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Servoing etalons

First make sure that the CS100 and its switchbox are switched on. These electronics units are both mounted on the side of TAURUS.

Now select which etalon is to be servo'd. The cabling from the etalons is routed to the switchbox, which selects which etalon is currently being controlled by the CS100. The CS100 itself can be controlled either locally, using the switches on the front panel, or from the system computer. Setting up the etalon it most straightforward when the CS100 is under local control. In order to servo the etalon in position number 1, and set the CS100 to local control, type the following commands:

The procedure for servoing an etalon is first to open the servo loop using the two-position switch on the CS100 panel. Then for each of the channels set the coarse, fine and resistive offsets, servo gain and time constant. Then close the servo loop again, and check that the etalon servo's without overloading.

A log of the offsets, time constant and gain for each etalon is kept on the VAX 8300 in the directory [TAURUS.INSTR.CS100]. These parameters do not change greatly with time, and so at the start of an observing run it should be possible to servo the etalon using the same values for these parameters as were used in the previous run.

If the values of these parameters are not known, the etalon should be servo'd using the following procedure:

  1. Set the servo gain for each channel to 8, and time constant to 1.6 ms.
  2. Ensure that the CLOSE LOOP and INTEGRATE switches are down, and the METER DISPLAY switch is up.
  3. Turn on power
  4. Null the X, Y and Z meters using the X, Y and Z COARSE and FINE PARALLELISM controls. There is some interaction between the channels, so it will be necessary to iterate. Do not aim to achieve an exact null.
  5. Switch the METER DISPLAY switch to RESISTIVE COMPONENT and null the readings using the R BALANCE control.
  6. Switch the METER DISPLAY back to ERROR SIGNAL and repeat step (4). Iterate between (4) and (5) until the meters read zero when switched to RESISTIVE COMPONENT and are on scale when switched to ERROR SIGNAL
  7. Set the gains to 32 and time constants to 250 s, and switch the CLOSE LOOP switch up. After a delay of about 1s, the LOOP CLOSED LED should light, indicating correct closed--loop operation.

The etalon is now under servo control, but the offsets, gain and time constamnt will not be optimised. The CS100 manual describes the process by which the optimum time constant and gain are determined. The normal values for TAURUS--2 etalons are a time constant of 250 seconds and a gain of 16. The process of optimising the offsets is described in Section 7.5. Note that the offsets should be re-optimised at least every few days, and preferably before each night's observing.

When the etalon cannot be servo'd, the problem is almost always the electrical connections to the etalon. Disconnect and reconnect all the cables.

Thu Apr 7 00:29:52 BST 1994