Sparse Aperture Masking (SAM) has recently been commissioned on SPHERE, the VLTs new adaptive optics high resolution imager. SAM extends the capabilities of SPHERE by providing high contrast measurements at and beyond the traditional diffraction limit. SAM can be used in conjunction with each of the SPHERE modules (IRDIS, IFS and ZIMPOL), allowing dual band imaging in the visible and near-infrared, near-infrared integral field spectroscopy, and polarized differential imaging in the visible and near-infrared. In this paper we report information relevant for observers as well as some commissioning observations.
We present several engineering and algorithmic aspects of non-redundant masking (NRM) as they pertain to the James Webb Space Telescope (JWST). NRM's fundamental data structures have multiple uses in wavefront sensing as well in as high resolution imaging. Kernel phases are a full aperture generalization of NRM applicable to moderate and high Strehl ratio images. Eigenphases, the complement to kernel phases, provide wavefront sensing with single in-focus images. Thus this set of phases is relevant to wavefront sensing with routine science images on any Nyquist-sampled science camera on JWST. We attempt to organize these apparently diverse aspects of such Fizeau interferometry into an inter-related picture in order to facilitate their development and potential use on JWST and future space telescopes.
Segmented mirrors have quickly become an integral part of large telescope design in optical astronomy. They mitigate many of the problems associated with monolithic mirrors, such as rigidity, fabrication and transport, and even allow for foldable primary mirrors such as for the James Webb Space Telescope (JWST). However, one significant disadvantage is the need to cophase the separate segments to ensure they conform to the optimum mirror shape. Many cophasing approaches have been proposed and employed in practice, but all suffer from significant problems. Most notably the introduction of non-common path error, a persistent problem plaguing both the fields of active and adaptive optics. One recent proposed cophasing algorithm eliminates non-common path error by removing the requirements for additional hardware and instead concentrating on measuring piston and tip/tilt aberrations by their effects on images. Fizeau Interferometric Cophasing of Segmented Mirrors (FICSM) yields a large capture range and allows phasing to interferometric precision; for these reasons it was recently selected as the backup phasing strategy for the JWST. Here we present an overview of numerical simulations and results of optical testbeds, including the first lab demonstration of FICSM which successfully phased a segmented mirror with more than 5 wavelengths of piston to an RMS of 25nm, a result consistent with the limit set by the accuracy of segment motion. These results suggest this approach is well suited to the task of segment cophasing for future space missions.
The Gemini Planet Imager (GPI) Extreme Adaptive Optics Coronograph contains an interferometric mode: a 10-hole non-redundant mask (NRM) in its pupil wheel. GPI operates at Y, J, H, and K bands, using an integral field unit spectrograph (IFS) to obtain spectral data at every image pixel. NRM on GPI is capable of imaging with a half resolution element inner working angle at moderate contrast, probing the region behind the coronagraphic spot. The fine features of the NRM PSF can provide a reliable check on the plate scale, while also acting as an attenuator for spectral standard calibrators that would otherwise saturate the full pupil. NRM commissioning data provides details about wavefront error in the optics as well as operations of adaptive optics control without pointing control from the calibration system. We compare lab and on-sky results to evaluate systematic instrument properties and examine the stability data in consecutive exposures. We discuss early on-sky performance, comparing images from integration and tests with the first on-sky images, and demonstrate resolving a known binary. We discuss the status of NRM and implications for future science with this mode.