In the far-infrared (FIR) / THz regime the angular (and often spectral) resolution of observing facilities is still very
restricted despite the fact that this frequency range has become of prime importance for modern astrophysics. ALMA
(Atacama Large Millimeter Array) with its superb sensitivity and angular resolution will only cover frequencies up to
about 1 THz, while the HIFI instrument for ESA'a Herschel Space Observatory will provide limited angular resolution
(10 to 30 arcsec) up to 2 THz. Observations of regions with star and planet formation require extremely high angular
resolution as well as frequency resolution in the full THz regime. In order to open these regions for high-resolution
astrophysics we present a study concept for a heterodyne space interferometer, ESPRIT (Exploratory Submm Space
Radio-Interferometric Telescope). This mission will cover the Terahertz regime inaccessible from the ground and outside
the operating range of the James Webb Space Telescope (JWST).
This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI), to be launched onboard of ESA's Herschel Space Observatory, by 2008. It includes the first results from the instrument level tests. The instrument is designed to be electronically tuneable over a wide and continuous frequency range in the Far Infrared, with velocity resolutions better than 0.1 km/s with a high sensitivity. This will enable detailed investigations of a wide variety of astronomical sources, ranging from solar system objects, star formation regions to nuclei of galaxies.
The instrument comprises 5 frequency bands covering 480-1150 GHz with SIS mixers and a sixth dual frequency band, for the 1410-1910 GHz range, with Hot Electron Bolometer Mixers (HEB). The Local Oscillator (LO) subsystem consists of a dedicated Ka-band synthesizer followed by 7 times 2 chains of frequency multipliers, 2 chains for each frequency band. A pair of Auto-Correlators and a pair of Acousto-Optic spectrometers process the two IF signals from the dual-polarization front-ends to provide instantaneous frequency coverage of 4 GHz, with a set of resolutions (140 kHz to 1 MHz), better than < 0.1 km/s. After a successful qualification program, the flight instrument was delivered and entered the testing phase at satellite level. We will also report on the pre-flight test and calibration results together with the expected in-flight performance.
In the far-infrared (FIR) / THz regime the angular (and often spectral) resolution of observing facilities is still very restricted despite the fact that this frequency range has become of prime importance for modern astrophysics. ALMA (Atacama Large Millimeter Array) with its superb sensitivity and angular resolution will only cover frequencies up to about 1 THz, while the HIFI instrument for ESA'a Herschel Space Observatory will provide limited angular resolution (10 to 30 arcsec) up to 2 THz. Observations of regions with star and planet formation require extremely high angular resolution as well as frequency resolution in the full THz regime. In order to open these regions for high-resolution astrophysics we propose a heterodyne space interferometer mission, ESPRIT (Exploratory Submm Space Radio-Interferometric Telescope), for the Terahertz regime inaccessible from ground and outside the operating range of the James Webb Space Telescope (JWST).
The far-infrared (FIR) wavelength regime has become of prime importance for astrophysics. Observations of ionic, atomic and molecular lines, many of them present in the FIR, provide important and unique information on the star and planet formation process occurring in interstellar clouds, and on the lifecycle of gas and dust in general.
As these regions are heavily obscured by dust, FIR observations are the only means of getting insight in the physical and chemical conditions and their evolution. These investigations require, besides high spectral, also high angular resolution in order to match the small angular sizes of star forming cores and circum-stellar disks. We present here a mission concept, ESPRIT, which will provide both, in a wavelength regime not accessible from ground by ALMA (Atacama Large Millimeter Array), nor with JWST (James Webb Space Telescope).
The Short-Wavelength Spectrometer (SWS) is one of the four focal plane instruments of ESA's Infrared Space Observatory (ISO). The satellite was launched on November 15, 1995 with a super fluid Helium content of about 2300 liters to keep the telescope, the scientific payload and the optical baffles at operating temperatures between 2 and 8 K. On April 8, 1998 the liquid Helium depleted and the instruments were switched-off when the focal plane reached a temperature of 4.2 K. A satellite engineering test program was conducted between April 20 and May 10. Timeslots before and during the test program were used to operate the InSb detectors of the SWS instrument while the temperature of the focal plane slowly increased up to 40 K. The instrument was used to record spectra of 260 stars between 2.36 and 4.05 microns at a resolution of 2000 and with high S/N. Goal of the program was to observe a set of stars covering the entire MK spectral classification scheme to extend this classification scheme to the infrared. We discuss changes in the instrument relevant for operating and calibrating the instrument at temperatures above 4K: changes in the InSb detector behavior (dark levels, noise, response, ...), behavior of the JFETs and geometry changes in the grating scanner mechanism. We also show that the calibration of the data obtained after Helium loss is accurate, resulting in a data set of great scientific value.
