Maihara, Toshinori

Okuda, Haruyuki

Sugiyama, Takuya

English

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:A Balloon Borne Liquid NitrogenCooled Infrared RadiometerMaihara, Toshinori; Okuda, Haruyuki; Sugiyama,TakuyaMaihara, Toshinori ...[et al]. A Balloon Borne Liquid Nitrogen Cooled Infrared Radiometer.Memoirs of the Faculty of Science, Kyoto University. Series of physics, astrophysics,geophysics and chemistry 1974, 34(3): 341-3511974-10http://hdl.handle.net/2433/257551Memoirs of the Facu}ty of Science, Kyoto Unirerersity, Series of Physics, Astrophysics,       Geophysics and Chemistry, Vo.l XXXIV, No. 3, Article 12, 1974.A BALLOON BORNE LIQUID NITROGEN   COOLED INFRARED RADIOMETERBYToshinori MAIHARA, Haruyuki OKUDA      and Takuya SUGIYAMADepartment of Physics, Faculty of Science, Kyoto University, Kyoto               (Receicred APril 19, 1974)                           ABSTRACT   An infrared radiometer has been built to observe the galactic diffuse light in near infraredregion. Whole telescope is cooled by liquid nitrogen so as to reduce the intense emission fromthe telescope. It was launched by a balloon on Oct. 9, 1971. Small size irregularities werefound to be present in the OH airglow emission and of great obstacle for the observation of thegalactic light.   [lrhese irregularities are due to winds, gravity wave and turbulence in the upper atrnos-phere, and hence the observation of the OH airglow emission would give a useful probe for thestudies of dynamic properties of the thermosphere.l. Intreduction    The globa} structure of our galaxy has been investigated by various methods ofobservat2ons. The observations of 21 cm radio ernission have revealed clearly the spiralpattern of interestellar hydrogen clouds.i) O and B type stars which are bright eltoughto be observed in long distances have been used to ipvestigate the stellar distribution.2)Several authors have measured diffuse integrated light from the stars which show somestructure in the intensity distribution.3),4> Since these observations by vis!ble light areseriously affected by the interstellar extinction, the observable distance ls badly limttedji.e. Iess than ! or 2 Kpc.    The gaiaxy would become relatively transparent to the infrared radiation, for theinterestellarextinctiondecreasesrapidlywithwavelength. Thediffuseinfraredlightwouldtherefore reveal the stellar distributlon in the galaxy to much more distaltt region. How-ever, the stellar spectra show a steep decrease in the infrared regien beyond l micron. Nearinfrared region is optimum to be observed.    We have built a radiometer to observe the diffuse infrared radiation of the galaxy whichis useful to know the giobal distribution of the ga}actic stellar components. But theobservation is strongly dlsturbed by the OH airglow and thermal emisslons of the atmos-phere and the telescope itself. The intensities of these background radiations are shownin Fig. 1. The lnfrared radiation from the galactic plane is roughly estimated and com-pared wlth the backgrounds in the same figure. The airg}ow dominates in the wavelengthsbetween 1 and 2 microns, while the therma} backgrouRds become serious above 2 microns.In order to avoid the atmospheric emission and absorption, the observation must be doRein upper or out of the atmosphere. The effect of OIE[ airglow can be elliminated only by342 T, MAIHARA, H. OKUDA and T. SUGIYAMA1o-7r.. 1diN=.L.e>)<U 1dilaszti ievlo-ilOHAV=3OHbV=2   N/"t'"Nt        -m,itom,OHAERGLOWAV=lG<(4c)!c      (tGlyh          (EXPECTEO)             1 2 3 4                    WAVELENGTh C }i ]Fig. 1. Estimated intensities of the galactic }ight and background radiations;      the OH airglosv and thermal emission from the telescope wall.the rocket bome experlment. However, a limited window is availabie in the OHairglow emission between the vibrat2on bands ofA v=:1 and A v=2, providing a emissionfree window from 2.2 to 2.7 microns through which we can observe the extraterrestrialradiation.    