U. Grözinger

Far-infrared photoconductor arrays for Herschel and SOFIA

We are presently developing large format photoconductor arrays for the Herschel Space Observatory and for the Stratospheric Observatory For Infrared Astronomy (SOFIA). These arrays are based on individual Ge:Ga detectors contained in integrating cavities which are fed by an array of light cones to provide for area-filling light collection in the focal plane of an instrument. In order to detect light at wavelengths > 120 μm, uniaxial stress has to be applied to each detector crystal. We have developed a method to efficiently stress an entire stack of detector elements which allows us to form two-dimensional arrays from an arbitrary number of linear detector modules. Each linear module is read out by a cryogenic readout electronics circuit which operates at 4 K and is mechanically integrated into the module. We have measured effective quantum efficiencies of the light cone / detector /read-out chain of > 30% under realistic background conditions. GaAs photoconductive detectors could extend the spectral response cut-off up to > 300 μm. In the past, a continuous progress in material research has led to the production of pure, lightly and heavily doped n-type GaAs layers using the liquid phase epitaxy technique (LPE). Sample detectors demonstrated the expected infrared characteristics of bulk type devices. Modeling of BIB detector types predicts an improved IR sensitivity due to the attainable higher doping of the infrared sensitive layer. However, the modeling gives also an estimate of the severe material requirements for the n-type blocking layer. With a new centrifugal technique for the LPE material growth we intend to achieve this goal. Technical details of this unique equipment, first results of the achieved material quality in the initial growth runs and future steps to optimize operational parameters are reported. If successful, this detector technology will be first implemented in our spectrometer FIFI LS for SOFIA.

Residual conductivity of stressed Ge:Ga photoconductors after low-dose gamma irradiation

Experimental results and a plausible physical model of the γ-induced conductivity changes in stressed Ge:Ga photoconductive detectors are presented. Detector samples with boron-implanted contacts operated under conditions of low-temperatures 1.6−2.3 K and low photon backgrounds 106–108 cm−2 s−1 showed a long-term increase of conductivity and a decrease of the breakdown field after being exposed to 1 MeV γ-quanta for a total dose <4 rad. The radiation-induced changes of the parameters are entirely suppressed by a temperature increase to 4.2 K, while an increase of the electric field above breakdown restores the parameters only partially. The effects are explained in terms of a change of the effective compensation ratio of the samples due to recharging of the donor and acceptor impurities by electrons and holes generated under the γ-irradiation, as well as taking into account that the boron-implanted contacts block minority carriers (electrons) in the semiconductor bulk and prevent their sweepout from the d...

The grating and filter wheels for the JWST NIRSpec instrument

The Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope can be reconfigured in space for astronomical observation in a range of filter bands as well as spectral resolutions. This will be achieved using a Filter wheel (FWA) which carries 7 transmission filters and a Grating wheel (GWA) which carries six gratings and one prism. The large temperature shift between warm launch and cryogenic operation (30K) and high launch vibration loads on the one hand side and accurate positioning capability and minimum deformation of optical components on the other hand side must be consolidated into a single mechanical design which will be achieved using space-proven concepts derived from the successful ISO filter wheel mechanisms which were manufactured and tested by Carl Zeiss. Carl Zeiss Optronics has been selected by Astrium GmbH for the implementation of both NIRSpec wheel mechanisms. Austrian Aerospace and Max-Planck-Institut fur Astronomie Heidelberg (MPIA) will contribute major work shares to the project. The project was started in October 2005 and the preliminary designs have been finalized recently. Critical performance parameters are properly allocated to respective hardware components, procurements of long-lead items have been initiated and breadboard tests have started. This paper presents an overview of the mechanism designs, discusses its properties and the approach for component level tests.

