Characterisation, mitigation and correction of telescope vibrations have proven to be crucial for the performance
of astronomical infrared interferometers. The project teams of the interferometers for the LBT, LINC-NIRVANA
and LBTI, and LBT Observatory (LBTO) have embarked on a joint effort to implement an accelerometer-based
vibration measurement system distributed over the optical elements of the LBT. OVMS, the Optical Path
Difference and Vibration Monitoring System will serve to (i) ensure conditions suitable for adaptive optics
(AO) and interferometric (IF) observations and (ii) utilize vibration information, converted into tip-tilt and
optical path difference data, in the control strategies of the LBT adaptive secondary mirrors and the beam
combining interferometers. The system hardware is mainly developed by Steward Observatory's LBTI team and
its installation at the LBT is underway. The OVMS software development and associated computer infrastructure
is the responsibility of the LINC-NIRVANA team at MPIA Heidelberg. Initially, the OVMS will fill a data archive
provided by LBTO that will be used to study vibration data and correlate them with telescope movements and
environmental parameters thereby identifiying sources of vibrations and to eliminate or mitigate them. Data
display tools will help LBTO staff to keep vibrations within predefined thresholds for quiet conditions for AO
and IF observations. Later-on real-time data from the OVMS will be fed into the control loops of the AO systems
and IF instruments in order to permit the correction of vibration signals with frequencies up to 450 Hz.
We review the status of hardware developments related to the Linc-Nirvana optical path difference (OPD) control. The
status of our telescope vibration measurements is given. We present the design concept of a feed-forward loop to damp
the impact of telescope mirror vibrations on the OPD seen by Linc-Nirvana. At the focus of the article is a description of
the actuator of the OPD control loop. The weight and vibration optimized construction of this actuator (aka piston
mirror) and its mount has a complex dynamical behavior, which prevents classical PI feedback control from delivering
fast and precise motion of the mirror surface. Therefore, an H-; optimized control strategy will be applied, custom
designed for the piston mirror. The effort of realizing a custom controller on a DSP to drive the piezo is balanced by the
outlook of achieving more than 5x faster servo bandwidths. The laboratory set-up to identify the system, and verify the
closed loop control performance is presented. Our goal is to achieve 30 Hz closed-loop control bandwidth at a precision of 30 nm.
The Large Binocular Telescope (LBT) uses two 8.4 meters active primary mirrors and two adaptive secondary
mirrors on the same mounting to take advantage of its interferometric capabilities. Both applications, interferometry
and AO, are sensitive to vibrations. Several measurement campaigns have been carried out at the LBT
and their results strongly indicate that a vibration monitoring system is required to improve the performance of
LINC-NIRVANA, LBTI, and ARGOS, the laser guided ground layer adaptive optic system.
Currently, a control software for mitigation and compensation of the vibrations is being designed. A complex set
of algorithms collects real-time vibration data, archiving it for further analysis, and in parallel, generating the
tip-tilt and optical path difference (OPD) data for the control loop of the instruments. A real-time data acquisition
device equipped with embedded real-time Linux is used in our systems. A set of quick-look tools is currently
under development in order to verify if the conditions at the telescope are suitable for interferometric/adaptive
LINC-NIRVANA is the near-infrared homothetic imaging camera for the Large Binocular Telescope. Once
operational, it will provide an unprecedented combination of angular resolution, sensitivity and field of view. Its
Fringe and Flexure Tracking System (FFTS) is mandatory for an efficient interferometric operation of LINC-NIRVANA.
It is tailored to compensate low-order phase perturbations in real-time to allow for a time-stable
interference pattern in the focal plane of the science camera during the integration. Two independent control
loops are realized within FFTS: A cophasing loop continuously monitors and corrects for atmospheric and
instrumental differential piston between the two arms of the interferometer. A second loop controls common
and differential image motion resulting from changing orientations of the two optical axes of the interferometer.
Such changes are caused by flexure but also by atmospheric dispersion.
Both loops obtain their input signals from different quadrants of a NIR focal plane array. A piezo-driven
piston mirror in front of the beam combining optics serves as actuator in the cophasing loop. Differential piston
is determined by fitting a parameterized analytical model to the observed point spread function of a reference
target. Tip-tilt corrections in the flexure loop are applied via the secondary mirrors. Image motion is sensed for
each optical axis individually in out-of-focus images of the same reference target.
In this contribution we present the principles of operation, the latest changes in the opto-mechanical design,
the current status of the hardware development.
The Large Binocular Telescope (LBT) is an international collaboration, with partners from the United States, Italy, and
Germany. The telescope uses two 8.4-meter diameter primary mirrors to produce coherent images with the combined
light along with adaptive optics.
The correct functioning and optimum performance of the LBT is only achieved through a complex interplay of various
optical elements. Each of these elements has its individual vibration behaviour, and therefore it is necessary to
characterize the LBT as a distributed vibration system.
LINC-NIRVANA is a near-infrared image-plane beam combiner with advanced, multi-conjugated adaptive optics, and
one of the interferometric instruments for the Large Binocular Telescope (LBT). Its spectral range goes from 1.0 μm to
2.45 μm, therefore the requirements for the maximum optical path difference (OPD) are very tight (λ/10 ~ 100 nm). <sup>1</sup>
During two dedicated campaigns, the vibrations introduced by various actuators were measured using different kinds of
sensors. The evaluation of the obtained data allows an estimation of the frequency and amplitude contributions of the
individual vibration sources.
Until the final state of the LBT is reached, further measurements are necessary to optimize and adapt the equipment and
also the investigated elements and configurations (measurement points and directions, number of sensors, etc.).