WARNING!

This procedure should only be performed by qualified Keck Instrument Specialists and Instrument Technicians.

Purpose

When a grating station refuses to home and the problem cannot be solved by cleaning the puka on the rotor, then the only option may be to disable the problem port and continue observing using the remaining grating stations.

Procedure

Check the turret status.
1. Start a motor crate session by issuing the following command on lrisserver:
```
  telnet tserver4 3009
```
2. Press <Enter> to get a VME prompt.
3. Type check_turret to get a status report on the turret and grating positions. Note:
  - that the detent pin is in;
  - that the grating position is valid;
  - that the grating brake is unclamped;
  - the grating port number for later reference;
  - the tilt encoder position for later reference.
4. If the grating brake is clamped, then unclamp it by typing:
```
  unclamp_grating_brake
```
Home the grating manually.
1. If not already done, LRIS must be pulled from the telescope and rotated to the ``grating changeout'' position. This puts the dewar at the 12:00 position.
2. Technician should open the inspection hatch to the optical port. When standing behind LRIS, this hatch is at 9:00 (i.e., on the opposite side of the instrument from the filter and slitmask jukeboxes).
3. Ask technician to check the tilt of the grating. The goal is to have the grating in the ``home position'' (i.e., with its face perpendicular to the grating turret axis of rotation) so that the turret can be rotated. If it is already well aligned, then proceed to the next series of instructions.
4. If the grating appears to be tilted such that reflected light would reach the filters, then the motor must be moved to lower angle. Otherwise, the angle must be increased.
5. From the motor crate session, run the api program and switch to API 0:
```
  api
  :0
```
6. While the technician watches the grating tilt, try making a small move in the intended direction. To reduce the angle, type
```
  mov -12000
```
  This should move the grating about one degree lower in angle.
7. If the direction moved was correct, then issue moves of about 10 degrees at a time until the grating is roughly vertical in the port using the command mov -120000. If the direction of the move was wrong, reverse the sign of the move (i.e., mov 120000).
8. Continue issuing moves, decreasing the amplitude as needed until alignment is achieved.
Disable software sensing of home switch.
1. Within the api session, type :0 to ensure that you are still connected to API 0.
2. Type set 1 to turn on all of the optical homing switches.
3. Type ver o1 to check the output word. Confirm that the first bit is now 1, indicating that the lights are on.
4. Type ver i1 to display the status of all bits on input word 1. The first five bits are the home bits for individual grating ports. One of them should be a zero, indicating a bad home signal. For example, if grating two is bad you may get 10111xxx, where the x's can be either 1 or 0.
5. Reset the input definition mask to invert the malfunctioning home bit. For example, if grating station 2 is the one giving problems, then issue the command
```
  id1=10111111
```
6. Type ver i1 to display the home bits status. You should now get 111111xx, indicating that all ports are homed and that bit 6, the ``all gratings homed'' switch, is lit and will allow the turret to be moved.
7. Type reset 1 to disable the homing switches; otherwise, the LEDs remain on and will contaminate images.
8. Type quit to return to the crate session.
Move grating to another port.
If the grating in the disabled port needs still needs to be used in the current night's observing, follow these steps to move the grating from non-functional port M to new port N:
1. Have technician again verify that the disabled grating is currently in the ``stow'' position so that the turret can be rotated.
2. On the motor crate session, type gr_service M to put grating port M in the service position.
3. Have technician remove the grating in port M via the service hatch.
4. Have technician install any available grating from the grating storage drawer into the the now-empty port M.
5. Identify another grating which can be removed so that the desired grating can be re-installed. Call this port N.
6. Ensure that techician is clear of the port.
7. Type gr_service N to move station N to the service port.
8. Have technician remove the grating from port N and store in grating storage drawer.
9. Have technician re-install grating originally from port M into port N.
10. Type command gr_optical N to put the desired grating into the observing position.
Disable broken grating station.
This step is recommended to reduce the risk of selecting the broken station and thereby getting hung up again.
1. From a lrisserver session, type the following command:
```
  configure -d graname grating_name
```
  where grating_name is the name of the grating which was removed from port N (e.g., 600/7500).
2. If the grating from port N was moved to new port M, then you must reconfigure the grating table so that the software can set the new grating correctly. From a lrisserver session, type the following command:
```
  change_grating M grating_name
```
  where M is the new port number and grating_name is the name of the grating now installed there (e.g., 600/7500).
3. Quit and restart the XLRIS widget to enable the change to the grating configuration table.