Collaboration Case:  Set FSS Translation Stage

Description:  Set the FSS Translation Stage position.
This action is used by the FSS Translation Stage Control Loop that updates periodically the XY position of the translation stage so as to keep on tracking the center of the APD device on the computed trajectory of the guide object (on-axis object itself or off-axis guide star).

Priority: Major

Performance: 

According to FISBA description of the FSS support (VLT-TRE-FIS-151100-032) and for a telescope plate scale of 318 um/arcsec:

• Positioning resolution: 1um, i.e. 0.0031 arcsec.
• Positioning range:
  • 20mm in each direction from the center, i.e. 62 arcsec in each direction
• Positioning accuracy:
  • 3um rms over 1 mm displacement, i.e.
    0.0094 arcsec over 3.14 arcsec displacement
  • 30um rms over 40 mm displacement, i.e.
    0.094 arcsec over 125.8 arcsec displacement
• Maximum translation speed: 1.6mm/s in X and Y, i.e 5 arcsec/s

Frequency: Pseudo periodically, up to 1 Hz

Package Assignment:  Field Stabilization Subsystem.

Preconditions: 

• FSS sub-system is ONLINE

Basic Course: 

• Send command SETXYT <X,Y> to move FSS translation stage to (X,Y) position
  Exception Course: Command failed
• ATCS sends command to FSS translation stage

  Interface:  IfFssTranslationStage

  Exception Course: Command failed
• ATCS returns OK reply

Exception Course: Command failed

• ATCS returns ERROR reply
  Postcondition: No motion

Postconditions: 

• FSS XY translation stage at given (X,Y) position

Issues to be Determined or Resolved: 

Notes: We consider a telescope plate scale of ps=318um/arcsec. Close to the zenith, the max. field rotation is r=24"/s=0.116 mrd/s thus the linear velocity at the edge of the translation stage (d=20mm from center, i.e. d/ps=62.9") is then up to v=d*r=2.3um/s (i.e. d*r/ps=0.0071"/s).
The half size of the image on the APD is approx. s=0.15", i.e. s*ps=50um (half the seeing).
The maximum shift allowed for the APD device to give a valid measure is half the image size (s=0.15", i.e 50um) around the center position.
For a star located d=10mm off the optical axis on the APD plane, the maximum field rotation is v=1.15um/s (0.0036"/s).
This means about t=s/v=40s before the off-axis star moves out of the APD measurement domain, under the condition that the APD domain is linear is this range.
For on-axis guiding, solely the differential refraction must be considered. The Translation Table position may then not need to get updated since all the compensations may be handled by computation.
The main goal of the prototype will be the evaluation of the types of motion to be used for the Translation Table tracking:

• Stepwise (in Position mode)
• Continuous (in Velocity Tracking mode)
  The velocity loop is closed on the LCU (as for Enchilada)

Last modified: Mon Feb 25 13:59:17 UTC 2002