ESO invested enormous effort in developing and commissioning the VLT-Interferometer (VLT-I), a unique facility
providing a spatial resolution equivalent to that of a 200m-telescope. Complementary to the regular VLT operations,
latterly additional 230 nights per year were scheduled to execute scientific observations with large VLT-I baselines. But
to the same degree as the VLT-I performance and stability were improving over the past years, likewise the vibration
sensitivity of the optical system was increasing and stricter requirements on mechanical stability were necessary. As a
consequence ESO started years ago an extensive program to identify and mitigate potential vibration issues. In the scope
of this work, the mechanical vibrations induced by cryo-coolers, widely used in ESO’s VLT instrumentation suite, were
diagnosed as one of the major disturbance sources. In order to be able to better control their impact, the development of a
more significant vibration specification for VLT instruments became essential.
In the course of preparing such a specification, we first followed an experimental approach where we installed a
dedicated dummy instrument equipped with current ESO standard cryo-coolers in different VLT foci configurations and
performed a comprehensive vibration measurement test campaign under real VLT/VLT-I operation conditions. All
obtained vibration measurement data were spectral analyzed with respect to the actual VLT-I optical path length
difference acceptance levels. This campaign gave valuable information about typical cryo-cooler induced vibration levels
and their consequence for VLT-I operations. It also enabled the release if novel conform cryo-cooler instrument design
and operation recommendations.
This paper describes the applied vibration measurement methodology on the basis of examples, the development and
description of the significant VLT instrument vibration specification, and a proposal for a generic verification procedure
for standalone instruments or sub-units prior final acceptance.