RESULTS OF THE PIROG 8 BALLOON FLIGHT WITH AN EMBARKED

EXPERIMENT BASED ON A 425/441 GH2 SIS RECEIVER FOR 02 SEARCH

by (alphabetical order):A. Deschamps l , P. Encrenaz l , P. Febvre l* , H.G. Flor6n2, S. George l , B. Lecomte l , B.Ljung3 , L. Nordh2, G. Olofsson2, L. Pagani l , J.R. Pardo l , I. Peron l , M. SjOkvist3,K. Stegner3 , L. Stenmark4 , J. Tauber5, and C. tillbere

1 DEMIRM - PARIS OBSERVATORY - 75014 PARIS - FRANCE - E-mail: Pascal_Febvre@univ-savoie.fr2 STOCKHOLM OBSERVATORY - S-13336 SALTSJOBADEN - SWEDEN3 SWEDISH SPACE CORPORATION - P.O. Box 4207 - 5-17104 SOLNA - SWEDEN4 ACR ELECTRONIC AB - BOX 99 - 61922 TROSA - SWEDEN5 ESTEC - ASTROPHYSICS DIVISION - 2200 AG - NOORDWLTK - THE NETHERLANDS

CNES - AIRE-SUR-L'ADOUR - FRANCE

* New address: LAHC - UN1VERSITE DE SAVOIE - 73376 LE BOURGET DU LAC CEDEX - FRANCE

Abstract. The swedish-french PIROG 8 project(Pointed InfraRed Observation Gondola) has beendevelopped in 1996/1997 in order to try to detect thepresence of molecular oxygen in two molecularclouds of the interstellar medium: NGC7538 aidW51. PIROG 8 consists of a 500 kg gondolacarrying a 60 cm diameter Cassegrain telescopeequipped with a 425/441 GHz SIS heterodynereceiver at its focus. This balloon-bornesubmillimeter SIS experiment successfully flew inseptember 1997 from the south-west of France. Anoverview of the technical experiment behaviourduring flight is presented along with some performedmeasurements.

I - Introduction

The scientific goal of the PIROG 8 project(Pointed InfraRed Observation Gondola) was toobserve simultaneously the 02 line at 425 GHz andthe 'CO3 line at 441 GHz in the molecular clouds ofthe interstellar medium by heterodyne spectrometry.The main scientific interest is to detect molecularoxygen which has never been observed in theinterstellar medium so far. The theoretical modelspredict' that the line emissivity is weak: about500 mK.km .s- 1 in dark molecular clouds.Consequently, it is necessary to use a sensitivereceiver with a good spectral resolution. Thesimultaneous observation of the 13C0 line allows toverify the correct operation of the receiver and thepointing of the telescope.

In order to be free from the Earth atmosphere,the experiment must observe at high altitude so ithas been embarked on the PIROG gondola, run by

the Swedish Space Corporation, which flew 7 timesin the past.

The heterodyne receiver developped for thePIROG 8 project consists of a sensitive front-endcooled at liquid helium temperature which feeds anautocoxrelator spectrometer (back-aid). The cryostatused to cool the receiver front-end operates at ambientpressure (p--- 3 mbar at 39 km altitude) so that theSIS mixer is at a physical temperature of about1.5 K. The cryostat has been provided by ESTECalong with the autocorrelator spectrometer. Both havebeen successfully used during former flights.

The PIROG 8 instrument has been developpedat the Observatory of Paris between september 95 andJune 96 in order to be ready for a flight initiallyscheduled for september 96 and postponed to may 97.This delay has allowed some additional tests to beperformed on the instrument. in particular some testsof the receiver behaviour in the real low-pressureenvironment (measurements of cryostat hold time,receiver noise temperature and of physicaltemperature profiles of most sub-systems) have beenmade during winter 96 at the Observatory of Paris.The integration of the instrument on the PIROG 8gondola took place in march 97 in Trosa (Sweden) atthe ACR facility where the gondola was designed artbuilt. Final integration including control of thereceiver by the PIROG ground station has been ma&at the CNES balloon base in Aire-sur-l'Adour(France) during april and early may 97. Payload checkand flight simulations procedures have been definedand tested during the same period. The flight, plannedin may 97 has been delayed to september 97 due tobad weather conditions and balloon-flight safetyregulations. Payload check procedures and flightsimulations have been fine-tuned in early september

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and the flight occurred on september 25th, 1997. Theflight duration was 11 hours from take-off to cut-down with more than 8 hours at the ceiling altitudeof about 39.5 km.

