Point-to-point laser metrology systems can be used to stabilize large structures at the nanometer levels required for
precision optical systems. Existing sensors are large and intrusive, however, with optical heads that consist of several
optical elements and require multiple optical fiber connections. The use of point-to-point laser metrology has therefore
been limited to applications where only a few gauges are needed and there is sufficient space to accommodate them.
Range-Gated Metrology is a signal processing technique that preserves nanometer-level or better performance while
enabling: (1) a greatly simplified optical head - a single fiber optic collimator - that can be made very compact, and (2) a
single optical fiber connection that is readily multiplexed. This combination of features means that it will be
straightforward and cost-effective to embed tens or hundreds of compact metrology gauges to stabilize a large structure.
In this paper we describe the concept behind Range-Gated Metrology, demonstrate the performance in a laboratory
environment, and give examples of how such a sensor system might be deployed.
This paper reviews recent progress with technology being developed for the Terrestrial Planet Finder Interferometer (TPF-I). TPF-I is a mid-infrared space interferometer being designed with the capability of detecting Earth-like planets in the habitable zones around nearby stars. TPF-I is in the early phase of its development. The science requirements of the mission are described along with the current design of the interferometer. The goals of the nulling and formation-flying testbeds are reviewed. Progress with TPF-I technology milestones are highlighted.
The Terrestrial Planet Finder Interferometer (TPF-I) is a space-based NASA mission for the direct detection of Earth-like planets orbiting nearby stars. At the mid-infrared wavelength range of interest, a sun-like star is ~107 times brighter than an earth-like planet, with an angular offset of ~50 mas. A set of formation-flying collector telescopes direct the incoming light to a common location where the beams are combined and detected. The relative locations of the collecting apertures, the way that the beams are routed to the combiner, and the relative amplitudes and phases with which they are combined constitute the architecture of the system. This paper evaluates six of the most promising solutions: the Linear Dual Chopped Bracewell (DCB), X-Array, Diamond DCB, Z-Array, Linear-3 and Triangle architectures.
Each architecture is constrained to fit inside the shroud of a Delta IV Heavy launch vehicle using a parametric model for mass and volume. Both single and dual launch options are considered. The maximum separation between spacecraft is limited by stray light considerations. Given these constraints, the performance of each architecture is assessed by modeling the number of stars that can be surveyed and characterized spectroscopically during the mission lifetime, and by modeling the imaging properties of the configuration and the robustness to failures. The cost and risk for each architecture depends on a number of factors, including the number of launches, and mass margin. Quantitative metrics are used where possible.
A matrix of the architectures and ~30 weighted discriminators was formed. Each architecture was assigned a score for each discriminator. Then the scores were multiplied by the weights and summed to give a total score for each architecture. The X-Array and Linear DCB were judged to be the strongest candidates. The simplicity of the three-collector architectures was not rated to be sufficient to compensate for their reduced performance and increased risk. The decision process is subjective, but transparent and easily adapted to accommodate new architectures and differing priorities.
The Terrestrial Planet Finder interferometer design concepts are large
and complex systems that must operate in environments that are impractical to reproduce in preflight testing. The structurally- connected design is 36 meters long - longer than all but one thermal vacuum chamber in existence. The formation flying design will be comprised of up to five separate spacecraft, each with a sunshield over 15 meters on a side, and is designed to operate with formation sizes spanning 60-100 meters. System-level verification of the performance of the designs will rely on analytical modeling. The effort to model the many physical aspects of the designs under study
is under way.
This paper describes the program of modeling for the TPF-I concepts.
The program includes a number of types of models, such as the standard
stand-alone optics, thermal, and structural models, as well as an end-to-end performance model of the project system called the Observatory Simulation. Aspects of each model are discussed including the purpose, methods of implementation (software applications), and approaches to validation. Program-level considerations (such as model-to-model integration and configuration management) are also discussed. Given that there are at least seven different organizations contributing to model developments and more than twenty separate models, these are special challenges.
The TPF interferometer family suppresses the stellar glare using a deep interferometric null, which for the planet can become constructive interference because of its angular offset. The null depth need not be as great as the star-planet contrast, but its systematic fluctuations must be perhaps 5 times better than the variations which constitute the planet's signature. We present an allocation of errors which meet these needs, and identify areas which need better definition.