Walfried Raab

The European contribution to the SPICA mission

The Japanese led Space Infrared telescope for Cosmology and Astrophysics (SPICA) will observe the universe over the 5 to 210 micron band with unprecedented sensitivity owing to its cold (~5 K) 3.5m telescope. The scientific case for a European involvement in the SPICA mission has been accepted by the ESA advisory structure and a European contribution to SPICA is undergoing an assessment study as a Mission of Opportunity within the ESA Cosmic Vision 1015-2015 science mission programme. In this paper we describe the elements that are being studied for provision by Europe for the SPICA mission. These entail ESA directly providing the cryogenic telescope and ground segment support and a consortium of European insitutes providing a Far Infrared focal plane instrument. In this paper we describe the status of the ESA study and the design status of the FIR focal plane instrument.

Realizing Integral Field Spectroscopy in the Far-Infrared

The optical design of an integral field spectrometer for far-infrared observations, the Far-Infrared Field-Imaging Line Spectrometer (FIFI LS), is presented. The instrument will fly on board the joint NASA/DLR airborne observatory SOFIA, observing in two nearly independent wavelength channels simultaneously: a blue channel (40-105 μm) and a red channel (105-210 μm). To achieve instantaneous integral field spectroscopy for the first time in the far-infrared, a novel reflective image slicer system is utilized that slices the 5 × 5 pixel, two-dimensional field of view into a pseudo-long slit of 25 × 1 pixels. The slicer assembly consists of three sets of five mirrors that have optical power, enabling a compact design. After the sky field has been optically rearranged to the pseudoslit, the image is spectrally dispersed in a standard Littrow-mounted reflective grating spectrometer. The practical concerns for the optical design in the far-infrared and, in particular, the significant effect of diffraction in the entire optical system is discussed.

The SOFIA far-infrared spectrometer FIFI-LS: spearheading a post Herschel era

FIFI-LS (Field-Imaging Far-Infrared Line Spectrometer) is an imaging spectrograph for SOFIA comprised of two medium resolution (R~2200) grating spectrometers feeding two 16x25 pixel detector arrays, which enable simultaneous line observations across two wavelength ranges (42-110 μm and 110-210μm) each across a field of view of 5x5 pixel. FIFI-LS will be the extragalactic spectroscopic workhorse for SOFIA. FIFI-LS has enough sensitivity to observe a substantial sample of nearby galaxies. It also has the right combination of wavelength range and spatial resolution to carry out unique new observations beyond those possible with Herschel, Spitzer, ISO and IRAS. As the effective sensitivity of FIFI-LS is only about a factor of 3-5 lower than the PACS spectrometer onboard Herschel, mainly due to an enhanced multiplexing advantage, FIFI-LS will build upon recent exciting scientific results and spearhead the post- Herschel far-infrared era. FIFI-LS is scheduled for commissioning onboard SOFIA in early 2014. An account on the instrument and its current stratus will be presented.

FIFI LS getting ready to fly aboard SOFIA

FIFI LS is the German far-infrared integral field spectrometer for the SOFIA airborne observatory. The instrument consists of two independent integral field spectrometers for two different wavelength bands (45-110 μm and 100-200 μm). A dichroic filter enables simultaneous observation of two different spectral lines in the same field-of view. This allows very efficient mapping of extended regions with FIFI LS in many important far-infrared cooling lines with line ratios sensitive to temperature and density. FIFI LS will become a facility instrument for SOFIA. In the next two years it will become a fully commissioned facility instrument. After its commission, FIFI LS will be available for general observing with a large science potential. In this paper, we will also discuss the science of FIFI LS.

The ARGOS laser system: green light for ground layer adaptive optics at the LBT

We report on the development of the laser system of ARGOS, the multiple laser guide star adaptive optics system for the Large Binocular Telescope (LBT). The system uses a total of six high powered, pulsed Nd:YAG lasers frequency-doubled to a wavelength of 532 nm to generate a set of three guide stars above each of the LBT telescopes. The position of each of the LGS constellations on sky as well as the relative position of the individual laser guide stars within this constellation is controlled by a set of steerable mirrors and a fast tip-tilt mirror within the laser system. The entire opto-mechanical system is housed in two hermetically sealed and thermally controlled enclosures on the SX and DX side of the LBT telescope. The laser beams are propagated through two refractive launch telescopes which focus the beams at an altitude of 12 km, creating a constellation of laser guide stars around a 4 arcminute diameter circle by means of Rayleigh scattering. In addition to the GLAO Rayleigh beacon system, ARGOS has also been designed for a possible future upgrade with a hybrid sodium laser - Rayleigh beacon combination, enabling diffraction limited operation. The ARGOS laser system was successfully installed at the LBT in April 2013. Extensive functional tests have been carried out and have verified the operation of the systems according to specifications. The alignment of the laser system with respect to the launch telescope was carried out during two more runs in June and October 2013, followed by the first propagation of laser light on sky in November 2013.