Development of a Si:As blocked impurity band detector for far IR detection

This paper reports on the fabrication and characterization of a linear array of Blocked Impurity Band (BIB) far infrared detectors and of the related Cryogenic Readout Electronics (CRE). It is part of the ESA DARWIN project which aims at the study of exoplanets by means of null interferometry and requires high performance infrared detector arrays in the 6 18μm range. Si:As BIB detectors have been fabricated on an infrared transparent Silicon substrate enabling backside illumination. The buried contact, the active and the blocking layers are deposited by epitaxy; the doping profile is controlled by adjusting the growth parameters. Access to the buried contact is provided by anisotropic silicon etch of V-grooves in the epi layers. Spray coating of photoresist is used for the lithography of the wafers with high topography. The CRE is composed of an input stage based on an integrating amplifier in AC coupled feedback with selectable integrator capacitors, of a sample and hold stage which provides isolation between input and sampling capacitance, and of an output buffer with multiplexing switch. The readout is optimized for low noise with minimum operating temperature of 4K. Linear arrays made of 42 and 88 detectors and having 30μm pixel pitch with various active areas are fabricated. Detector arrays are coupled to the CRE by Indium bumps using flip-chip technology. Measurements on the readout show reduced noise, good linearity and dynamic range. First detector characterization results are presented.

A filter wheel mechanism for the Euclid near-infrared imaging photometer

The Euclid mission is currently being developed within the European Space Agencys Cosmic Vision Program. The five year mission will survey the entire extragalactic sky (~ 20 000 deg2) with the aim of constraining the nature of dark energy and dark matter. The spacecrafts payload consists of two instruments: one imaging instrument, which has both a visible and a near-infrared channel, and one spectroscopic instrument operating in the near-infrared wavelength regime. The two channels of the imaging instrument, the Visible Imaging Channel (VIS) and the Near-Infrared Imaging Photometer Channel (NIP), will focus on the weak lensing science probe. The large survey area and the need to not only image each patch of sky in multiple bands, but also in multiple dithers, requires over 640 000 operations of the NIP channels filter wheel mechanism. With a 127 mm diameter and a mass of ~ 330 g per element, these brittle infrared filters dictate highly demanding requirements on this single-point-failure mechanism. To accommodate the large filters the wheel must have an outer diameter of ~ 400 mm, which will result in significant loads being applied to the bearing assembly during launch. The centrally driven titanium filter wheel will house the infrared filters in specially designed mounts. Both stepper motor and brushless DC drive systems are being considered and tested for this mechanism. This paper presents the design considerations and details the first prototyping campaign of this mechanism. The design and finite element analysis of the filter mounting concept are also presented.

High-precision cryogenic wheel mechanisms of the JWST/MIRI instrument: Performance of the flight models

The Mid Infrared Instrument (MIRI) aboard JWST is equipped with one filter wheel and two dichroic-grating wheel mechanisms to reconfigure the instrument between observing modes such as broad/narrow-band imaging, coronagraphy and low/medium resolution spectroscopy. Key requirements for the three mechanisms with up to 18 optical elements on the wheel include: (1) reliable operation at T = 7 K, (2) high positional accuracy of 4 arcsec, (3) low power dissipation, (4) high vibration capability, (5) functionality at 7 K < T < 300 K and (6) long lifetime (5-10 years). To meet these requirements a space-proven wheel concept consisting of a central MoS2-lubricated integrated ball bearing, a central torque motor for actuation, a ratchet system with monolithic CuBe flexural pivots for precise and powerless positioning and a magnetoresistive position sensor has been implemented. We report here the final performance and lessons-learnt from the successful acceptance test program of the MIRI wheel mechanism flight models. The mechanisms have been meanwhile integrated into the flight model of the MIRI instrument, ready for launch in 2014 by an Ariane 5 rocket.

Cryogenic wheel mechanisms for the Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST): detailed design and test results from the qualification program

The Mid-Infrared Instrument (MIRI) of the James Webb Space Telescope, scheduled for launch in 2013, will provide a variety of observing modes such as broad/narrow-band imaging, coronagraphy and low/medium resolution spectroscopy. One filter wheel and two dichroic-grating wheel mechanisms allow to configure the instrument between the different observing modes and wavelength ranges. The main requirements for the three mechanisms with up to 18 positions on the wheel include: (1) reliable operation at T ~ 7 K, (2) optical precision, (3) low power dissipation, (4) high vibration capability, (5) functionality at 6 K < T < 300 K and (6) long lifetime (5-10 years). To meet these stringent requirement, a space-proven mechanism design based on the European ISO mission and consisting of a central bearing carrying the optical wheels, a central torque motor for wheel actuation, a ratchet system for precise and powerless positioning and a magnetoresistive position sensor has been selected. We present here the detailed design of the flight models and report results from the extensive component qualification.

