Laura Needels

Modeling and optimization of a segmented reflector telescope

An optomechanical model of a segmented reflector telescope is developed. An image quality metric is used to optimize the placement of damping elements in the telescope backup truss. Comparisons with an alternative placement criteria are made.

Limits on adaptive optics systems for lightweight space telescopes

Future space telescopes seek to maximize collecting aperture for increased sensitivity and high spatial resolution yet are limited in mass due to launch weight restrictions. JPL has developed the Controlled Optics Modelling Package (COMP) to easily facilitate analyses of optical systems whose elements are perturbed. Development of the computer tool, IMOS (Integrated Modeling of Advanced Optical Systems) allows modeling of structurally and thermally induced deformations to interact with optical systems. Presented here are analyses on the Segmented Reflector Telescope, (SRT), to estimate and then minimize the effect of anticipated disturbances on the resultant optical performance. Such studies are a needed prerequisite for estimating the requirements for adaptive optics due to structural movements. A simulation study estimates the space-time power spectral density of the residual telescope phases from reaction wheel disturbances. Results show that significant disturbances are concentrated in the first few Zernike polynomials with 87% of all disturbances described by the first 11 terms leaving a 0.35 micrometers rms residual. The time bandwidth of the disturbances is between 20 - 25 Hz which placed the required corrections in the adaptive optics regime.

Integrated multidisciplinary analysis of segmented reflector telescopes

The present multidisciplinary telescope-analysis approach, which encompasses thermal, structural, control and optical considerations, is illustrated for the case of an IR telescope in LEO; attention is given to end-to-end evaluations of the effects of mechanical disturbances and thermal gradients in measures of optical performance. Both geometric ray-tracing and surface-to-surface diffraction approximations are used in the telescopes optical model. Also noted is the role played by NASA-JPLs Integrated Modeling of Advanced Optical Systems computation tool, in view of numerical samples.

Large-telescope natural guide star adaptive optics system

This paper presents an adaptive optics (AO) system for a large IR telescope. The Keck Telescope on Mauna Kea is used as a case study. The system is designed to provide low-order wavefront correction using faint natural guide stars. It is shown that two innovations, namely the use of an AO-optimized CCD detector, and use of a variable control bandwidth, result in correction to a Strehl ratio of about 0.1 using guide stars of visual magnitude 18. Sky coverage is approximately 20% at this level of performance. Sky coverage rises to 100% at a Strehl ratio of 0.05.

Wavefront Reconstruction Methods for a Natural Guide Star Adaptive Optics Application to the Keck Telescope

This paper investigates and compares wavefront sensing options for a proposed natural guide star adaptive optics upgrade for the Keck telescope. The methods of curvature sensing and slope sensing with a Hartmann sensor are compared. A curvature sensor model is developed that includes nonlinear, diffraction, and noise effects. The analytical model is validated in simulation using the Keck optical prescription parameters. A comprehensive treatment of estimation/reconstruction is presented and used for the Hartmann sensor/curvature sensor comparisons.

Time simulation of segmented reflector telescopes in multidisciplinary analysis

NASA has an ambitious plan for exploring the universe involving many space-based observatories. The CSI program (Control Structure Interaction) is a NASA funded project to design microprecision spacecraft observatories from a multidisciplinary approach by integrating the structural, control and optical subsystems of these spacecraft. A Focus Mission lnterferometer (FMI), representative of one of NASAs astrometric missions for extrasolar planet detection was selected as the focus for developing this multidisciplinary technology. Very early in this study it was discovered that integrated modeling tools were necessary to predict the on orbit performance of the FMI.