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Chapter 6. C3 Telescope
The C3 optical design derives from the NRL coronagraph flown on OSO-7 (1971-74), and the similar SOLWIND coronagraph flown on the P78-1 satellite (1979-84). It is similar in conceptual approach to C2 (externally occulted), and both ultimately derive from the fundamental work of B. Lyot and J.W. Evans. However, C3 is designed to function at extremely large solar elongation angles, and therefore differs in a number of respects that optimize it for this region. In particular, the field lens, which determines the radial extent of the field of view, is quite large compared to the other C2 optics.
6.1 C3 optics
The conceptual diagram for C3 illustrated in Figure 6-1 (low resolution) or Figure 6-1 (high resolution) is identical to the diagram for C2 in the previous chapter. The top diagram traces a selected ray bundle for the coronal image, while the lower diagram illustrates the optical elements and ray paths involved in the suppression of stray light. Beginning at the left of each diagram, a 110 mm diameter front aperture A0 admits the coronal light. The external occulter D1 shadows the small 9.6 mm entrance aperture A1 from direct sunlight. The D1 occulter is an assembly of three equally spaced disks on a common spindle, with each disk sized to intercept diffracted light from the edge of the previous one. This configuration reduces the amount of scattered light falling into the umbra where the A1 aperture is located. It is similar in concept to the C2 external occulter, whose threaded conical cylinder can be considered as a stack of closely spaced disks.
The objective lens O1 immediately behind the A1 entrance aperture is a superpolished, plano-convex singlet, to obtain absolutely minimal scattering. It forms the primary image of the corona in its rear focal plane. A field stop here defines the 32 Rsun outer field limit. This is followed by the internal occulter D2, which intercepts the image of the D1 external occulter and its halo of diffracted sunlight. Its supporting pylon intercepts diffracted light from the external occulter pylon. Behind this occulter is the field lens O2, whose large diameter is necessary to maintain the outer field limit of 32 Rsun. Because it operates at high relative aperture (f/1.5), it requires a number of elements of different optical glass type to keep aberrations under control. However, its function is the same as that in C2, to image the entrance aperture A1 upon a Lyot stop A3, in order to intercept diffracted light from the A1 entrance aperture edges. It also collimates the primary image, and presents it to a relay lens O3, placed behind the Lyot stop A3 to avoid diffracted light that would otherwise fall upon its surfaces. The relay lens is also a complex lens that controls the residual aberrations from the O1 singlet objective lens, and also provides a long working distance in which to install the shutter and the filter and polarizer wheels, while still maintaing a small image that will fit onto the 21.5 mm square CCD detector. Just before the front surface of the O3 relay lens is a glass plate with a small, opaque Lyot spot, which intercepts a small ghost image of D1 and its diffraction halo produced by interreflections in the O1 objective lens, a non- negligible potential source of stray light. The relay lens transfers the image onto the 1024x1024 pixel CCD camera. The 21 †m CCD pixel size subtends an angle of 56 arc seconds to the solar corona. The CCD square is circumscribed by a 30 Rsun radius image, so that portions of the 32 Rsun optical field-of-view are lost off the top, bottom, left, and right edges of the CCD surface (see Figure 3-5).
Several further design features are used in stray light suppression. The contribution to diffracted light from the out-of- field sunlit edge of the A0 front aperture is uncertain, but it has been provided with smooth-edged serrations to direct high order diffrated light away from the A1 entrance aperture. In addition, its out of focus image is intercepted by the field stop at the O1 objective lens image plane. Another stray light source is surface and volume scattered light from the O1 objective lens, which is superpolished to keep scattered light to a minimum. This light cannot be intercepted downstream in the optical path of the telescope, and appears at the focal plane. Stray light arises from internal interreflections in the O1 objective lens, which produce a small ghost image of the external occulter and its ring of diffracted sunlight. This light is propagated down the system, but is re-imaged at the front of the relay lens O3 and intercepted by the small, opaque Lyot spot on a glass plate. The remaining scattered light arises by reflections from edges and from walls. All edges are made sharp, polished to a 25 †m radius, and blackened. Baffles are located so that optical elements view only the rear surfaces of baffles, or shadowed walls.
After these efforts, stray light in the system is measured to be 10E-12 Bsun, where Bsun is the disk average solar brightness. This low level is partly achieved by over-occulting the solar disk and corona to 3.7 Rsun. The outer circular field, 32 Rsun, is slightly larger than the 30Rsun circular field which is circumscribed by the 1024x1024 CCD, as illustrated in Figure 3-6. Because of the relatively short distance between the external occulter D1 and the entrance aperture A1, the coronal image is strongly vignetted. A compensating factor is that the brightness range at the focal plane is reduced, so that the entire coronal image can be well recorded with a single exposure. The corona becomes unvignetted beyond 10 Rsun.
Filter and polarizer wheels, and the shutter, are located in the space between the relay lens and CCD camera. C3 does not have a narrowband, spectroscopic quality filter. As an aid to separation of F from K coronal light, however, it has broadband color filters and polarizers for polarization analysis, as does C2. A moderately narrow (2 nm) H-alpha filter is included. The filter wheel contains the blue, orange, deep red, and infrared filters, and a clear glass position. The polarizer wheel contains three polarizers at 120 degrees, the H-alpha filter, and a clear glass position. The four color filters are used with the clear glass polarizer wheel position or one of the three polarizers. The H€ filter on the polarizer wheel is used with the filter wheel clear glass position. Table 6-1 lists the bandpasses (FWHM) of these filters.
Internal calibration lamps (redundant) are located behind the shutter. When powered, light from the lamp will be reflected diffusely from the rear of the shutter blade, pass through a filter and a polarizer, and then illuminate the CCD. The signal level of the CCD detector can be monitored when the front door is closed. In this case, the coronagraph will see the rear of a small, Sun- illuminated diffuser set in the door. This patch will be out of focus at the CCD, but provides a known level of solar disk illumination.