Use Case:  Probe To Next Guide Star

Description:  The Guide Probe is moved to the Guide Star coordinates selected by the user in one of many possible ways:

• Interactively from the FAS display
• Interactively from a catalogue
• From the list of guide stars provided in the setup file or automatically selected by the system from the guide star catalogue at preset.
• Taking automatically the next un-tried guide star from the list of guide stars provided in the setup file or automatically selected by the system from the guide star catalogue at preset. Automatic catalogue search for guide star is done if the list is empty. The found guide stars are put in the internal catalogue for guide star candidates and the probe is positioned to the more suitable one. Optionally, specific criteria can be given. If not the setup values are used.If no more un-tried entry is available an error is returned.

Role(s)/Actor(s): 
Primary: Operator, Maintenance, VLTICS-ISS
Secondary: FAS, FSS Strap, FSS Translation Stage, FSS Filter Wheel,

Priority: Critical

Performance:  some seconds

Frequency:  Whenever a new guide star has to be selected and in particular when a new preset is done or the used guide star is lost during tracking.

Package Assignment:  Auto Guiding.

Preconditions:

• Preset coordinates are available (telescope is tracking or presetting to a new target).
• FS and AG are Idle

• Coordinates for the next guide star are selected.
  Variations: One of the following way of selection is chosen:
  • Interactively from the FAS display
    Any position in the FAS display can be selected.
  • Interactively from a catalogue
  • From the list of guide stars provided in the setup file or automatically selected by the system from the guide star catalogue at preset

    (command SELGS to select the entry for the guidestar followed by PRTOGS)

    Preconditions:

    • Internal catalogue contains proper guide stars for the target
  • Taking automatically the next un-tried guide star from the list of guide stars provided in the setup file or automatically selected by the system from the guide star catalogue at preset (command PRTOGS). Automatic catalogue search for guide star is done if the list is empty. The found guide stars are put in the internal catalogue for guide star candidates and the probe is positioned to the more suitable one. Optionally, specific criteria can be given. If not the setup values are used.If no more un-tried entry is available an error is returned.

    Preconditions:

    • Internal catalogue contains proper guide stars for the target
    Exception Course: No more guide stars
    Exception course: Command failed
• ATCS Move Probe to Guide Star (alpha, delta) coordinates and magnitude, if available.
  Exception Course: Command failed
  
• ATCS returns OK reply

Exception Course: No more guide stars

• ATCS returns ERROR reply.
• A catalogue search can be requested with the command SRCGS and the procedure repeated.

Exception Course: Command failed

• ATCS returns ERROR reply

Postconditions:

• Guide Reference Point set to the position of the selected guide star.
• FSS Translation Stage positioned and tracking to new Guide Reference Point

Issues to be Determined or Resolved:

Notes:

• Since at this stage we cannot know if Autoguiding or Field Stablization will be selected, the FSS Translation stage is always positioned and tracking at the new guide reference point.
• This Use Case is the merge of the "Identify Guide Star" and the Probe to Next Guide Star" Use Cases and adds some selection possibilities thaw where not included in these two Use Cases.
• See Move Probe to Guide Star for a discussion of the problems related to the availability of a magnitude estimate for the Guide Star.

Last modified: Fri Oct 26 13:42:55 UTC 2001