Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT
Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT Chapter 5 Robust Performance Tailoring with Tuning - SSL - MIT
the SCI, both the size and flexibility of the structure cause difficulties when creating a testing environment on Earth that is consistent with the operations environment in space. More importantly, a pseudo-star is necessary to inject collimated light across the entire baseline of the interferometer simulating the observation of a star many light years away. A pseudo-star of this type is currently used with the Microarecond Metrology Testbed which is a technology demonstrator for the Space Interferometry Mission [69]. In this case, only one pseudo-star is necessary to test the single baseline interferometer. However, to complete the objectives of the Origins missions a mul- tiple baseline interferometer is necessary. Therefore, one pseudo-star per baseline is required to test all baselines simultaneously. In addition, each pseudo-star must be more sensitive than the interferometer instrument that it is designed to test. Such a SIT is theoretically possible, but is cost-prohibitive. 1.1.4 Missions The Space Interferometry Mission (SIM) is the first interferometer in the suite of Ori- gins missions and is scheduled for launch in 2009 . The primary goal of SIM is planet detection and galaxy mapping through precision astrometric measurements. SIM is also a technology demonstrator for space-based interferometry and hopes to pave the way for future Origins missions. The current SIM design, shown in Figure 1-2(a) [1], is a structurally-connected Michelson interferometer with four individual parallel base- lines. Each baseline is approximately 10 meters long and consists of 35 cm diameter aperture telescopes that collect star light, compress it and direct it through the opti- cal train to the beam combiner. Two of the interferometers are guide interferometers and are pointed directly at guide stars to provide precise inertial reference data. The other two interferometers are science instruments and are used for observing science targets [69]. The Terrestrial Planet Finder (TPF) is a second generation Origins mission sched- uled for launch between 2012-2015. The science goals of the mission include detecting Earth-like planets in a habitable zone around nearby stars, searching for atmospheric signatures of life through spectroscopy and performing high-resolution imaging of 22
(a) (b) Figure 1-2: Artists’ concepts of Origins missions: (a) SIM [1] and (b) TPF [2]. astrophysical targets [2, 70]. The original baseline design for TPF is an infrared sep- arated spacecraft interferometer [13]. However, as the result of a series of industry studies at the time of this writing there are three competing architectures for TPF: an infrared (IR) structurally-connected interferometer, an infrared formation-flown interferometer and a visible light coronograph. Figure 1-2(b) shows an artist’s con- cept of the SCI design proposed by Lockheed Martin. For the purpose of the work in this thesis only the SCI architecture is considered. 1.2 Problem Statement One problem inherent in complex system design arises due to a trade-off that occurs over the development phase between design flexibility and the accuracy of performance predictions. Early in the mission development, models of the proposed design are created to assess system performance. At this stage, design alterations come at a modest cost, but the models used to predict performance are uncertain, resulting in low-confidence predictions. Designing the system to meet requirements across this large uncertainty space may not be possible. In the later phases of the program, flight components and limited integrated test data become available dramatically increasing the accuracy of the performance predictions. However, an unfortunate consequence 23
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the SCI, both the size and flexibility of the structure cause difficulties when creating<br />
a testing environment on Earth that is consistent <strong>with</strong> the operations environment in<br />
space. More importantly, a pseudo-star is necessary to inject collimated light across<br />
the entire baseline of the interferometer simulating the observation of a star many<br />
light years away. A pseudo-star of this type is currently used <strong>with</strong> the Microarecond<br />
Metrology Testbed which is a technology demonstrator for the Space Interferometry<br />
Mission [69]. In this case, only one pseudo-star is necessary to test the single baseline<br />
interferometer. However, to complete the objectives of the Origins missions a mul-<br />
tiple baseline interferometer is necessary. Therefore, one pseudo-star per baseline is<br />
required to test all baselines simultaneously. In addition, each pseudo-star must be<br />
more sensitive than the interferometer instrument that it is designed to test. Such a<br />
SIT is theoretically possible, but is cost-prohibitive.<br />
1.1.4 Missions<br />
The Space Interferometry Mission (SIM) is the first interferometer in the suite of Ori-<br />
gins missions and is scheduled for launch in 2009 . The primary goal of SIM is planet<br />
detection and galaxy mapping through precision astrometric measurements. SIM is<br />
also a technology demonstrator for space-based interferometry and hopes to pave the<br />
way for future Origins missions. The current SIM design, shown in Figure 1-2(a) [1], is<br />
a structurally-connected Michelson interferometer <strong>with</strong> four individual parallel base-<br />
lines. Each baseline is approximately 10 meters long and consists of 35 cm diameter<br />
aperture telescopes that collect star light, compress it and direct it through the opti-<br />
cal train to the beam combiner. Two of the interferometers are guide interferometers<br />
and are pointed directly at guide stars to provide precise inertial reference data. The<br />
other two interferometers are science instruments and are used for observing science<br />
targets [69].<br />
The Terrestrial Planet Finder (TPF) is a second generation Origins mission sched-<br />
uled for launch between 2012-2015. The science goals of the mission include detecting<br />
Earth-like planets in a habitable zone around nearby stars, searching for atmospheric<br />
signatures of life through spectroscopy and performing high-resolution imaging of<br />
22
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