David W. Gellatly

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.

Autofib--2: an automated fiber positioner for the prime focus of the William Herschel Telescope

We are currently testing an automated fibre positioner for the 4.2 m William Herschel Telescope (WHT). This instrument, known as Autofib-2, operates at the prime focus where it is able to utilize the full 1 degree field provided by the prime focus corrector (PFC). The robotic positioner is able to place 160 optical fibres in the focal plane of the WHT which feeds the light to a dedicated spectrograph (WYFFOS) located on the Nasmyth platform. This paper contains a description of the instrument which highlights the new techniques demanded by the prime focus plate scale and the scale distortions due to the PFC and its atmospheric dispersion compensator. These include robot vision to help achieve the high positioning accuracy and the use of two sky viewing probes to accurately determine the time dependent transformation from celestial coordinates to instrumental Cartesian coordinates. Also presented are the initial technical results on the performance of the instrument and the operational results of particular interest to the astronomical observer.

Low dispersion survey spectrograph for the William Herschel Telescope

This paper describes the Low Dispersion Survey Sectrograph (LDSS-2) currently under construction for the 4.2-m William Herschel Telescope. The instrument is a versatile highly efficient multiobject spectrograph with an 11.5-arcmin field of view. Around 100 objects can be observed simultaneously using multiaperture masks made at the telescope in real-time at dispersions selectable between 10 and 47 nm/mm. By removing the dispersing element, the instrument can act as a focal reducer with a final focal ratio of f/2. This allows the field to be imaged first in order to derive accurate instrumental coordinates for the manufacture of the aperture mask.

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.

Faint object spectrograph for the William Herschel Telescope

The design and performance of the FOS-2 faint-object spectrograph for the Herschel Telescope at the Observatorio del Roque de los Muchachos are described and illustrated with diagrams and drawings. The FOS-2 has an image scale of 223 microns/arcsec at the slit and features a cryostat-cooled 385 x 578-pixel CCD detector with 22-micron-sq pixels, operating with resolution 1.5 pixels and dispersion 8.73 A/pixel at 460-970 nm (first order) or 4.32 A/pixel at 350-490 nm (second order). The high-efficiency optical system is of the type described by Wynne (1982) and operates in the diverging f/11 Cassegrain beam without a collimator. Overall system efficiency in first order with a 5-arcsec slit has a peak at 16 percent at 700 nm and remains above 1 percent at all wavelengths imaged by the CCD; in second order the peak efficiency is about 3 percent at 490 nm and remains above 1 percent except below 370 nm. The mechanical system, the control and data acquisition procedures, and the data-reduction software are briefly discussed.