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GREAT: the SOFIA High-Frequency Heterodyne Instrument A&A 542, L1 (2012) Astronomy DOI: 10.1051/0004-6361/201218811 & c ESO 2012 Astrophysics GREAT: early science results Special feature Letter to the Editor GREAT: the SOFIA high-frequency heterodyne instrument S. Heyminck1,U.U.Graf2,R.Güsten1, J. Stutzki2,H.W.Hübers3,4, and P. Hartogh5 1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [email protected] 2 I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany 3 Deutsches Zentrum für Luft- und Raumfahrt, Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany 4 Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany 5 Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany Received 12 January 2012 / Accepted 1 March 2012 ABSTRACT We describe the design and construction of GREAT (German REceiver for Astronomy at Terahertz frequencies) operated on the Stratospheric Observatory For Infrared Astronomy (SOFIA). GREAT is a modular dual-color heterodyne instrument for high- resolution far-infrared (FIR) spectroscopy. Selected for SOFIA’s Early Science demonstration, the instrument has successfully per- formed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that – in various flight configurations, with three different detector channels – observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was ver- ified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT is a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis. Key words. techniques: spectroscopic – telescopes 1. Introduction to the NASA Dryden Aircraft Operations Facility (DAOF) in Palmdale, CA. On SOFIA’s base there, integration into the air- With SOFIA (Becklin & Gehrz 2009; Young et al. 2012)a craft and on-ground commissioning proceeded notably smooth, new platform for inter alia high-resolution far-infrared spec- and first light was recorded already on April 1, 2011, during troscopy has began science operation. Building on the legacy SOFIA’s observatory characterization flight OCF#4. Since then, of the Kuiper Airborne Observatory (Gillespie 1981), SOFIA GREAT has been operated for more than a dozen flights in with its projected 20 years operational lifetime will complement SOFIA’s Basic Science program. As a PI-class instrument and carry on the science heritage of Herschel (Pilbratt et al. GREAT is operated by the consortium only, but by agreement 2010). Since the first pioneering high-resolution FIR spectrome- with the SOFIA director the instrument has been made available ters were flown on the KAO (Betz & Zmuidzinas 1984; Phillips to the SOFIA communities on a collaborative basis. 1981; Röser et al. 1987, and references therein), new technolo- In the following we focus mainly on the configuration and gies have enabled the development of instruments with – in performance of the instrument as flown during the Short and those days – unobtainable performances and sensitivities. HIFI Basic Science periods of SOFIA. A more detailed technical de- (de Graauw et al. 2010), the heterodyne spectrometer currently scription of GREAT, including ongoing developments, will be flying onboard Herschel, and GREAT (German REceiver for published elsewhere (Heyminck et al., in prep.). Astronomy at Terahertz frequencies), the subject of this paper, are the most advanced implementations for actual science oper- ation. 2. Science requirements The development of GREAT1 with SOFIA, initially began in 2000, not least as pathfinder for the Herschel satellite mission. The users’ desire for wide radio frequency (RF) coverage Owing to severe SOFIA project delays, it was not un- (1–5 THz), with highest possible intermediate frequency (IF) til October 2010, however, that the instrument was shipped bandwidth (>3 GHz) and spectral resolution (<100 kHz) is con- fronted with current limitations of THz technologies. Despite 1 GREAT is a development by the MPI für Radioastronomie (Principal impressive advances during recent years, the operating band- Investigator: R. Güsten) and the KOSMA/Universität zu Köln, in coop- width of the local oscillator (LO) sources in particular is Δν/ν ∼ eration with the MPI für Sonnensystemforschung and the DLR Institut 0.1 only (and significantly less above 2 THz). Therefore, from für Planetenforschung. the beginning (Güsten et al. 2000) GREAT was designed as a Article published by EDP Sciences L1, page 1 of 7 A&A 542, L1 (2012) Fig. 1. Left: GREAT mounted to the instrument flange, in stow position (the telescope parks at 40◦ elevation). During observations GREAT (together with the counterweight at top right) rotates ±20◦ from the vertical. Right: basic structural components of GREAT. One of the two cryostats, the location of the LO compartments and position of the main optics benches are shown. A more detailed view of the