atcsucoffsetguideprobeonstarip

Use Case: Offset Guide Probe on star (Field Stab or Autoguiding loop closed)

Description: This use case is used to make small modifications to the tilt angles of the light beam arriving in the interferometric Laboratory while still being stabilised by the Field Stabilisation (or autoguiding) loop. The offsets shall be given as fixed offsets in the U-V-W site co-ordinate system. They shall be defined as tilt_around_U and tilt_around_W at the output of the M11 (i.e. they are not given in RA,dec and they are not subject to field rotation).
The Guide Probe (FAS guide window or FSS X-Y Table) is moved with a given offset step to a X-Y position while the loop is closed with M6 using the guide star light.
A Combined Offset is not suited for this purpose, because it implies too high time overhead and while it takes place the beam is not stabilized anymore.
This Use Case will be extensively used during VLTI operation (e.g. to optimise the flux injected in the Instrument fibre after each preset.

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

Priority: Major

Performance: Shall be limited by the guide-probe hardware. As a consequence the basic performance requirements are:

Range: > 1 arcsec/sky
Resolution: < 0.005 arcsec/sky
Repeatibility: < 0.01 arcsec/sky RMS
Absolute accuracy (over the whole range, including repeatibility errors): < +/- 0.01 arcsec/sky
Time response time/step (time required to perform the step with a final error less than 2 times the resolution: < 1 sec / 0.100 arcsec/sky

Frequency: Asynchronous, maximum 1 per second

Package Assignment: Auto Guiding.

Preconditions:

FAS sub-system is ONLINE (if selected)
FSS sub-system is ONLINE (if selected)
The telescope is tracking with Field Stabilisation (or Autoguiding) ON.

Basic Course:

Send command OFFSGUV to offset Guide Probe (while in closed loop)
ATCS Sets Guide Reference Point with new position

Exception Course: Command failed
Exception Course: Range of Guide Probe reached
Exception Course: Field Stabilisation loop is interrupted
ATCS sends OK reply when the X-Y position has been reached (i.e. the guide star is at the required position on the FAS or on the FSS - "on target" - and when the closed loop error signal is below a given threshold.

Exception Course: Command failed

ATCS returns ERROR reply

Postconditions:

The X-Y table (or Guide Window) has reached its offset position
The STRAP loop (or Autoguiding loop) is closed
The error signal of the loop is below given threshold

Issues to be Determined or Resolved:

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 specific "Set FSS Translation Stage" may be required.

Last modified: Thu Feb 28 10:19:16 UTC 2002