The UVCS characterization was accomplished in three stages. Throughout the UVCS development, optical components and mechanisms were characterized to ensure compliance with design goals. Optical characterization of the spectrometer assembly was performed at the time of its integration and alignment (Pernechele, C., Naletto, G., Nicolosi, P., Poletto, L., Tondello, G., Proc. SPIE, 2517, 1995). The performance tests of the integrated instrument included calibrations of the mechanisms, radiometric and photometric efficiencies, stray light suppression, spectral and spatial resolutions, spectral and spatial scales, and scattering of a spectral line into its line wing.
Figure 15 shows a schematic layout of the vacuum chamber used for the system level performance tests. The UVCS was supported on an optical bench that could be articulated to acheive alignment with the chamber optics. The chamber provided a solar divergent beam that was large enough to fill the UVCS aperture. This was accomplished by placing a circular field stop (4.2 cm in diameter) at the focal surface of the collimating mirror, and illuminating them with appropriate light sources. A light trap, similar to the one used in the UVCS instrument, simulated a very low radiance corona for the stray light tests. For calibrations in dispersed light, a monochromator could be placed with its exit slit at the collimator focus. Calibrated photodiodes mounted on translation stages were used to scan the incident beam and determine incident irradiance levels.
The results of the UVCS characterization are summarized in Table VII The efficiency values are the ratio of the instrument count rate to the measured incident photon rate. It includes the effects of the optical components and detector, but is independent of the instrument aperture and slit width. Off-band efficiency refers to detection of 2537 Å light when the spectrometer is set for 1216 Å. The stray light levels are provided as the ratio of the measured straylight level (described as an equivalent coronal radiance) to the radiance at disk center. Spectral and spatial resolutions are provided as the FWHM of the instrument response to a narrow spectral or spatial source. Because of technical limitations and time constraints, all calibrations were for the instrument aperture used for observations at a heliocentric height of 2.7 R. Because of light source limitations, the spectral resolution for the OVI channel is only an upper limit on the size of the resolution element. The stray light levels are all upper limits since there may be contributions from the test chamber. Also, because of light source limitations, the stray light level for the O VI channel was measured at 1236 Å using the redundant Ly- path. The value of was obtained before several refinements to the measurement that resulted in the value of 1x10^-8 for the Ly- channel. It is likely that the O VI channel also has a suppression of . All values provided in Table VII are representative of the instrument during the test, but must be updated throughout the mission.