MASCARA, the Multi-site All-Sky CAmeRA, is a project aimed at finding exoplanets transiting the brightest stars, in the V = 4 to 8 magnitude range, currently probed neither by space nor by ground based surveys. The target population for MASCARA consists mostly of hot Jupiters, for which the average transit depth is around 1%, and hot Neptunes. In order to achieve consistently a signal-to-noise ratio of better than 100 per hour at magnitude 8, MASCARA is based on three main concepts; simplicity stability and calibration.
MASCARA was designed with a minimum number of moving components. Five fixed, shutter-less, Peltier-cooled cameras, fitted with standard Canon 24 mm f/1.4 lenses are operating in a temperature controlled environment. Each camera constantly stares at the same patch of the sky. The exposure time is set to 6.4 seconds, keeping trailing of stars and saturation to a minimum while allowing for continuous exposures. Each camera is connected to its own control and data processing computer, allowing for fully independent operation of each of the cameras. Each camera takes between 4,000 and 7,000 exposures per night, which are reduced locally to produce un-calibrated light curves for the up to ~40,000 pre-selected stars, as well as image stacks of 50 images. For each set of 50 images, astrometry of the solution is verified to monitor drifts in the station. Currently both reduced data as well as raw data (~500 GB/night) are transferred to a central data repository, but for stations with less bandwidth, potentially only the reduced data could be transferred. MASCARA currently only permanently stores the reduced light curves and binned image stacks, deleting the raw images after one month.
After transfer, the raw light curves are self-calibrated in batches of 2-4 weeks, removing the spatially varying transmission of the camera, the impact of crowding and spatially variable PSF, and the time variable transmission of the atmosphere. Using a combination of SysRem and flagging of data points that are impacted by known artifacts (moon, sun, clouds, etc.), we have demonstrated a photometric stability of MASCARA down to 0.3% at magnitude V=7.7 within 5.3 minutes.
The next generation of extremely large telescopes (ELTs) have the potential to image habitable rocky planets, if suitably optimized. This will require the development of fast high order "extreme" adaptive optics systems for the ELTs. Located near the excellent site of the future GMT, the Magellan AO system (MagAO) is an ideal on-sky testbed for high contrast imaging development. Here we discuss planned upgrades to MagAO. These include improvements in WFS sampling (enabling correction of more modes) and an increase in speed to 2000 Hz, as well as an H2RG detector upgrade for the Clio infrared camera. This NSF funded project, MagAO-2K, is planned to be on-sky in November 2016 and will significantly improve the performance of MagAO at short wavelengths. Finally, we describe MagAO-X, a visible-wavelength extreme-AO "afterburner" system under development. MagAO-X will deliver Strehl ratios of over 80% in the optical and is optimized for visible light coronagraphy.
The direct detection of low-mass planets in the habitable zone of nearby stars is an important science case for future E-ELT instruments such as the mid-infrared imager and spectrograph METIS, which features vortex phase masks and apodizing phase plates (APP) in its baseline design. In this work, we present end-to-end performance simulations, using Fourier propagation, of several METIS coronagraphic modes, including focal-plane vortex phase masks and pupil-plane apodizing phase plates, for the centrally obscured, segmented E-ELT pupil. The atmosphere and the AO contributions are taken into account. Hybrid coronagraphs combining the advantages of vortex phase masks and APPs are considered to improve the METIS coronagraphic performance.
The Apodizing Phase Plate (APP) is a phase-only pupil-plane coronagraph that suppresses starlight in a D-shaped region from 2 to 7 λ D around a target star. Its performance is insensitive to residual tip-tilt variations from the AO system and telescope structure. Using liquid crystal technology we develop a novel and improved version of the APP: the broadband vector Apodizing Phase Plate (vAPP). The vAPP prototype consists of an achromatic half-wave retarder pattern with a varying fast axis encoding phase structure down to 25 microns. The fast axis encodes the required phase pattern through the vector phase, while multiple twisting liquid crystal layers produce a nearly constant half-wave retardance over a broad bandwidth. Since pupil phase patterns are commonly designed to be antisymmetric, two complementary PSFs are produced with dark holes on opposite sides.
We summarize results of the characterization of our latest vAPP prototype in terms of pupil phase reconstruction and PSF contrast performance. The liquid crystal patterning technique allows us to manufacture more extreme phase patterns than was possible before. We consider phase-only patterns that yield higher contrasts and better inner working angles than previous APPs, and patterns that produce dark regions 360 degrees around the PSF core. The possibility of including a phase ramp into the coronagraph is demonstrated, which simplifies the vAPP into a single optic. This additional phase ramp removes the need for a quarter-wave plate and a Wollaston prism, and enables the simplified implementation of a vAPP in a filter wheel at a pupil-plane location. Since the phase ramp is analogous to a polarization grating, it generates a (polarized) spectrum of a planet inside the dark hole, and thus allows for instantaneous characterization of the planet.
