TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 1
1.6.5 Sunshield
The telescope is protected from the Sun by a six-vane v-
groove radiator. This radiator rejects the heat from the Sun
so that the telescope can rotate about its observing axis
without significant deformation of the wavefront in the
telescope. Extreme thermal stability is required so the
optical wavefront will remain stable enough to allow for
subtraction of the diffracted speckle pattern created by the
telescope. The v-groove radiator is based on technology
developed for the James Webb Space Telescope. The
radiator must be deployed in space because it will not fit
into a launch vehicle. Deployment from the stowed
configuration behind the primary mirror is very challenging,
requiring both radial and axial motion and tensioning of the
thin vanes. The current concept for the sun shade
deployment is shown in Figure 1-9.
1.7 How TPF-C Detects Planets
Figure 1-9. Concept for the
TPF-C Deployable Sunshield
TPF-C is designed as a high-performance coronagraph. The ability of TPF-C to detect a planet
as separate from its host star is determined by the diffraction pattern of the primary mirror, as
viewed through the optics of the coronagraph. There are two issues to contend with: (1) the
primary mirror must be sufficiently large to provide the angular resolution necessary to resolve
the planet as separate from the star, and (2) the sidelobes, stray light, and speckles in the
diffraction pattern must be controlled and suppressed so that the faint planet light is detectable
above the otherwise bright glare of the background.
Diffraction effects are minimized by having an unobstructed primary mirror (using an off-axis
telescope design) and requiring the mirror surface to be sufficiently flat at spatial scales
corresponding to the angular separation of star and planet, thus reducing the halo of speckles
arising from imperfections in the mirror itself.
Scattered light is further controlled using two deformable mirrors that operate in concert to
correct for both amplitude and phase irregularities in the wavefront. The remaining scattered
light (after wavefront compensation) is blocked by a Lyot stop located in a pupil plane, and the
diffraction pattern is further tapered using an image-plane stop.
The key conflict arises that a larger telescope at least in principle improves the ability to separate
the planet light from the star, because the planet will appear further away from the image of the
star, but this makes the observatory larger, more technologically challenging, and more
expensive. The principal design trade is therefore to arrive at a primary mirror diameter that
does not impose unrealistic constraints on the coronagraph performance.
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