The 4th mirror rotary position is now set and read by stepper motor's absolute pulse counts due to the encoder failure. Its index position has been realigned so that an optimum setting for 230 GHz receiver is close to the home position. New optimum positions have been determined by TGP by setting the 4th mirror such that the beam fell symetrically on the secondary mirror:
| Receiver | Absolute Position (Oct 1999) |
Absolute Position (Current) |
|---|---|---|
| 230 | +2400 | +4325 |
| 345 | +1400 | +6000 |
| 492 | +5000 | +5890 |
| 690 | +2200 | +3600 |
You can override these default values by editing the file CANONICAL:SIDECAB_MIRRORS.TXT on alpha1. You have to reload the antenna computer in order for new mirror positions to become effective.
To set or fine-tune the 4th mirror rotary position manually, log on to the antenna computer and use the program smc1. First rotate the 4th mirror (motor 1) and minimize the total power from the receiver to get on to the sky. Once the beam is on the sky, point up the telescope to one of the strong point-like sources, fine-tune the mirror to maximize the efficiency. You have to repoint to the object every time you move the mirror. The following is a sample session, illustrating the use of smc1:
hau$ smc1 Opening SMC1... done smc1> send "1tp" // get absolute position of motor #1 01 0 // absolute position is 0 (= home) smc1> send "1ma-1000" // move to absolute position -1,000 smc1> send "1tp" // get absolute position 01 -1000 // should be -1,000 smc1> send "1mr100" // move +100 relatively to fine-tune smc1> send "1tp" // get absolute position 01 -900 // should be -900 smc1> exit hau$