The Northrup-Grummann/Ball/Kodak team is building the James Webb Space Telescope (JWST), scheduled for launch in 2011. Part of Ball’s responsibility is to develop the wavefront sensing and control (WFS&C) algorithms and software that will be used to provide the level of imaging performance needed to support the mission’s science objectives. Wavefront sensing on JWST differs from that performed on many ground-based telescopes in that it is conducted entirely within the focal plane of it’s chief science camera, the Near Infrared Camera (NIRCam). In a sense, the complexity of a conventional wavefront sensor is eliminated, in favor of rather complex image processing performed on the ground, to extract the wavefront information. This paper will describe the algorithms being developed for JWST. Specifically, we will describe algorithms for the coarse alignment of the primary mirror segments and the secondary mirror, the coarse phasing of the primary mirror segments, and the fine phasing of the entire telescope. We will also present algorithms for monitoring the wavefront quality throughout the JWST mission.
The Northrop-Grumman/Ball/Kodak team is building the JWST observatory that will be launched in 2011. To develop the flight wavefront sensing and control (WFS&C) algorithms and software, Ball is designing and building a 1 meter diameter, functionally accurate version of the JWST optical telescope element (OTE). This testbed telescope (TBT) will incorporate the same optical element control capability as the flight OTE. The secondary mirror will be controlled by a 6 degree of freedom (dof) hexapod and each of the 18 segmented primary mirror assemblies will have 6 dof hexapod control as well as radius of curvature adjustment capability. In addition to the highly adjustable primary and secondary mirrors, the TBT will include a rigid tertiary mirror, 2 fold mirrors (to direct light into the TBT) and a very stable supporting structure. The total telescope system configured residual wavefront error will be better than 175 nm RMS double pass. The primary and secondary mirror hexapod assemblies enable 5 nm piston resolution, 0.0014 arcsec tilt resolution, 100 nm translation resolution, and 0.04497 arcsec clocking resolution. The supporting structure (specifically the secondary mirror support structure) is designed to ensure that the primary mirror segments will not change their despace position relative to the secondary mirror (spaced > 1 meter apart) by greater than 500 nm within a one hour period of ambient clean room operation.
KEYWORDS: Actuators, Cryogenics, Aerospace engineering, Nanotechnology, Interferometers, Control systems, Mirrors, Temperature metrology, Photography, Space telescopes
An effort has been in place at Ball Aerospace & Technologies Corp. (BATC) for over three years to develop a mechanism for precise positioning of optical elements for such applications as the Next Generation Space Telescope (NGST). It is desired for such a mechanism to be of low mass, to have nanometer-level positioning capability over a comparatively large range of travel, to be both ambient and cryogenically capable, and to have high strength and stiffness capabilities. The development effort has resulted in a simple 288-gram mechanism that meets these requirements, and does so with a single stepper motor and a simple control system. Performance has been verified at both ambient and cryogenic temperatures, and the mechanism design is currently being implemented on BATC's Advanced Mirror System Demonstrator program (AMSD). The current design achieves steps of less than 10 nanometers per step over more than 20mm of travel. We will present an overview of the capabilities of the mechanism, as well as a discussion of the test results achieved to date. Test results will include both ambient and cryogenic performance, hysteresis and stiffness measurement, as well as verification of single-stepping capability.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.