Claude A. Roddier

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.

Large mirror testing facility at the National Optical Astronomy Observatories

A method for testing the surfaces of large mirrors has been developed at the National Astronomy Observatories (NOAO) to be used even when conditions ofvibration and thermal turbulence in the light path cannot be eliminated. The full aperture ofthe mirror under test is examined by means of a scatterplate interferometer that has the property of being a quasi-common-path method, although any means for obtaining interference fringes will do. The improvements in the NOAO method lie in the means for capturing the fringe pattern and the analysis that follows. The method uses a remotely operated CCD camera system to record the fringe pattern from the workpiece. Recording the spot pattern resulting from a Hartmann test screen placed over the workpiece is also possible, but the analysis methods will differ from those discussed. By operating the test equipment remotely, the optician does not cause unnecessary vibrations or heat in the testing area. The typical test is done with a camera exposure of about a millisecond to freeze the fringe pattern on the detector. Averaging up to 10 separate exposures effectively eliminates the turbulence effects. The analysis program is based on a method originally developed for examining telescope images affected by atmospheric seeing. From the intensity information, a phase map of the wavefront reflected from the surface is obtained using a new phase-unwrapping technique. The resolution is limited only by the number of fringes in the test aperture. The method described provides the optician with complete numerical information and visual plots for the surface under test and the diffracted image the method will produce, all within a few minutes, to an accuracy of 0.01 μm measured peak-to-valley. The method has been extensively used for a variety of tests of a 1.8-m-diam borosilicate glass honeycomb mirror, where the method was shown to have a sensitivity equal to a Foucault test. The test setup is described, including the analysis technique, and examples of the results obtainable are provided. The method continues to be used at NOAO for testing large optics (currently a 3.5-m honeycomb mirror).

First light for Hokupa'a 36 on Gemini North

The University of Hawaii adaptive optics program has recently moved its 36 actuators system, named Hokupaa 36, to the Gemini North Telescope. First light for Hokupaa 36 was in time for the dedication of this telescope during June 1999 and most of the images presented were taken with this adaptive optics system. This paper will cover the modifications to the CFHT, Hokupaa 36 system that were necessary to accommodate the larger 8 meter aperture of the Gemini Telescope. Performance at the telescope has now been measured and compares favorably with that predicted.

Deconvolution of adaptive optics images: from theory to practice

Practical guidelines are proposed for deconvolution of partially adaptive optics corrected images: from raw data to high photometric precision object restoration. Preliminary processing is discussed (detector calibration, background subtraction...). The deconvolution scheme itself is then presented. It takes into account the noise statistics in the image, the imprecise knowledge of the point spread function (PSF), and the available a priori information on the object (spatial structure, positivity...). This deconvolution scheme is first validated on simulated images of NAOS, the AO system of the VLT and then applied to astronomical images. In particular, an edge preserving regularization is tested on several solar system objects: Io, Uranus and Neptune.

Testing of telescope optics: a new approach

Results recently obtained for the use of the curvature-sensing method as a substitute for slope sensing in optical wavefront reconstruction, using long-exposure CCD images of the beam cross-section on either side of the telescope focal plane. A program based on the solution to the Poisson equation is then applied in order to reconstruct the wavefront. Relative to the existing Hartmann sensing methods, curvature-sensing yields sensitivity comparable to that of the Shack-Hartmann test. Additional optics and reference plane-based calibration are obviated. Tests of the new method on an 88-inch Ritchey-Chretien telescope have yielded a map of residual wavefront errors as a solution of the Poisson equation.

Reconstruction of the Hubble Space Telescope mirror figure from out-of-focus stellar images

Both the amplitude and the phase of the incoming wave front viewed from the focal plane are reconstructed from the illumination recorded in defocused stellar images taken in flight by the HST. The reconstructed wavefront surface shows very little aberration. Compared to geometrical optics methods, the technique produces high spatial resolution wavefront maps with images taken closer to the telescopes focal plane. For the HST, the quality of the wavefront reconstruction is limited by the telescope jitter. The technique cannot be applied to ground-based telescopes at optical wavelength where the image is heavily blurred by the atmosphere. It is argued to be promising as a diagnostic tool for optical telescopes in space.

National Optical Astronomy Observatories (NOAO) Infrared Adaptive Optics Program II: Modeling Atmospheric Effects In Adaptive Optics Systems For Astronomical Telescopes

However pertect an adaptive optical system can he, it win never tuiiy correct tne image. We discuss here the effects of amplitude errors due to stellar scintillation, the effects of chromatic errors due to both refraction and diffraction in a two-wavelength system, and the effects of non-isoplanicity. All these errors are directly related to the height of turbulence layers. An expression is derived for the scale height of turbulence which should be considered as an important parameter in selecting new sites for astronomica observations. The performances of adaptive optical systems are usually described in terms of Strehl ratios. We present here the results of computations of the whole transfer function for lon exposure compensated images. It is shown that the point-spread function for a partially compensated image generally consists of an Airy disk surrounded with a halo. The ratio of the energy in the Airy disk over the energy in the halo is independent of the telescope aperture and provides a better measure of the quality of the compensation.

Diffraction-Limited Imaging Through Aberrated Optics Using Pupil-Plane And/Or Image-Plane Information

Diffraction-limited imaging of an unknown, incoherently illuminated object is demonstrated through strongly aberrated optics with unknown aberrations. No reference source other than the object itself is used to calibrate the aberrations. Two different techniques are discussed and their merits are compared.

Image Reconstruction From Rotational Shearing Interferograms

Rotational shearing interferometers produce Fourier holograms of incoherent objects. Optical aberrations affect only the phase of the object Fourier transform. Phase errors can be corrected if interferograms are also taken either of a reference source with the same shear, or of the same source with a different shear.

Self-calibration in rotational shearing interferometry

Abstract not available.