The VISTA IR Camera has now completed its detailed design phase and is on schedule for delivery to ESO’s Cerro Paranal Observatory in 2006. The camera consists of 16 Raytheon VIRGO 2048x2048 HgCdTe arrays in a sparse focal plane sampling a 1.65 degree field of view. A 1.4m diameter filter wheel provides slots for 7 distinct science filters, each comprising 16 individual filter panes. The camera also provides autoguiding and curvature sensing information for the VISTA telescope, and relies on tight tolerancing to meet the demanding requirements of the f/1 telescope design. The VISTA IR camera is unusual in that it contains no cold pupil-stop, but rather relies on a series of nested cold baffles to constrain the light reaching the focal plane to the science beam. In this paper we present a complete overview of the status of the final IR Camera design, its interaction with the VISTA telescope, and a summary of the predicted performance of the system.
VISTA is a 4-metre survey telescope currently being constructed on the NTT peak of ESO’s Cerro Paranal Observatory. The telescope will be equipped with a dedicated infrared camera providing images of a 1.65 degree field of view. The telescope and camera are of an innovative f/3.26 design with no intermediate focus and no cold stop. The mosaic of 16 IR detectors is located directly at Cassegrain focus and a novel baffle arrangement is used to suppress stray light within the cryostat. The pointing and alignment of the telescope and camera is monitored by wavefront sensing elements within the camera cryostat itself. This paper describes the optical, mechanical, electronic and thermal design of the combined curvature sensor and auto-guider units positioned at the periphery of the camera field of view. Centroid and image aberration data is provided to the telescope control system allowing real time correction of pointing and alignment of the actively positioned M2 unit. Also described are the custom optics, mounted in the camera filter wheel, which are used to perform near on-axis high order curvature sensing. Analysis of the corresponding defocused images allows calibration tables of M1 actuator positions to be constructed for varying telescope declination and temperature.
VISTA is a wide-field survey telescope with a 1.6° field of view, sampled with a camera containing a 4 x 4 array of 2K x 2K pixel infrared detectors. The detectors are spaced so an image of the sky can be constructed without gaps by combining 6 overlapping observations, each part of the sky being covered at least twice, except at the tile edges. Unlike a typical ESO-VLT instrument, the camera also has a set of on-board wavefront sensors. The camera has a filter wheel, a collection of pressure and temperature sensors, and a thermal control system for the detectors and the cryostat window, but the most challenging aspect of the camera design is the need to maintain a sustained data rate of 26.8 Mb/second from the infrared detectors. The camera software needs to meet the requirements for VISTA, to fit into the ESO-VLT software architecture, and to interface with an upgraded IRACE system being developed by ESO-VLT. This paper describes the design for the VISTA camera software and discusses the software development process. It describes the solutions we have adopted to achieve the desired data rate, maximise survey speed, meet ESO-VLT standards, interface to the IRACE software and interface the on-board wavefront sensors to the VISTA telescope software.
The VISTA wide field survey telescope will use the ESO Telescope Control System as used on the VLT and NTT. However the sensors for both auto-guiding and active optics are quite different and so the ESO TCS will require some significant modifications. VISTA will use large format CCDs at fixed locations in the focal plane for auto-guiding and a pair of curvature sensors, also fixed in the focal plane, for wave-front sensing. As a consequence, three reference stars are required for each science observation in contrast to the VLT which uses a single star for both auto-guiding and active optics. This paper will outline the reasons for adopting this design, review how it differs from the VLT/NTT and describe the modifications that are being made to the ESO TCS to enable it to be used for VISTA. It will describe the software that implements auto-guiding and active optics in the VLT TCS and how the design has been adapted to the different requirements of VISTA. This will show how the modular and distributed design of the ESO TCS has enabled it to be adapted to a new telescope with radically different design choices whilst maintaining the existing architecture and the bulk of the existing implementation.
This paper describes the image analysis algorithm developed for VISTA to recover wavefront information from curvature wave front sensor images. This technique is particularly suitable in situations where the defocused images have a limited number of pixels and the intrinsic or null aberrations contribute significantly to distort the images. The algorithm implements the simplex method of Nelder and Mead. The simplex algorithm generates trial wavefront coefficients that are fed into a ray tracing algorithm which in turn produces a pair of defocused images. These trial defocused images are then compared against the images obtained from a sensor, using a fitness function. The value returned from the fitness function is fed back to the simplex algorithm, which then decides how the next set of trial coefficients is produced.
We describe a coronagraph facility built for use with the 4.2 metre William Herschel Telescope (WHT) and its adaptive optics system (NAOMI). The use of the NAOMI adaptive optics system gives an improved image resolution of ~0.15 arcsec at a wavelength of 2.2 microns. This enables our Optimised Stellar Coronagraph for Adaptive optics (OSCA) to null stellar light with smaller occulting masks and thus allows regions closer to bright astronomical objects to be imaged. OSCA is a fully deployable instrument and when in use leaves the focus of the NAOMI beam unchanged. This enables OSCA to be used in conjunction with a number of instruments already commissioned at the WHT. The main imaging camera to be used with OSCA will be INGRID; a 1024×1024 HgCdTe cooled SWIR detector at the NAOMI focus. OSCA can also be used in conjunction with an integral field spectrograph for imaging at visible wavelengths. OSCA provides a selection of 10 different occulting mask sizes from 0.25 - 2.0 arcsec and some with a novel gaussian profile. There is also a choice of 2 different Lyot stops (pupil plane masks). A dichroic placed before the AO system can give us improved nulling when occulting masks larger than the seeing disk are used. We also present results from initial testing and commissioning at the William Herschel Telescope.
An experimental Adaptive Optics system utilizing a low-altitude Rayleigh Laser Guide Star is presented. The current status of the project is described together with plans for future work, with the ultimate goal of providing demonstration Rayleigh LGS AO science on the 4.2m William Herschel Telescope. The implementation of the demonstration system is described in detail as are the technical challenges encountered during the development of the current shared-launch based system. Other papers in this conference describe a proposal for a facility class LGS system for the WHT and the performance of the observatory's NAOMI adaptive optics system.