1. Initialise the filter slide by typing ISIS_INIT BLUE_FILTER_B (or whatever) at ICL. This sets the step count to zero. 2. Goto the engineering desk opposite the observing console and set the engineering control selector switch to CASS INST. 3. On the engineering terminal (marked TERMINAL A), type BFB TT (if you want to move the BFILTB slide). For other slides, consult the list of pneumonics in the table below. 5. Now type UNSEAL. Note that the characters are not echoed to the terminal. 6. Type three question marks in a row (???). The last column of the resulting output shows the position of the slide in encoder units. This should be zero at this stage. 7. Type, for example, 5100. 4 INDEX This will move the BFILTB slide (SMDM number 4 - see the table below) 5100 encoder units from its current position to approximately three-quarters of the way between filter positions 1 and 2. This is the optimum position for this slide - for other filter slides consult the table below for the number of encoder units to move. 8. Type ??? and check the result in the last column. It should now say 5100 (or whatever). 9. Type 4MS to return to normal engineering control (at which you can then type BFB SEE at any time to check the position of the BFILFB filter slide, for example). Note that the ICL mimic will (wrongly) show that the filter is out of the beam. SLIDE PNEUMONIC SMDM IMPOL POSITION ---------------------------------------- RFILTA RFA 3 5500 RFILTB RFB 4 ? BFILTA BFA 3 ? BFILTB BFB 4 5100