WYFFOS is built on a dedicated optical table in the GHRIL cabin on one of the WHT Nasmyth platforms. It follows the white-pupil design of Baranne (in Very Large Telescopes and their Instrumentation, ESO Conference and Workshop Proceedings 30 (1988), 1195), in which a dioptric collimator is used in double-pass (near-Littrow) mode with a reflection grating to form an intermediate spectrum close to a concave spherical relay-mirror. The Schmidt-type camera works at finite conjugates to re-image the intermediate spectrum onto the detector, while the function of the relay mirror is to image the collimator pupil onto the camera pupil. The configuration offers considerable stability with respect to vignetting, and allows modification of the effective pupil to ease the camera design and reduce the effect of the central obstruction.
The reflection grating can be replaced with an echelle for higher-dispersion work. Introducing a second collimator (with the relay mirror and camera re-positioned) permits the use of a grism for high-throughput low-dispersion work.
The beam emerging from each fibre is matched to the f/8.2 collimator. Immediately beyond the fibre slit is the spectrograph shutter, whose rear surface is used as a reflector for the fibre back-illumination source (an array of IR LEDs). Two filter slides (for colour and ND filters) follow, while the beam is still relatively narrow.
The four-element collimator is wide-band coated, though its twin for use with the grism has a multi-layer coating. Mirrors are overcoated silver, while the doublet Schmidt corrector and field-flattener of the camera are also wide-band coated.
In its reflection-grating mode, WYFFOS will use existing gratings belonging to the ISIS and IDS spectrographs of the William Herschel and Isaac Newton Telescopes. Of the many possible grating configurations, just three are shown in Table . They assume the use of the 2.7 arcsec fibres and the Tektronix detector.
Table: Examples of WYFFOS dispersion options using ISIS gratings
WYFFOS is controlled using EPICS/VxWorks on a VME micro. Motor-drives operate slit translation and focus, filter slides and the grating table, while the Hartmann shutters on the camera are operated pneumatically. The spectrograph shutter is under the direct control of the CCD system. The VME communicates with the Observing System on the Vax, and thence with Autofib-2.
For wavelength calibration, it is planned to direct a near-collimated beam from calibration lamps on or near the GHRIL table (adjacent to WYFFOS) via the Nasmyth flat to the prime-focus corrector and thence to the fibres. A disadvantage of this is that the beam entering each fibre will have a very slow focal ratio, changing the effective resolution of the spectrograph.