Imagery collected from space provides very useful information about our planet. Today there are many Earth Observation satellites in orbit which allow us to collect information which is used for environmental monitoring, response to catastrophes, surveillance and security, urban planning, economic analysis and many other applications. Thus, there is a drive to improve the quality of this imagery, such as its resolution and the frequency with which it can be collected. The quantity of pictures taken will be increased by launching more systems, the quality of the pictures depends, amongst other factors, on the system’s physical size. To serve those needs, bigger telescopes have to be launchedmore often.....@en

Piezoelectric actuators are a very attractive technology for active optics mechanisms in space applications due to their very high precision and reliability. However, self-heating of these actuators may be limit their use in space or under high loads, due to the limited ability to evacuate heat. Test procedures that reproduce the operational conditions of these actuators are important to check these operational limits. Here an effort to characterise the thermal emission of pre-stressed piezoelectric actuators in operation is presented. The technique allows direct measurement of the power dissipated by the test item via the control of the different heat transfer mechanisms, using the fall in power provided as measure of power dissipated by the actuators, instead of relying on direct temperature sensors. This allows the construction of a thermal model with a dissipation term readily integrated in system-level modelling to account for the dissipated heat of the piezo. The technique may also be applied to other piezo low power applications in the order of 1 W of thermal emission, and is adaptable to emulate the boundary conditions encountered in operations.

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A new analytical procedure is developed for the deduction of the asymptotic series of the singular solutions in displacements and stresses near the vertex of the linear elastic isotropic corners with the Dirichlet–Robin (fixed-spring) and Neumann–Robin (free-spring) boundary conditions. Under the assumption of antiplane shear loading, the corresponding elastic problem reduces to the Laplace equation for the out-of-plane displacement. In the deduction of such singular solution, the complex variable is used to propose a harmonic function in the form of an asymptotic series including both power and logarithmic terms. This original procedure is suitable for its implementation in a computer algebra software which makes all the necessary symbolic computing, simplifications and rearrangements. This is a key issue due to the fact that the complexity of terms in these series may increase with increasing order of terms. These series are composed by the main terms (also called main singularities), solutions of the corresponding Dirichlet–Neumann or Neumann–Neumann problems, and the associated finite or infinite series of the so-called shadow terms (also called shadow singularities). These terms are determined by solving systems of recursive inhomogeneous Dirichlet–Neumann or Neumann–Neumann problems, respectively. A general classification of the behaviours of the asymptotic series covering all the considered corner problems is introduced. A few examples of the asymptotic series for corners with Dirichlet–Robin and Neumann–Robin boundary conditions are presented to illustrate the capabilities of this procedure.

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Deployable optics promise a revolution in the capability of observing the universe by delivering drastically reduced mass and volume needs for a desired level of performance compared to their conventional counterparts. However, this places new demands on the mechanical and thermal designs of new telescopes, essentially trading mass and volume for structural and control complexity. We compile the thermomechanical challenges that should be taken into consideration when designing optical space systems, as well as summarize 14 projects proposed to address them. Stringent deployment repeatability requirements demand low hysteresis, whereas stability requirements require high stiffness, proper thermal management, and active optics.

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The following research topics, running at the Delft University of Technology, aim to increase the spatial resolution of Earth Observation systems: 1. Small instruments and sensor development for PocketQubes and CubeSats 2. A Stable Highly Accurate Pointing EO (SHAPE) 6U spacecraft 3. The Stratospheric balloon project Stratocruiser 4. The Delft Deployable Space Telescope (DST) project The focus of this paper is topic 4 which is driven by the need for cheaper, lighter and smaller telescopes imposed by the on-going trend to deliver more refined Earth observation data at a lower price. Evident reasons to incorporate deployable telescope structures are firstly to fit in a launcher and secondly to decrease launch mass and volume. The Deployable Space Telescope (DST), being developed at the Delft University of Technology, aims to reduce volume (> 4 times) and mass (< 100 kg) by using innovative deployable optics. The WorldView-4 satellite was chosen as benchmark for its development. The DST overall systems design is driven by a strict bottom-up versus top-down systems engineering approach. The coarse alignment budget is treated as a one-off deployment precision performance, with the drift and stability budgets as low and high frequency stability margins. Most critical subsystem is the first DST mirror M1: its position has to be accurate up to 2 µm in all directions whilst the tilts shall be within 2 µrad. In orbit the dynamic thermo-mechanical conditions require these parameters to be within 5 nm position and 10 nrad as stability budget. The M1 calibration and actuation is controlled by a wave front error algorithm. The novel actuation system, implementing the active optics strategy, is mounted on the mirror support structure. The allowable deployment errors in tip, tilt and piston are 16, 10 and 13 µm whilst the actuation precision is 51, 32 and 10 nm. To support these critical budgets the development and testing of the first key DST hardware comprises a 3D printed COmpliant Rolling contact Element (CORE) hinge design. Hinges of this design were not applied in space so far. A good mechanical hinge design for high-precision deployment is identically one that exhibits low-hysteresis response to load cycling. The hysteresis was tested by a technique called Digital Image Correlation, which is normally used to detect micro-cracks in composite layers. The test setup proved to be very suitable for the hysteresis characterisation with a precision down to 100 nm. The maximum hysteresis found was 0.3 µm for over 50 load cycles. The CORE hinge design is currently tested for hysteresis response and thermal gradient behaviour. This paper describes the status of the optical, thermo-mechanical and active optics systems design. The concurrent design approach combined with a strict bottom-up and top-down compliant systems engineering approach shows that the DST is a healthy system concept.

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The increase of spatial and temporal resolution for Earth observation (EO) is the ultimate driver for science and societal applications. However, the state-of-the-art EO missions like DigitalGlobe's Worldview-3, are very costly. Moreover, this system has a high mass of 2800 kg and limited swath width of about 15 km which limits the temporal resolution. In this article, we present the status of the deployable space telescope (DST) project, which has been running for 6 years now at the Delft University of Technology, as a cutting-edge solution to solve this issue. Deployable optics have the potential of revolutionising the field of high resolution EO. By splitting up the primary mirror (M1) of a telescope into deployable segments and placing the secondary mirror (M2) on a deployable boom, the launch volume of a telescope can be reduced by a factor of 4 or more, allowing for much lower launch costs. This allows for larger EO constellations, providing image data with a much better revisit time than existing solutions. The DST specification baseline, based on Wordview-3, aims to provide images with a ground resolution of 25 cm (panchromatic 450-650 nm) from an orbital altitude of 500 km. In this paper, the current status of the optical, thermo-mechanical, and active optics systems design are described. The concurrent design approach combined with a strict bottom-up and top-down compliant systems engineering approach show that the DST is a healthy system concept.

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