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The design of the original FOS was adapted to the WHT by Sue Worswick. The main components are as follows:

Slit unit and dekker slide.
FOS-2 uses the same slit unit and dekker slide as ISIS (see Section gif).

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.

This has a focal ratio of f/1.4 giving a slit to detector reduction factor of 7.8. The dispersion at the detector is 400 Å mm in first order and 200 Å mm in second order. The camera consists of an aspheric corrector plate cemented to the underside of the grism/prism assembly and the silver-coated spherical mirror. A field flattening lens produces a flat focal plane at which the detector surface is located. The detector package is small enough to fit within the shadow of the telescope's secondary mirror to minimize obscuration losses. The optical system has a resolution of 33 m FWHM (minimum slit width) at the camera focus corresponding in the spectral direction to 13.2 Å in first order and 6.6 Å in second order, and in the spatial direction to 1.2 arcsec.

The detector for FOS-2 is a GEC(EEV) P8603 CCD coated by ESO to enhance the blue response. This chip is described in more detail in Chapter gif. The CCD is mounted inside the camera body, and is cooled via a copper cold finger leading to a cryostat bolted to the side of the instrument. There are plans to upgrade this to a thinned but otherwise similar chip.

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.


Figure: FOS

next up previous contents
Next: Performance Up: Faint Object Spectrograph Previous: FOS-1

Tue Aug 15 16:42:46 BST 1995