Wittrock, Ulrich
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- active optics, adaptive optics, sharpness metrics, aberration compensation, algorithm design (2)
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Further space exploration in the far-infrared (FIR) requires larger apertures in order to improve the spatial resolution of captured images. To this purpose, the Thinned Aperture Light Collector (TALC) concept of a deployable annular telescope has been recently developed at CEA, which offers novel perspectives for FIR space missions. The consortium ELICSIR consortium of European institutes and companies has been created to improve the technological readiness level (TRL) of its key systems and components.
Active optics is an enabling technology for future large space telescopes. Image-based wavefront control uses an image-sharpness metric to evaluate the optical performance. A control algorithm iteratively adapts a corrective element to maximize this metric, without reconstructing the wavefront. We numerically study a sharpness metric in the space of Zernike modes, and reveal that for large aberrations the Zernike modes are not orthogonal with respect to this metric. The findings are experimentally verified by using a unimorph deformable mirror as
corrective element. We discuss the implications for the correction process and the design of control algorithms.
With a view to future large space telescopes, we investigate image-based wavefront correction with active optics. We use an image-sharpness metric as merit function to evaluate the image quality, and the Zernike modes as control variables. In severely aberrated systems, the Zernike modes are not orthogonal to each other with respect to this merit function. Using wavefront maps, the PSF, and the MTF, we discuss the physical causes for the non-orthogonality of the Zernike modes with respect to the merit function. We show that for combinations of Zernike modes with the same azimuthal order, a flatter wavefront in the central region of the aperture is more important than the RMS wavefront error across the full aperture for achieving a better merit function. The non-orthogonality of the Zernike modes with respect to the merit function should be taken into account when designing the algorithm for image-based wavefront correction, because it may slow down the process or lead to premature convergence.
We present our latest results on a refined unimorph deformable mirror which was developed in the frame of the ESA GSTP activity ”Enabling Technologies for Piezo-Based Deformable Mirrors in Active Optics Correction Chains”. The identified baseline concept with the soft piezoceramic material PIC151 successfully sustained all vibration requirements (17.8 gRMS random and 20 g sine) and shock testing (300 g SRS). We cover the mirror design development which reduces the stress in the brittle piezo-ceramic by 90 % compared to the design from
a former GSTP activity. We briefly address the optical characterization of the deformable mirror, namely the achieved Zernike amplitudes as well as the unpowered surface deformation (1.7 µm) and active flattening (12.3 nmRMS). The mirror produces low-order Zernike modes with a stroke of several tens of micrometer over a correction aperture of 50 mm, which makes the mirror a versatile tool for space telescopes.
This paper presents the results of the technology development project “Enabling Technologies for Piezo-Based Deformable Mirrors in Active Optics Correction Chains” conducted by OHB System AG together with its partner Münster University of Applied Sciences (MUAS). The project was funded by ESA within their General Support Technology Programme
(GSTP).
We address in this paper mainly the definition, flow-down and verification of the requirements for the Deformable Mirror (DM). The requirements were derived from a set of real space mission applications. The deformation of the mirror is performed by piezo-ceramic actuators in an unimorph configuration. The finally developed DM is able produce Zernike modes with a stroke of several tens of µm over a clear optical aperture of 50 mm in diameter. It underwent successfully a full environmental qualification campaign including thermal cycling, shock- and vibration testing, as well as exposure to
proton and γ–ray radiation. Thermal and performance tests were performed in the temperature range from 100 K to 300 K.
Furthermore, the DM sustained all vibration (random 17.8 g RMS and sinus) and shock (300 g) testing. Thereby all criticalities which were identified a previous study have been overcome successfully.
A Technology Readiness Level (TRL) of 5 is reached, as the component has been validated in relevant environment. Based on the high level of maturity, this deformable mirror is now ready for the incorporation in future flight instruments. The achieved TRL of 5 is sufficient for the status of a PDR at payload level and gives thus a very good basis for all kinds of potential B2, C/D payload developments.