Ralph Hofferbert

The Mid-Infrared Instrument for the James Webb Space Telescope, II: Design and Build

The Mid-InfraRed Instrument (MIRI) on the James Webb Space Telescope (JWST) provides measurements over the wavelength range 5 to 28.5 μm. MIRI has, within a single ‘package’, four key scientific functions: photometric imaging, coronagraphy, single-source low-spectral resolving power (R ∼ 100) spectroscopy, and medium-resolving power (R ∼ 1500 to 3500) integral field spectroscopy. An associated cooler system maintains MIRI at its operating temperature of <6.7 K. This paper describes the driving principles behind the design of MIRI, the primary design parameters, and their realisation in terms of the ‘as-built’ instrument. It also describes the test programme that led to delivery of the tested and calibrated Flight Model to NASA in 2012, and the confirmation after delivery of the key interface requirements.

MICADO: first light imager for the E-ELT

MICADO will equip the E-ELT with a first light capability for diffraction limited imaging at near-infrared wavelengths. The instrument’s observing modes focus on various flavours of imaging, including astrometric, high contrast, and time resolved. There is also a single object spectroscopic mode optimised for wavelength coverage at moderately high resolution. This contribution provides an overview of the key functionality of the instrument, outlining the scientific rationale for its observing modes. The interface between MICADO and the adaptive optics system MAORY that feeds it is summarised. The design of the instrument is discussed, focusing on the optics and mechanisms inside the cryostat, together with a brief overview of the other key sub-systems.MICADO will be the first-light wide-field imager for the European Extremely Large Telescope (E-ELT) and will provide difiraction limited imaging (7mas at 1.2mm) over a ~53 arcsecond field of view. In order to support various consortium activities we have developed a first version of SimCADO: an instrument simulator for MICADO. SimCADO uses the results of the detailed simulation efforts conducted for each of the separate consortium-internal work packages in order to generate a model of the optical path from source to detector readout. SimCADO is thus a tool to provide scientific context to both the science and instrument development teams who are ultimately responsible for the final design and future capabilities of the MICADO instrument. Here we present an overview of the inner workings of SimCADO and outline our plan for its further development.

Development approach and first infrared test results of JWST/Mid Infra Red Imager Optical Bench

The present paper describes the different steps leading to the Flight Model integration of the Mid-Infra Red IMager Optical Bench MIRIM-OB which is part of the scientific payload of the JWST. In order to demonstrate a space instrument capability to survive the challenging space environment and deliver the expected scientific data, a specific development approach is applied in order to reduce the high level of risks. The global approach for MIRIM-OB, and the principal results associated to the two main models, the Structural Qualification Model for vibration and the Engineering and Test Model for optical performance measured in the infra red at cryogenic temperature will be described in this paper.

LINC-NIRVANA Pathfinder: testing the next generation of wave front sensors at LBT

LINC-NIRVANA will employ four wave front sensors to realize multi-conjugate correction on both arms of a Fizeau interferometer for LBT. Of these, one of the two ground-layer wave front sensors, together with its infrared test camera, comprise a stand-alone test platform for LINC-NIRVANA. Pathfinder is a testbed for full LINC-NIRVANA intended to identify potential interface problems early in the game, thus reducing both technical, and schedule, risk. Pathfinder will combine light from multiple guide stars, with a pyramid sensor dedicated to each star, to achieve ground-layer AO correction via an adaptive secondary: the 672-actuator thin shell at the LBT. The ability to achieve sky coverage by optically coadding light from multiple stars has been previously demonstrated; and the ability to achieve correction with an adaptive secondary has also been previously demonstrated. Pathfinder will be the first system at LBT to combine both of these capabilities. Since reporting our progress at A04ELT2, we have advanced the project in three key areas: definition of specific goals for Pathfinder tests at LBT, more detail in the software design and planning, and calibration. We report on our progress and future plans in these three areas, and on the project overall.

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.