II - Technical features of the PIROGheterodyne SIS instrument

11.1. General design

The PIROG 8 receiver is based on thetechnology of SIS mixers. The mixer block 2 uses areduced-height waveguide and has an integrated Potterhom3. It is sensitive to signals of horizontalpolarization at frequencies of 425 and 441 GHz on aninstantaneous bandwidth of 320 MHz. The SISmixer uses one mechanical contacting backshortwhich has been fixed after proper optimization in thelaboratory. It is cooled in a cryostat at liquid heliumtemperature, along with the HEMT amplifier Owedat the mixer output. The cryostat has one heliumtank of about 2 liters and one intermediate thermalshield, at a physical temperature of 20 K, screensthe helium tank from external thermal radiations. Ablock diagram of the receiver is shown in figure 1.

to room-tempeniureLOCAL I IF processor

OSCILLATOR CRYOSTAT g w signal •

The signal coming from the secondary minorof the 60 cm Cassegrain telescope is directly injectedin the cryostat through a quartz window with anantireflexion coating. Then it meets a biconvexteflon lens cooled at liquid helium temperamre whichcouples the secculary mirror with the mixer horn.The quasi-optical coupling has been calculated so thatthe beam-waist is located in the plane of the cryostatwindow in order to reduce the window size aidimpulse the hold time of the cryostat. Two one-wavelength-thick (450 gm) infrared filters made withfluorogold are installed on the beam path to preventinfrared radiations from heating the mixer. Tlx firstone is installed on the intermediate thermal shield atabout 20K, the second one is directly fixed in front ofthe lens and mixer on the stage at liquid heliumtemperature. A beamsplitter, made with a 9-pm-thickmylar sheet, is located at room-temperamre in frontof the cryostat window to glow the coupling of afew % of the local oscillator (LO) power in thecryostat. The remaining LO power is absorbed by asubmillimeter absorber.

II.2. Controlled phase-krked Local Oscillator

The local oscillator consists of a Gunnoscillator at 72 GHz stabilized in frequency by adedicated electronics with a phase-lock loop. Thefrequency of the Gunn output signal is multiplied by6 by a doubler followed by a frequency tripkr2 inorder to generate a few lamdrecis it.W around433 GHz. The center frequency of the Gtmnoscillator is electrically controlled as follows: a17-bit frequency synthesizer, centered at about 1053MHz, tunable from the ground, drives a PLLoscillator at about 3.8 GHz (x 36). The PLLoscillator output signal is injected with a tinyfraction of the Gunn signal in an harmonic mixer.The signal at the intermediate frequency of theharmonic mixer is generated at about 100 MHz bymixing the Gunn signal with the 19th harmonic ofthe PLL oscillator signal. It is compared with areference oscillator at 100 MHz by a dedicatedelectronics which reacts on the Gunn voltage. Thephase-lock electronics works over a 600 MHzbandwidth centered at the LA frequency of 433 GHz.This frequency allows the simultaneous detection inthe double side-band mode of the 02 line at 425 GHzand the CO13 line at 441 GHz with a receiverintermediate frequency of 8 GHz. The local oscillatorpower is also tenable from the ground through thebias voltage of the whisker-contacted varactor diodeof the frequency doubler. The local oscillator with itsphase-lock electronics is installed on a plate directlyfixed on the cryostat in order to simplify and ease thequasi-optical coupling. Moreover, this allows tomake the receiver quite compact which is necessaryfor a balloon-borne experiment where constraints inmass and volume are strong (see figure 1).

from cryostat8 Gib IF output

ROOM-TEMPERATURE IF PROCESSOR total IFoutput power

Vto autocorrelator

spectrometer system160 - 480 MHz

Fig. 1: General block diagram of the PIROG 8 receiver

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11.3. SIS mixer and SIS junction

The mixer uses a 2 gm 2 round-shapedsuperconducting tunnel junction made in NbiAl-Al203/Nb4. This junction has a high current density(about 13 kA/cm) which corresponds to an toRNCproduct of 6 at 433 GHz and eases the coupling ofthe junction with its microwave environment. Theassociated normal resistance is RN == 12 SI Moreover,the subgap leakage current is low: the leakageresistance at 2 mV is R,= 150 K2, which correspondsto a ratio Rst/RN = 12.5 and a factor of meritVm = 3 5.

