Grating Scan Mechanism Setup

The Grating Scan Mechanism (or GSM) needs to be carefully adjusted for proper operation. The main problem is that the sensor coils both inside the dewar and in the warm electronics are temperature sensitive. The inductance of the dewar coils changes significantly as the dewar cools from 77 K to 60 K when the LN2 can is pumped. If the electronics are not adjusted correctly to compensate for these drifts, the wavelength calibration of the GSM will be wrong, and in some cases the GSM will not function at all.

The necessary adjustments are made using the controls on the GSM electronics box and the PSYS software. The approximate potentiometer settings on the electronics box are given in Table 2.1, and the values of several software parameters in Table 2.2.

1. Preparation: The PC should be turned on and all cables in place as described in sections Electronics and Cabling and Detector Electronics Checkout. PSYS should be running, and the main menu should be visible. (Press Return to exit Listen mode and display the main menu if necessary.)
   1. Turn on the GSM sensor power only; leave the motor power off for now. The motor power should never be turned on before the GSM is initialized from the PC.

   2. Type im c on the PC to set the GSM to camera mode.

   3. On the GSM electronics panel, set the GSM Control switch to Manual, and set the Fine/Coarse switch to Coarse Mode.

   4. Turn on the GSM motor power switch on the power supply.

2. Calibrate the Coarse Sensor.
   1. Turn the Roto-matic counterclockwise a few turns until the GSM reaches the lower hard stop. The panel Monitor voltage should decrease as the GSM turns and a ``clunk'' should be heard as the carousel hits the stop. Make sure the carousel comes to rest against the stop by leaving the crank in the ``Lower Limit'' position, and check that the coarse voltage should stay at the lowest value it ever reaches.

   2. Adjust the Coarse Sensor Offset pot until the panel Monitor reads 1.25 volts.

   3. Turn the Roto-matic clockwise until the carousel comes to rest against the upper hard stop, with the crank in the ``Upper Limit'' position.

   4. Adjust the Coarse Sensor Gain pot to get 8.50 volts.

   5. Repeat steps (a) through (d) a few times until both voltages are correct to about +- 0.01 volt. It is important to adjust the Gain and Offset pots in the above order for the adjustment to converge.

   6. Set the GSM mode to Auto by setting the three toggle switches on the GSM electronics box panel to 1-1-1 (left-to-right).

   7. Set the GSM Motor Control switch to Automatic.

   8. The status light may or may not change from red to green depending on the GSM's position. If the red light stays on, type "GM C" on the PC to initialize the electronics. If the green light does not come on now, the electronics are probably not properly adjusted.

3. Fine Sensor Calibration: This step is usually not necessary. The fine loop is quite stable and any small adjustments are better made via software. Perform this next step only if the settings for the fine sensor pots given in Table 2.1 seem incorrect, otherwise proceed to step 4. If you need to calibrate the Fine Sensor, the coarse loop is used to rotate the carousel to two known positions, where the fine sensor voltage is then set to certain values. The positions correspond to the fine sensor coil being halfway between two tabs, or fully filled with a tab. At these positions the fine sensor voltage passes through a maximum (between tabs) or a minimum (filled with tab).
   1. Set the three toggle switches on the GSM electronics control panel to Coarse mode (0-1-1).

   2. On the PC, from the PSYS GSM Parameters menu (accessed by entering GP from the main menu), select mode CV. This mode allows you to set the coarse voltage directly. Enter 4.2.

   3. The panel Monitor should read Coarse Volts = 4.2 after it settles down. If you hear a ``clunk'' because the GSM has hit the hard stop, turn the power off and check all the connections. The coarse loop is not very stable and the Monitor voltage may wander up and down a bit.

   4. Switch the panel Monitor switch to Fine Volts and set the Fine Sensor Offset Pot to give a reading of 5.07 volts.

   5. From PSYS, set the Coarse Sensor volts to 5.5.

   6. Adjust the Fine Sensor Gain pot to read 9.72 volts.

   7. Repeat steps (1) through (5) until both Fine sensor voltages are correct. When you're close to a final solution, it may be wise to move the GSM slightly using PSYS to be sure the Fine Sensor voltage passes through a minimum voltage of 5.07 and a maximum of 9.72.

   8. Enter -10 to exit the PSYS CV routine and return to the GSM Parameters menu.

   9. Set the three GSM panel toggle switches to Auto (1-1-1).

   The hardware adjustments of the GSM system should now be correctly adjusted. However, the importance of monitoring the GSM voltages periodically to be sure that they have not drifted with temperature cannot be overemphasized!

4. GSM electronics temperature controller: The controller is used to stabilize the temperature inside the GSM electronics box. Ideally, the box should be left to warm up and stabilize with the main GSM power on, then the controller should be turned on and set to a value a few degrees higher. Push the toggle switch on the controller's front panel to the right and turn the black knob to set the temperature. This is a very important adjustment that may change if the outside or dome temperature varies, and is worth checking periodically.

5. Software checks: Once the hardware is set correctly, check the software zero point values in the GSM Parameters menu. They should approximately equal the values in Table 2.2.