Light from the corona (along with some diffracted solar-disk light from the edge of the external occulter) enters through the front of the UVCS instrument and travels 1.7 meters to the primary telescope mirrors at the back of the instrument. There are 3 mirrors positioned in a row and mounted on a common sub-assembly, and each mirror has a concave spherical shape with a radius of 1.5 meters (i.e., a focal length of 0.75 meters). The mirrors themselves are small and roughly rectangular: two of them are 7 cm by 5 cm, and the third is 7 cm by 3 cm. The mirrors can pivot back and forth in order to focus light from different heights in the corona into the rest of the UVCS instrument.
(NOTE: After this point in the tutorial we will talk about the three channels of UVCS, which are three independent paths taken by light that has reflected off of each of the three mirrors. )
The two mirrors that are designed to reflect UV photons are coated with layers of silicon carbide (which is the same substance as the green gemstone moissanite and the industrial abrasive carborundum). The remaining mirror is designed to reflect visible photons, and is coated with layers of pure silicon, silicon carbide, and chromium.
Positioned in front of the mirrors is a movable "blocker" called the internal occulter, made of stainless steel coated with nickel. The purpose of the internal occulter is to intercept a portion of the solar-disk light that was diffracted from the edge of the external occulter. Only the portion of this diffracted light that would otherwise be reflected by the mirrors directly into the rest of the instrument needs to be blocked here; the remainder of this light is blocked at the next stage by the slits and pinhole.
The internal occulter is movable, so that just enough of the diffracted edge-light is blocked without eliminating a large fraction of the light from the extended corona. The internal occulter and mirrors are typically moved together, so that the same relative amount of occulting is done when looking at different heights in the corona. We typically "over-occult" (i.e., block out a bit more coronal light than we need to) to be absolutely sure that we have minimized the "stray" contribution from diffracted solar-disk light. The length of the strip of illuminated area on the mirror (d) is given approximately by
where D is the distance from the external occulter to the mirror (1.7 meters), and b is the amount of over-occulting that is used (typically 1.5 or 2 millimeters). The angle theta is just the ratio of the radial distance in the corona for which rays can hit the mirror (r - 1.2, measured in units of solar radii) to the distance from SOHO to the Sun (1 AU = 215 solar radii = 150 million km). Theta is typically a very small angle.
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