D. S. Acton

PHASE-DIVERSITY WAVE-FRONT SENSOR FOR IMAGING SYSTEMS

A phase-diversity wave-front sensor has been developed and tested at the Lockheed Palo Alto Research Labs (LPARL). The sensor consists of two CCD-array focal planes that record the best-focus image of an adaptive imaging system and an image that is defocused. This information is used to generate an object-independent function that is the input to a LPARL-developed neural network algorithm known as the General Regression Neural Network (GRNN). The GRNN algorithm calculates the wave-front errors that are present in the adaptive optics system. A control algorithm uses the calculated values to correct the errors in the optical system. Simulation studies and closed-loop experimental results are presented.

Solar imaging with a segmented adaptive mirror

A 19-segment adaptive-mirror system is currently being used on the Sacramento Peak 76-cm Tower Telescope to remove wave-front distortions resulting from atmospheric turbulence. The system has proven to be capable of substantially improving the quality of an image, at times achieving 0.33-arcsec resolution in visible wavelengths under 1-3-arcsec seeing conditions. An improvement in resolution seems to occur across a large field of view that is, at times, 30 arcsec in diameter.

Wave-front tilt power spectral density from the image motion of solar pores

We have constructed an image-stabilization system that measures wave-front tilt over a telescope aperture that is due to atmospheric turbulence. This system uses small features on the Sun as point sources. The wave-front tilt power spectral density has been measured with this system out to more than 500 Hz. The spectra show three distinct asymptotic slopes that do not, in general, agree with theoretical predictions based on the Kolmogorov model.

Correction of static optical errors in a segmented adaptive optical system

Adaptive optical systems are designed to compensate for wave-front errors caused by atmospheric turbulence. In addition, they may also correct for wave-front errors associated with fixed optical aberrations in the host telescope. In general, however, adaptive optical systems cannot sense wave-front errors caused by imperfections in their own internal optical components. Consequently, these fixed internal errors will remain uncorrected. The problem of fixed internal aberrations has been noted in a segmented adaptive optics system designed for solar astronomy. This problem has been eliminated by measurement of the fixed errors, off line, through the use of a simple adaptation of a Hartmann test. Results from a recent experiment demonstrating the correction of the errors are presented.

Optical state estimation using wavefront data

The use of wavefront measurements to deduce the state of multiple optics in a telescope beam train - their misalignments and figure errors - can be confused by the fact that there are multiple potential sources for the same measured error. This talk applies Kalman filtering techniques as a tool for separating true telescope errors from artifactual testing errors in the alignment and testing of NASAs James Webb Space Telescope, a large segmented-aperture cryogenic telescope to be launched after 2010.

Integrated Telescope Model for the James Webb Space Telescope

Integrated modeling is a valuable tool for analyzing complex optical-mechanical systems such as the James Webb Space Telescope. An implementation, the Integrated Telescope Model (ITM), has been developed for JWST to analyze the performance of the Observatory. ITM is an end-to-end physical math model starting from stellar photons through the image produced by the science data pipeline. The model also includes all effects that contribute to the formation of the image including pointing errors, vibration and thermal distortions of the optical system, and the mechanical response of the mirrors and actuation devices. A time domain interface to the attitude control system rounds out the capabilities. The model is used over the life-cycle of the JWST program including: development, verification and on-orbit operation. ITM is used to perform verification analysis on the set of test data resulting in a statistical assessment of the expected observatory performance. This capability offers numerous advantages to the verification of the system, validation of the wavefront sensing and control (WFS&C) system along with system level studies for design assessments. ITM has been developed to interface to the ground control system in the same way as the actual observatory. This allows it to be used as substitute for the Observatory for training, mission planning and operational trades.

Simultaneous daytime measurements of the atmospheric coherence diameter r 0 with three different methods

The most common parameter used in characterizing atmospheric turbulence (seeing) is the atmospheric coherence diameter, or r(0). r(0) can be measured in many ways. Three such techniques that are useful when one is making daytime seeing measurements by observing the Sun are described. Results from an experiment in which r(0) was measured with all three methods are presented.