optical layout is given in Fig. 2. Table 1. Science opportunities with GREAT. RF tuning Astrophysical lines [GHz] + L1a 1252–1392 CO(11–10), CO(12–11), OD, SH, H2D , HCN, HCO+ (13) + L1b 1417–1520 [NII], CO(13–12), HCN, HCO 2 L2 1815–1910 NH3(3–2), OH( Π1/2), CO(16–15), [CII] 2 Ma 2507–2514 OH( Π3/2) Mb 2670–2680 HD(1–0) H 4750–4770 [OI] Notes. Suffixes a and b denote the different LO-chains available for this receiver channel to extend their RF-coverage. It is not possible to swap these LO-chains during flight. Status: channels L1 and L2 were oper- ated during Early and Basic Science, Ma during Basic Science only; Mb and H are under development (see the following sections). highly modular instrument: within the framework of a common Fig. 2. Optics layout: a central polarizer inside the optics compartment infrastructure a choice of two frequency-specific channels can splits and re-directs the incoming radiation to the two optics plates be operated simultaneously. For Early Science three detector of the instrument. A common beam scanning mechanism (for internal alignment) and the calibration loads complete the optical system. channels with a total of four LO sources were flown (Table 1), two more are being developed. The instrument configuration can be changed between flights, while GREAT stays installed on The mechanical support structure of GREAT (consisting of SOFIA. lightweight, certified aluminum Al-2024 plates) is connected to the science instrument flange of the telescope (Fig. 1). This 3. Instrument description frame carries the optics box with the common and channel- specific optical components to guide the signals from sky and GREAT is composed of a wide variety of common modules from the LOs to the mixers. The receiver cryostats and the cal- out of which, the desired flight configuration set-up is formed ibration unit are clamped to the upper side of the structure with together with the frequency specific subunits (Heyminck et al. tight tolerance, which allows a reproducible re-positioning of the 2008). units when the instrument configuration is changed. L1, page 2 of 7 S. Heyminck et al.: GREAT: the SOFIA high frequency heterodyne instrument The structure carries the front-end control electronics on the rooftop mirrors make the design more robust against vibrations forward side and the two LOs underneath the optics box (Fig. 1). and alignment errors. The cold optics only consists of one off- The optics compartment represents the pressure boundary to the axis mirror, matching the beam directly to the horn antenna of aircraft, is open toward the telescope cavity and hence oper- the mixer. ates at ambient pressure to minimize absorption losses. The FIR For all channels, the wavelength dependence of the diplexer windows maintain the LO compartments at ambient pressure. requires mechanical adjustment of its adjustable stage to sub-μm The total mass of the main structure, including the cryostats, positioning accuracy (see Sect. 3.4.2, for the implementation). is between 400 and 500 kg, depending on the actual instrument The L1 channel uses high-density polyethelene (HDPE) win- configuration. dows for the cryostat and the LO window in the optics compart- Parts of the IF processor and the digital spectrometer back- ment. Since absorption of the material increases with frequency, ends are located in the co-rotating counterweight rack. The so- we chose anti-reflection grooved silicon windows for the L2- and called PI-rack mounted to the aircraft cabin floor holds the the M-band channels (Wagner-Gentner et al. 2006). main IF-processor, the acusto-optical spectrometer (AOS) and the chirp transform spectrometer (CTS) back-ends, the power distribution, and the observing computer with ethernet hub. 3.2. IF processors and spectrometers All GREAT channels flown during Early Science used the same type of liquid nitrogen and liquid helium-cooled cryostat The GREAT IF system operates two IF-processors with remote- with a similar wiring scheme. Special attention was given to po- control attenuators and total power detectors, one for the AOS tential safety hazards, and detailed structural analysis was per- and the CTS, one for the fast-Fourier transform (FFT) spectrom- formed. Eight identical units were manufactured so far (Cryovac, eter. They shift the IF band (receiver output) to the input band of Germany), including those assigned for the high-frequency the individual spectrometer. Both IFs are equipped with remot- channel. Hold times of 20+ h are adequate for science flights control synthesizers for the internal mixing processes, allowing of not more than 10–11 h duration – typically a last top-up is the position of individual spectrometer bands within the IF band performed four hours before take-off.
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