Instrument Overview

The SOHO Ultraviolet Coronagraph Spectrometer consists of a telescope spectrometer unit (TSU) and a remote electronics unit (REU) (see Figure 3 ). The TSU is a triple telescope with external and internal occultation and a high resolution spectrometer assembly. The TSU has its own pointing mechanism, a roll mechanism, a telescope mirror mechanism, an internal occulter mechanism and an aperture door mechanism.

Electronics located in the TSU include image processors, detector power supplies and monitors, sun sensor electronics and position transducer electronics. Three spherical telescope mirrors focus co-registered images of the extended corona onto the three entrance slits of the spectrometer assembly. The spectrometer assembly consists of three channels:

• The Ly- channel is a toric grating spectrometer with an entrance slit mechanism, a neutral density filter inserter, a grating mechanism, and a windowed crossed delay line (XDL) detector (the Ly- detector). This channel is optimized for line profile measurements of H I 1216 and may also be used for other spectral lines in the 1145 Å to 1287 Å spectral range. An extended wavelength range (1100 Å to 1361 Å) may be observed by rotating the grating.

• The O VI channel is a toric grating spectrometer with an entrance slit mechanism, a neutral density filter inserter, a grating mechanism, and an open XDL detector (the O VI detector). The O VI detector is optimized for measurements of the O VI lines at 1032 Å and 1037Å\ and may also be used for other spectral lines in the 984 Å to 1080 Å (first order) and 492 Å to 540 Å (second order) spectral range. An extended wavelength range in first and second order (937 Å to 1126 Å and 469 Å to 563 Å, respectively) may be observed by rotating the grating. The O VI channel includes a convex mirror between the grating and the O VI detector to focus the HI 1216 Å (first order) and Mg X 610 Å and 625 Å (second order) radiations onto the O VI detector. That arrangement is called the redundant Ly- path.
• The white light channel (WLC) is a visible light polarimeter and consists of an entrance aperture, a polarimeter assembly, and a photomultiplier tube. The polarimeter assembly includes a rotatable half-wave plate, a fixed linear polarizer, a bandpass filter and a lens. The WLC measures polarized radiance in the 4500 Å - 6000 Å wavelength band.

The REU electronics include the central processing unit, low voltage power supplies, power switching, mechanism controller circuitry, housekeeping, thermal control and interface electronics.

Typical instrument parameters used for UVCS observations are summarized in Table I. The primary observational mode provides simultaneous and cospatial measurements of the line profile of HI 1216 Å, the integrated spectral radiance of OVI 1032 Å and 1037 Å, Mg X 610 Å and 625 Å, Si XII 499 Å and 521 Å, Fe XII 1242 Å, other spectral lines within the UVCS spectral ranges, and the visible polarized radiance. Other modes provide intensities of other lines, electron scattered HI Ly-, spectral line profiles for minor ions and higher spectral and/or spatial resolution, but require longer exposure times.

The field-of-view (FOV) of UVCS is illustrated in Figure 4 . The instantaneous FOV is the portion of the solar image that passes through the entrance slits of the ultraviolet channels. Spatial elements along the instantaneous FOV are resolved by the XDL detectors. The WLC has a 14 arcsecond x 14 arcsecond spatial field at the center of the instantaneous FOV. Internal mirror motions are used to step the instantaneous FOV across the 141 arcminute direction. Offset pointing adjustments with the UVCS pointing mechanism can be used to extend the FOV down to the solar limb and onto the disk or up to 12 solar radius. UVCS can rotate its FOV about Sun-center in order to observe the full corona.