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The slit area contains the dekker slide; the dichroic and folding flat slide; and the four ISIS filter slides (two in the blue beam after the blue fold, and two in the red and FOS beams between the blue and red folds. Slides not in use are kept in a steel cupboard behind the WHT engineering console, or else in the grating cupboard on the observing floor.
Care must be taken when working in the slit area because it is possible to knock the slit jaws out of alignment. To realign the slit jaws it is necessary to remove ISIS from the telescope, and this is equivalent to an extra instrument change. The slit area access door is interlocked with the dekker slide, it is only possible to open the slit door when the dekker is in position 0, when it offers some protection to the slit unit. However with the door open it is then possible to move the dekker away from position 0, and it would then be easy to disturb the slit alignment. Users must not do this, and if they require any mechanism moved when the slit access door is open they must contact RGO technical staff. If the IPCS EHT supply is on then only a minimum of light can be used in the slit area with the door open, and in the dark it is more likely that mechanisms will be disturbed by accident. Under these circumstances users must either switch the IPCS EHT to standby, or else contact RGO technical staff for assistance before opening the slit door.
Before the slit door is opened the dekker slide must be in position 0. At
the ICL interface type:--
DEKKER 0
wait for the dekker action to complete, this mechanism is slow.
SLIT_DOOR OPEN.
The slit access door can then be opened manually. The dekker slide is a horizontal slide at the top of the slit area, which can be withdrawn from an angled groove. When replacing the dekker slide care must be taken to locate the slide correctly in this groove.
The two blue beam filter slides are to the right of the slit area, and can also be withdrawn and replaced. However the grooves that the two slides run in are not identical and these slides cannot be interchanged. The same is true of the two red beam slides which are in the lower part of the slit area. All the filter slides are somewhat stiff, and changing these filters should normally be done by RGO technical staff.
The dichroic slide is in the centre of the slit area, and is rather larger than the filter slides. It contains the mounting cells for the dichroics and for the flat mirror, these have the shape of a right angled triangle. The dichroic slide is held in place by a sprung loaded locator pin, this can be released by lifting the plastic knob on the slide. The knob can be twisted through 90 degrees, when the locator pin will be held out, and the slide can then be withdrawn carefully through the slit access door. When the slide is replaced the user must ensure that the locator pin is correctly engaged.
The slit door should be closed and latched; and is then locked by the
command:--
SLIT_DOOR CLOSE.
The procedure CHANGE has been written to take the user through the process of changing the below slit filters. The procedure will move the dekker to position 0, unlock the slit door, and when the change has been completed will lock the door and update the database.