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.
