Euclid Structural-Thermal-Optical Performance

TL;DR

This paper presents a structural-thermal-optical performance (STOP) analysis for the Euclid VIS instrument to ensure weak-lensing image quality meets stringent PSF stability requirements. It integrates thermal, structural, and optical models (TMM, MSC NASTRAN, CODE V) via MaREA-Isight to predict IQ perturbations across 96 steady-state cases and a transient case, and validates these predictions against early in-orbit measurements. The study identifies the telescope baseplate temperature as the primary thermo-mechanical driver, with defocus and subtle mirror deformations shaping the PSF, and demonstrates that prelaunch STOP predictions closely track in-orbit behavior, though some discrepancies arise from instrument-driven heat and spectral/color effects. The results confirm excellent overall performance, provide quantitative correlations between baseplate temperature and IQ metrics, and show STOP as a valuable design and in-orbit diagnostic tool for maintaining WL science requirements over the mission lifetime.

Abstract

The Euclid system performance is defined in terms of image quality metrics tuned to the weak gravitational lensing (WL) cosmological probe. WL induces stringent requirements on the shape and stability of the VIS instrument system point spread function (PSF). The PSF is affected by error contributions from the telescope, the focal plane and image motion, and is controlled by a global error budget with error allocations to each contributor. Aims. During spacecraft development, we verified through a structural-thermal-optical performance (STOP) analysis that the built and verified telescope with its spacecraft interface meets the in-orbit steady-state and transient image quality requirements. Methods. For the purposes of the STOP analysis, a detailed finite-element mathematical model was set up and a standard set of test cases, both steady-state and transient, was defined, comprising combinations of worst-case boundary conditions. Results. The STOP analysis addressed the interaction of all spacecraft components in transmitting temperature-induced loads that lead to optical train deformation. The results of the prelaunch analysis demonstrated that temperature-induced optical perturbations will be well below the allowable limits for all permitted observing conditions. During the first year in orbit, we used the STOP analysis predictions to help interpret the measured performance as a function of environmental variables. Unpredicted disturbances were discovered and unexpected sensitivities were revealed. In-orbit temperature variations are small (<300 mK) and so are their effects on the telescope structure, but they are detected in the time histories of the image quality metrics and are a non-negligible factor in the PSF stability budget demanded by the WL science. Taking everything into account, our analysis confirms the excellent overall performance of the telescope.

Figures (15)

• Figure 1: Definition of Solar aspect angle (SAA) and $\alpha$ angle (AA) with respect to the spacecraft axes. SAA is the angle between the Sun direction vector and the Z axis of the spacecraft, while AA is the angle between the X axis and the projection of the Sun direction on the X-Y plane. The spacecraft is a schematic presentation to illustrate the configuration without the protective multi-layer insulation. Thermal disturbances occur whenever SAA $> 90\degree$ where the Sun illuminates the bottom of the spacecraft, or AA $> -2{\fdg}5$ where the Sun illuminates the micro-propulsion system (MPS) boom, encircled in the figure.
• Figure 2: VIS image plane field points. The Korsch telescope design provides an off-axis exit pupil. See \ref{['FigAtt']} for the definition of the spacecraft $X$, $Y$, and $Z$ coordinates.
• Figure 3: Baseplate average temperature versus STOP case number, see \ref{['subsub:steady-state-thermal']} for the sequencing description. Upper panel (a): full-length MPS boom, the topside can be illuminated by the Sun. Lower panel (b): half-length MPS boom, the topside cannot be illuminated by the Sun.
• Figure 4: Temperature transient analysis. Upper panel (a): baseplate temperature evolution over 96 hours. Lower panel (b): the temperature difference DT11000 derived from the temperature evolution in (a); the dashed curve shows a match of DT11000 with two exponential terms with $c_1=$ 168 mK and $c_2=$ 315 mK.
• Figure 5: M2 rotations (upper panel) and displacements (lower panel) around the spacecraft axes after a transition according to the parameters in Table \ref{['TabTtra']}.