Francois J. Roddier

Wavefront sensing and the irradiance transport equation

Phase retrieval has recently been experimentally demonstrated, using the irradiance transport equation. We show that this equation provides a unified description for incoherent wavefront sensing methods, bringing new insights and opening new prospects in this area.

Wave-front reconstruction from defocused images and the testing of ground-based optical telescopes

A new method has been developed for testing the optical quality of ground-based telescopes. Aberrations are estimated from wideband long-exposure defocused stellar images recorded with current astronomical CCD cameras. An iterative algorithm is used that simulates closed-loop wave-front compensation in adaptive optics. Compared with the conventional Hartmann test, the new method is easier to implement, has similar accuracy, and provides a higher spatial resolution on the reconstructed wave front. It has been applied to several astronomical telescopes and has been found to be a powerful diagnostic tool for improving image quality.

A SIMPLE LOW-ORDER ADAPTIVE OPTICS SYSTEM FOR NEAR-INFRARED APPLICATIONS

It is shown that low-order wavefront compensation can significantly improve astronomical images over most of the sky. A novel approach to wavefront sensing and compensation is described. It is optimized for low-order correction and high efficiency. Computer-simulation results show it can achieve the desired performance, and preliminary laboratory tests demonstrate its feasibility.

Discovery of a moon orbiting the asteroid 45 Eugenia

Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13 km, and an orbital period of about 4.7 days with a separation of 1,190 km from Eugenia. Using a previously determined diameter for Eugenia, we estimate that its bulk density is about 1.2 g cm-3, which is similar to that of the C-type asteroid Mathilde. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

One-dimensional spectra of turbulence-induced Zernike aberrations: time-delay and isoplanicity error in partial adaptive compensation

A simple derivation is given for the expression of one-dimensional spectra of turbulence-induced Zernike aberration terms in the case of a stellar source at infinity. Numerical results are presented for several terms. Expressions are given for the maximum acceptable delay and the maximum isoplanatic angle in a partially compensating adaptive optics system. Temporal-frequency spectra of the Zernike terms, recorded at the Canada–France–Hawaii telescope, are well reproduced when one convolves all the corresponding theoretical spectra with the same turbulence distribution. Only a few atmospheric layers are found to contribute to image degradation.

Combined approach to the Hubble Space Telescope wave-front distortion analysis

Stellar images taken by the Hubble Space Telescope at various focus positions have been analyzed to estimate wave-front distortion. Rather than using a single algorithm, we found that better results were obtained by combining the advantages of various algorithms. For the planetary camera, the most accurate algorithms consistently gave a spherical aberration of -0.290-µm rms with a maximum deviation of 0.005 µm. Evidence was found that the spherical aberration is essentially produced by the primary mirror. The illumination in the telescope pupil plane was reconstructed and evidence was found for a slight camera misalignment.

Curvature Sensing: A New Wavefront Sensing Method

A new wavefront sensing method based on local wavefront curvature measurements rather than tilt measurements is investigated. Performances are compared to that of a Shack-Hartmann sensor. Examples of experimentally reconstructed wavefronts are presented and several applications are discussed.

Images of Neptune's ring arcs obtained by a ground-based telescope

Neptune has a collection of incomplete narrow rings, known as ring arcs, which should in isolation be destroyed by differential motion in a matter of months. Yet since first discovered by stellar occultations in 1984, they appear to have persisted, perhaps through a gravitational resonance effect involving the satellite Galatea. Here we report ground-based observations of the ring arcs, obtained using an adaptive optics system. Our data, and those obtained using the Hubble Space Telescope (reported in a companion paper), indicate that the ring arcs are near, but not within the resonance with Galatea, in contrast to what is predicted by some models.

Variations on a Hartmann theme

We describe modifications of the classical Hartmann wavefront sensing technique which can be used to improve the accuracy, dynamic range, and spatial resolution of the technique.

Seeing at Mauna Kea: a joint UH-UN-NOAO-CFHT study

During two short campaigns intensive coordinated measurements have been performed to determine the various contributions to image degradation on Mauna Kea. Some of the results already obtained are presented here.