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1. Polarization properties of real aluminum mirrors; I. Influence of the aluminum oxide layer

Author

van Harten, G., Snik, F., and Keller, C. U.

Subjects
Astrophysics - Instrumentation and Methods for Astrophysics
Abstract

In polarimetry it is important to characterize the polarization properties of the instrument itself to disentangle real astrophysical signals from instrumental effects. This article deals with the accurate measurement and modeling of the polarization properties of real aluminum mirrors, as used in astronomical telescopes. Main goals are the characterization of the aluminum oxide layer thickness at different times after evaporation and its influence on the polarization properties of the mirror. The full polarization properties of an aluminum mirror are measured with Mueller matrix ellipsometry at different incidence angles and wavelengths. The best fit of theoretical Mueller matrices to all measurements simultaneously is obtained by taking into account a model of bulk aluminum with a thin aluminum oxide film on top of it. Full Mueller matrix measurements of a mirror are obtained with an absolute accuracy of ~1% after calibration. The determined layer thicknesses indicate logarithmic growth in the first few hours after evaporation, but it remains stable at a value of 4.12+/-0.08 nm on the long term. Although the aluminum oxide layer is established to be thin, it is necessary to consider it to accurately describe the mirror's polarization properties., Comment: accepted for publication in PASP

Published
2009
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4. The European Solar Telescope

