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Hardware description

FOS was an `add on' to the Cassegrain cluster of instruments on the INT, the IDS and A&G units having been designed prior to Wynne's paper. It was thus necessary to find a mounting point on the IDS which did not cause flexure of the structure (FOS weighs approximately 100kg). This, combined with the small clearance between the Cassegrain cluster and the dome floor meant that a folding flat mirror had to be introduced to accommodate the long path length of the f/15 beam. Figures 1.1, 1.2 and 1.3 show FOS and its relationship to the IDS.

The FOS frame

is constructed of 3mm steel plate, welded together to give a rigid structure. Flexure tests of FOS on the telescope show the rigidity is better than the design specification, the image of a spectral line shifts by only 0.03 pixels per hour in the dispersion direction as the telescope tracks through the zenith.

The folding flat

is placed approximately halfway along the light path from the slit to the grism. It has a plane glass surface, with an extended blue response silver coating.

The Hartmann shutters

are situated between the folding flat and the grism. They are only used in the initial focussing of the instrument, not during normal observing. They are pneumatically operated and controlled from the instrumentation computer via Camac.

The camera body

has the grism at one end, the mirror at the other, with the chip between them. The body is fitted to the frame on a 3-point seat which is adjusted so the spectrum falls on the correct part of the chip.

The grism

has a 150 lines/mm plane transmission grating on the front surface. A ray from the slit centre to the centre of the grating surface makes an angle of 6.892 degrees with the normal to the surface and with the normal to the grating rulings. The central ray at a wavelength of 8000Å is diffracted normal to the grating. The grism as a whole is a cemented assembly of the transmission grating, cross dispersing prisms of Schott SK5 and LF5 glass, and an aspheric corrector plate of Schott K10 glass.

The camera mirror

is coated with an extended blue response silver coating and is expected to keep its high reflectivity, as the system is not let up to atmospheric pressure, except for maintenance.

The CCD

is supported in the camera body by four slender arms attached to an outer ring, which is held off the camera mirror by three invar rods, resulting in an insignificant change in focus position with temperature. The focus position is initially set up by moving the outer ring in small increments, using motor micrometers controlled by a microprocessor. It is not necessary to adjust the focus position during normal observing. The camera contains an absolute minimum of electronic components, a temperature control circuit, and some capacitors for stabilisation. The pcb on which the chip sits is thus very small, minimising light losses within the camera.

The cryostat

contains sufficient liquid nitrogen to keep the CCD cool for more than 12 hours, and is bolted to the side of the FOS frame, connecting via a copper cold finger through the side of the camera body to the CCD. It could be cooled to 120K, but is normally operated at 150K, stability being achieved using a small heater.