Whole components of the telescope; the mirror, the telescope wall, the chopper asweii as the detector should be cooled by liguld nitrogen to reduce the thermal radiations tobe ineffective enough. Originally the radiometer was deslgned for a rocketobservation toescape from the airglow emission. We changed howevex to observe it at balloon altitude,discriminating the galactic component frorn the strong airglow bacl<ground by differentialmethod, i.e. detection of inhomogeneous component superposed on the uniform background.The balloon is more favorable for such an observation as to scan large area of the sky whichneeds long time.2. Liquid nitrogen ceoling of the telescope    Cryogenical devices for cooling the Åíe}escope by liquid nitrogen is illustrated in Fig. 2.The teiescope wall is made of a copper tube which is surrounded by liquid nitrogen vesse}.At the upper part of the vessel molecular sieves are packed to mal<e vacuum more completeby adsorption when liquid nltrogen is fiIIed.    The mirror made of Kanigen coated aiuminum is fixed at the bottom of the tube. Thedetector system, i.e. the detector, the filter, the chopper and its driving motor are mountedinside an alumiRum tube which is supported by two plates of aluminum to the telescopewall.A BALLOON BORNE INFRARED RADIOMETER 343cqpPotyprcpylene rnembraneFiUed witheyaporatedN2-gasK- Pyrex plate1(4.    MdectstarI,,: - sieve?    Staintess     steet     Motor       DeteCtOr liX N2 gas vent        FMer                              Copper tube       Chopper       t6crn JzSAt rnirrerFig, 2. General view of the radiometer (about 7e crn in height)    The whole system is hanged by a thin stainless steei cone frorn the lowest edge of theliquid nitrogen vessel to the upper edge ef the vacuum vessel. The heat flow can beeffectively insulated by its thinness and distance between the outer wall and the inner vessel.    The outer wa}l is tapered to the top to fit the shape of the rocket nose cone. In thecase of rocket experiment which was orlglnally planned, the telescope was expected to becovered at the top by a lid for keeping vacuum, which should be opened in the observation.    For the balloon borne observation several modifications are required. The top lid isreplaced by a transparent window, for the vacuum must be kept in the observation fromresidual atmosphere. A fused quartz of 6mrn thick had been first used for the window.It was crashed in the test operatlon just before the flight. We were compelled to replaceit by a pyrex glass of 11 mm thick.    To reduce the scatter-in of the atmospheric background emission from the ground, ahood blackened inside with Iusterless paint is extended from the top of the cryostat. Thehood is covered by a cap which will be opened by a spring at the beglnning of level flight.The front of hood is sealed by a thin polypropylene membrane to prevent water vaporfrom fiowing in and frosting on the giass window cooled by the cold cryostat vessel. Thevolume inside the hood is connected by avyni! tube to one of the pipe of vent gas tomal<e the pressure balance with environmental one wlthout introducing the atmospherecontaining water vapor into the inner space.    The whole components in the telescope; the mirror, the detector system and thetelescope wall can be cooled eficiently down to liquid nitrogen temperature with a delay ofabout one hour after fi11ing up liquid nitrogen. The coo}ing ls keptfor about 10 hours by6 litres liquid nitrogen, when it is put in the room temperature. At balloon altitudes, wherethe environmental temperature is as low as -500C, the keeping time would be considerablylengthened.344 T. MAIHARA, H. OKUDA and T. SUGIYAMA3. The optical systema) Light collectiRg mirror    For the purpose of observlng the galactic infrared radiation with about 2.50 beamwidth, a simple optical system has been adopted, i.e. PbS detectors are mounted on thedirect focal plane of a spherical mirror with 16 cm dlameter and with a rather smali focalratie of O.7. The mirror being made of Kanigen coated alurninum alloy has some meritssuch as mechanical strength for the launching sbock and the good thermal conductivity.The reflectivity of the Kanigen coated surface, however, has shown a rather low valuethan 700/o in near infrared waveiength, so the surface is aluminlzed to improve the' re-flectivlty up to 900/. or hlgher.    