There are two primary off-band scatter requirements for UVCS. Both are for UV wavelengths. The first to be discussed is scatter of near UV and visible radiations by the occulted telescope. Because of the high light levels in the near UV and visible, the combined stray light irradiance for these wavelengths at the entrance slit are much larger than the irradiances for a single UV spectral line. The dispersion of the grating and the spectral variation in the detector sensitivity is needed to further suppress the detected off-band stray light. In general, one can expect a reduction of better than 1x10 in the near UV and 1x10^-9 in the visible.
The other requirement is needed to suppress resonantly scattered HI Ly- radiation when observing the broad electron scattered line profile resulting from Thomson scattering of chromospheric HI Ly- in the corona.
The off-band scatter specifications of the UVCS Ly- spectrometer channel state that the integrated intensity in the central 5 Å band of the line (i.e., at the diffraction angle) should be or greater of the integrated intensity in any 5 Å band between to (x25) Å on either side of the primary diffraction angle.
In order to accomplish this requirement, the Ly- grating was produced by holography in order to obtain better stray light properties than those for mechanically ruled gratings. In addition, the Ly- resonant line is blocked by an opaque strip mounted in front of the Ly- detector face to avoid the saturation of the detector and scatter in the detector window. The blocker is placed in such a way that in the grating launch lock position it does not block any line of interest in the wavelength range of the Ly- channel, but, when the grating is moved to perform the electron scattered Ly- profile measurement, the strip completely blocks the Ly- resonant line for unvignetted telescope areas used for observations up to 4 R.