7.7 Steady-state

7.7.3 Monitoring while loops are closed

Exception and alarm handling are not discussed here. See "Exception and alarm handling" on page 7-23. Whenever a fault is detected, any closed loops will be opened and it will be necessary to re-initialize to recover from the fault. This will be fast but will nevertheless be inconvenient.

Various events can occur while the loops are closed which, while they require special action to be taken, are not regarded as faults. Examples include:

1. The DM or TT loops open for some reason (e.g. cloud or aircraft detected). If the DM loop opens, the SC will revert to the TT OPEN state. If the TT loop opens, the SC will open the DM loop and revert to the OPEN LOOP state. Re-closing of loops will initially be manual but one could imagine an automatic attempt to do this.

2. The astronomer wants to slew the telescope to a new target. This requires both loops to be opened.

3. The astronomer wants to offset (or nod[10]) the telescope. This requires both loops to be opened (for all but the smallest offsets) while various OBS mechanisms are moved. Then they can be closed again.

4. The astronomer wants to adjust the rotator angle. This is similar to a telescope offset.

These situations are all handled by the SC sequence and are illustrated in Figure 7-8 on page 7-20. The first case merely requires checking the k2:ao:wc:dm:state and k2:ao:wc:tt:state variables when in the TT + DM LOOP or TT LOOP states and taking appropriate action when they change. The others involve more complicated interactions which are described and illustrated in Chapter 9. See "Keck II Drive Control System (DCS)" on page 9-3.

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