The paper presents the metrological setups for the optical measurements of two high quality, large aperture, extremely compact, diffraction limited, relative aperture 1:2.4 free-form telescopes: the CHI and the TIR. The CHI “Compact and Highly Performing Hyperspectral Imager“, is based on a Three-Mirror Anastigmat (TMA) layout and works in the spectral range 0.4-2.5μm with 150 mm aperture. The Thermal Infrared (TIR) Telescope is based on a two-powered-mirror design and works in the spectral range 10-12 μm with 140 mm entrance aperture. Both free-form Telescopes have been designed by OHB Germany and are currently under production at SPACEOPTIX GmbH / Fraunhofer IOF and at VDL ETG / SUMIPRO, respectively. The measurement campaign consists in measuring the wavefront error all over the field of view at 633 nm, the focal length across- and along-track and the geometric distortion. The paper focuses attention on the TIR freeform Telescope. To support the optical tests preparation, the TIR free-form Telescope is analyzed with a pure geometrical optic approach, making also use of the Helmholtz-Lagrange Invariant. The definition of the Helmholtz-Lagrange Invariant for an axial field is recalled from the classic literature. Then the optical invariant of the paraxial optic is extended to the real ray-tracing of the TIR Telescope both in its nominal layout and when subjected to the manufacturing, alignment and integration (MAIT) tolerances. In this way, the optical invariant represents a method, complementary to the commonly used ones, for evaluating the quality of an optical design and for assessing its robustness to the MAIT tolerances.
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.
The Meteosat Third Generation (MTG) Programme will ensure the future continuity and enhancement of meteorological data from geostationary orbit as currently provided by the Meteosat Second Generation (MSG) system. The industrial prime contractor for the space segment is Thales Alenia Space (France), with a core team consortium including OHB System AG (Germany).
A novel isostatic mounting concept for a space born TMA of the Meteosat Third Generation Infrared Sounder is presented. The telescope is based on a light-weight all-aluminium design. The mounting concept accommodates the telescope onto a Carbon-Fiber-Reinforced Polymer (CRFP) structure. This design copes with the high CTE mismatch without introducing high stresses into the telescope structure. Furthermore a Line of Sight stability of a few microrads under geostationary orbit conditions is provided. The design operates with full performance at a temperature 20K below the temperature of the CFRP structure and 20K below the integration temperature. The mounting will sustain launch loads of 47g. This paper will provide the design of the Back Telescope Assembly (BTA) isostatic mounting and will summarise the consolidated technical baseline reached following a successful Preliminary Design Review (PDR).
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