Exo-C consists of the instrument payload attached to a spacecraft bus through a vibration isolation system. The payload includes the telescope assembly, the primary mirror support structure (PSS), the instrument bench with associated optics, and the star trackers (see figure). The interim Exo-C design is an unobscured Cassegrain telescope with a 1.4-m clear aperture. Two openings in the barrel give the payload radiators a view to cold space to cool the detectors. Additional cutouts allow the proper field of view for the star trackers. There is a scarfed baffle structure at the top in order to observe towards the Sun. Along the height of the barrel are thin cylindrical ribs to suppress stray light.
The spacecraft bus is a notional design based largely on the Kepler spacecraft bus. The orbit, lifetime, power and communications requirements are all very similar to Kepler, leading to similar power, communications, and attitude control systems.Payload Optical Configuration
The optical portion of the payload comprises the telescope and instrument bench. The instrument has two main subsections: the wavefront control optics and the coronagraph. The control subsection contains a fine-guidance sensor (FGS) and a low-order wavefront sensor (LOWFS) used for precision pointing and real-time wavefront error correction, respectively. The final focal planes are the imager and the integral field spectrograph (IFS).
The instrument bench is located laterally with respect to the telescope axis, in a plane parallel to the telescope axis and offset to one side. The volume available in this configuration for the packaging of the instrument assembly enables a minimum number of fold mirrors, and provides for low angles-of-incidence reflections that minimize adverse polarization effects.
The interim design supports three candidate coronagraph architectures that provide varying degrees of contrast, inner working angle, and throughput . The figure below shows an example for the baseline Hybrid-Lyot Coronagraph (HLC), however the Vector Vortex (VV) and phase-induced amplitude apodization (PIAA) architectures are still under consideration.
In the figure, light from the telescope M1 and M2 mirrors enters from the left, focuses at the field baffle, and is re-collimated by the telescope M3 to form a pupil image. This is followed by Deformable fold mirror 1/fine-steering mirror (DFM1/FSM) located at a pupil image, Deformable fold mirror 2 (DFM2), and Focusing mirror M4 which creates a stellar image on the coronagraphic mask. Starlight is split off to the FGS/LOWFS, while the planet light continues to collimating mirror M5. The beam then passes through a Lyot stop, which focuses the light onto either the imaging camera or the IFS (the specific destination being selected by a movable flip mirror)/. Optical filters upstream of the flip mirror select the observing bandpass. All mirrors within the instrument are either flat (DMs, flip mirror) or off-axis concave paraboloids. The DMs are the key element which corrects static wavefront errors down to the level needed for high contrast exoplanet imaging.