Use Case: Set Guide Reference Point

Description:  With this function the user can define the sky coordinates in (alpha,delta) of the reference point on the Field Acquisition Sensor used for Autoguiding. The reference point is mapped on the pixel on the CCD to be used as a reference for error vector calculations.
The same coordinates are used to position the FSS Translation table for Field Stabilization.
This reference point shall be updated automatically during observation to take into account changes in the differential atmospheric refraction
between the observing and the guiding wavelength.

Role(s)/Actor(s): 
Primary: Operator, Maintenance
Secondary: FAS, FSS Strap

Priority: Major

Performance:  1 second

Frequency: At any time

Package Assignment:  Auto Guiding.

Preconditions: 

• FAS is ONLINE
• FSS Strap is ONLINE

Basic Course: 

• Send new sky coordinates in (alpha, delta) for Guide Reference Point
  Exception course: Command failed
• ATCS stores and converts the Guide Reference Point coordinates to FAS / FSS coordinates
• ATCS Prepare FAS Setup with updated Guide Reference Point coordinates for the FAS
  Exception Course: Command failed
• ATCS Sends FAS Setup with updated values
  Exception Course: Command failed
• ATCS Prepare FSS Setup with updated Guide Reference Point coordinates for the FSS
  Exception Course: Command failed
• ATCS Sends FSS Setup with updated values
  Exception Course: Command failed
• ATCS Preset Probe to new sky position
  Exception Course: Command failed
• ATCS returns OK reply

Exception Course: Command failed

• ATCS returns ERROR reply

Postconditions: 

• New guide reference point coordinates are available for Autoguiding or Field Stabilization.

Issues to be Determined or Resolved: 

Notes: In the case of the Field Stabilization, the FSS Translation Table will be placed at the field position corresponding to the specified pixel in the Field Acquisition sensor. In this way, both FAS and FSS have the same object coordinates.

Notice that the FFS APD itself has a "reference point", but with a completely different meaning: in order to allow guiding for a while without moving the FFS Translation Table, it is possible to specify an offset between the center of the APD and the position of the guide star. The system will then do field stabilization keeping this offset, instead of centering the guide star on the ADP.

Last modified: Thu Jun 28 13:04:09 UTC 2001