- Servo system not locked in `fringe' mode - lock servo.
- The carriage was not advanced ahead of optical ZPD before the scan
started - relocate optical ZPD and remember to flag the `move carriage'
in the SCANS menu.
- The preamp setting is too high, and the detectors are saturated
(common problem when looking at the moon or very bright late-type stars).
Select a less sensitive gain setting.
- Carriage may not be moving in the correct direction - check lights
labelled `F' and `B' at the bottom of the right hand side of the main
- For faint sources, the divider may be on - turn divider off.