Andrew Kissil

Wavefront sensing and control for a Next-Generation Space Telescope

The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.

Fine pointing control for a Next-Generation Space Telescope

The Next Generation Space Telescope will provide at least ten times the collecting area of the Hubble Space Telescope in a package that fits into the shroud of an expendable launch vehicle. The resulting large, flexible structure provides a challenge to the design of a pointing control system for which the requirements are at the milli-arcsecond level. This paper describes a design concept in which pointing stability is achieved by means of a nested-loop design involving an inertial attitude control system (ACS) and a fast steering mirror (FSM). A key to the integrated control design is that the ACS controllers has a bandwidth well below known structural modes and the FSM uses a rotationally balanced mechanism which should not interact with the flexible modes that are within its control bandwidth. The ACS controller provides stable pointing of the spacecraft bus with star trackers and gyros. This low bandwidth loop uses nearly co-located sensors and actuators to slew and acquire faint guide stars in the NIR camera. This controller provides a payload reference stable to the arcsecond level. Low-frequency pointing errors due to sensor noise and dynamic disturbances are suppressed by a 2-axis gimbaled FSM locate din the instrument module. The FSM servo bandwidth of 6 Hz is intended to keep the guide star position stable in the NIR focal plane to the required milli-arcsecond level. The mirror is kept centered in its range of travel by a low-bandwidth loop closed around the ACS. This paper presents the result of parametric trade studies designed to assess the performance of this control design in the presence of modeled reaction wheel disturbances, assumed to be the principle source of vibration for the NGST, and variations in structural dynamics. Additionally, requirements for reaction wheel disturbance levels and potential vibration isolation subsystems were developed.

Exo-C: a Probe-Scale Space Mission to Directly Image and Spectroscopically Characterize Exoplanetary Systems Using an Internal Coronagraph

“Exo-C” is NASA’s first community study of a modest aperture space telescope designed for high contrast observations of exoplanetary systems. The mission will be capable of taking optical spectra of nearby exoplanets in reflected light, discover previously undetected planets, and imaging structure in a large sample of circumstellar disks. It will obtain unique science results on planets down to super-Earth sizes and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable exoplanets. We present the mission/payload design and highlight steps to reduce mission cost/risk relative to previous mission concepts. At the study conclusion in 2015, NASA will evaluate it for potential development at the end of this decade.

Integrated Modeling Applied to the Terrestrial Planet Finder Mission

Integrated Modeling is currently being used to assess the feasibility of a baseline design concept (pre-phase A), developed for the Coronagraph version of the Terrestrial Planet Finder (TPF) mission. This design concept incorporates many challenging design elements for a space-born observatory: including a monolithic 8 by 3.5 meter elliptical primary mirror; a 12 meter long deployable secondary mirror support structure; as well as a 14 meter long deployable, tensioned-membrane, V-groove sunshield. Unprecedented thermal and dynamic stability is required by this flight system to allow observation of enough contrast between planets and their parent stars. This stringent performance requirement necessitates a balanced system, designed to optimize the various interacting disciplines: optical, thermal, structural & control. To support design feasibility studies, a MATLAB-environment-based integrated modeling tool (IMOS: Integrated Modeling of Optical Systems) was employed for analyzing the end-to-end system performance for typical in-orbit maneuvers. Our integrated modeling goal is to use a single model definition file to specify the thermal, structural, and optical modeling and analysis parameters, improving results accuracy, configuration control and data management. In working towards that goal, we have had parallel efforts in IMOS capability development, as well as design concept modeling and analysis. Typical system performance metrics studied include the relative motions of the optical elements, as well as the deformation of individual optics, decomposed into best-fitting Zernike polynomials.

