The telescope mirrors for UVCS are of two types. The mirrors for the uv channels are SiC clad SiC and the visible mirror is Si clad SiC with a Cr overcoating. All three mirrors have spherical figures and radii of curvature of 1502 mm. The mirrors have rectangular cross sections with the uv mirrors being 72 mm by 50 mm, and the visible mirror being 72 mm by 30 mm (the smaller dimension parallel to the solar limb tangent).
The surface finish
is the most critical specification for the UVCS mirrors. Surface roughness
values of 8 Å rms were specified for the uv mirrors and 5 Å with a
goal of 3 Å for the visible mirror. Those values were most readily
achieved on spherical surfaces. The spherical aberration added 7 
m
to the total image spread and about 
/4 wavefront error compared to an
off-axis parabola. The distribution of scattered energy is important
because the light diffracted from the external occulter edge can be scattered (at small angles from the specular direction) by the telescope mirrors into the entrance slits. Hence, the autocovariance of surface roughness for large autocovariance
lengths should be very small and the autocovariance curve should approximate
a Gaussian. The quality of the edges around the front surface is also
important because aperture masks are not used to cover the edges. A small
polished radius (R < 0.1 mm) blending into the clear aperture is used to specularly reflect light away from the image.
It is recognized that the chosen surfaces do not provide optimal
reflectance at all wavelengths of interest. The SiC clad mirrors
provide about 45% reflectance at 1216 Å compared to a typical value of
78% for Al + MgF
. The Cr coating provides about 65% reflectance for visible
wavelengths. Nonetheless, the selected surfaces for the Ly-
and visible channel mirrors were accepted because of problems in manufacturing
and time constraints.