Final tolerancing approach and the value of short-cutting tolerances by measurement

Grupp, F.; Prieto, E.; Geis, N.; Bode, A.; Bodendorf, C.; Costille, A.; Katterloher, R.; Penka, D.; Bender, R. 

Proc. SPIE 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave, 99042M (29 July 2016); doi: 10.1117/12.2231362

Abstract: Within the ESAs 2015 - 2025 Cosmic Vision framework the 1.2 m aperture EUCLID space telescope addresses cosmological questions related to dark matter and dark energy. Being equipped with two instruments that are simultaneously observing patches of > 0.5 square degree on the sky EUCLID is aiming at major cosmological probes in a large seven years survey scanning the entire extragalactic sky. These two instruments, the visual light high spacial resolution imager (VIS) and the near infrared spectrometer and photometer (NISP) are separated by a dichroic beam splitter. Its huge field of view (FoV) - larger than the full moon disk - together with high demands on the optical performance and strong requirements on in flight stability lead to very challenging demands on alignment and post launch – post cool-down optical element position. The role of an accurate and trust-worthy tolerance analysis which is well adopted to the stepwise integration and alignment concept, as well as to the missions stability properties is therefore crucial for the missions success. While the previous contributions of this series of papers (e.g.[1])was addressing the technical aspects of tolerancing, the mechanical challenges and the answers of the NISP instrument to these challenges, this paper will focus on our concept of shortcutting the tolerance chain by measurement wherever useful and possible. The NISP instrument is only possible, due to the innovative use of technologies such as computer generated hologram (CGH) based manufacturing and alignment. Expanding this concept, certain steps in the assembly process, such as focal length determination before detector placement allow to reduce the overall tolerance induced imaging errors. With this papers we show three major examples of this shortcutting strategy.

Performance measurement of high precision optical assemblies for cosmological observations: Comparison of different approaches

Bodendorf, C.; Bode, A.; Geis, N.; Grupp, F.; Bender, R.

Proc. SPIE 10009, Third European Seminar on Precision Optics Manufacturing, 100090F (June 30, 2016); doi:10.1117/12.2235210

Abstract: Euclid is the name of a space telescope currently being developed in the framework of the ESA Cosmic Vision 2015-2025 Program. It addresses cosmological questions related to dark matter and dark energy. The lens system of one of the two scientific key instruments on board of Euclid (a combined near-infrared spectrometer and photometer) is designed and tested at the Max Planck Institute for Extraterrestrial Physics.
In this paper, we compare two complementary approaches to determine the imaging quality of the photometer. The first approach is based on a direct camera measurement of the point-spread function (PSF) while the second approach uses a Shack-Hartmann sensor to reconstruct the wave front of the system.
Both methods yield in principle largely overlapping information in terms of e.g. modulation transfer function (MTF), encircled energy (EE) or spot shape. However, the experimental requirements are quite different. Details like the spatial extent and coherence properties of the object have different impact and are essential for the degree of agreement between both approaches. Moreover, the disturbance due to measurement noise is quite unequal and requires different evaluation steps.
The pros and cons of both methods that actually complement one another are investigated and discussed.

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