Light from both the solar disk and the extended corona enters UVCS through a rectangular opening called the entrance aperture. A large fraction of light from the solar disk is blocked out by a "serrated" (i.e., sawtoothed) edge of stainless steel called the external occulter. The purpose of the external occulter is to put the remainder of the UVCS instrument in the shadow of the solar disk, while simultaneously letting in the light from the extended corona.
However, some solar-disk light makes it through because of the phenomenon of diffraction from an edge. The wave nature of light results in some "bending" of waves around the edge of an obstacle, as illustrated by the following theoretical calculation.
If light were not wave-like, the left side of the above plot would be exactly 0 (i.e., rays are fully blocked) and the right side would be exactly 1 (i.e., rays pass over the edge). The curve for the UVCS external occulter is slightly different than the above ideal curve because of the serration, but the above curve (computed for a perfectly straight "knife edge") illustrates the general principle of diffraction.
For UVCS, the diffracted solar-disk light "contaminates" the light from the extended corona, and, for the rays with both components, we want the transmission function above to be as close to zero as possible. For UVCS, the x-coordinates on the above plot where the shadowed solar disk enters the rest of the instrument are all less than about -5. (This number is in dimensionless units; it corresponds to an actual distance "below" the occulter of about 1.5 mm). At this position, about 0.001 times the fully-illuminated disk intensity gets transmitted. This level of "blockage" is nowhere near enough to allow the much dimmer extended corona (approximately a million times less bright than the solar disk) to be seen without contamination from the solar disk.
Thus, more occultation is needed . . . .
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