The aberration of the spherical mirror has been calculated by the light ray tracingmethod with an electronic computor. An example of the spot diagram of the mirror isshown in Fig. 3 in the case of an incident angle of 2.50 off axis. The estimated intensitydistribution on the focal p]ane by the same method is also illustrated in Fig. 4 in the caseof a unlformly extended source with a radius of 2.50.    Dimensions of two detectors are 5Å~ 5 mm2 and 4Å~ 6 mm2 respectively. The measuredpatterns of their field of view are roughly 2.50x2.50 and 2aÅ~30 as shown in Fig. 5. Theproduct of the effective so!id angle and area is about O.3 cm2•ster.b) Choppingsystem    The observation in the near infrared region is seriously suffered from the intenseemisslon of the airglow even at the balioon altitude. In order to cancel this strong emissionby comparing with the neighboring sky, we mount on the focal plane two detectors 50apart each otker. A rotating blades chop the incomlng beam onto the two detectors witha phase difference ofa halfwave. Signals from the two detecÅíors are added after adjustingtheir response level, then the uniform background radiation can be eliminated and onlydifferential component of the two neighboring sky is scooped up. We had, however, toabandon the method of this sky cancellation because one of the pair detectors was collapsedby an accldent and the two detectors used had quite dlfferent characteristics concerning       'wavelength dependences and temperature dependences. '    The chopper plate is driven by a motor which has been specially treated for low tem-.'  . .....,.tt. .-- .. '......il..,,i'`,, ' ...•t-t  t l-t-t : :       -l  t" --}-i -: ':': :"':';' :: :''  :1.:':•::;as.,,1•l•.I  -" tt- -t-- -  -- -t- "i - -- - t -I- --: :., ttt {t       .    --- ---i    Srmern. '  .t..+ Focal pointFig. 3. Aberration pattern of the 16 cm (F/O.7) spherical mirror for 2.50 off axis rays,A BALLOON BORNE INFRARED RADIOMETER 345xill'tLk,'6e5040302010o-ll loFig. 4. .16 .32 .48 .64 .80Di$tancG frorn focal point (cm)Intensity distribution on the focal plane for a diffuse object withangular diameter of 2.5e,.96:NORTH kloelDETECTO 4Å~5mm DE ECTOR5Å~5mrriZ4321o.o.05 3251O.5tlaFig. 5. Measured pattern of the field of views of the detector systern.346 T. MAIHARA, H. OKUDA and T. SUGIYAMAil.z(mA)  15ut5io"o"""'E    56co•--r" tiBeO5gil1  eO1No•3No•1No.2No.3No2No•1oOc-- 50 -1oo                      Temper&ture("C )Fig. 6. Temperature dependence of driving currents and fluctuating currents of      MeS2 }ubricated motors. No. 1 xvas used in the observation.perature operation by the procedure; micro size powder of MoS2 was chemicaily coatedon the bearing, after the oil was washed out. Low temperature characterlstics have beenchecked by measuring the dr2ving current and its stability. (see Fig. 6).    The motor was thermally insulated by the tefion supporter not to be coo!ed to too lowtemperature, in balance with Jou}e heat of driving current. The shaft connecting themotor axis to the chopper plate is iong enough for the choppper plate not to be warmed bythe keat from the motor.(c) PbS Detectors    Two PbS detectors (PbS-l, and PbS-2) are set at the focal plane of the sphericalmirror, as was described in tlte preceding section. The PbS-1 has 5Å~5mm2 effectivearea, whose optimum operating temperature is about 77eK (the liquid nitrogen tempera-ture) and the PbS-2 is a detector for the room temperature use having 4Å~ 6 mm2 effectivearea. We were obliged to choose these different types of PbS cells, because one of theformer type detectors was collapsed by an accident in the experiment.    