The short wavelength spectrometer (SWS) is one of the four instruments on-board of ESA's IR SPace Observatory (ISO), launched on 15 November 1995. It covers the wavelength range of 2.38-45.2 microns with a spectral resolution ranging from 1000-2000. By inserting Fabry-Perot filters the resolution can be enhanced by a factor 20 for the wavelength range from 11.4-44.5 microns. After the successful launch the instrument was tested and calibrated during a period of spacecraft checkout and performance verification. The opto- mechanical construction of the instrument appears to behave extremely well. The instrument performance is on all aspects as expected, except for the detector sensitivity where the noise is dominated by effects of particle radiation. We given here an overview of the in-orbit performance, discuss the calibration and present some result from trend analysis of the most important instrument and detector parameters.
We describe the preliminary design of the proposed Heterodyne Instrument for FIRST (HIFI). The instrument will have a continuous frequency coverage over the range from 480 to 1250 GHz in five bands, while a sixth band will provide coverage for 1410 - 1910 GHz and 2400 - 2700 GHz. The first five bands will use SIS mixers and varactor frequency multipliers while in the sixth band a laser photomixer local oscillator will pump HEB mixers. HIFI will have an instantaneous bandwidth of 4 GHz, analyzed in parallel by two types of spectrometers: a pair of wide-band spectrometers (WBS), and a pair of high- resolution spectrometer (HRS). The wide-band spectrometer will use acousto-optic technology with a frequency resolution of 1 MHz and a bandwidth of 4 GHz for each of the two polarizations. The HRS will provide two combinations of bandwidth and resolution: 1 GHz bandwidth at 200 kHz resolution, and at least 500 MHz at 100 kHz resolution. The HRS will be divided into 4 or 5 sub-bands, each of which can be placed anywhere within the full 4 GHz IF band. The instrument will be able to perform rapid and complete spectral line surveys with resolving powers from 103 up to 107 (300 - 0.03 km/s) and deep line observations.
In 1992, the European Southern Observatory (ESO) committed a phase A study of a mid- infrared instrument for the 2nd unit VLT telescope, to a consortium of laboratories (SAp at Saclay, France; SRON at Groningen, Germany; and the Kapteyn Observatory at Roden, Netherlands). The results of the study are presented. One key scientific objective for this instrument is foreseen to be the study of dust. The required observing modes are (1) diffraction limited imaging both at 10 and 20 microns, and (2) spectroscopy at low resolution (R approximately equals 500) both at 10 and 20 microns. Another key domain is the study of atomic, molecular, and ionic lines observable in the atmospheric window at 10 and 20 microns. Given the various environments where the lines originate, medium (approximately equals 5000) to high (approximately equals 30,000) spectral resolution is needed. The optical design, as well as a mechanical layout, incorporating the various modes is described. The imaging and spectroscopic channels are separated. The spectrometer is based on a long slit all reflective design. Two optical configurations have been studied in detail. Because of the need for variable magnifications, the imager is based on refractive optics.
The ISO Short Wavelength Spectrometer will open the full 2.4-45 micrometers wavelength range for astronomical spectroscopy. At spectral resolving power approximately equals 1000, most sources detected by IRAS will be within reach. In addition, spectral resolving power approximately equals 30000 is provided for the 12-45 micrometers range. Examples of proposed observations illustrate the SWS capabilities.
The European Space Agency's ISO satellite is a liquid helium cooled space observatory for infrared astronomy. It will be launched for an 18 month mission in 1995 by the Ariane 4 rocket. The payload module contains a 60 cm telescope and 4 focal plane instruments covering the wavelength range 2.4 to 240 micrometers . During the first cold tests early in 1994, ISO's 2300 1 tank was partially filled with superfluid helium. The main purpose of this test was to check all the functions of the instruments and their compatibility in ISO's cryovacuum environment. In addition the straylight level caused by thermal emission of the cryostat's interior was measured by the focal plane instruments.
The Short Wavelength Spectrometer is one of the four instruments for the Infrared Space Observatory. The instrument operates at about 3 K. Employing diffraction gratings, if offers a resolving power between 1000 and 2000 in the wavelength range 2.45 to 45 micrometers . An additional Fabry-Perot interferometer offers resolutions between 25,000 and 30,000 in the range 12 to 45 micrometers . The instrument employs arrays of discrete detectors: InSb photo- diodes and Si:Ga, Si:Sb and Ge:Be photo-conductors. In the course of 1992, the flight unit was tested and characterized, with interruptions for minor modifications. The paper discusses the SWS design and its performance.
Results of the ground test and characterization program show that the performance of the SWS is well within its specifications. The procedures for ground testing and calibration are tuned to achieve commonality with in-orbit procedures. This strategy optimizes the development of data analysis procedures and allows a fast in-orbit check-out and update of the calibration.
The Short-Wavelength Spectrometer (SWS) for ISO operates in the wavelength range from 2.4 to 45 micrometers. It consists of two, almost identical, grating spectrometers that provide resolving powers varying between 1000 to 2000. In the wavelength region from 12 to 45 micrometers a much larger (>20.000) resolution can be obtained with a pair of Fabry-Perot interferometers. This paper describes the design of the SWS.