Author

Quintero Noda, C., primary, Schlichenmaier, R., additional, Bellot Rubio, L. R., additional, Löfdahl, M. G., additional, Khomenko, E., additional, Jurčák, J., additional, Leenaarts, J., additional, Kuckein, C., additional, González Manrique, S. J., additional, Gunár, S., additional, Nelson, C. J., additional, de la Cruz Rodríguez, J., additional, Tziotziou, K., additional, Tsiropoula, G., additional, Aulanier, G., additional, Aboudarham, J., additional, Allegri, D., additional, Alsina Ballester, E., additional, Amans, J. P., additional, Asensio Ramos, A., additional, Bailén, F. J., additional, Balaguer, M., additional, Baldini, V., additional, Balthasar, H., additional, Barata, T., additional, Barczynski, K., additional, Barreto Cabrera, M., additional, Baur, A., additional, Béchet, C., additional, Beck, C., additional, Belío-Asín, M., additional, Bello-González, N., additional, Belluzzi, L., additional, Bentley, R. D., additional, Berdyugina, S. V., additional, Berghmans, D., additional, Berlicki, A., additional, Berrilli, F., additional, Berkefeld, T., additional, Bettonvil, F., additional, Bianda, M., additional, Bienes Pérez, J., additional, Bonaque-González, S., additional, Brajša, R., additional, Bommier, V., additional, Bourdin, P.-A., additional, Burgos Martín, J., additional, Calchetti, D., additional, Calcines, A., additional, Calvo Tovar, J., additional, Campbell, R. J., additional, Carballo-Martín, Y., additional, Carbone, V., additional, Carlin, E. S., additional, Carlsson, M., additional, Castro López, J., additional, Cavaller, L., additional, Cavallini, F., additional, Cauzzi, G., additional, Cecconi, M., additional, Chulani, H. M., additional, Cirami, R., additional, Consolini, G., additional, Coretti, I., additional, Cosentino, R., additional, Cózar-Castellano, J., additional, Dalmasse, K., additional, Danilovic, S., additional, De Juan Ovelar, M., additional, Del Moro, D., additional, del Pino Alemán, T., additional, del Toro Iniesta, J. C., additional, Denker, C., additional, Dhara, S. K., additional, Di Marcantonio, P., additional, Díaz Baso, C. J., additional, Diercke, A., additional, Dineva, E., additional, Díaz-García, J. J., additional, Doerr, H.-P., additional, Doyle, G., additional, Erdelyi, R., additional, Ermolli, I., additional, Escobar Rodríguez, A., additional, Esteban Pozuelo, S., additional, Faurobert, M., additional, Felipe, T., additional, Feller, A., additional, Feijoo Amoedo, N., additional, Femenía Castellá, B., additional, Fernandes, J., additional, Ferro Rodríguez, I., additional, Figueroa, I., additional, Fletcher, L., additional, Franco Ordovas, A., additional, Gafeira, R., additional, Gardenghi, R., additional, Gelly, B., additional, Giorgi, F., additional, Gisler, D., additional, Giovannelli, L., additional, González, F., additional, González, J. B., additional, González-Cava, J. M., additional, González García, M., additional, Gömöry, P., additional, Gracia, F., additional, Grauf, B., additional, Greco, V., additional, Grivel, C., additional, Guerreiro, N., additional, Guglielmino, S. L., additional, Hammerschlag, R., additional, Hanslmeier, A., additional, Hansteen, V., additional, Heinzel, P., additional, Hernández-Delgado, A., additional, Hernández Suárez, E., additional, Hidalgo, S. L., additional, Hill, F., additional, Hizberger, J., additional, Hofmeister, S., additional, Jägers, A., additional, Janett, G., additional, Jarolim, R., additional, Jess, D., additional, Jiménez Mejías, D., additional, Jolissaint, L., additional, Kamlah, R., additional, Kapitán, J., additional, Kašparová, J., additional, Keller, C. U., additional, Kentischer, T., additional, Kiselman, D., additional, Kleint, L., additional, Klvana, M., additional, Kontogiannis, I., additional, Krishnappa, N., additional, Kučera, A., additional, Labrosse, N., additional, Lagg, A., additional, Landi Degl’Innocenti, E., additional, Langlois, M., additional, Lafon, M., additional, Laforgue, D., additional, Le Men, C., additional, Lepori, B., additional, Lepreti, F., additional, Lindberg, B., additional, Lilje, P. B., additional, López Ariste, A., additional, López Fernández, V. A., additional, López Jiménez, A. C., additional, López López, R., additional, Manso Sainz, R., additional, Marassi, A., additional, Marco de la Rosa, J., additional, Marino, J., additional, Marrero, J., additional, Martín, A., additional, Martín Gálvez, A., additional, Martín Hernando, Y., additional, Masciadri, E., additional, Martínez González, M., additional, Matta-Gómez, A., additional, Mato, A., additional, Mathioudakis, M., additional, Matthews, S., additional, Mein, P., additional, Merlos García, F., additional, Moity, J., additional, Montilla, I., additional, Molinaro, M., additional, Molodij, G., additional, Montoya, L. M., additional, Munari, M., additional, Murabito, M., additional, Núñez Cagigal, M., additional, Oliviero, M., additional, Orozco Suárez, D., additional, Ortiz, A., additional, Padilla-Hernández, C., additional, Paéz Mañá, E., additional, Paletou, F., additional, Pancorbo, J., additional, Pastor Cañedo, A., additional, Pastor Yabar, A., additional, Peat, A. W., additional, Pedichini, F., additional, Peixinho, N., additional, Peñate, J., additional, Pérez de Taoro, A., additional, Peter, H., additional, Petrovay, K., additional, Piazzesi, R., additional, Pietropaolo, E., additional, Pleier, O., additional, Poedts, S., additional, Pötzi, W., additional, Podladchikova, T., additional, Prieto, G., additional, Quintero Nehrkorn, J., additional, Ramelli, R., additional, Ramos Sapena, Y., additional, Rasilla, J. L., additional, Reardon, K., additional, Rebolo, R., additional, Regalado Olivares, S., additional, Reyes García-Talavera, M., additional, Riethmüller, T. L., additional, Rimmele, T., additional, Rodríguez Delgado, H., additional, Rodríguez González, N., additional, Rodríguez-Losada, J. A., additional, Rodríguez Ramos, L. F., additional, Romano, P., additional, Roth, M., additional, Rouppe van der Voort, L., additional, Rudawy, P., additional, Ruiz de Galarreta, C., additional, Rybák, J., additional, Salvade, A., additional, Sánchez-Capuchino, J., additional, Sánchez Rodríguez, M. L., additional, Sangiorgi, M., additional, Sayède, F., additional, Scharmer, G., additional, Scheiffelen, T., additional, Schmidt, W., additional, Schmieder, B., additional, Scirè, C., additional, Scuderi, S., additional, Siegel, B., additional, Sigwarth, M., additional, Simões, P. J. A., additional, Snik, F., additional, Sliepen, G., additional, Sobotka, M., additional, Socas-Navarro, H., additional, Sola La Serna, P., additional, Solanki, S. K., additional, Soler Trujillo, M., additional, Soltau, D., additional, Sordini, A., additional, Sosa Méndez, A., additional, Stangalini, M., additional, Steiner, O., additional, Stenflo, J. O., additional, Štěpán, J., additional, Strassmeier, K. G., additional, Sudar, D., additional, Suematsu, Y., additional, Sütterlin, P., additional, Tallon, M., additional, Temmer, M., additional, Tenegi, F., additional, Tritschler, A., additional, Trujillo Bueno, J., additional, Turchi, A., additional, Utz, D., additional, van Harten, G., additional, van Noort, M., additional, van Werkhoven, T., additional, Vansintjan, R., additional, Vaz Cedillo, J. J., additional, Vega Reyes, N., additional, Verma, M., additional, Veronig, A. M., additional, Viavattene, G., additional, Vitas, N., additional, Vögler, A., additional, von der Lühe, O., additional, Volkmer, R., additional, Waldmann, T. A., additional, Walton, D., additional, Wisniewska, A., additional, Zeman, J., additional, Zeuner, F., additional, Zhang, L. Q., additional, Zuccarello, F., additional, and Collados, M., additional

