optics, the losses by diffraction, the fluctuation in time of the response of the detection system quickly build up errors of lO-50% on the measurements existing today. All of these issues have been carefully addressed for the SPM instrument. Distant sidelobes of the cryostat were measured and found consistent with an Airy pattern energy distribution. Transmission of the optics and diffraction within SPM are taken into account in the calibration process: an extended and modulated blackbody is placed in front of the SPM and completely fills the beam. This blackbody is the primary standard of the system. It is designed for the purpose of submillimeter calibrations and has been used for the Emilie experiment.‘*” The signal of this standard is directly compared to the internal calibrator signal. All calibrations may now refer to the internal calibrator. It consists of two blackbodies (310 and 370 K) seen alternately thanks to a reflection chopper. Optical coupling is made by the removable mirror MCI with a dilution factor of about 3%. The remaining part of the beam comes directly from the sky. This allows calibrations of the photometer, with small changes in the background and therefore in the operating point and the responsivity of the bolometers. The 60 K temperature difference between the two blackbodies has a stability better than 20 mK for a time scale of 1 h and 1 mK for a time scale of 1 min. The internal calibration has been operated under a large number of experimental conditions such as the background flux (with an extended cold-77 K-blackbody in front of the SPM), temperature of the detectors, and presence or not of the atmosphere.
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10
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Integration time (s) Fig. 6. Allan variance of the four channels with internal calibration as the optical source. These curves show that even with a strong source, the gain of the detection chain is stable enough to improve the S/N ratio for continuous integration times as long as several hundreds of seconds.
200.0
SIGNAL FILTRE
100.0 0.0 -100.0 -200.0
Fig. 7. Detection of Jupiter from the ground in the presence of strong atmospheric noise and absorption. Dotted lines are the outputs of channels 3 and 4 with effective atmospheric transmissions of lo-* and 5.10m4. The solid line is the result of the optimized difference between 3 and 4, which removes most of the atmospheric noise. Units are 0.5 s and SPM digital output. Jupiter transit in the field of view is visible at ~270 time units with an antenna temperature of 2mK. This type of measurement was used for alignments of the star sensor.
1000
J. M. LAMARREet al.
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INSTRUMENTAL
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ANALYSIS
The ultimate limit on a flux measurement is set by the photon noise of the total incoming radiation on the detectors. Instrumental noise must be added to the photon noise to give the real limit on the measurement. Instrumental noise includes detector noise, thermo-optical noise on the modulated signal and the noise induced by the fluctuations of the pointing system. SPM in-flight sensitivity is expected to be photon noise limited for the short wavelength channels and detector noise limited for the long wavelength channels. Therefore the experimental design must minimize both thermo-optical and pointing related noise. Thermo-optical noise results from thermal fluctuations or inhomogeneity of the parts seen directly or by diffraction, fluctuations of the modulation parameters (amplitude, offset) and fluctuations of the detection electronics. The first term is minimized by a careful thermal control of the structure and mirrors of the warm optics, and by severe control of the diffracted fluxes. The surface of M3 and its environment are a key in this process because the beam scans M3 during modulation. A dedicated modulation scheme (between the on-axis beam and a variable position on M3) gives a direct measure of the spurious fluxes as a function of the modulation amplitude. Other parts such as the edges of the primary mirror may also create a modulated-and varying-signal which adds to the astrophysical signal and cannot be eliminated. Achieving a highly stable modulation system needed intensive tests and work on the driving electronic of the wobbling mirror scanner. Appropriate dynamics for the complete detection system (typically 104), prevents artificial noise generation in the presence of constant parasitic signals. This is demonstrated, as well as the stability of the whole system, by the Allan variance(28~29) of measurements obtained with an internal calibration source (Fig. 6). A high linearity and stability is an efficient tool that makes difficult data reduction possible. The example of the detection of Jupiter from Toulouse (France) with an atmospheric effective transmission of 5.104 and strong atmospheric noise (Fig. 7) is an illustration of the efficiency of good quality multi-channel photometry. Acknowledgements-The development of SPM is funded by CNES and CNRS. We would like to thank Fran9ots Buisson, Jean Audoubert, Bernard Boullet, Jean-Pierre Diris, Andre Laurens, Roger Roulet-Matton, and the teams of CNES who played a decisive part during the integration and tests of SPM with the PRONAOS telescope.
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