Stray light suppression is the most demanding technical requirement for a coronagraph, and a successful design allows UVCS to convey on the entrance slits of the spectrometer a stray light level that is less than most of the coronal signals planned to be detected.
In addition to the occulted telescope stray light suppression, there are stringent stray light requirements on the Ly- channel of the spectrometer. UVCS plans to measure the electron scattered Ly- profile in order to determine the coronal electron temperature. The electron scattered Ly- profile is a broad line ( Å FWHM) superposed on the much stronger and narrower (~ 1 Å FWHM) Ly- resonant line. Therefore a grating with a good stray light rejection is needed, together with a blocker strip mounted on the detector face that prevents the Ly- resonant line from reaching the detector.