Published
2022
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7. Observations of Aerosol-Radiation-Cloud Interactions in the South-East Atlantic: First Results from the ORACLES Deployments in 2016 and 2017

Author

Redemann, Jens, Wood, R, Zuidema, P, Diner, D, Van Harten, G, Xu, F, Cairns, B, Knobelspiesse, K, and Segal Rozenhaimer, M

Subjects
Earth Resources And Remote Sensing
Abstract

Southern Africa produces almost a third of the Earths biomass burning (BB) aerosol particles. Particles lofted into the mid-troposphere are transported westward over the South-East (SE) Atlantic, home to one of the three permanent subtropical stratocumulus (Sc) cloud decks in the world. The SE Atlantic stratocumulus deck interacts with the dense layers of BB aerosols that initially overlay the cloud deck, but later subside and often mix into the clouds. These interactions include adjustments to aerosol-induced solar heating and microphysical effects, and their global representation in climate models remains one of the largest uncertainties in estimates of future climate. Hence, new observations over the SE Atlantic have significant implications for regional and global climate change predictions.The low-level clouds in the SE Atlantic have limited vertical extent and therefore present favorable conditions for their exploration with remote sensing. On the other hand, the normal coexistence of BB aerosols and Sc clouds in the same scene also presents significant challenges to conventional remote sensing techniques. We describe first results from NASAs airborne ORACLES (ObseRvations of Aerosols Above Clouds and Their IntEractionS) deployments in September 2016 and August 2017. We emphasize the unique role of polarimetric observations by two instruments, the Research Scanning Polarimeter (RSP) and the Airborne Multi-angle SpectroPolarimeter Imager (AirMSPI), and describe how these instruments help address specific ORACLES science objectives. Initial assessments of polarimetric observation accuracy for key cloud and aerosol properties will be presented, in as far as the preliminary nature of measurements permits.

Published
2017

8. SPEX: the Spectropolarimeter for Planetary Exploration

Author

Rietjens, J. H. H., primary, Snik, F., primary, Stam, D. M., primary, Smit, J. M., primary, van Harten, G., primary, Verlaan, A. L., primary, Laan, E. C., primary, Navarro, R., primary, Wielinga, K., primary, Voors, R., primary, Keller, C. U., primary, ter Horst, R., primary, and Moon, S. G., primary

Published
2017
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10. Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters

