Sensing starlight rejected from a coronagraph is essential in stabilizing the telescope pointing and wavefront drift, but performance is degraded for dim stars. Laser Metrology (MET) provides a different, complementary sensing method, one that can be used to measure changes in the alignment of the optics at high bandwidth, independent of the magnitude of the host star. Laser metrology measures changes in the separation of optical fiducial pairs, which can be separated by many meters. The principle of operations is similar to the laser metrology system used in LISA-Pathfinder to measure the in-orbit displacement between two test masses to a precision of ~10 picometers. In closed loop with actuators, MET actively maintains rigid body alignment of the front-end optics, thereby eliminating the dominant source of wavefront drift. Because MET is not photon starved, it can operate at high bandwidth and feed-forward secondary-mirror jitter to a fast-steering mirror, correcting line-of-sight errors. In the case of a segmented, active primary mirror, MET provides six degrees of freedom sensing, replacing edge sensors. MET maintains wavefront control even during attitude maneuvers, such as slews between target stars, thereby avoiding the need to repeat time-consuming speckle suppression. These features can significantly improve the performance and observational efficiency of future large-aperture space telescopes equipped with internal coronagraphs. We evaluate MET trusses for various proposed monolithic and segmented spacebased coronagraphs and present the performance requirements necessary to maintain contrast drift below 10-11.
The Planetary Imaging Concept Testbed Using a Rocket Experiment (PICTURE 36.225 UG) was designed
to directly image the exozodiacal dust disk of ǫ Eridani (K2V, 3.22 pc) down to an inner radius of 1.5 AU.
PICTURE carried four key enabling technologies on board a NASA sounding rocket at 4:25 MDT on October
8th, 2011: a 0.5 m light-weight primary mirror (4.5 kg), a visible nulling coronagraph (VNC) (600-750 nm), a
32x32 element MEMS deformable mirror and a milliarcsecond-class fine pointing system.
Unfortunately, due to a telemetry failure, the PICTURE mission did not achieve scientific success. Nonetheless,
this flight validated the flight-worthiness of the lightweight primary and the VNC. The fine pointing system,
a key requirement for future planet-imaging missions, demonstrated 5.1 mas RMS in-flight pointing stability.
We describe the experiment, its subsystems and flight results. We outline the challenges we faced in developing
this complex payload and our technical approaches.
The canonical Zernike phase-contrast technique transforms a phase object in one plane into an intensity object in the
conjugate plane. This is done by applying a static π/2 phase shift to the central core (~ λ/D) of the PSF which is
intermediate between the input and output planes. Here we present a new architecture for this sensor. First, the optical
system is simple and all reflective. Second, the phase shift in the central core of the PSF is dynamic and or arbitrary size.
This common-path, all-reflective design makes it minimally sensitive to vibration, polarization and wavelength. We
review the theory of operation, describe the optical system, summarize numerical simulations and sensitivities and
review results from a laboratory demonstration of this novel instrument.
We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed
in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror
in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the
deformable mirror to order of 1 nm rms.
Direct detection of exo-planets from the ground will become a reality with the advent of a new class of extreme-adaptive
optics instruments that will come on-line within the next few years. In particular, the Gemini Observatory will be
developing the Gemini Planet Imager (GPI) that will be used to make direct observations of young exo-planets. One
major technical challenge in reaching the requisite high contrast at small angles is the sensing and control of residual
wave front errors after the starlight suppression system. This paper will discuss the nature of this problem, and our
approach to the sensing and control task. We will describe a laboratory experiment whose purpose is to provide a means
of validating our sensing techniques and control algorithms. The experimental demonstration of sensing and control will
be described. Finally, we will comment on the applicability of this technique to other similar high-contrast instruments.
We describe the advantages of a nulling coronagraph instrument behind a single aperture space telescope for detection and spectroscopy of Earth-like extrasolar planets in visible light. Our concept synthesizes a nulling interferometer by shearing the telescope pupil into multiple beams. They are recombined with a pseudo-achromatic pi-phase shift in one arm to produce a deep null on-axis, attenuating the starlight, while simultaneously transmitting the off-axis planet light. Our nulling configuration includes methods to mitigate stellar leakage, such as spatial filtering by a coherent array of single mode fibers, balancing amplitude and phase with a segmented deformable mirror, and post-starlight suppression wavefront sensing and control. With diffraction limited telescope optics and similar quality components in the optical train (λ/20), suppression of the starlight to 10-10 is readily achievable. We describe key features of the architecture and analysis, present the status of key experiments to demonstrate wide bandwidth null depth, and present the status of component technology development.