Use Case:  Telescope Offset

Description:  Offset the telescope position of the given (alpha,delta) step.
The offset value is simply added to the reference position of the telescope. The telescope axes move to (alt,az) values corresponding to the new sky coordinates.
It is seen by the user as a change of the actual position with respect to the last preset (or rather the last position).
An offset step is always relative to the current position of the telescope (i.e . consecutive steps are accumulated). To issue a new offset it's not necessary to wait until a previous offset has completed. Consecutive offsets are therefore not lost if one does not exceed the defined performance limits.
This command CANNOT be used while guiding or doing field stabilization, since the guiding system is not aware of the offset and the behaviour of the system is unpredictable. Use Combined Offset instead.
This UC uses Offset Alpha/Delta

Role(s)/Actor(s): 
Primary: Operator/Maintenance
Secondary: Alt/Az Axes

Priority: Critical

Performance: up to 1.5 deg/sec in Alt and 2 deg/sec

Frequency: Asynchronous up to 10 Hz

Package Assignment:  Tracking.

Preconditions: 

• ATCS is ONLINE Tracking

Basic Course: 

• Send command OFFSAD to offset the telescope of (alpha,delta) in arcsec
  Exception Course: Command failed
• ATCS sends the (alpha,delta) offset to the axes by Offset Alpha/Delta
  Exception Course: Command failed
• ATCS updates telescope position
  Exception Course: Command failed
• ATCS returns OK reply

Exception Course: Command failed

• ATCS returns ERROR reply
  Postcondition: ATCS state ONLINE TRACKING

Postconditions: 

• Alt/Az axis are offset of the (alpha,delta) amount
• Telescope position is updated

Issues to be Determined or Resolved: 

Notes:See TCS for UTs

Last modified: Thu Jun 28 13:58:31 UTC 2001