A tuning circuit', using niobiumsuperconducting electrodes, is integrated to thejunction in order to resonate out, at the frequency ofoperation, the high intrinsic capacitance of thejunction estimated6 to 95 fF4un2. The tuning circuitis made of a microstrip line terminated by a 90-degreeradial stub and provides, at the frequency ofoperation, an inductance in parallel with the SISjunction to compensate for its capacitance (see figure2). The SIS junction, with its integrated tuningcircuit, has been fabricated with a low-passmicrostrip filter which propagates the mixer outputsignal at the intermediate frequency of 8 GHz butrejects the mixer input signals at 425, 433 and441 GHz. All these elements are deposited on aquartz substrate polished at a thickness of 50 gm atdiced at the final dimension of 200 grn by 2 mm.The substrate is installed transversally in thewaveguide of dimensions 120 by 700 gm.

substrat

Fig. 2: SIS tunnel junction with an integrated parallelmicrostrip tuning circuit and low-pass filter. a) generalview showing quartz substrate, low-pass filter, SISjunction and tuning circuit. b) Top view showing tuningcircuit dimensions. c) Cross section view showing filmtopology.

The low-pass filter, connected at one end tothe mixer ground, is wire-bonded to the input of an

impedance transformer at the intermediate frequency.The transformer is realized in microstrip technologyon a 1.27 mm-thick Duroid substrate (relativedielectric constant = 10.2), it is used to transform themixer output impedance to 50 O. It also allows theSIS junction to be DC biased.

On the other hand, a Helmholtz coil, madewith niobium-titane superconducting wire, is able toproduce a tunable magnetic fiekl in the plane of theSIS junction (up to 1000 Gauss). This is in order tosuppress the Josephson currents which all somenoise and create instabilities on the receiver. Themagnetic field is also tunable from the ground.

11.4. SIS junction fabrication process

The SIS junction has been fabricated' bysputtering a inlayer of Nb/Al-A1,0 3/Nb on a flisedquartz substrate. The thickness of the niobiumelectrodes is about 2000 A. Then, the low-pass filteris shaped by Reactive Ion Etching of the trilayer in aplasma of SF6. In a third step, the shape of thejunction is defined by photolithography and etched byReactive Ion Etching in a plasma of SF6 and 02. Twogold pads are deposited by evaporation on the ends ofthe low-pass filter in order to ensure good electricalcontacts with the mixer. Then, a 2000 A-thick SiOlayer is evaporated so that the junction perimeter iselectrically insulated, this layer is also used as adielectric for the integrated tuning circuit of thejunction. At last, a niobium counter-electrode, whichconnects the top electrode of the junction to the low-pass filter, is sputtered. This counter-electrode is alsoused as the top electrode of the integrated tuningcircuit.

11.5. Processing of the mixer output signal

The signal coming out of the impedancetransformer at the intermediate frequency is amplifiedby about 20 dB by a low-noise two-stage cryogenicHEMT amplifier located on the cold stage of thecryostat (at the temperature of liquid helium). Theaverage noise temperature of the amplifier is 9 Kover a 320 MHz bandwidth centered at 8 GHz. Theoutput signal of the amplifier propagates through acoaxial cable out of the cryostat and is amplified atroom-temperature, filtered over a 500 MHz bandwidthand down-converted to the center frequency of thespectrometer, which is 320 MHz. The room-temperature amplification is tunable from the groundfrom 55 to 70 dB by steps of 1 dB in order to obtainan output level of about -7 dBm, which is theoptimal level for the autocorrelator. Atelemeasurement of the total output power is alsomade to control the correct behaviour of the receiverfrom the ground during the flight and to determinethe beam pattern with moon measurements.

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11.6. Control electronics

A specific control electronics has beendevelopped to interface the receiver with the gondolaelectronics. It consists of 5 boards which allow onone hand to teleconunand the different modules of thereceiver SIS junction bias voltage, superconductingcoil current, HEMT amplifier status (on/off), gain ofthe room-temperature chain at the intermediatefrequency and gain and frequency of the localoscillator.

On the other hand, some telemeasurements atmade through this electronics: current and voltage ofthe SIS junction, total output power, control of thestate (on or off) of every module and check of thelocal oscillator phase-lock status. A portablecomputer allows to teleconunand and control thestatus of the receiver during the flight.

