Use Case:  Offset Guide Probe on Beacon (closed loop on Beacon)

Role(s)/Actor(s):
Primary: User (Operator, Maintenance, VLTI-ISS)
Secondary:  FSS subsystems,  FAS.

Priority: Major

Performance: A couple of seconds (10 to 15 seconds)

Frequency: Asynchronous, during Optical Alignment

Package Assignment:  Optical Adjustments.

Preconditions:

• FAS sub-system is ONLINE (if selected)
• FSS sub-system is ONLINE (if selected)
• Nasmyth beacon ON
• "Guide Probe to Beacon" has been done and beacon is still in the Field of View

• Send command OFFSGUV to Offset Guide Probe on Beacon with FSS
• ATCS Offsets Guide Probe on star

Postconditions:

• The X-Y table (or Guide Window) has reached its offset position
• The STRAP loop (or Autoguiding loop) is closed

Notes:
Since the FSS is moved with the loop closed, it may be necessary to limit the speed/acceleration of the trajectory to avoid that the loop open.It means that a specifuc "Set FSS Translation Stage" use case may be required. Note that this remark is also (even more) true for "offset guide probe" on a star for which the STRAP closed loop bandwidth can be lower (less flux).

It may be better to define this Use Case as part of the "Field Stabilization" (and "Autoguiding") Use Cases since it can be used for other purpose than image alignment. For example, ISS will need, after a relocation, to cross reference the co-ordinate systems of each AT Coudé with the VLTI Image Alignment Detector located in the Lab (VINCI TCCD). For this purpose, the "Guide Probe to Beacon (acquire beacon)" and the "Offset Guide Probe on Beacon" can be used to place the beacon image at two positions along X and Y and to calibrate the rotation and the scale factors between the AT coudé sensor(s) and the VINCI TCCD. In any case, these two commands must be made available as "atomic functions".

Last modified: Mon Feb 25 12:33:29 UTC 2002