Optical Layout

It can be seen in Figure 3 that the UVCS telescope consists of three very similar channels, the principal differences being in the size and surface finishes of the telescope mirrors and in the design of the entrance slit baffles. Figure 5 is an illustration of the optical layout of the H I Ly- channel which will serve here to represent all three telescope channels. The optical rays are for the case where the center line and roll axis of UVCS are pointed at Sun-center.

Figure 5: Optical Layout of the H I Ly- Channel

The primary optical components of each occulted telescope are the entrance aperture, the telescope mirror, the internal occulter, the entrance slit baffle, the entrance slit, and the sunlight trap. There are also a series of telescope baffles and a Sun sensor consisting of four shadow edge sensors.

The entrance aperture is rectangular and consists of three knife edges and a serrated edge that acts as a linear external occulter which shields the telescope mirror from direct sunlight; the knife edges serve to limit the FOV and the amount of solar disk light entering the instrument. The approximately 32 arcminute divergent beam from the solar disk enters the instrument through the entrance aperture, passes through a series of three baffles and within 1.6 mm of the telescope mirror edge. That light then enters the sunlight trap where it is attenuated.

The telescope mirror has a spherical figure with a focal length of 750 mm (see section 4.2). It is placed such that coronal light passing through the entrance aperture from 1.2 , just reaches the edge of the mirror and coronal light from 10 completely fills it. This mirror focusses the coronal light onto the spectrometer entrance slit, which accepts a segment of the coronal image thus defining the instantaneous FOV (see Figure 4 ). The mirror is rotated to scan the coronal image across the entrance slit. The telescope mirror also images the external occulter inside the spectrometer. This results in an out-of-focus image of the external occulter on the entrance slit. Most of the solar disk light diffracted and scattered by the external occulter onto the telescope mirror is blocked by the entrance slit jaws.

The purpose of the internal occulter is to intercept that portion of the diffracted and scattered light from the external occulter that would otherwise be specularly reflected by the telescope mirror through the entrance slit. The internal occulter must over-occult in order to compensate for alignment tolerances. Hence, in blocking the light from the external occulter, the internal occulter also blocks that coronal light from the instantaneous FOV that passes near the external occulter and toward the innermost edge of the unvignetted telescope mirror surface. This over-occulting nominally covers a 1 mm wide strip of the otherwise unvignetted mirror area. The unvignetted area of the telescope mirror is given by the following:

The entrance slit baffle limits the FOV of the entrance slit to an area that is slightly larger than the telescope mirror, thereby only passing light from the mirror surface, the internal occulter, narrow strips of the mirror masks near the mirror edges that are adjacent to the internal occulter, and a narrow open strip just off the edge of the mirror that is opposite the internal occulter. The sunlight trap must be positioned so the view of the slit through the narrow open strip is into a dark and un-illuminated region of the sunlight trap.

The sunlight trap consists of three regions: two regions which are illuminated by direct solar disk radiation and a dark region between them. One of the illuminated regions is placed so that the line-of-sight from it to the entrance slit is blocked by the telescope mirror and the line-of-sight to the other illuminated region is blocked by the entrance slit baffle. The design of the sunlight trap is discussed in Section 4.3.