Use Case:  Set Tracking Wavelength

Description:  Set Tracking Wavelength of the target

Role(s)/Actor(s): 
Primary: Operator,Maintenance
Secondary: Tracking axes, FAS, FSS

Priority: Major.

Performance: 1 second typ.

Frequency: Asynchronous, typically before starting tracking.

[Design Description Entry]

Package Assignment:  Tracking.

Preconditions: 

Basic Course: 

Exception Course: Command failed

Postconditions: 

Issues to be Determined or Resolved:  A detailed formula collection to be setup.

Notes:

It is necessary to take into account the differential refraction
in the computation of the Reference point trajectory in the FAS and/or FSS image space since the field rotation makes the tracking look like an ellipse on the image.
This effect is probably even stronger when guiding not on the object but on a nearby star together with the beam splitting effect (visible part on FAS/FSS and IR part to Combined Focus).

TCS currently does not support the possibility of setting the spectral types of the Object and of the Guide Star in the Set-up File instead of the wavelength. In the current implementation, the user specifies the Object wavelength while the guiding wavelength is well defined by a narrow-band filter. Narrow-band filter is not desirable in VLTI where the guiding/Field Stabilisation will often be done one the object itself (to avoid atmospheric anisoplanatic effects). In addition the differential pointing errors due to refraction are more stringent for VLTI. This issue will be further addressed in the review of the ICD ISS-Instrument.

The formula to convert from spectral class to wavelength would be something like (B.Koehler):

star spectrum * optics transmission (incl. M9) * detector QE

and find the wavelength corresponding to the the barycenter of the above intensity courve versus atmospheric position error.

Last modified: Thu Jun 28 13:58:27 UTC 2001