11.7. Autocorrelator spectrometer

The spectrometer used was an autocorrelator ofthe hybrid type, and was developed and built atOnsala Space Observatory in Gothenburg for theAstrophysics Division of the European SpaceAgency. "Hybrid" refers to the fact that the IF band isinitially split into several subbands by a set ofdownconversions using tunable local oscillators.Each subband is then fed to an autocorrelator basedon the chip developed7 by the Netherlands Foundationfor Research in Astronomy (NFRA). The maximumbandwidth of the IF is 320 MHz in the range 160-480 MHz; the user may select configurations withbandwidths of 20, 40, 80, 160 and 320 MHz, coveredby either 200 or 400 channels. The spectrometer isbuilt into a vessel pressurized with nitrogen gas at0.5 bar, and cooled by a circulating liquid system. Itsmass is around 9 kg, and it consumes about 120 Wat peak power. It is controlled by telemetrycommands from a dedicated PC (on the ground),which also processes the data from the level of lagsup to that of a spectrum.

11.8. Thermal control

Several temperature sensors have been fixedon different subsystems of the receiver to monitorreceiver components during flight. Two heatingresistors have been installed in order to warm up thereceiver if necessary during the balloon ascent whenair temperature drops below -50°C.

11.9. Power, volume and mass budgets

III. Ground measurements

IBA. Noise temperature & cryostat hold time

The cryostat hold time has been measured onthe ground (Thth. = 4.2 IC) and was about 20 hourswith receiver off and longer than 14 hours withreceiver on. A 180 K DSB receiver noise has beenmeasured using the Y-factor technique between roomand liquid nitrogen temperatures (see figure 3).

At a helium temperature of 1.5 K. obtained onthe ground by pumping on the cryostat helium tankto simulate an external pressure of about 3 mbar, thecryostat hold time was longer than 11.5 hours with =8.5 hours at ceiling. The best measured DSB receivernoise temperature was 130 K in the laboratory.

Velocity (cm/s)0 -50 -100

-•

• •424706 •- .• 424806 • 424900Rest Frequency (MHz)

Fig. 3: DSB receiver noise temperature versus lowersideband frequency at 4.2 K helium temperature.

Remark: the variation of the receiver noisetemperature in the useful bandwidth, seen in fig. 3, isdue to an unfortunate resonance in the impedancetransformer at the intermediate frequency, whichappears only at low physical temperatures. This doesnot pose any problem during operation since thelines to observe are located "in the edges" of thebandwidth and take advantage of the lowest receivernoise.

A DSB receiver noise temperature of about270 K has been measured on the ground before flight.And a noise temperature of about 200 K DSB wasexpected during flight from the measurements at lowpressure (= 3 mbars) performed in the laboratory.Note: The increase of the receiver noise temperature(from about 180 K to 270 IC) has been observed inthe end of 1996 and is likely due to some slightchange of the mixer backshort position caused byrepeated thermal cycles between 4 K and room-temperature.

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Sideband ratioThe total receiver consumption has been

measured to be about 45 watts, the receiver mass,excluding spectrometer, was lower than 28 kg. Thefront-end is 60 cm high x 23 cm x 30 cm and theelectronics boxes overall volume is 19 cm x 18 cm x18 cm.

Measurements in the laboratory have shownthat the average resonance frequency of the SISjunction with its microwave environment (at whichthe mixer is supposed to better work) is about

256

400 GHz, i.e. lower that the normal 433 GHz cent

frequency of operation.Some measurements of DC I-V curves

pumped at frequencies in the lower and upper mixersidebands have been performed. The I-V curves havebeen compared with theoretical predictions based onTucker's theory in order to determine the mixer gainin each sideband. It appeared that the sensitivity ofthe mixer is better in the lower side-band where theweak 02 signal is supposed to be seen. But thesideband gain ratio could not be determined accuratelyand has been estimated to be between 0.5 dB to 3 dB(gain ratio between 1.1 and 2).

