The UVCS spectrometer assembly has been designed to cover a suitable spectral range according to the scientific requirements. For several reasons, e.g., reflectivity of the optical coatings, large differences in the emitted intensity among the elements of the solar corona and others, it has been designed to accomodate two very similar UV channels. One is optimized at 1216 Å for the observations of the HI Ly- line and a second one at 1035 Å for the OVI lines at 1032 Å and 1037 Å. Their optical design has also been driven by mechanical constraints on the accommodation of the main optical elements: entrance slits, gratings and detectors. Both spectrometer channels use toric gratings and thus assure stigmatic spectral imaging: a single, albeit aspheric optical surface provides diffraction, reflection and focusing, as well as correction for astigmatism. Given the relatively low reflection coefficients in the vacuum ultraviolet wavelength domain, a single optical surface is optimum with respect to efficiency. Each UVCS grating is used in a Rowland-circle mounting, with a diffraction angle, , near the grating normal. The horizontal (i.e., spectral) focus of a spherical concave grating lies on the Rowland circle, and the vertical (i.e. spatial) focus lies outside this circle. The Rowland circle has a diameter equal to the horizontal radius of curvature, R, of the grating surface. When an originally spherical grating surface is transformed into a toric one by making the vertical radius of curvature, R, smaller than R, then the vertical (i.e. the stigmatic) focal plane begins to approach the Rowland circle. Eventually, the two focal surfaces touch at and then intersect each other at = . Both, astigmatism correction and spectral focus occur at the so-called stigmatic points, i.e. where the stigmatic focusing-surface intersects (or touches) the Rowland circle (see Figure 8 ). The relation between vertical and horizontal radii of curvature of the grating surface, the angle of incidence, , and the angles of diffraction, , where the astigmatism is corrected, is (see Figure 9 )
In the vicinity of the astigmatic foci, i.e., near the angles of diffraction, , the separation of the spectral and spatial foci along a bundle of light rays is small. Here, the blur remains near the detector resolution and effective stigmatic imaging occurs. As long as is kept small, effective stigmatic imaging can be maintained on both sides of the grating normal over a section of the Rowland circle that slightly exceeds . In order to reduce the aberrations, it was necessary to keep the angle of incidence on the gratings small and the diffraction angle as near as possible to zero. The parameters of the optical design are summarized in Table II.
In order to scan a suitable spectral range through the FOV of the detector, both the gratings can rotate, being mounted in a Johnson (1952)-Onaka (1958) configuration. The best center of rotation of the grating is located at 127 mm from its vertex along a direction nearly perpendicular to the bisector line of the angle between the incident and the diffracted rays. In this way, the grating translates when rotating, always keeping the best spectral focus on the detector. In the OVI channel, a grazing incidence mirror has been mounted (not shown in Figure 9 ) in order to deviate and focus the Ly- line radiation on the detector. In this way redundancy is provided for the Ly- observations.
A preliminary evaluation of the optical performances of the spectrometer has been done through extensive ray-tracings for both the Ly- and the OVI channels. In Figure 10 some ray-tracing results for the Ly- channel are reported as examples. The spectral and spatial blurs vs. wavelength for various positions of a point source with respect to the dispersion plane are shown.
From the analysis of these ray-tracings, the following conclusions can be derived regarding the predicted optical performance: