In 2011 NASA and ESA plan to launch the James Webb Space Telescope (JWST) as dignified successor of the Hubble Space Telescope. Three scientific instruments will cover the wavelength regions in the near-infrared (0.6-5μm, NIRCam and NIRSpec) and in the mid-infrared (5-28μm, MIRI), respectively. The ESA-led multi-object spectrograph NIRSpec as major European contribution is presently entering the detailed design phase in a collaboration between European space industries, scientific institutes, ESA and NASA. To allow for various operational modes in the instrument’s optical train several cryo-mechanisms are required, i.e. wheels for exchanging optical elements like filters and gratings as well as linear actuators on refocusing mirrors. We will give an overview on the detailed design, the prototyping and the testing of those mechanisms comprising highest reliability in the cryo-vacuum (~ 35K) combined with minimal power dissipation (~ 5mW on average), ultimate position accuracy (~ 0.5 - 1arcsec) combined with high launch vibration capability (ARIANE 5, ~ 60g) and a very long lifetime (~ 15 years) for ground tests and space operation under various environmental conditions. To reach this goal in a low cost and risk approach we rely on the heritage from ESA's earlier infrared missions, i.e. ISO and HERSCHEL.
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 goal of the Extra Large Telescope Actuator (ELTA) development project was to demonstrate operation of a relatively high stiffness, single stage optical positioning actuator capable of achieving diffraction-limited performance (<10 nm) in the visible optical band for weeks at a time while consuming no electrical power and dissipating no heat.
The design challenge was to develop a linear positioning mechanism exhibiting high stiffness, low power, zero backlash, and thermal stability over extended time periods. The key to achieving high resolution, and stability with low power is to eliminate the closed-loop control system that is normally employed to overcome the nonlinearities and hysteresis inherent in some technologies, such as piezoelectric and magnetostrictive transducers. This was accomplished by using the patented elastic transducer developed by Alson E. Hatheway (AEH Inc.) This device consists of two elastic elements; a soft spring and a stiff flexural member. Deflection of the soft spring applies a force input to the stiff flexure, which responds with a proportionally reduced output deflection. To maintain linearity, the displacements, and hence the stresses, developed in both elastic members are kept below the micro-yield strength of the material. The AEH transducer is inherently linear and hysteresis free. The unique design features of this actuator which contribute to its extremely precise motion capability include an electric motor driving a leadscrew through a zero backlash harmonic drive gear reduction. The already fine incremental motion of the leadscrew nut is further attenuated by the elastic action of the AEH transducer, to provide output motion with resolution <10 nm.
The Ultra-Precision Linear Actuator presented in this paper was developed for the Next Generation Space Telescopes' (NGST) primary mirror surface figure control. The development was a joint effort between Alson E. Hatheway, Inc (AEH) and Moog, Schaeffer Magnetics Division (SMD). The goal of this project was to demonstrate an extremely light weight, relatively high stiffness actuator capable of operating uniformly well over the range of 20°K to 300°K and achieving diffraction-limited performance (±10 nm) in the optical band for weeks at a time, while consuming no electrical power and dissipating no heat. The essence of the design challenge was to develop a lightweight, high stiffness, low power, thermally stable linear positioning mechanism. Actuation systems with resolutions comparable to that of this design normally are operated in a closed-loop control system to compensate for any non-linearities and hysteresis inherent in their enabling technologies, such as piezoelectric and magnetostrictive transducers. These technologies require continuous application of power and therefore are not low power consumption devices. The development challenge was met through the use of Alson E. Hatheway's (AEH) patented Rubicontm elastic transducer which consists of two elastic elements; a soft spring and a stiff flexural member. Deflection of the soft spring applies a force input to the stiff flexure, which responds with a proportionally reduced output deflection. To maintain linearity, the displacements, and hence the stresses, developed in both elastic members are kept well below the elastic yield strength of the material. The AEH transducer is inherently linear and hysteresis free.