Susan P. Worswick

Fiber-fed spectrograph for the 4.2-m William Herschel Telescope

WYFFOS is a spectrograph, currently being assembled, for multi- object spectroscopy using fiber-optic inputs. Initially 126 fibers, later 150 finer fibers (described in a separate paper) will enter it from the prime focus of the 4.2-meter William Herschel Telescope. WYFFOS uses the Baranne white-pupil, Littrow- mount optical system, given a wide 2D filed of view at the grating, high efficiency and a wide range of spectral resolutions. It has a twice-through lenticular collimator and a Schmidt-type cryogenic camera, operating from 350 nm to one micron. We shall install the spectrograph on an optical bench on one of the Nasmyth platforms of the telescope. One advantage of using the optical bench is that the spectrographs components can be re-arranged, converting the reflection-grating instrument to a transmission-grating instrument. Possible future enhancements include increasing the number of fibers to 300 in the transmission grating mode, and adding a dioptric camera.

NAOMI adaptive optics system for the 4.2-m William Herschel telescope

NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) is a recently completed and commissioned astronomical facility on the 4.2m William Herschel Telescope. The system is designed to work initially with Natural Guide Stars and also to be upgradeable for use with a single laser guide star. It has been designed to work with both near infrared and optical instrumentation (both imagers and spectrographs). The system uses a linearised segmented adaptive mirror and dual-CCD Shack-Hartmann wavefront sensor together with a multiple-DSP real-time processing system. Control system parameters can be updated on-the-fly by monitoring processes and the system can self-optimize its base optical figure to compensate for the optical characteristics of attached scientific instrumentation. The scientific motivation, consequent specification and implementation of NAOMI are described, together with example performance data and information on future upgrades and instrumentation.

Integration and testing of the WYFFOS fiber-fed spectrograph for the William Herschel Telescope

The prime focus of the William Herschel telescope (WHT) provides a field of one degree which is to be used for fiber spectroscopy. The WYFFOS spectrograph, based on a Baranne white pupil design, is located on one of the Nasmyth platforms of the telescope and is fed from prime focus by 126, 26 meter long fibers. The system is designed for a wavelength range of 350 nm to 1.1 microns using both transmission and reflection gratings. This paper describes the integration and testing of the spectrograph undertaken in the laboratory. The image quality and spectral resolving power have been measured. The scattered light and amount of cross-talk between adjacent fibers has been assessed. The provision of calibration illumination and facilities for back illumination of the fibers, a requirement needed by the fiber positioner, is discussed.

Engineering and testing of prime focus fiber feeds on the William Herschel Telescope

The prime focus of the William Herschel telescope provides a wide field (one degree) for fiber spectroscopy. The production of the fiber feeds, which run from the prime focus to one of the Nasymyth platforms, has required the development of a multiple fiber to fiber connector together with techniques for mounting micro-lenses at the input to the spectrography. The protection of the fibers on the telescope, coupled with careful routing, has led to good focal ratio degradation (FRD) performance. A system for measuring the variation in transmission of the fibers on the telescope has been produced; the technique allows these measurements to be made under normal daytime lighting conditions.

Optical design concept of the 4-m visible and infrared survey telescope for astronomy

This paper describes the optical design of VISTA (Visible and Infrared Survey Telescope for Astronomy) in the infrared and visible configurations. The design is based on a fast quasi Ritchey-Chretien Telescope with an f/1 primary and an f/3 secondary. The large field of views available: 1.65 degrees in the IR and 2.1 degrees in the visible, makes use of the latest technology in optical materials, active optics and large arrays of detectors. The residual third order spherical aberration for on-axis images introduced in the two-mirror design is used to compensate the residual spherical aberration in the field corrector lenses. The infrared camera is included in the telescope optimization, letting the radii of curvature and conic constants of the two mirrors in the telescope vary in order to get the best performance across the entire IR detector array. It also contains an innovative cold baffle with a special black coating. The visible camera contains an ADC incorporated in the field corrector lenses. The acquisition, guiding and wavefront sensing of this telescope is integrated in the instruments.

Integral approach to the design of a 100-m telescope: its adaptive optics and instrumentation

The design of any modern astronomical telescope requires close interaction between the science requirements, the optical and mechanical design of the telescope and its instrumentation. In addition new, large aperture, telescopes will need to have adaptive optics as an integral part of the concept. This paper discusses optical concepts for the telescope and instruments, highlighting technology challenges.

Performance criteria for the optical designs for VISTA

The Visible and Infra-red Survey Telescope for Astronomy, or VISTA, is a UK funded four meter class wide-field, infrared and optical survey telescope to be situated in Chile. The telescope, which can be regarded as a two-channel camera, provides a one-degree, infrared field and a two-degree, optical field. The project goal of low running cost requires minimal intervention by support staff. The optical configuration of the telescope must meet the scientific requirements while delivering a stable instrument profile for the 12-year duration of the survey. Accurate calibration of throughput, image quality and field distortion will be essential to the photometric and astrometric quality of that survey. The process for selection of optical design options is described and discussed with particular reference to meeting the functional and performance requirements, determined from the scientific specification, and to achieving a predictable and reliable image quality. The practical limitations imposed by budgetary and operational requirements are assessed for their role as contributory design drivers.

Optical design of a visible and infrared survey telescope for astronomy

A number of preliminary optical designs for a new generation of 4 meter astronomical survey telescopes have been investigated. These have large fields of view and operate in both the visible and near IR astronomical wavebands. Typical requirements for dual band systems of this type are presented. Two designs for prime focus refractive field corrector systems are presented, with and without aspheric surfaces. The use of aspheric surfaces on the field corrector lenses is shown to allow a large field of view to be achieved. A design for a three mirror reflective system is presented which also allows a 2 degree field to be achieved. An IR imager, based on a modified Schmidt camera, is presented which allows a 1 degree field of view to be achieved. Additionally, a modified Ritchey-Chretien telescope, incorporating refracting field corrector lenses is presented. This design provides a large field of view over both the visible and IR wavebands. The mechanical constraints of combining these systems into dual channel systems are also discussed.

Progress on the WHT natural guide star AO system (NAOMI)

A conceptual design for the WHT Natural Guide Star Adaptive Optics system, now called NAOMI (Nasmyth Adaptive Optics for Multi-purpose Instrumentation) was presented at the 1995 SPIE meeting1. Although the general principle of using off-axis paraboloidal mirrors (OAP) as a collimator- camera combination, with a deformable mirror located between them in the collimated beam, remains the same many features of the design have been improved.