A cooled telescope has been designed and prepared for measurements of both local and cosmological Near InfraRed Backgrounds in the waveband 1–5 mm at balloon altitude (40 km). The detection system consists of a linear array with

32 InSb pixels placed in the focal plane of the telescope, cryogenically cooled and thermally controlled. A short description is given of the main characteristics of the

instrument and of the expected performances. The experiment, repeatedly tested in laboratory, flew in the 1995 summer from Palestine (Texas, USA)

Attolini, M. R.

Boscaleri, A.

brighenti, A.

Calzolari, P.

Cazzola, P.

Cortiglioni, S.

Giovannini, G.

Malaspina, M.

Mandolesi, N.

Morigi, G.

Palazzi, E.

ventura, G.

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IL NUOVO CIMENTO Settembre-Ottobre 1997VOL. 20 C, N. 5A cooled telescope for measurements of the Near InfraredCosmological Background at balloon altitude(TRIP Experiment) (*)M. R. ATTOLINI (1), A. BOSCALERI (2), A. BRIGHENTI (1), P. CAZZOLA (1), P. CALZOLARI (1)S. CORTIGLIONI (1), G. GIOVANNINI (1), M. MALASPINA (1), N. MANDOLESI (1)G. MORIGI (1), E. PALAZZI (1) and G. VENTURA (1)(1) Istituto TESREOCNR - Via Gobetti 101, 40129 Bologna, Italy(2) IROEOCNR - Via Panciatichi 64, 50127 Firenze, Italy(ricevuto il 5 Dicembre 1996; approvato il 25 Febbraio 1997)Summary. — A cooled telescope has been designed and prepared for measurementsof both local and cosmological Near InfraRed Backgrounds in the waveband 1–5 mmat balloon altitude (40 km). The detection system consists of a linear array with32 InSb pixels placed in the focal plane of the telescope, cryogenically cooled andthermally controlled. A short description is given of the main characteristics of theinstrument and of the expected performances. The experiment, repeatedly tested inlaboratory, flew in the 1995 summer from Palestine (Texas, USA).PACS 94.40 – Cosmic rays.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe origin and source of the Near Infrared Cosmological Background (NIRB) is ofgreat interest to cosmology as it is likely to originate at a redshift of 5EzE1000.Physical processes taking place at that epoch, expecially galaxy formation, remainpoorly understood. A large theoretical effort has been devoted to this subject, howeveran adequate number of observations to probe this epoch has not followed. The lack ofobservations is mainly due to the fact that it is practically impossible to perform themfrom ground due to strong atmospheric thermal and line emission. The total NIRBintensity flux is a combination of the extragalactic background and some local back-grounds such as zodiacal light (ZL), interplanetary dust (IPD), integrated star light(*) Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.G Società Italiana di Fisica 713M. R. ATTOLINI, A. BOSCALERI, A. BRIGHENTI, P. CAZZOLA, ETC.714Fig. 1. – The expected IR local background emissions.(SL), atmospheric emission and OH emission. Hence it is necessary to distinguishbetween each different component to detect the cosmological extragalacticbackground.Observations in the waveband 1–5 mm are easier for two reasons: first of all, thecontribution by zodiacal light and star light decreases rapidly as the wavelengthincreases, while the cosmological background probably has a flat spectrum due to theredshift; second, the contribution of nearby galaxies, unlike that of the NIRB whichoriginates at high redshift, falls at optical wavelengths. The scenario which appears toobservers is represented in fig. 1. Estimates for the local background are availablefrom the models developed by different authors on the basis of existing data [1-5]. Thedetection of a NIRB was first reported by Matsumoto et al. from his rocket experi-ments [6]. The measured intensity is I4105 Jy sr21 at l42.2 mm and it is consistentwith a 1500 K black body diluted by a factor 10211 .This intensity value exceeds by a sizeable factor the predictions based on sourcecounts and on current galaxy evolution models.At the moment further observations are needed to confirm the existence of anexcess in the diffuse NIRB over the local emission.To perform the measurement the use of satellite-, rocket- or balloon-borneinstruments is required. The first solution, although the best, would imply high costsand a long time of realization. Due to the short life of a rocket flight, the secondsolution implies short observing time and a poor sky coverage, with the additionalproblem of pollution from the engine exhaust gas which is hard to evaluate.Stratospheric balloons, on the other hand, permit long flights with fair sky coverage ata lower manufacturing cost, the only problem being the residual atmosphere at about40 km altitude.A COOLED TELESCOPE FOR MEASUREMENTS ETC. 715We have chosen a balloon-borne experiment, because we think that, in order to finda compromise between the costs and the scientific aims, it is the most suitable solutionamong the three mentioned above. The TRIP balloon successfully flew in