111.3. In-flight calibration setup

A hot and cold load calibration setup has beeninstalled between the telescope and the cryostat tocalibrate the receiver in flight. The "cold" load is atambient temperature and the "hot" load is heated atabout 70 °C by a resistor fixed on its backside. Bothare made of a submillirneter absorber and can beinserted in the beam path any time during the flight.The low temperature difference (� 50 K) between thetwo loads did not allow us to make accurate noisecalibrations but was sufficient to verify the correctbehaviour of the receiver. Moreover, groundcalibrations showed that the measured hot loadtemperature had to be decreased by 12 K to accountfor the "real" receiver noise temperature (measuredmore accurately with a 77 K liquid nitrogen and a300 K room temperature set of loads and always usedin the past to calibrate the receiver).

III.4. Influence of Josephson currents

One of the main problems encountered withSIS mixers is the presence of Josephson currents8 inthe SIS junction. Also, they depend on the magneticflux trapped inside the junction. The phenomenom offlux trapping is random and can be provoked byelectromagnetic or electrical perturbations. WhenJosephson currents are present in the junction, someadditionnal Josephson noise add to the receiver noiseand some instabilities in the output signal make theoperation difficult or impossible. Usually, amagnetic field is applied in the plane of the junctionin order to suppress the Josephson currents and makethe receiver less noisy and more stable. Nevertheless,the currents reappear randomly and the magnetic fieldneeds to be reajusted, which is done on ground orairborne receivers by the operator. In our case, for aballoon-borne experiment, or for some future spaceapplications, this problem becomes overwhelmingsince there is no operator aboard. It becomesnecessary to fully understand the interaction ofJosephson currents with the quasiparticle mixing inorder to suppress them in a safe way. Consequently,PIROG 8 is a good test bed for such an objective.

Figure 4 displays unpumped and pumped I-V

curves along with DSB receiver noise temperatureversus the SIS junction bias voltage at 42 Kphysical temperature.

00 1 2 3 4 5

V (my)Fig. 4: Unpumped and pumped /-V curves andreceiver noise temperature versus bias voltage

Though the Josephson currents have beensuppressed the best we could by applying the propermagnetic field of about 250 Gauss, one clearly seesthat the noise is higher by about 100 K arotmd1.8 mV. Moreover, the receiver is unstable aroundthis voltage which corresponds to the second Shapirostep at 433 GHz. It turns out that, at 433 GHz, theoptimum voltage for lowest receiver noisetemperature is about 2.3 mV, i.e. not too close tothe Shapiro step. Consequently, the presence ofJosephson currents does not hurt too much theoperation of the receiver for this particular biasvoltage and we observed a very good stability withlow receiver noise for more than 8 hours in thelaboratory. Nevertheless, the stability depends on theperturbations around the receiver and may be differentduring the flight clic to the different receiverenvironment (motors, RF transceivers on thegondola, ...)

III.5. Problems with telemetry

Full experiment has exhibited ccrrectbehaviour on the ground EXCEPT in presence of realTeleMetry antenna which caused a big perturbationon the front-end and the back-end. Front-end wasunstable and could not be operated properly. A fewtests with microwave absorbers close to the antennaand with gondola outside the building lead to theconclusion that the receiver should work at ceilingdue to the absence of TM power reflected on theexperiment

III.6. Telescope alignment and attitude control

The telescope was aligned with the two TVcameras (a fixed one with a 2° field of view and amovable one with a 0.5° field of view) using a laserand remote light sources. The radioastronomical (RA)focus (' waist') of the telescope was defined using the

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257

optical focus as a reference and then relying on thecalculated difference. The optical/radio alignment ofthe telescope was done using a 30 cm collimator.The set-up was not fully satisfactory and thealignment was judged to be within 2 arcminutes. TheRA beam was measured using the 30 cm collimatorand it was in agreement with expectations - basicallydetermined by the small size of the collimator (it wasplanned to use the 1m collimator at Toulouse, but itwas occupied by ODIN). The angular position of thesun sensor relative to the telescope was measuredusing a theodelithe. The software and hardwarecontrolling the azimuth position of the gondola AVM

checked by daytime "observations" of Polaris. Theelevation software and hardware were checked byguiding on a star for a couple of hours. The gondolareference horizontal plane deviates from the true one,and two inclinometers are used to measure thisdeviation. The software is constructed to take care ofthis effect and this was confirmed by looking at adouble star, while tilting the gondola.

IV - Test procedures

Some payload check procedures have beendefined during the integration period to verify thecorrect operation of the receiver, including backend.Some specific routines have been implemented in thesoftware which controls the front-end PC: calibrationroutines, routines to digitize I-V curves of SISjunctions, routines to use the frequency switchobservation mode.

