The slit is common to the two arms of ISIS, as well as FOS. The slit length is 53 mm (4 arcmin), and the width is continuously variable between 30 m (0.14 arcsec) and 5 mm (22.6 arcsec). The slit is polished and aluminised and inclined to the optical axis of the telescope at an angle of 7.5 degrees to allow viewing of the reflected image in the A&G box TV camera. The width is driven by a linear motor, and encoded. The slit unit is a two position carriage, one position contains the conventional long slit, and the other position is a two position cross slide, containing a wide aperture which is used for mounting multi-slit masks, and the slit end of the ISIS fibre system. This cross slide is itself remotely driven from the ISIS 4ms microprocessor.
Dekkers are mounted in 8 position slides, which are inserted in a driven mechanism immediately above the slit. The slides are interchangeable. At present there are two dekker masks, one for general use and one for spectropolarimetry (Table ).
The long slit unit has gaps at each end, and it is important to use a long slit dekker (position 6 or 7) when observing, and not to observe with the dekker out (position 0). Ghosting is reduced considerably by use of the long slit dekkers.
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Figure: The Polarimetry optics, slit area components, folds, and below slit filters of ISIS and FOS-2.
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Table: ISIS/FOS Dekker Slides