June 1995.2. – Payload configurationThe gondola is a prism of about 1.7 m32.0 m31.0 m with a weight of about 850 kg.The central supporting column is fixed to the azimuth-pointing system and then to theballoon flight chain. The inner volume of the gondola is divided into three parts: one(gondola north) is reserved for the IR telescope and the zenith system, the oppositeside (gondola south) is occupied by the electronics and the batteries, the central part isreserved for the LHe dewar, the pressure control system and the telemetrypackage.3. – The optics and the filter systemTo observe the IR background in the wavelength range 1–5 mm at about 40 kmaltitude, one has to use a cooled apparatus to minimize the thermal emission from themirrors and the telescope mounting. This strongly limits the size of the primary mirrorsince a large diameter implies a big cryostat not allowed for a balloon experiment.Background observations need in general a quite large field of view to permit large skycoverage and a small angular scale at the focal plane to discriminate against point-likesources. These considerations, together with the constraints imposed by detector sizeand geometry, led to design a double Cassegrain optics system with an FO2 focal ratio.Figure 2 shows the cross-sectional view of the telescope. The detector optics assembly,including the filter wheel carrying six filters, is placed inside the cryostat and operatesat temperatures in the range 45 KETE55 K.4. – The cryogenic apparatusThe need of a cryogenic apparatus is mainly dictated by two reasons:– The InSb detector noise performance improves at low temperatures (B50 K );also the exposure time needed to reach the detector saturation lengthens as thetemperature is lowered.– The materials surrounding the detector (detector housing, detector assembly,optics and so on) give a contribution to the overall system noise due to their thermalemission which tends to mask the IR background to be measured. This «instrumentnoise component» is almost negligible if the surfaces which are in the field of view ofthe detector are cooled below 120 K.In the storage dewar, liquid He is boiled by controlling the current through fourheating elements, so that an increase of the internal pressure forces the He flow intothe transfer line. Figure 3 schematically shows the system used to control and stabilizethe temperature during the flight [7].M. R. ATTOLINI, A. BOSCALERI, A. BRIGHENTI, P. CAZZOLA, ETC.716Fig. 2. – Cross-sectional view of the telescope.5. – The detector and associated electronicsThe focal plane detector is a linear array consisting of 32 InSb integratingphotodiodes on which the IR signal is detected and stored during the exposition period.A shift register multiplexes the detectors onto a single output line connected to a JFETpreamplifier acting as output buffer while a MOSFET switch is used for resetting andbiasing the array. A temperature sensor gives the operating temperature of thedetector [8, 9].The main noise components which limit the detector response in the presentapplication are the dark current (which adds to the signal charge stored in the detectorduring the exposition; when long integration times are used to detect weak IR signals,the detector is «background limited» by this noise contribution) and kTC reset noiseA COOLED TELESCOPE FOR MEASUREMENTS ETC. 717Fig. 3. – Cryogenics control system.(which sets a limit on the minimum detectable signal when short exposure times areused in the presence of high IR backgrounds).The detector electronics (NIME-100) performs the following functions:– Timing generation for the detector clocks.– Power supplies to bias the detector chip.– Amplification of the detector signal.– Compensation for the temperature drifts of the video line (detector signal) andstabilization of the detector temperature at 5060.1 K.The detector electronics receives the ReadOut command from the on-board CPUand sends the amplified detector signal to the A-to-D electronics: the signal is hereconverted to 16-bit data words and sent to the CPU for data transmission. Theexposure time is set by the time between two successive readout commands. Since thesignal accumulated in each pixel is given by the difference between the signal portionof the video line and the zero voltage level following the application of the reset pulse,each pixel data is represented by two 16-bit words which are separately transmitted toground. The AOD converter is periodically recalibrated during the flight.6. – The azimuthal stabilization systemSince the infrared telescope elevation is independently controlled by the mainexperiment CPU, the pointing system can simply be referred to as a single axisM. R. ATTOLINI, A. BOSCALERI, A. BRIGHENTI, P. CAZZOLA, ETC.718stabilization. This implies that the system is able to drive the platform around thegravity vector only.Like many pointing systems it uses a relatively cheap position sensor, i.e. amagnetometer, mounted on a motorized platform: this permits the