Collaboration Case: Autoguider Offset

Description:  Offsets the telescope position of the given (alpha,delta) step, but without updating the (alpha,delta) values of the actual telescope position as set at the last preset (or rather the last position), i.e. the telescope is moved to the new position but without any update of pointing coordinates: the new physical position is a corrected value for the previous coordinates.
This offset is internally used to correct for telescope pointing errors.
In particular the guiding systems (autoguiding and field stabilization) use this command to correct pointing errors dynamically measured and typically due to wind and athmospheric effects and to minor imperfections in the installed telescope pointing model.
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 is 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.

Role(s)/Actor(s): 
Secondary: Alt/Az Axis

Priority:  Major

Performance: 0.5 second

Frequency: Asynchronous, up to 10 Hz

Package Assignment:  Tracking.

Preconditions: 

• ATCS is Online Tracking
• AG/FS Offset is available

Basic Course: 

• Send command OFFSAG with Alpha/Delta offset (in arcsec)
  Exception Course: Command failed
• ATCS performs Offset Alpha/Delta to the main axes
  Exception Course: Command failed
• ATCS returns OK reply

Exception Course: Command failed

• No action, log error

Postconditions: 

• Alt/Az axis tracking positions are offset of the (alpha,delta) amount
• Telescope position is not updated

Issues to be Determined or Resolved: 

Notes:

Last modified: Thu Jun 28 13:59:59 UTC 2001