The UVCS gratings are the dispersive optical elements in the two channels of the instrument that give access to the Ly-
line at 1216 Å and to the O VI line at 1032 Å and 1037 Å. Accordingly, the two gratings are designated `Ly-
' and `O VI'.
They were fabricated, respectively, by holography and on a mechanical ruling engine. The original grating rulings could be produced on spherical surfaces, since the toric surfaces were generated as part of the grating replication process. At one stage of this process the originally spherical grating was replicated onto an elastic substrate, subsequently ``bent'' into a toric grating through controlled deformation, and then replicated in this state.
The performance of toric gratings, produced in this fashion, at extreme ultraviolet (EUV) wavelengths had been verified in the laboratory, by use of MAMA detectors, by Huber et al. (1988), following the original work by Haber (1950), Lemaître (1978) and Timothy et al. (1989).
Each grating is mounted in a cell under spring-loading so that displacements under vibration and within the acceptable temperature range during flight remain well below one arc-sec. To facilitate the adjustment of the spectrometer before it is put into a vacuum chamber, both the Ly-
and O-VI gratings are equipped with two sets of alignment rulings which are placed on both sides of the main ruling.
The alignment rulings make the visible (5460.7 Å) and ultraviolet (2537 Å) Hg-I spectral lines appear at the location of the prime solar ultraviolet lines. In this way, the alignment of the grating can be tested under atmospheric conditions both visually and by use of a uv-sensitive detector. Special diaphragms, that are part of the grating cells, cover the alignment rulings in the flight configuration, so that the main ruling only is exposed to solar radiation.
The Ly-
and the O-VI gratings are coated with Al+MgF
and Ir, respectively. The O VI grating is also used in 2nd order so that the radiation from minor ions---in addition to O VI --- like Mg X and Si XII become accessible to measurement.
Furthermore, the straylight properties of the holographic Ly-
grating are such that a direct measurement of the coronal electron temperature appears feasible. This measurement is taken from the shape of the very wide, weak line-wings of the fluorescent coronal Ly-
line, that are caused by Thomson scattering of the Ly-
radiation emerging from the chromosphere.
Measurements during laboratory tests indicate overall efficiencies of 23% and 10% for the Ly-
and O-VI gratings, respectively; Fineschi et al. (1994).