Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.
SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.
The SPICA instrument complement offers spectral resolving power ranging from ~50 through 11000 in the 17-230 µm domain as well as ~28.000 spectroscopy between 12 and 18 µm. Additionally, SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ~5x10-20 W/m2 (5σ/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.
The European/Japanese SPace Infrared telescope for Cosmology and Astrophysics, SPICA, will provide astronomers with a long awaited new window on the universe. Having a large cold telescope cooled to less than 8K above absolute zero, SPICA will provide a unique environment where instruments are limited only by the cosmic background itself. A consortium of European, north American and Asian institutes has been established to design and implement the SpicA FAR infrared Instrument SAFARI, an extremely sensitive spectrometer designed to fully exploit this extremely low far infrared background environment provided by the SPICA observatory.
SAFARI’s extremely sensitive Transition Edge Sensing detectors will allow astronomers to very efficiently obtain moderate to high resolution spectra of many thousands of obscured celestial objects in the far infrared, allowing a full spectroscopic characterisation of this objects. Efficiently obtaining such a large number of complete spectra will be essential to address several fundamental questions in current astrophysics: how do galaxies form and evolve over cosmic time?, what is the true nature of our own Milky Way?, and why and where do planets like those in our own solar system come into being?
The basic SAFARI instrument is a highly sensitive Grating Spectrometer with a spectral resolution R of about
300 and a line sensitivity of a few x 10^-20 W/√Hz (5σ-1h). By routing the signal through a Martin-Puplett interferometer a high resolution mode is implemented providing R~11000 at 34 μm to R~1500 at 230 μm.
The instrument operates in four wavelength bands, simultaneously covering the full 34-230μm range. Each band has three arrays of about 300 TES sensors providing three spatial and 300 spectral outputs. To limit the number of signal wires between the cold focal plan and the warm electronics units a 160 pixel/channel Frequency Domain Multiplexing scheme is employed.
SAFARI is a point source spectrometer for the SPICA mission, which provides far-infrared spectroscopy and high sensitivity. SPICA mission, having a large cold telescope cooled to 6K above absolute zero, will provide an optimum environment where instruments are limited only by the cosmic background. SAFARI is a grating-based spectrometer with two modes of operation, Low Resolution (LR), or nominal mode (R~300) and High Resolution, (HR) (R~2000-11000). The SAFARI shall provide point source spectroscopy with diffraction-limited capability in four spectral bands over 34-230μm and a field of view (FoV) on sky over 2’×2’. Due to the complexity of the optical design of the SAFARI instrument a modular design was selected. Four principal modules are defined: Calibration Module (CS), Input Optics Module (IOM), Beam and Mode Distribution (BMDO) and Grating Modules (GMs). The present work is focused in the last module. Dispersive optical systems inherently demand the need of volume allocation for the optical system, being this fact somehow proportional to the wavelength and the required resolving power. The image sampling and the size of the detector elements are key drivers in this optical modular design. The optimization process has been performed taking into account the conceptual design parameters obtained during this phase such as collimator and camera optics focal lengths, subsystem diameters and periods and AOIs of the diffraction gratings.
SpicA FAR infrared Instrument, SAFARI, is an imaging spectrometer which is being designed to map large areas of the sky in the far infrared. The SPICA mission, having a large cold telescope cooled to 6K above absolute zero, will provide an optimum environment where instruments are limited only by the cosmic background itself.
SpicA FAR infrared Instrument, SAFARI, is one of the instruments planned for the SPICA mission. The SPICA
mission is the next great leap forward in space-based far-infrared astronomy and will study the evolution of galaxies,
stars and planetary systems. SPICA will utilize a deeply cooled 2.5m-class telescope, provided by European industry, to
realize zodiacal background limited performance, and high spatial resolution. The instrument SAFARI is a cryogenic
grating-based point source spectrometer working in the wavelength domain 34 to 230 μm, providing spectral resolving
power from 300 to at least 2000.
The instrument shall provide low and high resolution spectroscopy in four spectral bands. Low Resolution mode is the
native instrument mode, while the high Resolution mode is achieved by means of a Martin-Pupplet interferometer.
The optical system is all-reflective and consists of three main modules; an input optics module, followed by the Band
and Mode Distributing Optics and the grating Modules. The instrument utilizes Nyquist sampled filled linear arrays of
very sensitive TES detectors.
The work presented in this paper describes the optical design architecture and design concept compatible with the
current instrument performance and volume design drivers.
