|
Quick Technical Facts
Instrument Overview
SUMMARY
- The SIM Lite Astrometric Observatory is a 6-meter baseline optical Michelson interferometer.
- The observatory uses a 4.2 meter guide interferometer and a 30 cm guide telescope.
- Two multi-arm laser metrology systems permit the high precision necessary for microarcsecond measurement accuracy.
- The spacecraft carrying the instrument is 3-axis stabilized and suitable for SIM’s Earth-trailing solar orbit.
- Details of the design and in-flight operation of SIM Lite can be found in Chapter 20 and Chapter 17, respectively, of the SIM Book.
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 bulk of the flight segment consists of a single instrument. 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 Support Structure (PSS) and controlled by the Real-Time Control (RTC) subsystem.
One of the two Michelson stellar interferometers has a baseline of 6 meters between 50 cm siderostats and serves as the science interferometer. The other interferometer has a somewhat shorter baseline (4.2 m, 30 cm siderostats) and serves as the Guide-1 interferometer co-bore sighted with the center of the science interferometer field of regard (FOR). A 30 cm Guide-2 Telescope is oriented at 90° to the plane defined by the look direction of the Science and Guide-1 interferometers and the science baseline and is contained on the same optical bench as one end of the Guide-1 interferometer.
The guides essentially serve as a microarcsecond star tracker which is used to stabilize the science interferometer during long, dim-star integrations. The guides track relatively 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 Support Structure (PSS) serves as both an optical bench and provider of structural support during launch. It is a composite truss structure. 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). Instrument electronics are mounted on the opposite side of the PSS from the spacecraft bus.
Details of the design and in-flight operation of SIM Lite can be found in Chapter 20 and Chapter 17, respectively, of the SIM Book.
Spacecraft
 Flight segment configuration SIM Lite will launch on an intermediate class launch vehicle, Atlas V 521 or similar, from the Kennedy Space Center, Cape Canaveral, Florida, USA. Selection of the particular vehicle will be made at the start of full-scale development.
|
