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
1.1 Space-Based Interferometry NASA’s Origins program is an on-going effort aimed at exploring both the nature of the universe and its inhabitants. The specific goals of the program are to answer questions about the origin of our universe and to search for other Earth-like plan- ets in nearby solar systems. The program consists of a family of missions that span over twenty years (Figure 1-1). The earliest telescopes, including the Hubble Space Telescope (HST), are in operation and provide valuable information to scientists and astronomers at the time of this writing. Upcoming missions include SIM, JWST, and TPF (interferometer and coronograph). All three of these missions have ambi- tious science goals that push the boundaries of engineering across many disciplines, including optics, structures and control. Figure 1-1: Timeline of Origins missions [3]. 1.1.1 Astrometry and Imaging In order to achieve the first of Origins’ goals, mapping the universe and exploring its origins, a telescope with very high angular resolution is necessary. Angular resolution 18
is an optical metric that describes the accuracy of an optical system. It is defined as the minimum resolvable angular separation between two objects [54]. Consider two objects located far from a telescope, but very close to each other. A low-angular resolution telescope is unable to resolve the two objects, and they appear to an observer as a single blurred object. A telescope with high angular resolution, however, presents an image of two distinct light sources. One challenge faced in the design of these telescopes is the limitations imposed by a trade-off between system mass and optical performance. In the case of a monolithic telescope, such as HST, the angular resolution of the system scales proportionally with the diameter of the primary mirror. However, as the primary mirror becomes larger it also becomes more massive and more expensive to manufacture and launch. Therefore mass and cost budgets limit the angular resolution that can be achieved with a monolithic system. One alternative to the monolithic telescope design that has generated much inter- est over the past few decades is astronomical interferometers [105, 8]. These instru- ments provide high resolution imaging and astrometry at a significant savings of mass and volume compared to a monolithic design. Interferometers function by combining the light gathered by two or more smaller apertures separated by a large baseline. The angular resolution of an interferometer increases proportionally to the length of this baseline. Therefore, it is not necessary to manufacture and launch very large mirrors. Instead, it is only necessary to place collecting mirrors sufficiently far away from each other to achieve the desired performance. A number of ground-based stellar interferometers are currently in operation. These include the Keck Observatory [29] on Mauna Kea in Hawaii, the Sydney University Stellar Interferometer [33, 34] (SUSI) in Australia, and the Navy Prototype Optical Interferometer [9] (NPOI) in Flagstaff, Arizona. These systems have already provided valuable astronomical data [58, 59, 30, 38, 77, 32] such as stellar images, astromet- ric measurements of stellar positions and stellar angular diameters. However, the optical performance of these ground interferometers is limited by atmospheric distor- tions. The next logical step is to place these systems in space where they can operate 19
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is an optical metric that describes the accuracy of an optical system. It is defined as<br />
the minimum resolvable angular separation between two objects [54]. Consider two<br />
objects located far from a telescope, but very close to each other. A low-angular<br />
resolution telescope is unable to resolve the two objects, and they appear to an<br />
observer as a single blurred object. A telescope <strong>with</strong> high angular resolution, however,<br />
presents an image of two distinct light sources.<br />
One challenge faced in the design of these telescopes is the limitations imposed by<br />
a trade-off between system mass and optical performance. In the case of a monolithic<br />
telescope, such as HST, the angular resolution of the system scales proportionally<br />
<strong>with</strong> the diameter of the primary mirror. However, as the primary mirror becomes<br />
larger it also becomes more massive and more expensive to manufacture and launch.<br />
Therefore mass and cost budgets limit the angular resolution that can be achieved<br />
<strong>with</strong> a monolithic system.<br />
One alternative to the monolithic telescope design that has generated much inter-<br />
est over the past few decades is astronomical interferometers [105, 8]. These instru-<br />
ments provide high resolution imaging and astrometry at a significant savings of mass<br />
and volume compared to a monolithic design. Interferometers function by combining<br />
the light gathered by two or more smaller apertures separated by a large baseline.<br />
The angular resolution of an interferometer increases proportionally to the length of<br />
this baseline. Therefore, it is not necessary to manufacture and launch very large<br />
mirrors. Instead, it is only necessary to place collecting mirrors sufficiently far away<br />
from each other to achieve the desired performance.<br />
A number of ground-based stellar interferometers are currently in operation. These<br />
include the Keck Observatory [29] on Mauna Kea in Hawaii, the Sydney University<br />
Stellar Interferometer [33, 34] (SUSI) in Australia, and the Navy Prototype Optical<br />
Interferometer [9] (NPOI) in Flagstaff, Arizona. These systems have already provided<br />
valuable astronomical data [58, 59, 30, 38, 77, 32] such as stellar images, astromet-<br />
ric measurements of stellar positions and stellar angular diameters. However, the<br />
optical performance of these ground interferometers is limited by atmospheric distor-<br />
tions. The next logical step is to place these systems in space where they can operate<br />
19
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