Utilizing the so-called vector phase of polarized light, both focal-plane coronagraphs (e.g. the Vector Vortex Coronagraph) and pupil-plane coronagraphs (e.g. the vector Apodizing Phase Plate) are powerful components for high-contrast imaging. These coronagraphs can be built and optimized with polarization techniques and liquid crystal technology, that enable patterning at the micron level and furnish broad-band performance. The contrast between the residual starlight and the (polarized) reflected light off exoplanets can be further bridged by incorporating sensitive, dual-beam imaging polarimetry. As vector-phase coronagraphs use polarizers to enhance their performance, we introduce optimally integrated solutions that combine advanced coronagraphy and polarimetry. For both the VVC and the vAPP we present polarization beam-splitting concepts, with polarization analyzers either behind or in front of the coronagraphic optics. We discuss design solutions for the implementation of polarization optics, and set the stage for a trade-off between the improvement of coronagraphic and polarimetric performance and the ensuing degradation on the high-contrast imaging performance due to wavefront errors.
We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working with an extremely high resolution (larger than 150x150 correcting elements). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. Different approaches are considered in numerical simulations, and laboratory experiments are shown to confirm the results. We compare the performances of the standard Gerchberg-Saxton algorithm, Fast and Furious (use of small-phase assumption to take advantage of linearisation) and recently proposed phase-retrieval methods based on convex optimisation. The results indicate that the calibration task is easiest with algorithms similar to Fast and Furious, at least in the framework we considered.
The Multi-site All-sky CAmeRA MASCARA is an instrument concept consisting of several stations across the globe,
with each station containing a battery of low-cost cameras to monitor the near-entire sky at each location. Once all
stations have been installed, MASCARA will be able to provide a nearly 24-hr coverage of the complete dark sky, down
to magnitude 8, at sub-minute cadence. Its purpose is to find the brightest transiting exoplanet systems, expected in the
V=4-8 magnitude range - currently not probed by space- or ground-based surveys. The bright/nearby transiting planet
systems, which MASCARA will discover, will be the key targets for detailed planet atmosphere observations. We
present studies on the initial design of a MASCARA station, including the camera housing, domes, and computer
equipment, and on the photometric stability of low-cost cameras showing that a precision of 0.3-1% per hour can be
readily achieved. We plan to roll out the first MASCARA station before the end of 2013. A 5-station MASCARA can
within two years discover up to a dozen of the brightest transiting planet systems in the sky.
Non-common path (NCP) errors that lie downstream from the wavefront sensor (WFS) in an AO setup can’t
be directly corrected by the WFS and end up altering the science images by introducing quasi-static speckles.
These speckles impose limits to the direct imaging of exoplanets, debris disks and other objects for which we
require high contrast. Phase-sorting interferometry (PSI) uses WFS residuals as interferometric probes to the
speckles. With the retrieved amplitude and phase the deformable mirror can be adjusted to remove the speckles.
Previously PSI has been demonstrated to correct -to first order- the non-common path error on-sky at the MMTO
in Arizona. We present an AO laboratory testbed and the techniques used to determine the properties of PSI;
the influence of the time synchronisation between WFS and science camera, the achromacity of the atmosphere
and other limiting factors. Furthermore we test the performance of the PSI method when coronagraphs such
as apodizing phase plates, Lyot masks and 4QPMs are introduced to the setup. Lastly this setup enables us to
rapidly prototype high-contrast imaging techniques.
The apodizing phase plate (APP) is a solid-state pupil optic that clears out a D-shaped area next to the core
of the ensuing PSF. To make the APP more efficient for high-contrast imaging, its bandwidth should be as
large as possible, and the location of the D-shaped area should be easily swapped to the other side of the PSF.
We present the design of a broadband APP that yields two PSFs that have the opposite sides cleared out.
Both properties are enabled by a half-wave liquid crystal layer, for which the local fast axis orientation over
the pupil is forced to follow the required phase structure. For each of the two circular polarization states, the
required phase apodization is thus obtained, and, moreover, the PSFs after a quarter-wave plate and a polarizing
beam-splitter are complementary due to the antisymmetric nature of the phase apodization. The device can be
achromatized in the same way as half-wave plates of the Pancharatnam type or by layering self-aligning twisted
liquid crystals to form a monolithic film called a multi-twist retarder. As the VAPP introduces a known phase
diversity between the two PSFs, they may be used directly for wavefront sensing. By applying an additional
quarter-wave plate in front, the device also acts as a regular polarizing beam-splitter, which therefore furnishes
high-contrast polarimetric imaging. If the PSF core is not saturated, the polarimetric dual-beam correction can
also be applied to polarized circumstellar structure. The prototype results show the viability of the vector-APP