Structural Modeling for the Terrestrial Planet Finder Mission

The Coronagraph version of the Terrestrial Planet Finder (TPF) mission relies on a large-optics, space-born observatory, which requires extreme stability of the optics in the presence of thermal and dynamic disturbances. The structural design requires balancing of stringent constraints on launch packaging with unusually tight response requirements for thermal and dynamic environments. The minimum-mission structural model (pre-phase A, point design) includes a deployable, pre-tensioned membrane sun-shield and solar-sail, a 10m long deployable secondary support structure, and a light-weighted 6m diameter monolithic glass primary mirror. We performed thermal distortion and dynamic response analyses in order to demonstrate feasibility, quantify critical sensitivities, and to identify potential problems that might need to be addressed early on.

Demonstration of spectral calibration for stellar interferometry

A breadboard is under development to demonstrate the calibration of spectral errors in microarcsecond stellar interferometers. Analysis shows that thermally and mechanically stable hardware in addition to careful optical design can reduce the wavelength dependent error to tens of nanometers. Calibration of the hardware can further reduce the error to the level of picometers. The results of thermal, mechanical and optical analysis supporting the breadboard design will be shown.

Starshade design for occulter based exoplanet missions

We present a lightweight starshade design that delivers the requisite profile figure accuracy with a compact stowed volume that permits launching both the occulter system (starshade and spacecraft) and a 1 to 2m-class telescope system on a single existing launch vehicle. Optimal figure stability is achieved with a very stiff and mass-efficient deployable structure design that has a novel configuration. The reference design is matched to a 1.1m telescope and consists of a 15m diameter inner disc and 24 flower-like petals with 7.5m length. The total tip-to-tip diameter of 30m provides an inner working angle of 75 mas. The design is scalable to accommodate larger telescopes and several options have been assessed. A proof of concept petal is now in production at JPL for deployment demonstrations and as a testbed for developing additional elements of the design. Future plans include developing breadboard and prototype hardware of increasing fidelity for use in demonstrating critical performance capabilities such as deployed optical edge profile figure tolerances and stability thereof.

Engineering trade studies for the Terrestrial Planet Finder Coronagraph primary mirror

The Terrestrial Planet Finder Coronagraph (TPF-C) is conducting pre-formulation design and analysis studies based on a 8x3.5m elliptical aperture, light-weight primary mirror feeding an internally occulted (Lyot) coronagraph. The primary mirror has challenging static and dynamic performance requirements. We report on recent trade studies and concepts including open- and closed-back mirror blank designs and comparisons of thermal and mechanical performance; aperture shape alternatives to better match the coronagraph application with weight, packaging, and fabrication constraints; and mirror material trades.

High Precision Thermal, Structural and Optical Analysis of an External Occulter Using a Common Model and the General Purpose Multi-Physics Analysis Tool Cielo

The efficient simulation of multidisciplinary thermo-opto-mechanical effects in precision deployable systems has for years been limited by numerical toolsets that do not necessarily share the same finite element basis, level of mesh discretization, data formats, or compute platforms. Cielo, a general purpose integrated modeling tool funded by the Jet Propulsion Laboratory and the Exoplanet Exploration Program, addresses shortcomings in the current state of the art via features that enable the use of a single, common model for thermal, structural and optical aberration analysis, producing results of greater accuracy, without the need for results interpolation or mapping. This paper will highlight some of these advances, and will demonstrate them within the context of detailed external occulter analyses, focusing on in-plane deformations of the petal edges for both steady-state and transient conditions, with subsequent optical performance metrics including intensity distributions at the pupil and image plane.

Development of the Terrestrial Planet Finder Coronagraph Membrane V-Groove Sunshield

An innovative architecture for a V-groove membrane sunshield, which serves as both a thermal shield and a light baffle, has been developed for the Terrestrial Planet Finder Coronagraph. A preliminary design analysis has been conducted to determine the major design parameters and to verify the existence of the boom technologies. A packaging and deployment study has been accomplished. A thermal analysis has been carried out to verify the thermal performance of this architecture. A dynamic analysis of the membrane V-groove sunshield has also been performed to validate its dynamic characteristics. Consequently, the feasibility of this architecture has been confirmed.