Optical systems, which operate over a wide range of Fresnel numbers, are often times performance-limited by diffraction effects. In order to characterize such effects at the 40-100 picometer level, a diffraction testbed has been built which has the capability of measuring diffraction effects at this level. Concurrently, mathematical diffraction modeling tools have been developed that propagate an input wavefront through an optical train, while retaining amplitude and phase information at a grid resolution sufficient for yielding picometer-resolution diffraction test data. This paper contains a description of this diffraction hardware testbed, the diffraction modeling approach, and a comparison of the modeled and hardware test results, which then serves as validation of the diffraction modeling methodology.
The Space Interferometry Mission (SIM), planned for launch in 2009, will measure the positions of celestial objects to an unprecedented accuracy of 4.0 microarcseconds. In order to achieve this accuracy, which represents an improvement of almost two orders of magnitude over previous astrometric measurements, a ten-meter baseline interferometer will be flown in space. NASA challenges JPL and its industrial partners, Lockheed Martin and TRW, to develop an affordable mission. This challenge will be met using a combination of existing designs and new technology. Performance and affordability must be balanced with a cost-conscious Systems Engineering approach to design and implementation trades. This paper focuses on the Lockheed Martin-led Starlight (STL) and Metrology (MET) subsystems within the main instrument of SIM. Starlight is collected by 35cm diameter telescopes to form fringes on detectors. To achieve the stated accuracy, the position of these white-light fringes must be measured to 10-9 of a wavelength of visible light. The STL Subsystem consists of siderostats, telescopes, fast steering mirrors, roof mirrors, optical delay lines and beam combiners. The MET Subsystem is used to measure very precisely the locations of the siderostats with respect to one another as well as to measure the distance traveled by starlight from the siderostat mirrors and reference corner cubes through the system to a point very close to the detectors inside the beam combiners. The MET subsystem consists of beam launchers, double and triple corner cubes, and a laser distribution system.
KEYWORDS: Space operations, Control systems, Sensors, Mathematical modeling, Optical components, Systems modeling, Motion models, Device simulation, Control systems design, Space telescopes
This paper contains a presentation of the techniques needed to develop simulations for large scale systems which are composed of control systems, optics, and structures. A case study is presented which consists of a generic, controlled specraft whose purpose is to maintain a precise line-of-sight (LOS) in the presence of disturbances and structural deformations. The spacecraft model consists of a six degree-of-freedom (DOF) rigid body with superimposed small structural deformaüons obtained from high-order finite element model. The optical system consists of a telescope with a controlled Fast Steering Mirror (FSM) for maintaining an accurate line-of-sight. Reaction wheels and an Inertial Measurement Unit (IMU) are used for spacecraft attitude control. Disturbances consist of onboard payload motions, sensor noises and reation wheel imbalance. Results of the simulation are shown.
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