Receiver noise calibrations were performedboth by the autocorrelator PC, using power levelsmeasured by the correlator, and the front-end PC,measuring the total output power versus hot and coldloads. Also, a cold against sky calibration routine hasbeen implemented.

Payload check procedures have been defined toverify the correct receiver operation at differentfrequencies of interest and for different resolutions andbandwidths of the autocorrelator mainly forobservation of 02 for all scheduled astronomicalsources, for observation of ozone lines in order tocalibrate the mixer sideband gain ratio, and withfrequency offset for sky dip.

Also, routines for frequency switch aidposition switch modes of observation have beenimplemented. Several flight simulations have beenperformed to verify the correct operation of the entireexperiment including ground control station aid"human interfaces".

V - PIROG 8 flight

V.1. General considerations

The preparations before launch wentsmoothly, but due to a delay we decided to refill the

cryostat in order not to run out of helium in case of along-duration flight. The Launch was smooth.

The duration of the flight was unexpectedlylong, leaving 8 hours of observation at ceiling. But,because of repeated TM drop-outs caused by the longdistance between gondola and ground base in the erdof the flight, the last hour of observations at altitudewas essentially lost.

Laimch occurred at 5:38 am on september25th, 1997 from Aire-sur-l'Adour, ceiling wasraiched at about 8:30 am. Cut-down was decided byCNES at 4:48 pm. Gondola landed in the trees inMassif Central 45 mn after cut-down and wasbrought back to the base the day after in fairly goodconditions.

V2. Receiver behaviour in flight

The cryostat stayed cold up to the end of theflight. It started to warm up during the descent aftercut-down of the balloon. The hold time, higher than11.5 hours, was a little longer than expected and justenough not to shorten the long flight

The receiver, front-end and back-end, behavedvery well during the entire flight and the fact that itwas fully remote-controlled was totally Imharmfulduring the whole flight. Nevertheless a few problemshave been observed, fortunately they caused only veryminor troubles. First, there has been a fewunexpected total power shut-down of the receiver onthe ground pal (gondola on batteries) before launch,during the ascent and during the flight when an I-Vcurve was being digitized. The origin of theshut-down is not known so far but it seems to becorrelated with a big number of commands sent tothe receiver in a row. Hopefully, the receiver waspowered again every time with no damage for the SISjunction.

An expected failure of the phase-lock loop ofthe synthesized local oscillator happened during theascent during 1 hour and 40 mn between 10 kmaltitude (6:20 am local time) and 31 km altitude(08:00 am local time). The outside air temperaturewas between -40°C and -60°C. The outer shield of thegondola went down to -50°C and the local oscillatorplate, which was heated during the ascent, went downto 0°C. The phase-lock stopped functioning below+8°C at 10 km altitude and worked again when itreached +7°C at 31 km altitude. Consequently, thephase-locked system behaved better than expected andmade the operation of the receiver possible beforereaching ceiling. Figures 5 and 6 show the generaltemperature variations during flight for different partsof the receiver and gondola. In particular, one can seein figure 6 that heating of the Local Oscillator baseplate during ascent has been quite helpful inpreventing the phase-lock electronics from coolingdown below 0°C, which could have harmed thereceiver operation at the beginning of theobservations.

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during flight Just before launch

At last, the total output power value at theintermediate frequency was corrupted during the sameperiod, very likely by a malfunction of the associatedambient temperature electronics (different gain foroperational amplifiers, etc...). It recovered at about08:15 am local time, again before reaching ceiling.

Fig. 5: Temperature variations on different gondolaplaces and altitude profile during PIROG 8 flight.

Fig. 6: Temperature variations of receiver componentsduring PIROG 8 flight.

V.3. Stabilizing at altitude

The azimuth stability was achieved as plannedand a bright star was quickly found. However, apendulum motion - not seen in previous flights - wasnoted. As the inclinometers were useless to trace thismotion it could not be compensated for by thecontrol system. We had just to wait for a slowdecline. Post-flight analysis with statistical methodsof the fme pointing phase shows that pointing wasachieved with a RMS value of 0.4 arcminute.

V.4. Alignment

Once the gondola was stabilized, the guidecamera was confirmed to be aligned with the fixedcamera.

