We describe a new technique to reduce tip and tilt vibrations via the design of adaptive optics controllers in a frequency framework. The method synthesizes controllers by minimizing an H2 norm of the tip and tilt residuals. In this approach, open loop slopes (pseudo-open-loop) are reconstructed from on-sky data and input into off-line simulations of the adaptive optics system. The proposed procedure executes a sequence of off-line closed-loop runs with increasing controller complexity and searches for the controller that minimizes the variance of residuals. Although the method avoids any identification of the vibration and turbulence models during the controller synthesis, the actual models are indirectly constructed as a by-product of the H2 norm minimization. The technique has been implemented on and tested with two operational instruments, namely Paranal's NACO and Gemini-South's GeMS, showing an effective rejection of the main vibrations in the loop and also improving the overall performance of the system over varying turbulence conditions. It is shown that a superior performance is obtained when compared to the standard integrator controller.

Mitigation of vibrations in adaptive optics by minimization of closed-loop residuals

Vibrations are detrimental to the performance of modern adaptive optics (AO) systems. In this paper, we describe new methods tested to mitigate the vibrations encountered in some of the instruments of the Gemini South telescope. By implementing a spectral analysis of the slope measurements from several wavefront sensors and an imager, we can determine the frequencies and magnitude of these vibrations. We found a persistent vibration at 55 Hz with others occurring occasionally at 14 and 100 Hz. Two types of AO controllers were designed and implemented, Kalman and H∞, in the multiconjugate AO tip–tilt loop. The first results show a similar performance for these advanced controllers and a clear improvement in vibration rejection and overall performance over the classical integrator scheme. It is shown that the reduction in the standard deviation of the residual slopes (as measured by wavefront sensors) is highly dependent on turbulence, wind speed, and vibration conditions, ranging—in terms of slopes RMS value—from an almost negligible reduction for high speed wind to a factor of 5 for a combination of low wind and strong vibrations.

Comparison of vibration mitigation controllers for adaptive optics systems.

https://www.jstage.jst.go.jp/article/mer/1/2/1_2014se0010/_pdf

Microvibration in spacecraft

In the ground-based astronomical telescope, the imaging quality is usually affected by tip-tilt vibrations induced by the telescope structure. Long exposure time of the image sensor is usually required for a good signal-to-noise ratio, which brings the amount of time delays into the control loop of the tip-tilt mirror. Thus, a low control bandwidth is insufficient to compensate telescope vibrations. In this paper, an improved disturbance observer (DOB) control based on only the image sensor is proposed to reject structural vibrations. This new DOB controller is added into the original loop in the tip-tilt mirror control system, so the closed-loop performance can be optimized by designing an appropriate Q-filter. An improved Q-filter is presented to reject large-magnitude vibrations and also to reduce amplifications induced by this controller at nondisturbance frequencies. This proposed controller can achieve a satisfying result in vibration mitigations when vibration frequencies are provided to design the Q-filter. Furthermore, in the tilt–tilt mirror control system, this improved DOB is not restricted by an accurate model such that noise in the loop cannot provide serious influence on vibration mitigation. Both simulations and extensive experiments are carried out to demonstrate that the DOB can earn an extra improvement on the control performance in comparisons with the conventional feedback controller.

Disturbance Observer-Based Control of Tip-Tilt Mirror for Mitigating Telescope Vibrations

Vibration rejection of Tip-Tilt mirror using improved repetitive control

The Gemini Planet Imager (GPi) is an instrument that will mount to either of two nominally identical Telescopes,
Gemini North in Hawaii and Gemini South in Chile, to perform direct imaging and spectroscopy of extra-solar planets.
This 2,000-kg instrument has stringent mass, center-of-gravity, flexure, and power constraints. The Flexure Sensitive
Structure (FSS) supports the main opto-mechanical sub-systems of the GPi which work in series to process and analyse
the telescope optical beam.
The opto-mechanical sub-systems within the FSS are sensitive to mechanical vibrations, and passive damping strategies
were considered to mitigate image jitter. Based on analysis with the system finite element model (FEM) of the GPi, an
array of 1-kg tuned mass dampers (TMDs) was identified as an efficient approach to damp the first two FSS flexural
modes which are the main sources of jitter. It is estimated that 5% of critical damping can be added to each of these
modes with the addition of 23 kg of TMD mass. This estimate is based on installing TMD units on the FSS structural
members. TMD mass can be reduced by nearly 50% if the units can be installed on the opto-mechanical sub-systems
within the FSS with the highest modal displacements.
This paper describes the structural design and vibration response of the FSS, modal test results, and plans for
implementation of the TMDs. Modal measurements of the FSS structure were made to validate the FEM and to assess
the viability of TMDs for reducing jitter. The test configuration differed from the operational one because some
payloads were not present and the structure was mounted to a flexible base. However, this test was valuable for
understanding the primary modes that will be addressed with the TMDs and measuring the effective mass of these
modes.

Timothy J. Pargett

Papers

Vibration suppression for the Gemini Planet Imager