16 x 25 Ge:Ga detector arrays for FIFI LS

We are developing 2D 16 X 25 pixel detector arrays of both unstressed and stressed Ge:Ga photoconductive detectors for far-infrared astronomy from SOFIA. The arrays, based on earlier 5 X 5 detector arrays used on the KAO, will be for our new instrument, the Far Infrared Field Imaging Line Spectrometer (FIFI LS). The unstressed Ge:Ga detector array will cover the wavelength range from 40 to 120 micrometers , and the stressed Ge:Ga detector array from 120 to 210 micrometers . The detector arrays will be operated with multiplexed integrating amplifiers with cryogenic readout electronics located close to the detector arrays. The design of the stressed detector array and results of current measurements on several prototype 16 pixel linear arrays will be reported. They demonstrate the feasibility of the current concept.

Status of the ARGOS project

ARGOS is the Laser Guide Star and Wavefront sensing facility for the Large Binocular Telescope. With first laser light on sky in 2013, the system is currently undergoing commissioning at the telescope. We present the overall status and design, as well as first results on sky. Aiming for a wide field ground layer correction, ARGOS is designed as a multi- Rayleigh beacon adaptive optics system. A total of six powerful pulsed lasers are creating the laser guide stars in constellations above each of the LBTs primary mirrors. With a range gated detection in the wavefront sensors, and the adaptive correction by the deformable secondary’s, we expect ARGOS to enhance the image quality over a large range of seeing conditions. With the two wide field imaging and spectroscopic instruments LUCI1 and LUCI2 as receivers, a wide range of scientific programs will benefit from ARGOS. With an increased resolution, higher encircled energy, both imaging and MOS spectroscopy will be boosted in signal to noise by a large amount. Apart from the wide field correction ARGOS delivers in its ground layer mode, we already foresee the implementation of a hybrid Sodium with Rayleigh beacon combination for a diffraction limited AO performance.

Development of a far-infrared Ge:Ga monolithic array for a possible application to SPICA

We present the current status of the development of a far-infrared monolithic Ge:Ga photoconductor array proposed for the SAFARI instrument onboard SPICA, which is a future infrared space mission. SPICA has a large (3-m class) cooled (<6 K) telescope, which enables us to make astronomical observations with high spatial resolution and unprecedented sensitivity in the mid- and far-infrared wavelength. As a candidate detector to cover the 45-110 μm band of a far-infrared focal plan instrument of SAFARI, we are developing a large format monolithic Ge:Ga array. The monolithic Ge:Ga array is directly connected to cryogenic readout electronics (CRE) using the Au-Indium bumping technology. Our goal is to develop a 64×64 Ge:Ga array, on the basis of existing technologies and experience in making the 3×20 Ge:Ga monolithic arrays for the AKARI satellite. In order to realize a larger format array with better sensitivity than that of the AKARI array, we have been making some technical improvements; (1) development of the Au-In bumping technology to realize the large format array, (2) optimization of the structure of the transparent electrode to achieve the better sensitivity, (3) development of an anti-reflection coating to reduce interference fringe between the Ge substrate, and (4) Use of the low-noise cryogenic readout electronics with low power consumption. We fabricated the prototype 5×5 Ge:Ga arrays to demonstrate and evaluate the properties of monolithic array. We demonstrate experimentally the feasibility of these elemental technologies, and also show the results of performance measurements for the prototype Ge:Ga arrays.

Early laser operations at the Large Binocular Telescope Observatory

ARGOS is the GLAO (Ground-Layer Adaptive Optics) Rayleigh-based LGS (Laser Guide Star) facility for the Large Binocular Telescope Observatory (LBTO). It is dedicated for observations with LUCI1 and LUCI2, LBTOs pair of NIR imagers and multi-object spectrographs. The system projects three laser beams from the back of each of the two secondary mirror units, which create two constellations circumscribed on circles of 2 arcmin radius with 120 degree spacing. Each of the six Nd:YAG lasers provides a beam of green (532nm) pulses at a rate of 10kHz with a power of 14W to 18W. We achieved first on-sky propagation on the night of November 5, 2013, and commissioning of the full system will take place during 2014. We present the initial results of laser operations at the observatory, including safety procedures and the required coordination with external agencies (FAA, Space Command, and Military Airspace Manager). We also describe our operational procedures and report on our experiences with aircraft spotters. Future plans for safer and more efficient aircraft monitoring and detection are discussed.

FIFI LS: the far-infrared integral field spectrometer for SOFIA

FIFI LS is a far-infrared integral field spectrometer for the SOFIA airborne observatory. The instrument is designed to maximize the observing efficiency by simultaneous and nearly independent imaging of the field of view in two medium spectral resolution bands. Both spectral channels - covering a wavelength range of 42 to 110 microns and 110 to 210 microns respectively - allow diffraction limited spectral imaging. Reflective image slicers rearrange the 5x5 pixel field of view into the 1x25 pixel entrance slit of a grating spectrograph. Littrow mounted gratings with anamorphic collimators are used for spectral multiplexing with a spectral resolution between R = 1400 - 6500, depending on observing wavelength. Each spectral band employs two large format 25x16 pixel Ge:Ga detector arrays, axially stressed for the long wavelength band to achieve a longer wavelength response and slightly stressed for the short wavelength band. For each of the 25 spatial pixels, we are able to cover a velocity range of approximately 1500 km/s around a selected far-infrared line. This arrangement provides good spectral coverage with high responsivity. We present a summary of the FIFI LS design and the current status of instrument integration.