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Instrument Overview
SIM Lite will be the first space-based long baseline Michelson interferometer designed for precision astrometry.
As a pathfinder in the development of space-based interferometry -- a technology development is important to NASA's strategic planning -- it will open the frontiers of telescopic resolving power beyond the sizes of single apertures that can be launched. It will lead the way for future space interferometers that operate at wavelengths from infrared to X-rays, and for instruments being studied for several future missions.
In 2005, SIM Lite completed the final gate in an ambitious 10-year technology program. Results were reviewed and approved by both internal and external review boards.
Flight and mission system
 SIM Lite instrument optical configuration The SIM Lite instrument consists of two visible-wavelength Michelson (pupil plane) stellar interferometer sensors (each like Figure 2), one 30 cm Guide telescope, and one external metrology sensor, all supported by a Precision Structure subsystem (PSS) and controlled by the Real-Time Control (RTC) subsystem.
One of the two Michelson stellar interferometers has a baseline of 6 meters and serves as the science interferometer. The other interferometer has a somewhat shorter baseline (4.2 m) and serves as the Guide-1 interferometer co-bore sighted with the center of the science interferometer field of regard (FOR). A Guide-2 Telescope is oriented at 90° to the look direction of the science and Guide-1 interferometers and is contained on the same optical bench as one end of the Guide-1 interferometer. The Guides track relative bright stars, accurately estimating the interferometric baseline orientation change in inertial space and generating stabilizing feed forward control information for the science interferometer (to enable making science measurements on dim stars down to visual magnitude 19 or 20 in brightness).
The External Metrology sensor is a laser truss that measures the baseline length and relative orientation of the interferometers. The truss consists of ten laser beams launched by 10 metrology gauges that are each capable of measuring the distance between optical corner cube fiducials to an accuracy of a few picometers. This truss is used in non-real-time to reconstruct on the ground where the optical elements were located relative to each other as a function of time, correlated to science measurements.
The Precision Structure Subsystem (PSS) is a composite truss structure that supports the components of the interferometer. Instrument optical elements (collector telescopes and astrometric beam combiners) are connected to the PSS via support struts. Thermal radiators are mounted on the top surface of the PSS that is always facing away from the sun (the top surface of the PSS is a sun exclusion zone).
The instrument electronics and metrology laser source are mounted in two enclosures located on each side of the PSS.
Spacecraft
 Flight segment configuration SIM Lite will launch on intermediate class launch vehicle similar to an Atlas V 531 from the Kennedy space center, Cape Canaveral, Florida, USA. Selection of the particular vehicle will be made at the start of full-scale development.
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