ISIS and FOS-2 comprise three spectrographs sharing a common slit unit, dekker slide, optics for spectropolarimetry, and some filter slides. FOS-2 lies directly below the optical path, and is designed to provide the highest throughput at a dispersion of 400 Å/mm in first order, and 200 Å/mm in second order. Each of the two arms of ISIS is a conventional spectrograph, with interchangeable reflection gratings, and a horizontal optical layout. The optical components of the two arms, and the anti-reflection coatings on those components, are optimised for specific wavelength ranges. The upper arm is optimised for the range 3000 - 6000 Å, and is called the BLUE arm, whilst the lower arm is optimised for the range 5000 - 10000 Å, and is called the RED arm. Light is fed into the two arms of ISIS by 45 folding mirrors or prisms on the optical axis of the telescope and at the levels of the respective collimators. The folding mirror for the blue arm can be replaced by one of a range of dichroic filters, which reflects blue light and transmits red light, allowing simultaneous use of the blue arm of ISIS with either the red arm or FOS-2.
[ TIFF ]
Figure: Exploded view of the optical components of ISIS and FOS-2, showing the light paths through the components.