Collaboration Case:  FSS Control Loop

Description:  The FSS XY-Translation Stage control loop is responsible for the tracking of the FSS Translation Stage and FAS on a Guide Star. It is triggered by a timer positions the XY-table so that the center of the APD sensor is kept on the trajectory of the object in the Coudé focal plane. At every iteration, the reference position of the FSS XY Translation Stage in (alpha, delta) sky coordinates is converted in (X,Y) coordinates taking into account the current telescope position, the pointing model and differential refraction effects due to the difference in wavelength between the object and the guide star. The same is done to set the reference point of the FAS to do centroids. Also the Interaction matrix is updated accordingly.

Priority: Critical

Performance: 10ms

Frequency: 10Hz

[Design Description Entry]

Package Assignment:  Field Stabilization Subsystem.

Preconditions: 

Basic Course: 

Exception Course: FSS Translation Stage in limit vicinity

Exception Course: FSS Translation Stage on limit

Exception Course: Command failed

Postconditions: 

Issues to be Determined or Resolved: 

Notes: The Translation Stage is a tracking axis and its behavior is very similar to the adapter/rotator of the Unit Telescopes: the difference between the sky positions of the telescope (i.e. of the observed object) and of the guide star are used to calculate the (x,y) position of the table with respect to the center of the field.

The XY table positioning accuracy and positioning speed should be good enough for all our guide star tracking needs (see Set FSS Translation Stage Use Case for details on requirements and expected performance).

If not, we have to exploit the possibility of applying an offset to the APD center position, so that we:

This feature is not foreseen in the current STRAP and can be asked as a new feature in an already planned extension of the contract with the manufacturer of the system.

The problem is how to calculate the unbalancing in the APD reads to match a defined offset. These values depend strongly on the observing condition and will require some development. Also a calibration procedure will be necessary. All this cannot be foreseen for the current release of STRAP.

In the next iteration, the UC responsible of catching and handling the alarm FSS Translation Stage in limit vicinity shall be introduced.
Preliminary Algorithm for anisotropy correction:
* We assume here conversion sky ==> Coude X-Y
* Notation:
[d_alpha, d_delta]: position vector in SKY coordinate (wrt to center of
field, including refraction correction)
[d_X, d_Y]_isotrop: position vector at Coudé focus without anisotropy
[d_X, d_Y]_anisotrop: position vector at Coudé focus in the real case
with anisotropy

* Algorithm
1- convert [d_alpha, d_delta] into "perfectly isotropic" Coudé coordinates
[d_X, d_Y]_isotrop. For that purpose, the formulae given in "Field & Pupil
rotations..."G. Avila, K. Wirenstrand and its complement VLT-INS-93-0069 can
be used.

2- transform (by a rotation matrix) these coordinates into a reference frame
which is aligned (and rotates) with the Fork (i.e. X_fork,Y_fork,Z_fork).
The angle between the two reference frame is the Azimuth angle (+ possibly a
fixed misalignment angle depending how these reference frames are exactly
defined)

3- apply, in the "Fork frame", a correction formula of the type:
d_x= polynomial([d_X, d_Y])
d_y= polynomial([d_X, d_Y])
then
[d_X, d_Y]_new=[d_X, d_Y]
where polynomial is a 2 variable polynomial (max order 3) whose coefficient
shall be defined in a setup file or database
4- transform back (by the inverse rotation matrix) these new coordinates
into the selected Coude X-Y reference frame (which should normally be the
global site U_V coordinate).

Last modified: Mon Feb 25 14:05:38 UTC 2002