The next generation of large monolithic mirror space telescopes will use aberration correction to ensure resolution performance is maintained throughout their mission. This is due to the use of ever thinner and lighter primary mirrors, which are susceptible to deformation due to a range of effects such as mechanical stress and thermal changes. In this work, we outline our space-telescope design and its corresponding active optics system to correct for these aberrations. We also describe our laboratory system for testing the wavefront sensing and aberration correction capabilities of the active optics components, along with some preliminary experimental results.
Manufacturing telescopes with 4, 8 or 16 meter apertures is the most effective way to gather the light of faint exoplanets or look back towards the Big Bang. However, ultra-high optical quality large mirrors drastically increase the mass of such instruments - if made conventionally. Thinner, and hence lighter, primary mirrors suffer from gravity release, temperature changes and misalignment during launch. The resulting surface distortions as well as inherent surface errors which arise during manufacturing can be reduced by the implementation of active optics. We designed an active metal mirror as a key element for active optics in space. Our goal was to develop an ultra-stable, set-and-forget, lightweight active mirror with good wavefront correction performance. A simulation routine was developed to investigate the dependency between geometric parameters of the Deformable Mirror (DM) and the residual surface error after correction of Zernike modes. With the final 25 actuator mirror design we can achieve residual errors of less than 10 nm RMS for individual Zernike modes for an optical pupil of 103 mm diameter. It is able to withstand quasi-static launch loads, is insensitive to temperature changes and we can limit the overall weight to 2500 g including actuators and mirror mount.
While ambitious plans are being developed for giant, segmented telescopes in space, we feel that a large monolithic mirror telescope would have several advantages in the near term. In particular, the risk involved in deploying the optics will be significantly reduced, and the telescope can provide excellent image quality without the need for precise segment alignment and phasing.
An active support for large reflective optics is suitable to compensate for manufacturing-induced deformations and (re-)positioning-induced deformations such as induced by slewing of earth-based telescopes.
The High-Power Focus Mirror we present in this paper gives access to dynamic focus position adaptation along 3.6 mm in high-power laser manufacturing. We developed and tested a new thermo-mechanical design for a unimorph deformable mirror that provides an extensive focal length range down to -2 m focal length. Moreover, the mirror’s unique thermal characteristics enable high-power applications up to 6.4 kW (2000 W/cm²) with stable optical beam quality as thermal lensing is successfully suppressed. Thus, the laser’s optical beam quality M² is stable over the entire actuation and thermal range.
We will describe the design and the characterization of the High-Power Focus Mirror. The mirror setup is based on a unimorph concept using a piezoelectric actuator and a thin glass substrate with a highly reflective multilayer coating. An integrated copper layer improves the heat dissipation. Providing maximum stroke, as well as excellent dynamic properties, the deformable mirror substrate is mounted by our established compliant cylinders .
Furthermore, we investigate the incorporation of the High-Power Focus Mirror into a commercial laser-cutting system. We set up a laser-cutting test bench including a multimode laser source, the focus mirror, a commercial laser processing head, and measuring instruments. In this assembly, we measure the achievable focus position range as well as the laser beam quality.
With this focus mirror, we want to encourage new, innovative high-power application fields in 3D laser processing such as laser cutting, welding, and structuring.
The next generation of UVOIR space telescopes will be required to provide excellent wavefront control despite
perturbations due to thermal changes, gravity release and vibrations. The STOIC project is a response to an ESA
Invitation to Tender to develop an active optics correction chain for future space telescopes. The baseline space telescope
being considered is a two-mirror, 4m telescope with a monolithic primary mirror – we refer to this concept as Hypatia.
The primary mirror diameter could be extended, but is limited in the near future by launch vehicle dimensions. A
deformable mirror (pupil diameter 110mm) will be an integral part of the telescope design; it is being designed for high
precision and the ability to maintain a stable form over long periods of time. The secondary mirror of the telescope will
be activated to control tip-tilt, defocus and alignment with the primary. Wavefront sensing will be based on phase
diversity and a dedicated Shack-Hartmann wavefront sensor.
