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Proceedings Paper • Open Access

Evaluation of the refocusing system of the polarimetric helioseismic imager/full disk telescope of the solar orbiter mission
Author(s): M. Silva-Lopez; J. A. Bonet-Navarro; A. Núñez; A. Álvarez-Herrero

Paper Abstract

The Full Disk Telescope is part of the Polarimetric Helioseismic Instrument on board the future Solar Orbiter ESA/NASA mission. The Full Disk Telescope will provide for full-disk measurements of the photospheric vector magnetic field and line-of-sight velocity, as well as the continuum intensity in the visible wavelength range. Along this mission, it is expected that thermal drifts will induce image focus displacements. Consequently, providing an autofocus system is mandatory to prevent image degradation. The refocusing system is based on an autonomous image quality analysis and it allows for a lens displacement in order to locate the best focus position. In this document the system and the figure of merit chosen for the image quality evaluation is presented. The algorithm is a gradient based contrast and it is adapted to the object features. Moreover, the telescope is not equipped with image stabilization system and therefore attitude stability must be assessed. Jitter is modelled as a circular normal distribution of the pointing direction. A stability of 0.5 arcsec (σρ) during frame accumulation is simulated and its impact on image contrast evaluated. Considering this jitter, image contrast is only reduced to 99.8% the contrast of an unaffected image. Finally, to estimate the effect in the process performance, the mechanical uncertainties expected for the linear actuator were added to the defocus due to the lens excursion. The results show that the proposed refocusing system is sufficiently robust against the expected image shifts and mechanical instabilities.

Paper Details

Date Published: 17 November 2017
PDF: 8 pages
Proc. SPIE 10563, International Conference on Space Optics — ICSO 2014, 1056360 (17 November 2017); doi: 10.1117/12.2304142
Show Author Affiliations
M. Silva-Lopez, INTA Instituto Nacional de Técnica Aeroespacial (Spain)
J. A. Bonet-Navarro, Instituto de Astrofísica de Canarias (Spain)
A. Núñez, INTA Instituto Nacional de Técnica Aeroespacial (Spain)
A. Álvarez-Herrero, INTA Instituto Nacional de Técnica Aeroespacial (Spain)

Published in SPIE Proceedings Vol. 10563:
International Conference on Space Optics — ICSO 2014
Zoran Sodnik; Bruno Cugny; Nikos Karafolas, Editor(s)

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