Characterization and performances of the primary mirror of the PILOT balloon-borne experiment

• Published in: Experimental astronomy 2013-08, Vol.36 (1-2), p.21-57
• Main Authors: Engel, C., Ristorcelli, I., Bernard, J.-Ph, Longval, Y., Marty, C., Mot, B., Otrio, G., Roudil, G.
• Format: Article
• Language: English
• Subjects: Astronomy; Chemistry and Earth Sciences; Computer Science; Observations and Techniques; Physics; Physics and Astronomy; Review Article; Statistics for Engineering
• ISSN: 0922-6435; 1572-9508
• DOI: 10.1007/s10686-013-9332-7

PILOT is a balloon-borne experiment designed to perform large-scale surveys of the polarized interstellar emission in the submillimeter. It is based on the use of an off-axis Gregorian type telescope, with a 1 m diameter primary mirror, and a large focal plane equipped with detectors arrays providing a field of view. All optical elements except the primary mirror are located inside a large liquid He cryostat, cooled down to 3 K. Strong constraints are then imposed on the alignment between the primary mirror and the cold optics. The characterization and optimization of the optical system performances are critical to the success of the mission. In this paper, we present the modelling and measurements performed on the primary mirror for this purpose. The optical and mechanical parameters of the as-built primary mirror have been determined using a method based on 3D measurements of the mirror surface. The deformations expected under flight conditions due to temperature variations and flexion under gravity have been estimated. We have also performed measurements using a submillimeter test bench in order to control the image quality and derive the main optical parameters. The parameters derived from the modeling using 3D measurements are in agreement with the requirements except for the conic constant. The best positioning of the mirror has been optimized consequently. The modeling has also allowed us to determine the pre-flight alignment parameters of the mirror as a function of the expected structure temperature at ceiling altitude. We have shown that this adjustment will enable to keep the tight requirements on the focus position ( 600 μm) within a range of C around the ceiling temperature value. The submillimeter measurements have validated the results derived from the 3D measurement based modeling. The image quality was investigated by performing a spatial exploration in azimuth and elevation around the nominal focus position, and along the optical axis. The deviation between the predicted and measured positions of the best focus are 80 μm and 11″ in translation along the optical axis and in rotation respectively. The best image pattern is also close to the nominal one, with a sphericity deviation lower than 2 μm RMS. These results will be used for the end-to-end tests of the integrated instrument, and for the optimization of the alignment before flight.