The project will develop a laboratory prototype to demonstrate key aspects of the active correction chain. We present the
current state of the preliminary design for both the Hypatia space telescope and the laboratory breadboard.
We report on the development of an active mount with an orthogonal actuator matrix offering a stable shape optimization for gratings or mirrors. We introduce the actuator distribution and calculate the accessible Zernike polynomials from their actuator influence function. Experimental tests show the capability of the device to compensate for aberrations of grating substrates as we report measurements of a 110x105 mm2 and 220x210 mm2 device With these devices, we evaluate the position depending aberrations, long-term stability shape results, and temperature-induced shape variations. Therewith we will discuss potential applications in space telescopes and Earth-based facilities where long-term stability is mandatory.
Deformable mirrors can be used in cryogenic instruments to compensate for temperature-induced deformations. A
unimorph-type deformable mirror consists of a mirror substrate and a piezoelectric layer bonded on substrates rear
surface. A challenge in the design of the deformable mirror is the lack of knowledge about material properties.
Therefore, we measured the coefficient of thermal expansion (CTE) of the substrate material TiAl6V4 between 295 K
and 86 K. The manufactured mirror is characterized by an adaptive optical measurement setup in front of a test cryostat.
The measured mirror deformations are feedback into a finite element model to calculate the CTE of the piezoelectric
layer. We compare our obtained results to other published CTE-values for the piezoelectric material PIC151.
Laser-induced mirror deformation and thermal lensing in optical high power systems shall be compensated by a
thermally-piezoelectric deformable mirror (DM). In our device, the laser-induced thermal lensing is compensated by
heating of the DM as previously described with compound loading. We experimentally show the capability of this mirror
for wavefront shaping of up to 6.2 kW laser power and power densities of 2 kW/cm2. The laser-induced defocussing of
the membrane is compensated by mirror heating. We introduce a new mirror setup with buried heater and temperature
sensor elements. Therewith, the compensation of laser-induced mirror deformation is possible within the same time
scale. The piezoelectric stroke of the single actuators depends on their position on the membrane, and is not affected by
the reflected laser power.
The speed of real-time adaptive optical systems is primarily restricted by the data processing hardware and computational aspects. Furthermore, the application of mirror layouts with increasing numbers of actuators reduces the bandwidth (speed) of the system and, thus, the number of applicable control algorithms. This burden turns out a key-impediment for deformable mirrors with continuous mirror surface and highly coupled actuator influence functions. In this regard, specialized hardware is necessary for high performance real-time control applications. Our approach to overcome this challenge is an adaptive optics system based on a Shack-Hartmann wavefront sensor (SHWFS) with a CameraLink interface. The data processing is based on a high performance Intel Core i7 Quadcore hard real-time Linux system. Employing a Xilinx Kintex-7 FPGA, an own developed PCie card is outlined in order to accelerate the analysis of a Shack-Hartmann Wavefront Sensor. A recently developed real-time capable spot detection algorithm evaluates the wavefront. The main features of the presented system are the reduction of latency and the acceleration of computation For example, matrix multiplications which in general are of complexity O(n3 are accelerated by using the DSP48 slices of the field-programmable gate array (FPGA) as well as a novel hardware implementation of the SHWFS algorithm. Further benefits are the Streaming SIMD Extensions (SSE) which intensively use the parallelization capability of the processor for further reducing the latency and increasing the bandwidth of the closed-loop. Due to this approach, up to 64 actuators of a deformable mirror can be handled and controlled without noticeable restriction from computational burdens.
The testing of a lightweight unimorph-type deformable mirror (DM) for wavefront correction in cryogenic instruments is reported. The presented mirror manufactured from the titanium alloy TiAl6V4 with a piezoelectric disk actuator was cooled to 86 K and characterized for thermally induced deformation and the achievable piezoelectric stroke between room temperature and 86 K. Through a finite element analysis, we obtained a first approximation in determining the exact temperature-dependent coefficient of thermal expansion (CTE) of the piezo material PIC151. Simulations were based on dilatometer measurements of the CTE of the TiAl6V4 mirror base between room temperature and 86 K. These investigations will enable the improvement of the athermal design of a unimorph-type DM.