V.5. In-flight receiver calibration

Calibration in flight between sky and ambientcalibration load showed that Tr (DSB) was about295 K. The system DSB noise temperature(including the telescope) was about 500 IC Thereceiver noise temperature, higher than expected, islikely due to the presence of additional Josephsoncurrents which were hard to suppress. Also, onepumped I-V curve has been digitized at ceiling, itshows that there is a low frequency noise on the SISjunction which smooths the I-V curve non-linearity,hence reducing the performance of the mixer (seefigures 7 and 8). Moreover, this noise, which islikely due to the TM power emitted by the antennaand reflected on the gondola structure, may increasethe influence of the remaining Josephson currentsand, then, contribute to the lower mixer sensitivity.We had to increase the magnetic field in the plane ofthe SIS junction to about 500 Gauss during flight, tobe compared to 250 Gauss on the ground.

Just before launch

during flight

1 2 3 4 5Voltage (mV)

Fig. 7: Experimental pumped I-V curves digitized onground and during flight.

Fig. 8: Experimental output power versus bias voltagecurves digitized on ground and during flight. Smoothingof I-V curve is clearly seen. It resulted in degradedperformance of the SIS mixer.

V.6. Dip scan.

As planned, the atmospheric emission wasmeasured at a few different elevations. The expectedlines were seen, verifying the frequency calibration ofthe receiver. The dip scan confirms that there is nocontinuous atmospheric extinction, which means thatthe low elevation of our main target (down to 18

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3

degrees) does not imply any additional damping ofthe signal to that given by the telluric 02 line wing.

V.7. Ozone line fit

Another way of measuring the sideband gainratio in order to improve the accuracy obtained bylaboratory measurements has been suggested'. It isbased on the observation of two atmospheric ozonelines, one in each sideband, for which emissiontemperatures are blown (or at least for which theratio of their emission temperatures is known). Ittuns out that the tunability of the local oscillatorallows the observations of two ozone lines,respectively centered at about 425.16 and 441.34GHz. With this technique, the sideband ratio has beenestimated to range between 1.2 and 1.25 which is abig improvement compared to the 1.1-2 rangeobtained in the laboratory. A fit of observations ofmeasured ozone lines at ceiling with theoreticalmodels based on different sideband ratios is shown infigure 9.

3292: 4 OZONE 03(425+441) PtROG8 C9 9< 0-. 25—SER-1997 R: 4— MR- 1990.0 0.0 Ea8.1237E-02 El: 34.59

.5640 LSR441240.775

0Velocity (km/s)

Fig. 9: Experimental spectrurn obtained with PIROG 8receiver at ceiling altitude. Three theoretical fits withdifferent LSB/USB sideband ratios (0.8, 1 and 1.25) arealso shown (dashed lines). Best fit is for LSB/USB gainratio of 1.25

In fact, when the Local Oscillator is tuned toobserve the two ozone lines, the ratio is in the range1.2-1.25 but, once the receiver is tuned at the 02frequency, the sideband ratio seems to change but vvehave no direct evidence of this (no 03 line in theupper sideband). Indeed, we need a ratio of I toexplain the telluric 02 line intensity. Thediscrepancy may come from the ATM model beingnot precise enough, from the receiver behaviour itselfor from some other reason we could have overlooked.Thus, the overall uncertainty is between 1 and 1.25.It is not due to the method but on the way we caninterpret the results in this particular case.

V.8. Calibrations on the moon.

The purpose of the moon observation was todetermine the beam efficiency, the beam profile and ifpossible, to confirm the alignment of radio/opticalbeams. For some reason - not yet identified - it tooka while to find the moon and then in order to gaintime, the pL3nned step-by-step scan across the moonwas replaced by a few continuous scans, leaving theoptical/radio correlation for the post-analysis. Thetotal output power at the intermediate frequency ofthe receiver has been measured versus moon position.In order to relate the observations with a model, thebrightness temperature variation (at 0.7 mmwavelength) across the moon has been derived aniconvolved with a gaussian beam with a width of 5arcminutes. The observed data agrees very well withthe observations (see figure 10).