In the last two decades, Spain has built up a strong IR community which has successfully contributed to space instruments, reaching Co-PI level in the SPICA mission (Space Infrared Telescope for Cosmology and Astrophysics). Under the SPICA mission, INTA, focused on the SAFARI instrument requirements but highly adaptable to other missions has designed a cryogenic low dissipation filter wheel with six positions, taking as starting point the past experience of the team with the OSIRIS instrument (ROSETTA mission) filter wheels and adapting the design to work at cryogenic temperatures. One of the main goals of the mechanism is to use as much as possible commercial components and test them at cryogenic temperature. This paper is focused on the design of the filter wheel, including the material selection for each of the main components of the mechanism, the design of elastic mount for the filter assembly, a positioner device designed to provide positional accuracy and repeatability to the filter, allowing the locking of the position without dissipation. In order to know the position of the wheel on every moment a position sensor based on a Hall sensor was developed. A series of cryogenic tests have been performed in order to validate the material configuration selected, the ball bearing lubrication and the selection of the motor. A stepper motor characterization campaign was performed including heat dissipation measurements. The result is a six position filter wheel highly adaptable to different configurations and motors using commercial components. The mechanism was successfully tested at INTA facilities at 20K at breadboard level.
This paper describes the optical design of the far infrared imaging spectrometer for the JAXA’s SPICA mission. The SAFARI instrument, is a cryogenic imaging Fourier transform spectrometer (iFTS), designed to perform backgroundlimited spectroscopic and photometric imaging in the band 34-210 μm. The all-reflective optical system is highly modular and consists of three main modules; input optics module, interferometer module (FTS) and camera bay optics. A special study has been dedicated to the spectroscopic performance of the instrument, in which the spectral response and interference of the instrument have been modeled, as the FTS mechanism scans over the total desired OPD range.
The Japanese SPace Infrared telescope for Cosmology and Astrophysics, SPICA, aims to provide astronomers with a truly new window on the universe. With a large -3 meter class- cold -6K- telescope, the mission provides a unique low background environment optimally suited for highly sensitive instruments limited only by the cosmic background itself. SAFARI, the SpicA FAR infrared Instrument SAFARI, is a Fourier Transform imaging spectrometer designed to fully exploit this extremely low far infrared background environment. The SAFARI consortium, comprised of European and Canadian institutes, has established an instrument reference design based on a Mach-Zehnder interferometer stage with outputs directed to three extremely sensitive Transition Edge Sensor arrays covering the 35 to 210 μm domain. The baseline instrument provides R > 1000 spectroscopic imaging capabilities over a 2’ by 2’ field of view. A number of modifications to the instrument to extend its capabilities are under investigation. With the reference design SAFARI’s sensitivity for many objects is limited not only by the detector NEP but also by the level of broad band background radiation – the zodiacal light for the shorter wavelengths and satellite baffle structures for the longer wavelengths. Options to reduce this background are dedicated masks or dispersive elements which can be inserted in the optics as required. The resulting increase in sensitivity can directly enhance the prime science goals of SAFARI; with the expected enhanced sensitivity astronomers would be in a better position to study thousands of galaxies out to redshift 3 and even many hundreds out to redshifts of 5 or 6. Possibilities to increase the wavelength resolution, at least for the shorter wavelength bands, are investigated as this would significantly enhance SAFARI’s capabilities to study star and planet formation in our own galaxy.
The Japanese SPace Infrared telescope for Cosmology and Astrophysics, SPICA, will provide astronomers with a long
awaited new window on the universe. Having a large cold telescope cooled to only 6K above absolute zero, SPICA will
provide a unique environment where instruments are limited only by the cosmic background itself. A consortium of
European and Canadian institutes has been established to design and implement the SpicA FAR infrared Instrument
SAFARI, an imaging spectrometer designed to fully exploit this extremely low far infrared background environment
provided by the SPICA observatory.
SAFARI’s large instantaneous field of view combined with the extremely sensitive Transition Edge Sensing detectors
will allow astronomers to very efficiently map large areas of the sky in the far infrared – in a square degree survey of a
1000 hours many thousands of faint sources will be detected, and a very large fraction of these sources will be fully
spectroscopically characterised by the instrument. Efficiently obtaining such a large number of complete spectra is
essential to address several fundamental questions in current astrophysics: how do galaxies form and evolve over cosmic
time?, what is the true nature of our own Milky Way?, and why and where do planets like those in our own solar system
come into being?