We evaluate in detail the stability requirements for a band-limited coronagraph with an inner working angle as small as 2 λ/D coupled to an off-axis, 3.8-m diameter telescope. We have updated our methodologies since presenting a stability error budget for the Terrestrial Planet Finder Coronagraph mission that worked at 4 λ/D and employed an 8th-order mask to reduce aberration sensitivities. In the previous work, we determined the tolerances relative to the total light leaking through the coronagraph. Now, we separate the light into a radial component, which is readily separable from a planet signal, and an azimuthal component, which is easily confused with a planet signal. In the current study, throughput considerations require a 4th-order coronagraph. This, combined with the more aggressive working angle, places extraordinarily tight requirements on wavefront stability and opto-mechanical stability. We find that the requirements are driven mainly by coma that leaks around the coronagraph mask and mimics the localized signal of a planet, and pointing errors that scatter light into the background, decreasing SNR. We also show how the requirements would be relaxed if a low-order aberration detection system could be employed.
The Terrestrial Planet Finder Coronagraph (TPF-C) demands extreme wave front control and stability to achieve its goal of detecting earth-like planets around nearby stars. We describe the performance models and error budget used to evaluate image plane contrast and derive engineering requirements for this challenging optical system. We show that when the coronagraph is coupled to an 8 th -order band-limited mask, the performance is limited by shearing of the starlight beam across imperfect optics (a.k.a. beam walk), and that this in turn demands tight rigid body pointing, submilliarcsecond fine guiding, high-quality optics, and sub-micron positional stability of the optics including the secondary mirror. Additionally we show that the stability of low-order aberrations (focus, astigmatism, coma, and trefoil) is required to be ~ 2-4 Angstroms, while higher-order modes must remain stable to a few picometers.
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