PIROG8-moon

From this observation, it is also possible toderive the main beam efficiency. If one assumes thatthe brightness temperature of the hot load is 12 Kless than the measured one (as was found on theground) one calculates a main beam efficiency equalto 0.42. This is slightly lower than our goal (0.6),but in view of our crude ways to align and focus thesystem, it is not bad. The most probable reason forthe low efficiency is that the secondary minor is abit over-illuminated (half of the integrated beam fromthe receiver passing the edge of the mirror). This isin fact not so bad as the sky is transparent and oneonly sees the cosmic background outside the mirror.This would also mean a slightly lower tapering thanusual (10 dB at the edge), which in turn would give aslightly worse side-lobe situation.

VI - Astronomical observations

Two sources have been observed: N007538and W51. A map of the 441 GHz 4-3 13C0 line ofNGC7538 in the upper sideband has been made.Molecular oxygen has not been detected.Observations will be detailed elsewhere.

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RA: 0000:00.000 DEC: 00:00100.00 (1050.0) Offs:Unknown Tau: 0.0000E+00 Ter: 1200. lime:N: 400 10: 200.5 VO: —45.84 Dy:

FO: 425222.775 IX —.8000 F.:

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Some attempts to observe W51 have beenmade in the end of the flight but, because of repeatedTM drop-outs, the observations were essentially lost.We could, however, confirm the presence of the13C0 line (brighter than in the case of NGC7538),and it may become possible to recover somethingfrom the analog recordings of the TM data

VII. Conclusion

The SIS heterodyne receiver developped for thePIROG 8 balloon project is the first fullyremote-controlled submillimeter SIS receiver to havesuccessfully flown on a balloon gondola. It behavedvery well during the flight (except for 1 hour and 40mn during the ascent), which is above expectations.The receiver noise temperature was slightly degradedcompared to what was expected from measurementsperformed on the ground (= 300 K instead of 200 KDSB). Some investigations are necessary to safelyassess the real causes of this degradation but theyseem to be partly due to low-frequency noise in theSIS mixer generated by the TeleMetry antenna.

Project organization - Acknowledgements. PIROG 8 hasbeen developped in the frame of a swedish-frenchcollaboration between the Observatory of Stockholm(PI: Lennart Nordh, Instrument Scientist GtiranOlofsson) and the Observatory of Paris (coPI: PierreEncrenaz, Instrument Scientist: Laurent Pagani). Theproject has been managed by Bo Ljung from the SwedishSpace Corporation (SSC). The SIS instrumentdevelopment has been managed by P. Febvre from ParisObservatory. The swedish company ACR was in chargeof the gondola and of the interfaces with the receiver.The french space agency (CNES) was in charge of theflight campaign. The Observatory of Paris hasdevelopped the instrument with some contributions ofthe Observatory of Stockholm. The receiverdevelopment has been funded by CNES (213) and by SSC(1/3). We are very grateful to S. Lebourg andF. Pelletier from Meudon Observatory for their decisivecontribution by micromachining the most sensitiveparts of this receiver. We also wish to thank J.P.Ayache, V. Thevenet, P. Barroso, J.M. Munier, A.Maestrini and G. Santarelli, from Paris Observatory, forthe technical support that they brought to thedevelopment of the receiver. Also, we are very gratefulto Philippe Goy: indeed, the determination of thesideband ratio in the laboratory has been made possibleby using a tunable local oscillator of AB Millimetre.

References

1 P. MarOchal, vs'. Viala, and J.J. Benayotm,Astronomy and Astrophysics, vol. 324, pp. 221-236,1997.2 Made by Radiometer Physics, BergerwiesenstraBe15, 5309 Meckenheim, Germany.3 H.M. Pickett, J.C. Hardy, J. Farhoomand, TREFTrans. Microwave Theory Tech., vol. MTT-32, pp.936, 1984.

P. Feautrier, M. Hanus and P. Febvre, Supercond.Sci. Technol., vol. 5, pp. 564-568, 1992.

P. Febvre, C. Routez, S. George and G. Beaudin,Proceedings of the International Conference onMillimeter and Submillimeter Waves artApplications II, vol. SPIE 2558, pp. 136-147,San Diego Convention Center, 9-14 July 1995.6 P. Febvre, W.R. McGrath, P. Batelaan, B. Bumble,

LeDuc, S. George, P. Feaurier, InternationalJournal of Infrared and Nfillimeter Waves, vol. 15,no. 6, pp. 943-965, June 1994.7 A. Bos, "The NFRA correlator chip", NFRA ITR176, 1986.

B.D. Josephson, Phys. Lett., vol. 1, pp251-253,July 1962.'Jose Cemicharo (private communication).

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