Optical and system performance of SPEXone, a multi-angle channeled spectropolarimeter for the NASA PACE mission

Abstract

SPEXone is a multi-angle channeled spectropolarimeter that is developed by a Dutch consortium consisting of SRON and Airbus Defence and Space Netherlands with support from TNO. SPEXone will fly together with the Ocean Color Instrument (OCI) and the Hyper-Angular Rainbow Polarimeter-2 (HARP-2) on the NASA Plankton, Aerosol, Clouds and ocean Ecosystem (PACE) mission, which has a notional launch in 2023. SPEXone will deliver high quality hyperspectral multi-angle radiance and polarization products that, together with products from OCI and HARP2, enable unprecedented aerosol and cloud characterization from space.SPEXone employs dual beam spectral polarization modulation, in which the state of linear polarization is encoded in a spectrum as a periodic variation of the intensity. This technique enables high polarimetric accuracies in operational environments, since it provides snapshot acquisition of both radiance and polarization without moving parts. SPEXone has five viewing angles that are realized using a novel three-mirror segmented telescope assembly. The telescope focuses light captured by the five viewing angles onto a single image plane consisting of five stacked sub-slits. This multi-slit forms the entrance slit of a reflective grating spectrometer that consists of freeform mirrors and an order-sorting filter close to the focal plane, yielding an intrinsic spectral resolution of 2 nm and 5.4 km spatial resolution across the 100 km swath. The spectrometer re-images two spectral images per viewing angle following a dual beam spectral polarization modulation implementation.In this contribution, the optical performance of the telescope and spectrometer will be presented by means of star stimulus measurements at the slit plane and at the spectrometer focal plane. Measurements of the optical spot quality and preliminary measurements of stray light are compared with the optical design and with stray light simulations. We find that the measured optical performance of the telescope and spectrometer is better than modelled, showing higher resolution and lower slit keystone, thereby meeting all spatial and spectral resolution requirements. Also, preliminary stray light results indicate a higher diffuse but lower ghost contribution to the total stray light, which is in general beneficial for implementing stray light correction, which will enhance the polarimetric accuracy in inhomogeneous scenes.

Optoelectronics parts NewSpace qualification aboard FOLC2 optical modem mission

“…The principle of SPEX has been proven by several instruments: iSPEX, SPEX-airborne, and recently the SPEXone instrument for the NASA PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) mission, to be launched early 2024, has been delivered. More on the SPEXone instrument for PACE can be found in [6][7], its calibration in [8], and its data retrieval in [5] and [9].…”

Detector characterization and upcoming instrument calibration of an improved version of the five-angle spectro-polarimeter SPEXone

“…15 The linear-polarization state (degree and angle) of the scattered sunlight is encoded directly in the continuous wavelength spectrum by making use of polarization-modulation optics (PMO; patent P121636NL00). [10][11][12] This approach is applied for five distinct viewing angles (nadir, 20, and 50 degree angles forward and backward), by means of five three-mirror telescopes (three monolithic parts with five mirror segments each; patent US20200319026A1), projecting five push-broom swaths on a single spectrometer entrance slit, separated by masked-out areas (five sub slits in a slit plate). A polarizing beam splitter as part of the PMO splits each modulated signal in two perpendicularly polarized channels, transforming the polarization modulation into amplitude modulation.…”

Sub-percent characterization and polarimetric performance analysis of commercial micro-polarizer array detectors

“…• Atmospheric aerosol characterization through multi-angle and multi-wavelength polarimetric observations of scattered sunlight. Currently, only the SPEXone multi-angle spectropolarimeter [64][65][66] instrument onboard the PACE satellite achieves the ∼10 −3 polarimetric accuracy necessary to retrieve the full aerosol microphysical parameters, including complex refractive index. 67,68 The PACE/HARP2 instrument 69 implements full hyperangular imaging over a large field-of-view, at the cost of polarimetric accuracy.…”