Use Case:  Combined Offset

Description:  Perform a combined offset (alpha/delta).
A combined offset step is needed to perform an (alpha,delta) telescope offset while guiding.
Before actually offseting the telescope position, guiding is interrupted so that there is no unexpected intervention by the guiding system while the telescope is moving to the new position.
The telescope position is adjusted with the requested alpha/delta and the guide probe (XY-Translation Table of FSS or TCCD Window of FAS) is adjusted such that the guide star maintains its position on the guide probe. This includes correcting the guiding reference point to take into account the performed offset.
When all these steps are completed, guiding is resumed. An offset step is always relative to the current position of the telescope (i.e . consecutive steps are accumulated) (see Notes).
Extends Telescope Offset.

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

Priority: Major

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

Frequency: Asynchronous

[Design Description Entry]

Package Assignment:  Auto Guiding.

Preconditions:

Basic Course: 

Exception Course: Command failed

Exception Course: Offset failed

Postconditions:

Issues to be Determined or Resolved: ...

Notes:

The current implementation, actually running on the VLT Unit Telescopes, does not support accumulation of combined offset commands (see Spr 980583). This means that a combined offset must be completed before another one can be accepted. A first fix to the Spr will just reject multiple combined offsets. A complete implementation will have to be done at a later stage.

Last modified: Thu Jun 28 13:03:25 UTC 2001