Qualification status of the stressed photoconductor arrays for the PACS instrument aboard Herschel

The photoconductor detector arrays for the PACS instrument (Photoconductor Array Camera and Spectrometer) aboard the future ESA telescope Herschel have been developed during the engineering phase in 1999. In early 2000 the construction of the qualification models began for both, the highly and low stressed Ge:Ga arrays, which consist of 12 linear modules each. These two types of photoconductor arrays are dedicated for different wavelengths bands in the spectrometer section of the instrument. While the performance of a few engineering arrays has been studied and presented earlier, additional data are meanwhile available on the absolute responsivity and quantum efficiency of the detectors. Furthermore, experience has been obtained during manufacture of a larger series of arrays giving better statistics on performance aspects, such as uniformity of the cutoff wavelengths and of the responsivity or the maximum stress obtainable within such arrays. Considerable progress has also been made in the development and manufacture of the 4 Kelvin Cold Read-out Electronics (CRE), which will integrate and multiplex the signals generated in each linear array with its 16 detector pixels. Manufacture of the detector arrays for the qualification model is scheduled to be completed by this summer, and manufacture of the flight model has already started. The qualification model will be delivered to the test facilities, where absolute spectral performance of the 24 linear modules will be determined. In this paper we give a summary of the related activities and results as obtained during manufacturing and testing.

Lessons learnt and implemented: from ISO- to HERSCHEL- and JWST-instrumentation

Cost and schedule risks in space projects can be minimized by applying technical concepts successfully flown previously to the subsequent missions. Examples from the development of three cold infrared instruments are discussed: ISOPHOT on ISO, PACS on HERSCHEL and MIRI on JWST. The progress achieved over two decades in the development of opto-mechanical elements like cold focal plane chopper and cold filter wheels will be demonstrated. While the instruments became larger and more complex, eventually the development consortia grew up to 20 institutes involved per instrument, requiring increasing resources for interface handling and coordination activities.

Cryomechanisms for positioning the optical components of the mid-infrared instrument (MIRI) for NGST

Mechanisms operating in the cryovacuum are required to rotate filter and dichroic wheels, to tilt gratings and to flip in the beam of an internal calibration source. The design proposed here is based on similar mechanisms flown successfully on the liquid helium cooled European ISO-satellite and being presently under qualification for ESAs cooled HERSCHEL-satellite. Their main characteristics are high reliability during the 10 year lifetime in space, high precision and low heat dissipation in the cryovacuum.

Cryomechanisms for the instruments MIRI and NIRSpec on the James Webb Space Telescope (JWST)

The Mid-Infrared Instrument (MIRI) and the Near-Infrared Spectrograph (NIRSpec) of the JWST require various mechanisms for positioning optical elements in cryo-vacuum environment (7K resp. 35K): Wheels for exchanging filters, gratings and prisms, a flip mirror for switching between the sky and internal calibration sources and a linear actuator for refocusing purposes will have to be developed. In order to fulfill the stringent requirements of the mission, comprising to survive a warm ARIANE 5 launch, to guarantee high accuracy positioning in the cryovacuum with minimal power dissipation, to be operational with high reliability during 10 years of lifetime and to be testable under various environmental conditions, we propose a low cost and low schedule risk approach, based on the successful flight experience and qualification heritage from ESA’s infrared missions ISO and HERSCHEL.

The MICADO first light imager for ELT: derotator design status and prototype results

MICADO is the Multi-AO Imaging Camera for Deep Observations, a first light instrument for the Extremely Large Telescope (ELT). It will provide the ELT with diffraction limited imaging capacity over a ~53-arcsec field of view, while operating with the Multi-Conjugate Adaptive Optics (MCAO) module MAORY (0.8-2.5 μm). Here, we present the design status of the MICADO derotator, which at the same time serves (i) as crucial mechanical interface between the cryo-opto-mechanical camera assembly and the instrument support structure and (ii) as high-precision image and wavefront sensor derotator to allow for 50 µas astrometry over the entire MCAO corrected field. Additionally, first test results are presented which were obtained with a derotator prototype based on a scaled 1:2 test bearing. The derotator test stand is essential to explore the limitations of the preferred bearing type in the context of the given requirements. The technical difficulties addressed by the design include: (i) design of adequate mechanical interfaces to minimize mass, deformation and the effect of the warping moment on the bearing and (ii) analysis of the friction-related stick-slip effects at low tracking velocities for the implementation of a suitable position-velocity closed-loop control system. Furthermore, our prototype setup is used to develop and test the required control concept of this high-precision application.