ACCESS is one of four medium-class mission concepts selected for study in 2008-9 by NASA's Astrophysics Strategic
Mission Concepts Study program. ACCESS evaluates a space observatory designed for extreme high-contrast imaging
and spectroscopy of exoplanetary systems. An actively-corrected coronagraph is used to suppress the glare of diffracted
and scattered starlight to contrast levels required for exoplanet imaging. The ACCESS study considered the relative
merits and readiness of four major coronagraph types, and modeled their performance with a NASA medium-class space
telescope. The ACCESS study asks: What is the most capable medium-class coronagraphic mission that is possible with
telescope, instrument, and spacecraft technologies available today? Using demonstrated high-TRL technologies, the
ACCESS science program surveys the nearest 120+ AFGK stars for exoplanet systems, and surveys the majority of
those for exozodiacal dust to the level of 1 zodi at 3 AU. Coronagraph technology developments in the coming year are
expected to further enhance the science reach of the ACCESS mission concept.
ACCESS (Actively-Corrected Coronagraph for Exoplanet System Studies) develops the science and engineering case for
an investigation of exosolar giant planets, super-earths, exo-earths, and dust/debris fields that would be accessible to a
medium-scale NASA mission. The study begins with the observation that coronagraph architectures of all types (other
than the external occulter) call for an exceptionally stable telescope and spacecraft, as well as active wavefront
correction with one or more deformable mirrors (DMs). During the study, the Lyot, shaped pupil, PIAA, and a number
of other coronagraph architectures will all be evaluated on a level playing field that considers science capability
(including contrast at the inner working angle (IWA), throughput efficiency, and spectral bandwidth), engineering
readiness (including maturity of technology, instrument complexity, and sensitivity to wavefront errors), and mission
cost so that a preferred coronagraph architecture can be selected and developed for a medium-class mission.
Advances in the fabrication of adaptive ceramic modules result in compact components that can be adapted to a variety of applications. The adaptive tertiary is a unique component that can provide a variety of functions. Individual modules integrated with a tip/tilt stage can provide auxiliary phasing control of future large segmented telescopes. In addition, each module can contain hundreds of individual actuators allowing the devices to also act as high-spatial frequency deformable mirrors. Measurements have shown that pathfinder devices can be assembled well within the capture range
of the devices. This demonstrates the overall feasibility of the concept.
We present a study of the operating characteristics of the Xinetics Inc. deformable mirror and the driver electronics built by the Jet Propulsion Laboratory for Palomar Observatory’s adaptive optics project. This mirror, the first of its type built by Xinetics Inc., contains 349 PMN actuators which control a 2 millimeter thick mirror surface coated with protected silver. Measurements are separated into static and dynamic categories. The static tests determine the unpowered figure of the mirror surface, the influence of solitary actuators on the mirror surface and how the actuators move as a function of voltage applied, including considerations of hysteresis and creep. We have been able to flatten the mirror surface to an rms value of 19 nanometers. In the dynamic tests, we have resolved the motion of individual actuators whose voltages were changed at frequencies up to 1.5 kHz. The purpose of this study is to show that this deformable mirror has specific characteristics that must be determined in order to optimize its control.
Unique as an optical component, the deformable mirror must have a quality optical surface which can be contoured and controlled to a fraction of wavelength of light. The key element is the actuator. The actuator must provide precision motion for nanometer wavefront control and provide a stable structure for high quality optical figure. It must be dimensionally stable in terms of both time and temperature. Being in a dynamic environment, it must also withstand high tensile loads and high cycle fatigue. Despite its mechanical, optical, and electrical functions, it simply must work when the voltage is applied during any season and for any reason.