change of azimuthobservation direction either by remote commands or by on-board software.The system has been designed to stabilize payloads with weight up to 2500 kg with asafety factor of 10 g, is fully under digital control and oversees both the gondolamovements and the magnetometer platform changes [10]. The overall system pointingaccuracy was measured during integration tests to be of the order of 615 arcmin.Furthermore, the aspect field of view will be controlled in quas-real time by using aCCD camera mounted on-axis with the IR telescope.7. – The flight control system (FCS)The FCS is built around two CPUs, the Main CPU (a VME commercial board fromDynatem based on a 68030 CMOS processor) and the Detector CPU (also a VMEDynatem board based on a 68000 CMOS processor). The VME crate also includes someboards specifically designed and built to perform dedicated functions [11].8. – The ground programThe ground software equipment runs on several different CPUs. The REAL-TIMECPU (Motorola VME133 at 16 MHz) is the input and output interface with the flyingexperiment. Through a synchronous serial ISR controlled flow, it acquires the formatsFig. 4. – The expected sky coverage for a single run of measurements. This example refers to alaunch on May 30, 1995 at 20:00 (LT), from the NSBF base of Palestine (Texas, USA).A COOLED TELESCOPE FOR MEASUREMENTS ETC. 719and makes them available on the dual-ported memory. Furthermore, it provides theparallel strobed output for the telecommands which are to be sent to the experiment.Several PCs can be connected via LAN as clients. There is a general-purpose software,running under Windows 3.1, which performs data analysis and controls graphicaldisplay, dialog-boxes, etc. This software has been developed using C11 and allows theoperator to have complete control of the whole experiment in real-time [12].9. – Observational techniques and calibrationsThe observations consist of a series of scans in azimuth and zenith with all filters(see fig. 4 for a typical observation run). At the end of each zenith scan (five positions)and before changing the filter, a calibration routine for testing and monitoring thesystem stability will take place in the observing program. This routine consists essen-tially of an ONOOFF switching sequence of internal lamps and dark current measurements.Considering the characteristics of both the optics and the detector and thecapability of the pointing system to look for point-like sources, absolute in-flightcalibrations will require a consistent lot of time and a dedicated routine. Depending onthe flight site, date and time, a list of observable known sources will be prepared withrelated sky positions, fluxes and proper integration times. After a calibration source isselected, the calibration routine will perform a sort of raster scan around the expectedsource positon. The very thin detector field of view (1 .483498) forces us to thisprocedure, even if very time expensive. After the recognition of the first source it willbe possible also to set up the pointing system offsets, thus making the acquisition ofnext calibration sources easier. Due to its complexity, one or possibly two absolutecalibration procedures are planned at the start of each flight, before running theobserving program.Precise on-ground calibrations are difficult because it is impossible to open the lid ofthe cryostat when the telescope is cold, so it is not possible to illuminate the wholeprimary mirror from the outside. For on-ground calibrations we use the internal wall ofthe lid which was black painted so as to behave as a black body with emissivity of about97% in the K band. The non-uniformity of the temperature on the surface introduces anerror that we estimate to be not more than 10%.R E F E R E N C E S[1] MURDOCK T. L. and PRICE S. D., Astron. J., 90 (1985) 375.[2] BAHACALL J. N. and SONEIRA R. M., Astrophys. J. Suppl., 44 (1980) 73.[3] MAMON A. G. and SONEIRA R. M., Astrophys. J., 255 (1982) 181.[4] FRANCESCHINI A. et al., Astron. Astrophys. J. Suppl., 89 (1991) 285.[5] MACCLATCHEY R. A. et al., AFCRL-TR-55-213 (1972).[6] MATSUMOTO T. et al., Astrophys. J., 332 (1988) 575.[7] CORTIGLIONI S. et al., TESREOCNR Int. Rep. N. 119 (1986).[8] DAVIES R. M. and NYBLACK C. A., Proc. SPIE, 627 (1986) 438.[9] NYBLACK C. A., Proceedings of the 28th Midwest Symposium on Circuits and SystemsConference, Louisville, edited by J. D. COLE and E. J. MILLER (University of Louisville,Kentucky) 1985, pp. 4-8.[10] BOSCALERI A. et al., Meas. Sci. Technol., 5 (1994) 190.[11] VENTURA G. et al., TESREOCNR Int. Rep. N. 169 (1994).[12] BRIGHENTI A. et al., TESREOCNR Int. Rep. N. 148 (1990).

A cooled telescope for measurements of the Near Infrared Cosmological Background at balloon altitude (TRIP Experiment)

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A cooled telescope for measurements of the Near Infrared Cosmological Background at balloon altitude (TRIP Experiment)

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