This paper is a presentation of the so-called COME-ON adaptive optics prototype system developed jointly by four European institutions. This system has been tested on the 1.52m telescope of the Observatoire de Haute Provence on October 12 to 23 and November 13 to 24, 1989. Diffraction-limited infrared imaging has been achieved during these first tests. The adaptive optics system consists of a 19 actuator deformable mirror and a Hartmann-Shack type wavefront sensor. In this instrument the wavefront sensing is performed at visible wavelengths while the correction is performed for near infrared imaging (1.2 to 5 rim). Specialized computers drive the deformable mirror and a tip-tilt mirror. The bandwidth of the servo-loop is 9 Hz at 0 dB point in open-loop. The results obtained with this instrument will be very useful for the design of the future adaptive optics system for the ESO Very Large Telescope (VLT). . INTRODUCTIONAtmospheric turbulence severely limits the angular resolution for ground based telescopes. Whatever their aperture diameter, the angular resolution is rarely less than one arcsecond. In the past twenty years, new techniques have come to light in order to compensate the effects of atmospheric turbulence on astronomical images : They involve off-line digital image processing as in "Speckle Interferometry"12, or real time wavefront control as in "Adaptive Opticstt3,4. In the perspective of the future ESO Very Large Telescope (VLT), it is obvious that the adaptive optics will become an essential instrument both for high resolution infrared (IR) imaging and for long base-line interferometry.A collaboration of four European institutions has built an adaptive optics prototype system, the so-called COME-ON projcet. The instrument is aimed to demonstrate the performance of this technique for imaging at near infrared wavelengths (1.2 to 5 xm) at a 4 m class telescope. First results have recently been obtained at the 1.52 m telescope of the Observatoire de Haute Provence (OHP) (CNRS, France). Nearly diffraction-limited imaging has been achieved at 2.2 jim and above for average seeing conditions6. In section two, the principle and the scaling parameters of this instrument are reminded. In section three, the main components and their characteristics are described. Section four discusses the performance and the limitations obtained during the first observatory tests. Section five summarizes the potential of adaptive optics for Astronomy. THE INSTRUMENTAn adaptive optics system is composed of three main components (Fig. i): 1) An adaptive optical element which can introduce on the beam variable phase shifts in space and time. Very powerful phase correctors are the continuous thin-plate deformable mirrors8. For instance, discrete piezoelectric actuators push and pull on the rear face of an initially plane thin mirror. Moreover, a tip-tilt mirror is generally used in addition for the compensation of the overall wavefront tilt which is the largest perturbation generated by the turbulence9.2) A wavefront sensor (WFS) ...

The limitations of the coherent field of view in Optical Space Interferometry are presented. The size of the field required is inferred from the number of photoevents and from stellar density. Then we examine the limitations of the FOV in both "Michelson" -or planar arrayand "Fizeau" -the equivalent of a masked giant telescope-cases in order to assess their maximum size; it is shown that in all cases, the field is too small to include a reference star in the "Michelson" final image field (< a few arcsec) when the input pupil is too diluted; conversely this is not the case for the "Fizeau ", but alignment tolerances are extremely severe. The two main limitations in the Michelson type, field curvature effect and pupil geometry conservation, are then derived in terms of tolerances on the optics. The problem of spatial frequency plane coverage is addressed and we finally propose a mission concept that accomodates most of the problems raised in this paper and nevertheless makes use of two off -axis stars for tracking purposes by splitting the fields before recombination: a "Michelson" with flat collectors, making use of fiber optics in the reference channels.

No abstract

No abstract

The last results obtained with the adaptive optics (AO) system COME-ON are reported here. A new set-up, pushing the servo-loop bandwidth up to 25 Hz, allowed to get diffraction limited long exposure images down to 1.7 μm on a 3.6-m telescope (i.e. 96 milliarcsec). Compared with the turbulent case, gains in Strehl ratio ( S ) up to 13 are reported. Profile of partially corrected images are shown, which confirm partly the predicted profiles. These results are based on data collected at the ESO 3.6-m telescope, La Silla, Chile, on April 1991.