The VLT Interferometer - ESO

3.7. "BLIND FRINGE ACQUISITION AND MAINTENANCE" 85
• Accelerometers : they may help for short term drifts and vibrations but
will not provide the low frequency information.
3.7.4.4 Path Length Control in the Optical Train: Needs
• requirements: 1 to 10 pm
• need for frequent sampling (vibrations)
3.7.4.5 Path Length Control in the Optical Train: Methods
Once the" center of telescope" point is precisely located, the pathlength from
that point to the beamcombiner remains to be measured. The simplest way
would be to use a metrology laser beam in the optical train itself. In order to
prevent the problems that would arise, as a 10 9 brightness factor is to be expected
with the stellar light, a pulse mode may be envisioned. A more realistic,
though less precise solution, would be to measure only a few critical segments
in the optical train with well separated laser beams. Here again, accelerometers
will be useful, but cannot be used alone.