A carbon res!stor is used as a thermister to moniÅíor the temperature of the detectors.The calibrated curve of temperature-voitage relation is shown iR Fig. 7, in which themeasuring circuit is also inserted.    At the room temperature, the ohnic resistance of the detector for the Iliquid nitrogentemperature use (PbS-1) and that fer the room temperatuye detector use (?bS-2) are 200kstA BALLOON BORNE INFRARED RADIOMETER 347->-1.0 ous O.5g,'oSo[ldR(?s"isterfor qSensorpas100kn1.3vtMercuryx   x Battery Vo EoOP Arftp-200Fig. 7.-100Tempereture (oC >Calibration curve of the thermister.o10CXDM1ooM M 10(2    M        .i'1CÅík10k/8         o/      fbS-2      o,.x//'c@)'!/lbgiSg/i/tl.,/:e,g'1'i""'"ig,a,)o -50 -100Temperqture ("C )-150Fig. 8. Temperature dependences of ohmic resistances of the PbS detectors.348 T. MAMARA, H. OI<UDA and T. SUGIYAMA100 xo!oll'50o------ -------       s        s        N- --- - -- -I- x s tl:,,i,il NSixN----"'  UV Filter (uv 25)    (coolod >-----  Py rex g[assInlerferoncoFi[ter (Lo$B7o-( cooied )!XiNN  Wi  Nx   I t-i    t 1{ Nl tt t   llt N    !1.X N    IN    N N.         '1Fig. 9.       2 3 4           Wavelength (M)Transmissions of the filters and the pyrex glass window.5and 50K9 respectively. When they are cooled, however, their values are invertedbelow the temperature of -500C as shown in Fig. 8.    As the operating bias current to the detectors is supplied through a 20 M9 resistorfrom a 90 volts battery, and as the input impedance of the pre-amplifier is about 10 M9the PbS-2 would become use]ess below about -800C by impedance mismatching. Butwe could avoid the dithculty by, reducing the detector's resistivity by means of a subsidiaryirradiation onto the detector. For this purpose, the detector was illuminated from thebackside by a GaAs-emission diode. The detector resistivity was successfully reduced to10 M9(see Fig. 8) and the noise !evel could be also reduced greatly. The dlede emitsnear lnfrared light havlng a sharp maximum at about O.9 pa, completely ineffective for ourobservation.d) Filters    Considering the wavelength dependence of the detector sensitivity and the spectraldistribution of the airglow emission, the observed wave}ength is selected by two kinds offilters, one is an interference-fiter and the other is a UV glass filter, whose specÅíral trans-missioR curves are presented in Flg. 9, The transmission ofthe Pyrex glass of il mm thickis also shown in the same figure. By the combination of the filter transmlssions and thedetector sensitivity, the center of the effective wavelength is 2.2p and the bandwidth isabout O.8 ge for both detectors. The OH airglow window was given up to aim because ofinsuMcient sensitivity of the detector for the Galactlc Iight intensity.A BALLOON BORNE INFRARED RAD!OMETER 3494, ElectroRics    There were five signals to be transmitted by a telemeterJ those of the two PbS deÅíectors,the thermister, the atmospheric pressure gauge and the geomagnetic aspectometer (G.A.).The PbS-1 was to measure the absolute brightness of the airglow emission with relativelylow gain and the PbS-2 was to detect the detailed variat!on ef the emission with much highergain. One of the telemeter channels was occupied by the hlgh gain signal from the PbS-2.The other channel was used for the G.A. signal, and the last one was time-shared by thePbS-1, the thermister and the pressure gauge.    Three channels were available for the remote control. One was used to start and stepthe rotation of the gondola and also to light the standard lamp for the calibration of thedetector sensitivities. The other was used to shift the offset voltage of the DC-amplifierfor the high gain detector so as to set its slgnal Ievel in the dynamic range of the telemeter.