Stray-light calibration and correction for the MetOp-SG 3MI mission

Abstract

The MetOp-SG 3MI mission is part of the EUMETSAT Polar System Second Generation (EPS-SG), an Earth observation Program for Operational Meteorology from Low Earth Orbit. It consists of two multi-spectral cameras, one operating in VNIR and one in SWIR. With 13 spectral channels between 410nm and 2130nm, including polarized channels, the instrument covers a semi-field of view of 57°. Due to tight stray-light specifications, on-ground calibration and post-processing correction are required. This paper covers the stray-light correction and calibration methods. The correction is indeed based on the on-ground measurement of Spatial Point Source Transmittance (SPST) maps. Due to the limited amount of maps which can actually be calibrated within a reasonable amount of time, an interpolation method was developed to deduce the stray-light behavior in the whole field of view of the instrument. Furthermore, dynamic range decomposition was required during the acquisition of the maps to get a high signal to noise ratio. Ray-tracing data from the 3MI optical model were used to evaluate the performance of the correction algorithm, including the contribution of SPST maps interpolation and acquisition errors.

Stray Light Correction Algorithm for High Performance Optical Instruments: The Case of Metop-3MI

“…This situation is encountered in the Metop-3MI Earth observation instrument [4][5][6][7]. The scientific objective of the mission is to study the chemical composition of the Earth's atmosphere, analyze clouds properties, and measure the Earth's albedo [4][5][6][7].…”

“…This situation is encountered in the Metop-3MI Earth observation instrument [4][5][6][7]. The scientific objective of the mission is to study the chemical composition of the Earth's atmosphere, analyze clouds properties, and measure the Earth's albedo [4][5][6][7]. With an instrument in an on-axis refractive configuration [4], its many lenses are necessary for correcting optical aberrations over a wide field of view (FOV) of ±57 • .…”

Stray Light Correction Algorithm For High Performance Optical Instruments: The Case Of Metop-3MI

“…This situation is encountered in the Metop-3MI Earth observation instrument [4][5][6][7]. With an on-axis refractive configuration, its many lenses are necessary for correcting optical aberrations over a wide field-of-view (FOV) of ±57°.…”

Post-processing correction of stray-light in space instruments: application to the 3MI earth observation instrument

“…Many methods exist to reduce the level of stray-light as far as it concerns hardware [1]: optimizing the optical setup, applying anti-reflections coatings on the lenses, using baffles to block unwanted paths and using black coatings to reduce the scattering on the non-optical surfaces. The capabilities of hardware as a mean to reducing straylight is however limited [2]. No black coating perfectly absorbs light, no anti reflection completely avoids ghost reflections and optical surfaces always have some roughness and are subject to contaminations.…”

“…This was for example the case of the METOP-SG 3MI instrument, consisting of two multispectral instruments operating in the VNIR and SWIR spectral ranges [3]. For that reason, a stray-light correction algorithm was developed to reduce the stray-light by about 2 orders of magnitude compared to the initial level [2]. More specifically, the performance requirement is that for an extended scene with half the FOV illuminated at Lmax and the other half at Lref=Lmax/10, the corrected signal should present a residual stray-light error below 0.17% of the Lref signal (equation 1).…”