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The original `FOS' concept for a high-throughput, cross-dispersed spectrograph (FOS-I on the INT) was described by Breare et al. ( MNRAS, 227, 909, 1987). FOS-II for the WHT follows the same basic design, but with a higher dispersion, and is described by Allington--Smith et al. ( MNRAS, 238, 603, 1989). In brief, it covers a wavelength range from 4600-9700Å in 1st order and 3500-4900Å in 2nd order, with dispersions of 8.7 and 4.3Å per CCD pixel, respectively. The optical design of FOS-II is due to S.P. Worswick, and is based upon the design by C.G. Wynne for FOS-I.
The optical design is based upon an F/1.4 Schmidt camera, working without a collimator in the diverging f/11 Cassegrain beam. The dispersion is provided by a 150 l/mm transmission grating, blazed at 7300 Å, together with a cross-dispersing prism. The camera has an aspheric corrector plate cemented to the underside of the grism/prism assembly and a silver-coated spherical mirror. This optical arrangement gives a dispersion of 400Å/mm in 1st order and 200Å/mm in 2nd order. A field flattening lens produces a flat focal plane at the detector surface. The detector package is small enough to fit within the shadow of the telescope's secondary mirror and so minimize obscuration losses. The optical system has a resolution of 33 microns FWHM (zero slit width) at the camera focus, corresponding to 13Å in first order in the spectral direction, and 1.2 arcsec along the slit.
The electronic iris shutter is located just below the interface between FOS-II and ISIS, and a pair of Hartmann shutters, which can be inserted into and removed from the beam either by operation of a manual lever, or by motors controlled by the FOS 4ms controller The focus position and tilt of the detector can be remotely set by the 4ms controller. The focus and CCD tilt are defined by the position of three push rods on which the ring bearing the CCD is supported. The push rods are moved along axis by three motor-micrometers. The 4ms microprocessor is located in one of the Cassegrain cubicles, and is interfaced to the WHT Utility Network, and can accept commands over the Utility Network or from a local terminal.
The current detector for FOS-II is a GEC P8603 CCD, with a dye coating to enhance the blue response, and the format provided by the spectrograph is shown superimposed upon an outline of this CCD in Figure 8. Because the inter-order separation at the blue end of the CCD is small FOS-II is used either with a 20 arcsec dekker to limit the spatial coverage, or with a GG495 filter in the below slit filter slide to eliminate second order for longslit work. The readout noise of the current FOS-II CCD chip is 8e r.m.s.
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