atcsucacquirebeaconip

Use Case: Guide Probe to Beacon (acquire beacon)

Description: This use case is used for various Optical Alignment purposes (Image Alignment, Calibrate M6, Focus Alignment, ...). The Guide Probe (FAS guide window or FSS X-Y Table) is moved so that the image of an artificial light source (beacon) is centered and the corresponding Guide Probe position is returned. During this operation, the STRAP loop (Field Stabilisation) is NOT closed, i.e. the M6 does NOT move.

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)
CBSD at the right position (FSS or FAS)
Nasmyth wheel in Beacon position
Nasmyth Beacon ON
STRAP loop open
M6 at a given fixed position (not necessarily centered -e.g. for "calibrate M6"-)

Basic Course:

Send command GP2BEAC to send Guide Probe to beacon image with FSS or FAS

Exception Course:

ATCS identifies beacon image with FAS taking one exposure and retrieving centroid position in CCD X-Y coordinates.

Exception Course:

ATCS Set FSS Translation Stage (X,Y) coordinates.

Alternate Course

Exception Course

Exception Course:

ATCS centers the image on the detector using the X-Y Table (not the M6!!) untill the position error (averaged over a given time -typically 1 sec-) is less than a given threshold.
ATCS sends back the X-Y position to the caller.

Alternate Course:When FAS is selected.

ATCS sends back final reply with the corresponding centroid position in CCD X-Y coordinates. The Use Case is complete.

Exception Course: Command failed

ATCS returns ERROR reply

Postconditions:

Image of beacon has been detected
Image has been centered on FSS to the required accuracy (FSS option)
Image centroid has been computed (FAS option)
X-Y value has been returned to the caller

Issues to be Determined or Resolved:

Notes:

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.
Last modified: Thu Jun 28 13:39:21 UTC 2001