Dark hole maintenance with the Self Coherent Camera

The goal of this proposal is to improve the exoplanet imaging performance of large space telescopes by using active wavefront control during the actual science observations. The current operational baseline for direct imaging missions is to create a dark hole on a bright reference star, and then slew back to the target of interest for long science exposures. However, the dark hole must be regenerated occasionally during an observing sequence due to drifts in the system. This reduces the effective on-target time and furthermore, the telescope must be ultra-stable to guarantee similar dark hole performance on the science target and the reference star. We propose to use recent developments around the self-coherent camera (SCC) concept for active wavefront control during the science observations to alleviate both challenges. Not only will it increase the duty cycle of the science observations leading to a larger number of observed targets during the mission lifetime, but active wavefront control will also relax the telescope's stability requirements. To achieve active compensation, we need to be able to sense the wavefront at the science focal plane. The SCC is an integrated coronagraph and wavefront sensor (WFS) that uses a spatial modulation of the stellar speckles to estimate the complex electric field. The spatial modulation is achieved by adding an off-axis reference hole in the Lyot stop (LS) of the coronagraph. Only an on-axis source that hits the coronagraph's focal plane mask will scatter light outside the geometric pupil and therefore into the reference hole. This creates Fizeau fringes for the on-axis source, which allows us to measure the incoming wavefront in both phase and amplitude with a single measurement. However, the reference hole must be placed far away from the pupil edge to create a high enough frequency interference fringe such that it can be separated from the stellar speckles. This has several disadvantages for volume constrained instruments, which is a typical situation for space telescopes. The large distance of the pinhole requires significantly oversize optics, which make it difficult to fit the SCC in standard optical layouts. Secondly, at large distances from the pupil, the throughput for the intensity modulation becomes quite weak and difficult to detect. And finally, because the speckles are modulated at high frequency, the focal plane must be sampled with at least 5 pixels per spatial resolution element. This makes it difficult to use the SCC in combination with an Integral-Field Spectrograph (IFS) for exoplanet characterization because those have limited number of spatial pixels. In the past couple of years, several new variations on the SCC have been created that remove the restrictions of the classic SCC. To go from design to practice, work is needed in developing optimal system configurations, quantifying, and demonstrating the performance improvements in hardware in the loop experiments in a more relevant environment, and determining feasibility given available resources. We propose to conduct laboratory tests in existing coronagraphic wavefront control testbeds at the University of Arizona to demonstrate the modified SCC for coronagraphic wavefront control. We will demonstrate the modified SCC on two testbeds that emulate different telescope architectures: an off-axis monolithic telescope with a vector vortex coronagraph (VVC) and, an on-axis telescope with a phase induced amplitude apodization complex mask coronagraph (PIAACMC). Significance: Analysis and simulations have shown that active wavefront control can relax the stability requirements by orders of magnitude. This will enable future large space-based segmented telescopes to achieve the ~1e-10 contrast necessary to spectroscopically characterize Earth-like planets orbiting Sun-like stars.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground-based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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