Use Case:  Start Field Stabilization

Description:  The Field Stabilization Loop is started with the current configuration parameters, after having checked that all start conditions are satisfied. Optionally, a preliminary exposure can be requested to automatically measure the position of the guide star and move the XY Translation Stage to the measured position.
If the system is already in closed loop when a START is received, it continues with no error indication.

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

Priority: Major

Performance: A couple of seconds

Frequency: After each preset when guiding is enabled with field stabilization, or at any time manually during operation while tracking or for test purpose.

Package Assignment:  Field Stabilization.

Preconditions: 

• FS is Idle
• FSS Strap is ONLINE
• Coude Beam Switching device allows light path to FSS
  

  Interface:  IfBeamSwitchingDevice.Encoder.ReadoutPosition

• Guide star available
• Guiding Reference point is set
• M6 is ONLINE

Basic Course: 

• Send command to start Field Stabilization.

  Variations: Use STARTAG command to immediately start Field Stabilization Use BOX2GS measure the error vectors and counter offset the XY Translation Stage of the same amount.

  Exception Course: Command failed
• ATCS Checks Autoguiding Conditions at start for field stabilization
  Exception Course: Command failed
• ATCS Prepares FSS Setup with selected setup configuration in continuous closed loop and M6 correction
• ATCS send the command CHP/ENAFS to Enable Field Stabilisation
  Exception Course: Command failed
• ATCS launches a TRS timer with the given period for the Field Stabilization Loop
  Exception Course: Command failed
• ATCS waits for the loop to be stable (see Issue 1).
• Optionally (BOX2GS)
• retrieve the M6 offset from center position
• counter offset the x-y table of the same amount. The M6 is kept in close loop and goes back automatically to center position. In this way the probe is centered on the guide star.
• ATCS returns OK reply

Exception Course: Command Failed

• ATCS returns ERROR reply
  Postcondition: FS is Idle

Postconditions: 

• FS is active
• TRS timer is running
• M6 is receiving tilt corrections from FSS Strap

Issues to be Determined or Resolved: 

Notes:

• Autoguiding and Field Stabilization are mutually exclusive, since only one detector is used for both. The Auto Guider sends the corrections to the Axis while alternatively FS sends corrections to M6 that in turn recenters itself on 10 second period by sending the corresponding corrections to the Axis.
  It is the same scheme as for the UTs, having M6 playing the same role as M2 for the UTs (see M6 Control Loop).
• START and STOP commands are used during nodding and during combined offsets, since I all these cases FS must be paused while the telescope and the XY table move to the new position. They have then to be restarted and we assume that probe positioning during these telescope movements is very good and no new guide star acquisition is necessary.
• See AG/FS Analysis for UTs The algorithm for differential refraction handling (different wavelength of target and guide star) could be taken from the UT adapter/rotator implementation.
• The BOX2GS option can be used to correct for catalogue error, but probably the telescope pointing will not be accurate enough to ensure the tight pointing requirements (some 0.02-0.05 arcsec) needed for the interferometric instrument. It will be then necessary to use for VLTI reference stars which coordinates are very well known (Hypparcos catalogue) and rely on these coordinates rather than on the telescope pointing. More over, the object itself will be used as much as possible to Field Stabilize and in that case the probe position shall not be adjusted. (Option is Desirable).

Last modified: Thu Jun 28 13:15:19 UTC 2001