•The last one was used to adjust the phase of the reference signals. The block diagram ofthe e}ectronics system is shown in Fig. 10.5. Flight    The radiometex was iaunched te observe the near infrared galactic light from theSanriku Baiioon Base (of the Institute of Space and Aeronautical Science, Tokyo Unlver-sity) at 18 : 30 on October 9, 1971, by a balloon of 50eO m3. The balloon reached thece!ling altitude of 27 Km after about an hour and l<ept its Ievel flight for 11 hours until itwas cut down at the sunrise in the next morning.lof{set Vottl:Cotnmand 1L-"----.--"    IPbS-2fVe-Atap' AC -A,TLp.Pb S-1Pte-Af"pi Ac-Amp•Ph cts e Sh lft er  ctndReÅí - WaveS}LqPeT va-D{viUerThermisterPS. D.ÅÄ--- 7-l). S. D,:s DC-Atsp•ID C- A,Tt p.PtessureGauge       Ie>c-Amp, :       l       1ISIS6DsG, A.,"ripaer Reta7------ E------------•{l c&t{L-----           el           t      r----- ---H- -t t   IPhase SelectI : Com-fTLaTxd 2      i-m"-------"--------m---------- --b-r-a-tl, g-nm-Li-g-lt-t. .-.j, : Co,vx ra ahd 3Fig. 10. Block diagram of the electronics systern.l-.-.-- ;i Clt.1 lltllitttlI TTanstaitterl <.FM;FM)1lltltl'llII Ch.2 litttttlllI[l      I      I350 T. MAIllARA, H. OI<UDA and T. SUGIYAMA    Since the radiometer had been first designed for a rocket borne experiment, the mech-anicai strength for the iaunching shocl< was more than enough. On the level fiight, theradiometer was fixed at an eievation angle of 300, and rotated azimuthally with a constantspeed; one rotation in about 2 minutes. The attitude of the radiometer was moRitored bythe geomagnetic aspectometer (G.A.).    Whole system operated normally except a few troubles. The phase locking of thereference signal was slightly unstable. The temperature in the cryostat arose to -500Cin the !evei flight, the slgnal level of the iow gain channel (PbS-1) became too low to detectthe intensity variation.    Unexpectedly, the signals of the high gain channel (PbS-2) have shown signlficantfluctuations in the airgiow intensity. The observed results are shown in Fig. 11. Thefluctuation patterlts show good recurrency with the rotatlon of the radiometer ln azimuth,and change gradually with time. There are various scales ofirregularities from relativelyiarge variations with angular sizes of several tens degrees to smali ripples wlth sizes of theorder of 3ev5 degrees. These fiuctuations have failed the detection of the galactic light.    These fiuctuations kave been also found bv Peterson and Kieffaber5),6> from tlteground based photometric and photographic obselvations, which rnay not be free from thedisturbances in the mesosphere such as clouds or haze. The details of the observationshas been discussed elsewhere,7) in which it was suggested that the observed fiuctuationsare caused by the motions of the upper atmosphere i.e. winds, gravity waves and turbulence.The observations of such fiuctuations in the O}I airglow intensity will be used as a goodi v-as:oo- 05NStMtix.    'N.•'"vn "Zuf- 4045- 50North South10South West East             ,2foi ls"/--2530/;Fig. 11. Observed intensity fiuctuations of the OH airglow ernission duringthe period from 26:OO to 26:50 local time.              ABALLOON BORNE IATFRARED RADIOI,IETER 351probe for extensive studies in space and time of the dynamical properties near the base ofthe thermosphere.   We thank the staff of the Sanriku Balloon Center for their 1<ind assistance in the flight.                         REFERENCES1) Kwee, I<. K., .A,Iuller, C. A., and Westerhout, G. 1954, B. A. N., 12, 211.2) Elsasser, H., and Haug, U. 1960, Zs. f. Ap., 50, 121.3) Roach, F. E., and Megill, Lawence R. 1961, Ap. J., l33, 228.4) Roach, F. E., and Smith, L. L. 1968, Geophys, J., i5, 227.5) Peterson, A. W., and Kieffaber, L. IÅí, J. Atmosph. Terr. Phys. 34, 1357 (1972).6) Peterson, A. W., and Kieffaber, L. M., Nature 242, 321 (1973).7) Sugiyama, T., Maihara, T. and Okuda, H., Nature 246, 57 (1973).

A Balloon Borne Liquid Nitrogen Cooled Infrared Radiometer

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A Balloon Borne Liquid Nitrogen Cooled Infrared Radiometer

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