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Proceedings Paper • Open Access

Long-wave infrared digital holography for the qualification of large space reflectors
Author(s): J.F. Vandenrijt; C. Thizy; M. P. Georges; P. Queeckers; F. Dubois; D. Doyle

Paper Abstract

Deformation metrology of complex and large space reflectors is a recurrent problem addressed by ESA. The challenging tasks of on-ground qualification and verification testing are to achieve the required accuracy in the measurement of these reflectors deformation and to verify their performance under simulated space conditions (vacuum, low temperature).

A long-wave infrared digital holographic interferometer for the verification and validation of this type of reflector in a space environment is presented. It has been developed to fill the gap between holography/interferometry techniques in the visible wavelengths and methods based on structured light illumination like videogrammetry, stereocorrelation, and fringe/pattern projection. The former provide a good measurement uncertainty but the displacements are often too large to be measured and they require a very stable environment, while the latter provide large measurement range but with higher measurement uncertainty.

The new instrument is based on digital holography and uses a CO2 lasers emitting at 10.6μm combined with a commercial thermographic camera. A diffuser is illuminated by the laser beam, producing a speckle wavefront which is observed after reflection on the reflector surface. This reflected speckle wavefront behaves exactly as if the reflector was a diffusive surface, producing its own speckle, allowing the measurement of its deformation. The advantage of this configuration compared to a classical interferometer working at 10.6μm, is that it requires no specific optics such as a null lens (in the case of parabola) or expensive illumination/collection optics (in the case of ellipse).

The metrological certification of the system was performed in the laboratory by measuring the tilts of a 1.1 meter diameter parabolic reflector. The displacements are measured in parallel with a Doppler effect interferometer and the measurement uncertainty is estimated. The technique has been certified during a thermal-vacuum test. The deformation of the parabolic reflector is measured for a temperature variation from 288 K down to 113 K. The results are compared to previous results obtained on the same reflector with a high spatial resolution infrared interferometer, also developed at CSL.

Paper Details

Date Published: 20 November 2017
PDF: 5 pages
Proc. SPIE 10564, International Conference on Space Optics — ICSO 2012, 1056403 (20 November 2017); doi: 10.1117/12.2309074
Show Author Affiliations
J.F. Vandenrijt, Univ. of Liège (Belgium)
C. Thizy, Univ. of Liège (Belgium)
M. P. Georges, Univ. of Liège (Belgium)
P. Queeckers, Univ. Libre de Bruxelles (Belgium)
F. Dubois, Univ. Libre de Bruxelles (Belgium)
D. Doyle, European Space Research and Technology Ctr. (Netherlands)

Published in SPIE Proceedings Vol. 10564:
International Conference on Space Optics — ICSO 2012
Bruno Cugny; Errico Armandillo; Nikos Karafolas, Editor(s)

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