The production of high-precision aspheres typically involves at least one extensive chemo-mechanical polishing process to remove subsurface damage after grinding. Especially when machining glasses that are prone to fracture from grinding, subsurface damage can be substantial. As a result, subsequent polishing times are high and it is challenging to maintain a reasonably low surface form error. In order to reduce processing times and increase process stability SwissOptic established an ultra-precision grinding process. This process operates in a ductile grinding mode that not only minimizes subsurface damage but also leads to surfaces with form errors well below one micrometer. The high surface quality after grinding makes it possible to omit chemo-mechanical polishing processes. Instead, we polish and form correct the ultraprecision ground aspherical surfaces in a single process step by applying magnetorheological finishing (MRF). In this paper, we demonstrate the feasibility of this approach based on the production of a fused silica asphere and a steep S-PHM52 asphere. We utilize QEDs Q-Flex 300 for MRF polishing. Very aggressive process parameters are used to keep processing times as low as possible. We find that polishing and form correction to an irregularity below 300 nm is feasible in less than 30 minutes. Depending on the desired quality of the aspheres, MRF polishing parameters can be adjusted.
State of the art laser-based refractive surgery instruments require precise optical beam steering and focusing capabilities to actively control an operational laser beam during eye surgery. As part of an industrial collaboration, Celera Motion and SwissOptic AG worked on a novel design approach to realize a compact, actively controlled lens moving mechanism for beam focusing. This system incorporates a closed loop voice coil driven air bearing that integrates a focusing lens of 10mm in diameter in a low module footprint. The application driven design approach, modularity, and scalability of the solution will be presented together with results of the practical system evaluation. Thereby, the achieved optical alignment accuracy of the moving lens of only a few microns along a moving range of up to 80mm in combination with a lateral movement bandwidth up to 100 Hz indicate best agreement with the design requirements for the driving unit. Since the developed driving mechanism offers high system performance for additional applications, the authors will give further application examples and show scalability approaches for various lens diameters and moving ranges for customized z-scan units in optical systems. As part of future developments, design modifications for applications with higher bandwidth requirements and reduced module footprint with lower weight will be discussed.
We dynamically model the Large Binocular Telescopes optical path using a linear system approach. The model is
derived from experiments conducted at the telescope. These experiments will be described and we will explain the
possibilities and difficulties in extracting a simulation model from measured data. The model also incorporates
disturbances, such as wind forces and single excitations induced by vibrating machinery. We will show, why it is
necessary to measure structural vibrations at the LBT and why we follow a model based approach in estimating
the mirror’s oscillatory motions. Some simulation results will be presented and compared to measured time series
and a conclusion will be drawn. An outlook will be given on how the observer can be implemented.
MPIA leads the construction of the LINC-NIRVANA instrument, the MCAO-supported Fizeau imager for the
LBT, serves as pathfinder for future ELT-AO imagers in terms of size and technology. In this contribution,
we review recent results and significant progress made on the development of key items of our stratgey to
achieve a piston stability of up to 100nm during a science exposure. We present an overview of our vibration control strategies for optical path and tip-tilt stabilization, involving accelerometer based real-time vibration measurements, vibration sensitive active control of actuators, and the development of a dynamical model of the LBT. MPIA also co-develops the E-ELT first-light NIR imager MICADO (both SCAO and MCAO assisted). Our experiences, made with LINC-NIRVANA, will be fed into the MICADO structural AO design to reach highest on-sky sensitivity.
The linearity and accuracy of holography-based modal wavefront sensing (HMWS) is reduced when large aberrations
are present in the incoming wavefront. In this contribution, a combination of HMWS and a low-resolution Shack-
Hartmann sensor (LRSHS) is introduced to extend the dynamic range of HMWS via a compact holographic design. The
typically dominating low-order modes in the incoming wavefront are first corrected by the LRSHS. The system then
switches to HMWS after one or two corrections to obtain better sensor sensitivity and accuracy. First experimental
results are shown for validating the method.
We present our recently started eort to realize feedforward vibration control loops with a full adaptive optics
(AO) testbed in the laboratory. A piezo-driven tip-tilt mirror unit introduces an arbitrary, but controllable, vibration power-spectrum to simulate telescope mirror vibrations of any kind on the wavefront sensor. Our ultimate goal is to demonstrate in realistic laboratory tests, how telescope vibrations faster than atmospheric tip-tilt can be measured by accelerometers, and controlled in real-time feedforward to allow for longer and more sensitive wavefront sensor (WFS) integrations.
To improve the mechanical characteristics of actively controlled continuous faceplate deformable mirrors in adaptive optics, a strategy for reducing crosstalk between adjacent actuators and for suppressing low-order eigenmodes is proposed. The strategy can be seen as extending Saint-Venant's principle beyond the static case, for small local families of actuators. An analytic model is presented, from which we show the feasibility of the local control. Also, we demonstrate how eigenmodes and eigenfrequencies are affected by mirror parameters, such as thickness, diameter, Young's modulus, Poisson's ratio, and density. This analysis is used to evaluate the design strategy for a large deformable mirror, and how many actuators are needed within a family.
The crosstalk problem inherent in holography based modal wavefront sensing (HMWS) becomes more severe with
increasing aberrations of the incident beam. In this paper, the cause of crosstalk is theoretically revealed and then
demonstrated using simulations. For extending the use of HMWS in correcting atmospheric turbulence introduced
aberration, the sensor response is statistically analyzed with random aberrations created in accordance with the
atmosphere turbulence model. The system parameters are optimized considering the turbulence strength and calibrated
response curves are further used to improve the sensor performance. The simulation and first preliminary experimental
results are shown for validating the method.
For high accuracy simulation of Adaptive Optics (AO), multi-conjugate AO (MCAO), and ground layer AO (GLAO)
analytic models have proven to be of significant importance. Usually, these models employ a finite set of Zernike basis
functions that allow replacing point-by-point computation of phase maps by algebraic manipulation of basis function
coefficients. For closed loop simulation of AO systems it is essential to consider the spatial and temporal dynamics of
deformable mirrors and wavefront sensors. In this case, simulations with Zernike basis functions have several drawbacks.
First of all, they become computationally intractable when high order and high frequency behavior is analyzed. Additionally,
the spatial dynamics of deformable mirrors cannot be approximated well by Zernike functions when mechanical
constraints are considered.
In this paper, a set of orthogonal basis functions formed by spatial eigenmodes of deformable mirrors is proposed for
simulation of large scale AO systems. It is shown that an analytic approximation of deformable mirror bending modes can
be derived by solving a partial differential equation (PDE) and an inclusion of appropriate boundary conditions. Three sets
of basis functions from different boundary conditions are studied in detail: the cases of a clamped edge, free edge, and
flexible support of a circular mirror plate. The basis functions are compared to the Zernike functions and their mathematical
properties are discussed.
Shack-Hartmann sensors are commonly used wavefront sensors in a large field of applications, like adaptive optics, beam characterization and non-contact measurements. They are popular because of the ease of use and the robustness of the sensor. We introduce a new way to improve the performance of miniaturized and mass-producible optical wavefront sensors for industrial inspection: A sensor design due to an aperiodic diffractive element working as microlens array allows the use of small and cost-efficient detector chips. The diffractive element was optimized using raytracing and thin element approximation (done in Zemax). As an example, we present the design and realization of a sensor for laboratory use with a measurement diameter of 20mm. We show an example measurement and results concerning dynamic range. The measurement accuracy was determined by measuring spherical waves.