The Laser Interferometer Space Antenna (LISA), with its extreme distance measurement requirements (pm over arm lengths of 2.5 million km), imposes many stringent requirements on the laser sources used for the distance metrological measurements. In particular, meeting the frequency noise, power stability and side band phase noise requirements reliably for multiple laser systems over the mission lifetime presents a considerable technical challenge. These constraints demand a robust state-of-the-art laser design and a particular attention to reliability and procurement strategy, which all pose a significant challenge. Relying on its strong metrology expertise, CSEM, Swiss Center of Electronics and Microtechnology, is, in the frame of an ESA activity, upgrading all of the metrology techniques and hardware, used to characterize a previously developed non NPRO laser system for the LISA mission. These metrology systems are the baseline for assessing the performance of LISA mission laser heads, developed by NASA. Novel metrology techniques have been developed to assess the challenging laser head specifications. The measurement of frequency stability requires combining different frequency references to cover the full frequency range spanning over more than 10 decades. The measurement of power stability requires combining several metrology approaches to cover the full frequency range and dedicated development, in collaboration with NASA, to improve the long-term measurement capability. As already demonstrated in the previous CSEM activity, sideband phase noise measurement is very sensitive to the environment and complex to perform. A dedicated and improved test setup has been implemented. After a presentation of the NASA laser head, dedicated testing philosophy approaches, encountered technical challenges and obtained test results are presented.
The Laser Interferometer Space Antenna (LISA), with its extreme distance measurement requirements (pm over arm lengths of 2.5 Mio km), imposes many stringent requirements on the laser sources used for interferometry. Frequency and power stability, as well as the side band phase noise represent considerable technological challenges, that must be maintained over the full 12.5 years mission duration. These constraints demand a streamlined laser design and a particular attention to reliability and procurement strategy, which poses a significant challenge. The main requirements for the laser critical sub-system have been analyzed. The Centre Suisse d’Electronique et de Microtechnique (CSEM), in the frame of a European Space Agency activity, was mandated to demonstrate a laser head for the LISA mission based on an alternative laser oscillator approach that does not rely on the LISA-baseline technology (i.e. Nd:YAG NPRO laser). The activity was named MONALISA. After a presentation of the key laser head requirements, the laser head design is described. A comprehensive test campaign was performed, and test results are presented.
A MEMS scanner with a high level of motion freedom has been developed. It includes a 2D mechanical tilting capability of +/- 15°, a piston motion of 50μm and a focus/defocus control system of a 2mm diameter mirror. The tilt and piston motion is achieved with an electromagnetic actuation (moving magnet) and the focus control with a deformation of the reflective surface with pneumatic actuation. This required the fabrication of at least one channel on the compliant membrane and a closed cavity below the mirror surface and connected to an external pressure regulator (vacuum to several bars). The fabrication relies on 3 SOI wafers, 2 for forming the compliant membranes and the integrated channel, and 1 to form the cavity mirror. All wafers were then assembled by fusion bonding. Pneumatic actuation for focus control can be achieved from front or back side; function of packaging concept. A reflective coating can be added at the mirror surface depending of the application. The tilt and piston actuation is achieved by electromagnetic actuation for which a magnet is fixed on the moving part of the MEMS device. Finally the MEMS device is mounted on a ceramic PCB, containing the actuation micro-coils. Concept, fabrication, and testing of the devices will be presented. A case study for application in an endoscope with an integrated high power laser and a MEMS steering mechanism will be presented.
The Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) achieved first light on the W. M. Keck Observatory’s Keck I telescope on 4 April 2012 and quickly became the most popular Keck I instrument. One of the primary reasons for the instrument’s popularity is that it uses a configurable slitmask unit developed by the Centre Suisse d’Electronique et Microtechnique (CSEM SA) to isolate the light from up to 46 objects simultaneously. In collaboration with the instrument development team and CSEM engineers, the Keck observatory staff present how MOSFIRE is successfully used, and we identify what contributed to routine and trouble free nighttime operations.
KEYWORDS: Mirrors, Data modeling, Actuators, Linear filtering, Device simulation, Telescopes, System identification, Space telescopes, Observatories, Aerodynamics
The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope build into a Boeing
747SP. During observations the telescope will not only be subject to aircraft vibrations and maneuver loads - by
opening a large door to give the observatory an unhindered view of the sky, there will also be aerodynamic and
aeroacoustic disturbances. A critical factor in the overall telescope performance is the SOFIA Secondary Mirror
Assembly. The 35cm silicon carbide mirror is mounted on the Secondary Mirror Mechanism, which has five
degrees-of-freedom (rotation about line of sight is blocked) and consists of two parts: The slow moving base for
focusing and centering, and on top of that the Tilt Chop Mechanism (TCM) for chopping with a frequency of up
to 20Hz and a chop throw of up to 10arcmin. A new controller for the TCM is introduced in this paper in order
to meet the stringent performance requirements for the chopper. A state space controller is chosen that combines
a feedback path for steady state behavior with a model-based feed forward controller for improved settling time
performance. The paper explains the modeling of the TCM via a grey box model approach optimized with system
identification data and compares simulated with measured data. Then the structure of the controller is explained
and Matlab/Simulink simulations are presented. The simulation results are compared to measurements taken
with the real system on ground and finally flight test results with open and closed door are discussed.
A Configurable Slit Unit (CSU) has been developed for the Multi-Object Spectrometer for Infra-Red Exploration
(MOSFIRE) instrument to be installed on the Keck 1 Telescope on Mauna Kea, Hawaii. MOSFIRE will provide NIR
multi-object spectroscopy over a field of view of 6.1' x 6.1'. The reconfigurable mask allows the formation of 46 optical
slits in a 267 x 267 mm2 field of view. The mechanism is an evolution of a former prototype designed by CSEM and
qualified for the European Space Agency (ESA) as a candidate for the slit mask on NIRSpec for the James Webb Space
Telescope (JWST). The CSU is designed to simultaneously displace masking bars across the field-of-view (FOV) to
mask unwanted light. A set of 46 bar pairs are used to form the MOSFIRE focal plane mask. The sides of the bars are
convoluted so that light is prevented from passing between adjacent bars. The slit length is fixed (5.1 mm) but the width
is variable down to 200 μm with a slit positioning accuracy of ± 18 μm. A two-bar prototype mechanism was designed,
manufactured and cryogenically tested to validate the modifications from the JWST prototype. The working principle of
the mechanism is based on an improved "inch-worm" stepping motion of 92 masking bars forming the optical mask.
Original voice coil actuators are used to drive the various clutches. The design makes significant use of flexure
structures.
A mechanical slit mask mechanism has been designed for the Near Infrared Spectrograph of the James Webb Space Telescope. This mechanism was successfully tested at a cryogenic temperature of 30K, in vacuum. The reconfigurable mask allows to form 24 optical slits in a 137 x 137 mm2 field of view. The slit length is fixed (4.8 mm) and their width can range from 50 μm to 137 mm. The slit positioning accuracy is ± 5 μm and the slit width accuracy is ± 8 μm. The working principle of the mechanism is based on an improved "inch-worm" stepping motion of 48 masking bars forming the optical curtain. Voice coil actuators are used to drive the various clutches and the principal mobile stage. Ratchets which engage in the teeth of a rack machined on the bars allow to cancel the accumulation of motion errors as steps succeed one another. The design makes significant use flexure structures. Cryogenic performance, life and vibration tests have been performed successfully on subassemblies of the mechanism and a full-scale prototype.
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