Decreasing the Inner Working Angle in High-Contrast Imaging by Pupil Replication

摘要

Imaging for exo-planet detection requires both high contrast and a small inner working angle. We show that, for several of the techniques proposed so far to achieve this, the inner working angle can be reduced by adding pupil replication between the telescope and the high contrast imaging system. Using pupil replication, the on-axis image of the star is decreased to a size smaller than the diffraction limit of the telescope, and off axis the point spread function of the planet undergoes minor changes, contained within the envelope of the point spread function of the telescope; the spectrum remains unchanged. The principle of pupil replication was proven experimentally and can be effected by a small-sized, high throughput optical system added between the telescope and the high contrast imaging system. High contrast imaging systems to which pupil replication has been found to be applicable so far include apodisation techniques like pupil apodisation, aperture masks, image plane masks, coronagraphs and combinations. Mathematical assessment and simulations of the sensitivity of pupil replication to optical errors show that the requirements for this system are the same as those for the primary telescope - pupil replication effectively remaps the output pupil of the telescope to the input pupil of the high contrast imaging system. Our results in this paper aim to show, in a realistic set-up, the feasibility of an improvement of the inner working angle by a factor of 4 using four-fold replication optics while maintaining the contrast performance. We do this through analysis of the pupil replication principle including off axis behavior when applied to high contrast imaging systems using pupil apodisation or a shaped mask. We specifically look at the situations similar to that of the Terrestrial Planet Finder Coronagraph and Darwin. We found that an inner working angle of 30 mas can be achieved with a contrast of 10~(-10) and a large field of view without increasing the requirements except for the pointing.