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We present the catadioptric optical design solution for the stereo channel of the imaging system SIMBIOSYS for the BepiColombo European Space Agency mission to Mercury. The main scientific objectives of the instrument are the three-dimensional global mapping of the entire surface of Mercury in the panchromatic band and imaging of selected areas in four broad colored bands; both tasks have to be accomplished with a scale factor of 50 m per pixel at periherm. The system consists of an original compact layout in which the two stereo subchannels share a common detector; also, the optical components are common to the two subchannels, with the exception of the first element, which is a rhomboid prism. The field of view of each subchannel is about 5[degrees] x 5[degrees] with a scale factor of 23 arcsec/pixel. The ray- tracing simulation of the system shows that the design guarantees optimal aberration balancing over the entire field of view and the entire wavelength range covered by the instrument, with ensquared energy of the order of 80% in one pixel. OCIS codes: 110.6770, 120.4570, 120.0280, 350.1260, 350.6090.

Adaptive optics (AO) has been recently used for the development of ophthalmic devices. Its main objective has been to obtain high-resolution images for diagnostic purposes or to estimate high-order eye aberrations. The core of every AO system is an optical device that is able to modify the wavefront shape of the light entering the system; if you know the shape of the incoming wavefront, it is possible to correct the aberrations introduced in the optical path from the source to the image. The aim of this paper is to demonstrate the feasibility, although in a simulated system, of estimating and correcting an aberrated wavefront shape by means of an iterative gradient-descent-like software procedure, acting on a point source image, without expensive wavefront sensors or the burdensome computation of the point-spread-function (PSF) of the optical system. In such a way, it is possible to obtain a speed and repeatability advantage over classical stochastic algorithms. A hierarchy in the aberrations is introduced, in order to reduce the dimensionality of the state space to be searched. The proposed algorithm is tested on a simple optical system that has been simulated with ray-tracing software, with randomly generated aberrations, and compared with a recently proposed algorithm for wavefront sensorless adaptive optics. OCIS codes: 010.1080, 220.1000, 170.4470, 110.0110.

The description of an adaptive optics (AO) system with no wavefront sensor to correct primary aberrations is presented. This system is based on closed loop software that iteratively analyzes a point source target image on the instrument focal plane and suitably modifies the AO device. The performed tests with a pull-only deformable mirror (DM) have shown that the system works very well, reaching an optimal focusing condition in a few seconds using standard components. Such a system can be conveniently applied in all the fields in which a not very fast optical adaptation is acceptable. OCIS codes: 010.1080, 220.1000, 230.4040.

The design of a quasi-null lens system for the fabrication of an aspheric oblate convex ellipsoidal mirror is presented. The Performance and tolerance of the system have been analyzed. The system has been applied successfully for the fabrication of the primary mirror of the Wide Angle Camera (WAC), the imaging system onboard the Rosetta, the European Space Agency cornerstone mission dedicated to the exploration of a comet. The WAC is based on an off-axis two-mirror configuration, in which the primary mirror is an oblate convex ellipsoid with a significant conic constant. OCIS codes: 350.1260, 220.1000, 120.4610.

A spectroheliograph dedicated to the observation of the solar disk in the extreme-ultraviolet OV spectral line at 62.97 nm is described. As demonstrated in the Skylab SO-82A spectroheliograph [Appl. Opt. 16, 870 (1977)], this line is uniquely suited to characterize solar plasma in the important 250, 000 K temperature regime. No multilayer coating or suitable filter is yet available to select this wavelength, so an optical design based on a double spectrograph with a spatial filter to remove the unwanted radiation has been developed. Analysis of the optical design shows that this instrument can obtain a 1 arc-sec spatial resolution (two pixels) with a relatively high image-acquisition cadence. A preliminary tolerance analysis has been performed. A simple method of instrument alignment in visible light is also described. OCIS codes: 110.6770, 120.6200, 220.4830, 260.7200, 350.1260, 350.6090.

The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) instrument on board the Rosetta spacecraft, a mission of the European Space Agency to comet P/Wirtanen, includes two cameras for acquiring images of the comet. A set of interference filters deposited upon glass and fused-silica substrates will be added to the cameras for wavelength tuning. For this mission of more than 10/years in an interplanetary environment, the requirement of preserving the optical characteristics of the filters is a critical one. We checked the variation in the transmission of some filter substrates after proton irradiation that simulated the solar wind. To produce a situation that is representative of the interplanetary environment, we irradiated proton fluences at three energies: 1.5 x [10.sup.11] protons/[cm.sup.2] at 4 MeV, 1.9 x [10.sup.10] protons/[cm.sup.2] at 8 MeV, and 7.1 x [10.sup.9] protons/[cm.sup.2] at 18 MeV. Seven substrates were tested: three Suprasil-1; three colored glasses, namely, OG590, KG3, and RG9; and one quartz. In addition, two interference filters were checked. The results obtained show that Suprasil-1 is rather insensitive to this irradiation, whereas very small reductions in transmission, of the order of a few percent, occur for colored glasses. The transmission of these filters was remeasured 2 years after the irradiation, and showed a general decrease in the transmission reduction. OCIS codes: 120.2440, 160.2750, 160.4670, 350.2450, 350.5610, 350.6090.

The final optical design of the Wide Angle Camera for the Rosetta mission to the P/Wirtanen comet is described. This camera is an F/5.6 telescope with a rather large 12 [degrees] x 12 [degrees] field of view. To satisfy the scientific requirements for spatial resolution, contrast capability, and spectral coverage, a two-mirror, off-axis, and unobstructed optical design, believed to be novel, has been adopted. This configuration has been simulated with a ray-tracing code, showing that theoretically more than 80% of the collimated beam energy falls within a single pixel (20' x 20') over the whole camera field of view and that the possible contrast ratio is smaller than 1/1000. Moreover, this novel optical design is rather simple from a mechanical point of view and is compact and relatively easy to align. All these characteristics make this type of camera rather flexible and also suitable for other space missions with similar performance requirements. OCIS codes: 110.6770, 120.4570, 220.2740, 220.4830, 350.1260, 350.6090.

We report on thin-film photodetectors optimized for detecting the vacuum UV and rejection of the visible spectrum of electromagnetic radiation. The devices are made of hydrogenated amorphous silicon and silicon carbide on a glass substrate. At room temperature the photodetectors exhibit quantum efficiencies of 52% at [Lambda] = 58.4nm, 1% at [Lambda] = 400nm, and 0.1% at [Lambda] = 650nm. The response time for UV pulses from an [N.sub.2] laser gives signals of 6-[[micro]seconds] full width at half-maximum and 500-ns rise time.

The optical performances of the spectrometer assembly for the Ultraviolet Coronagraph Spectrometer of the Solar and Heliospheric Observatory mission have been tested. The flight unit of the spectrometer assembly, consisting of the structure equipped with the entrance slits assembly, the grating drive mechanisms mounting two toroidal gratings, and the photon-counting detectors, has been integrated and aligned; also the flight unit of the White Light Channel has been integrated and aligned in the spectrometer assembly. Tests with both visible and UV radiation have been performed. Aberration and stray-light measurements have shown that the instrument performs satisfactorily, almost in compliance with the scientific requirements; also some measurements of the polarimeter modulation curve and the relative error have shown performances within the specified requirements. Key words: VUV spectroscopy, visible polarimetry, space instruments.