Preparing Herschel's commissioning phase: Ge:Ga detector tuning

The Photodetector Array Camera and Spectrometer (Pacs) instrument aboard the Herschel space observatory contains an integral field spectrometer with two camera channels which consist of 25 linear arrays of 16 stressed Gallium doped Germanium crystals (Ge:Ga) each. The space radiation environment induces changes in the detector performance. Therefore, testing the Ge:Ga detectors under space radiation environment during the commissioning phase (CP) is important for optimization of later detector operation in orbit. The test program for Ge:Ga detector tuning during this phase has been designed according to findings obtained in laboratory experiments: Protons as well as a 137Cs-γ-source have been used to simulate the space radiation environment and to induce the radiation impacts on the photoconductor arrays. From comparison of the performance of the detectors during CP versus laboratory tests the best strategy for operating the detectors during scientific observations will be derived. This includes annealing, proposals for on-board data reduction algorithms and the best estimated strategy for well-calibrated scientific measurements.

A test setup for the characterization of far-infrared filters under cryogenic conditions

The characterization and calibration of far-infrared (FIR) detectors is a delicate task that requires good knowledge of the incident flux and its spectral composition. In many test setups the FIR flux to the detectors is provided by means of an external or internal black body and a set of cold attenuation, band pass, and blocking filters. For scientific instruments (e.g. PACS aboard ESAs Herschel satellite) band pass and blocking filters are used to achieve the desired spectral throughput either as order sorting filters in spectrometers or for selecting a wavelength range in imaging cameras. In all cases a detailed knowledge of the spectral transmittance of the used filters is mandatory for an accurate calibration of the system. We have build a test platform that allows to measure the transmission of cold (T ~ 4K) filters in the far-infrared. The setup uses a dual grating monochromator with excellent spectral purity and a resolution up to 800, which is operated under a dry nitrogen atmosphere to eliminate water vapor absorption bands. An Si-bolometer is used as detector and is read out by a cryogenic low noise trans-impedance amplifier circuit with common mode rejection and a warm electronics using a lock-in amplifier and a 22 bit analog-to-digital converter. A cryogenic filter slider in the setup allows for differential measurements between filters and the use of cold order sorting filters. We present initial results for FIR cut-on and attenuation filters, demonstrating that our setup is suited to measure transmissions as low as 10-4 over the covered wavelength range.

Cryogenic filter- and spectrometer wheels for the Mid Infrared Instrument (MIRI) of the James Webb Space Telescope (JWST)

Following a warm launch in 2013 the MIRI instrument aboard JWST will be operated for a lifetime of 5-10 years in the L2-orbit at a temperature of ~6 K. The main requirements for its three wheel mechanisms include: (1) reliability, (2) optical precision, (3) low power dissipation, (4) high vibration capability, (5) functionality at 4 < T < 300 K. The filter wheel carries broad and narrow band spectral filters, coronographic masks and a prism on its 18 positions. Each of the two spectrometer wheels is equipped with two disks on both sides of a central torque motor, one of them carries 6 gratings, the other a dichroic/mirror arrangement. The optical positions are defined by a ratchet mechanism. No closed loop control is required; therefore the long time average heat dissipation is negligible. A new ratchet mechanism had to be developed to satisfy a 120° increment of only three positions for the spectrometer wheels. Extensive cold and warm tests were performed on the development models of the filter and spectrometer wheels at MPIA. These results stimulated numerous improvements in the mechanical and thermal design which are now to be implemented in the qualification and flight models developed jointly with Carl Zeiss. Synergies are expected from a similar development of the NIRSPEC wheels, in which MPIA and Carl Zeiss are involved.