We present the development and testing of focal plane wavefront control techniques that utilize microwave kinetic inductance detectors (MKIDs) as a focal plane IFU. MKIDs are ideally suited to this application, as they are energy resolving, and have single photon sensitivity, zero read noise, and microsecond time resolution. These characteristics enable much higher feedback rates than conventional systems; for the first time, focal plane measurements may be used to correct atmospheric aberrations in addition to quasistatics. A variety of approaches are under development, including conventional speckle nulling, as well as more advanced approaches such as linear dark field control.
We have developed an MKID-specific speckle nulling code for DARKNESS, a 10,000 pixel MKID IFU behind the stellar double coronagraph (SDC) and PALM-3000 (P3K) AO system at Palomar observatory. Our code implements the algorithm described in (Bottom, et. al, 2016) with minor modifications. To facilitate high feedback rates on sky, the code is optimized for computational speed, and implements low-latency communications to both P3K and the MKID readout. It is also capable of integrating with P3K in closed loop operation. Using our code, we have demonstrated quasi-static speckle nulling at a 1 Hz feedback rate in the laboratory. We hope to demonstrate rates ~10-100 Hz in the near future.
In addition to continuing our work with DARKNESS, we will adapt our code to MEC, a 20,000 pixel MKID IFU behind the Subaru coronagraphic extreme adaptive optics system (SCExAO) at Subaru observatory. MEC is scheduled to commission in January 2018.
Direct Imaging of exoplanets is one of the most technically difficult techniques used to study exoplanets, but holds immense promise for not just detecting but characterizing planets around the nearest stars. Ambitious instruments at the world’s largest telescopes have been built to carry out this science: the Gemini Planet Imager (GPI), SPHERE at VLT, SCExAO at Subaru, and the P1640 and Stellar Double Coronagraph (SDC) at Palomar. These instruments share a common archetype consisting of an extreme AO system feeding a coronagraph for on-axis stellar light rejection followed by a focal plane Integral Field Spectrograph (IFS). They are currently limited by uncontrolled scattered and diffracted light which produces a coherent speckle halo in the image plane. A number of differential imaging schemes exist to mitigate these issues resulting in star-planet contrast ratios as deep as ~10^-6 at low angular separations. Surpassing this contrast limit requires high speed active speckle nullification from a focal plane wavefront sensor (FPWS) and new processing techniques.
MEC, the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera, is a J-band IFS module behind Subaru Telescope’s SCExAO system. MEC is capable of producing an image cube several thousand times a second without the read noise that dominates conventional high speed IFUs. This enables it to integrate with SCExAO as an extremely fast FPWS while eliminating non-common path aberrations by doubling as a science camera. Key science objectives can be further explored if longer wavelengths (H and K band) are simultaneously sent to CHARIS for high resolution spectroscopy. MEC, to be commissioned at Subaru in early 2018, is the second MKID IFS for high contrast imaging following DARKNESS’ debut at Palomar in July 2016.
MEC will follow up on young planets and debris disks discovered in the SEEDS survey or by Project 1640 as well as discover self-luminous massive planets. The increased sensitivity, combined with the advanced coronagraphs in SCExAO which have inner working angles (IWAs) as small as 0.03” at 1.2 μm, allows young Jupiter-sized objects to be imaged as close as 4 AU from their host star. If the wavefront control enabled by MEC is fully realized, it may begin to probe the reflected light of giant planets around some nearby stars, opening a new parameter space for direct imaging targeting older stars. While direct imaging of reflected light exoplanets is the most challenging of the scientific goals, it is a promising long-term path towards characterization of habitable planets around nearby stars using Extremely Large Telescopes (ELTs). With diameters of about 30-m, an ELT can resolve the habitable zones of nearby M-type stars, for which an Earth-sized planet would be at ~10^-7 contrast at 1 μm. This will complement future space-based high contrast optical imaging targeting the wider habitable zones of sun-like stars for ~10^-10 contrast earth analogs.
We will present lessons learned from the first few months of MEC’s operation including initial lab and on-sky (weather permitting) results. We already have preliminary data from Palomar testing a new statistical speckle discrimination post-processing technique using the photon arrival time measured with MKIDs. Residual stellar light in the form of a speckle masquerading as a planetary companion is pulled from a modified Rician distribution and can be statistically discerned from a true off-axis Poisson point source. Additionally, the progress of active focal plane wavefront control will be briefly discussed.