Author

Snik, F., Rietjens, J.H.H., Apituley, A., Volten, H., Mijling, B., Di Noia, A., Heikamp, S., Heinsbroek, R.C., Hasekamp, O.P., Smit, J.M., Vonk, J., Stam, D.M., Van Harten, G., De Boer, J., Keller, C.U., and Earth and Climate

Subjects
citizen science, aerosols, smartphones, SDG 11 - Sustainable Cities and Communities, polarimetry
Abstract

To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low-cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud-free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground-based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts. Key Points The iSPEX add-on turns smartphones into aerosol measurement devicesThousands of iSPEX measurements across the Netherlands form AOT mapsThe iSPEX AOT data match MODIS and AERONET data and have 2 km resolution

Published
2014
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12. A multi-domain full-Stokes polarization modulator that is efficient for 300-2500nm spectropolarimetry

Author

Snik, F, Van Harten, G, Alenin, AS, Vaughn, IJ, Tyo, JS, Snik, F, Van Harten, G, Alenin, AS, Vaughn, IJ, and Tyo, JS

Abstract

We present the design and prototyping results for an ultra-wideband rotating polarization modulator that consists of a stack of quartz plates. The plate thicknesses and orientations were optimized such that after rotation of the modulator to 6 different angles before a polarization analyzer, the full Stokes vector can be optimally determined at all wavelengths from 300 to 2500 nm. Additional optimization parameters include minimal variation of the retardance with incidence angle and temperature, and the suppression of polarized spectral fringes for a spectral resolution of 10,000. The prototype modulator's design was re-optimized after the production and measurement of each individual quartz plate. We present the performance of the as-built prototype. To eliminate aliasing with inherent temporal variations of the source, the modulator can be used together with a polarizing beam-splitter (ual-beam approach). Because of the large sinusoidal spectral variations of the polarization modulation, this modulator can also be considered a spectral modulator for channeled spectropolarimetry. Therefore, at each modulation state, spectrally resolved polarization information can also be extracted directly, although at limited spectral resolution. We use this modulator as an example of a multi-domain polarization modulator, and outline a general approach for optimally storing polarization information in all available measurement dimensions (temporal, spatial, spectral), and rendering the overall polarization measurement independent from systematic effects in any of these dimensions.

Published
2015

14. Design of a full-Stokes polarimeter for VLT/X-shooter

Author

Snik, F., van Harten, G., Navarro, R., Groot, P., Kaper, L., de Wijn, A., McLean, I.S., Ramsay, S.K., Takami, H., and Low Energy Astrophysics (API, FNWI)

Subjects
Savart, SPIE Proceedings, Astronomy, Polarimetry, FOS: Physical sciences, Astrophysics::Cosmology and Extragalactic Astrophysics, symbols.namesake, Optics, Stokes parameters, Astrophysics::Solar and Stellar Astrophysics, Spectrograph, Instrumentation and Methods for Astrophysics (astro-ph.IM), Astrophysics::Galaxy Astrophysics, Solar and Stellar Astrophysics (astro-ph.SR), Physics, business.industry, Astrophysics::Instrumentation and Methods for Astrophysics, Cassegrain reflector, Polarimeter, Polarization (waves), Stars, Astrophysics - Solar and Stellar Astrophysics, symbols, ComputingMethodologies_DOCUMENTANDTEXTPROCESSING, Astrophysics - Instrumentation and Methods for Astrophysics, business
Abstract

X-shooter is one of the most popular instruments at the VLT, offering instantaneous spectroscopy from 300 to 2500 nm. We present the design of a single polarimetric unit at the polarization-free Cassegrain focus that serves all three spectrograph arms of X-shooter. It consists of a calcite Savart plate as a polarizing beam-splitter and a rotatable crystal retarder stack as a "polychromatic modulator". Since even "superachromatic" wave plates have a wavelength range that is too limited for X-shooter, this novel modulator is designed to offer close-to-optimal polarimetric efficiencies for all Stokes parameters at all wavelengths. We analyze the modulator design in terms of its polarimetric performance, its temperature sensitivity, and its polarized fringes. Furthermore, we present the optical design of the polarimetric unit. The X-shooter polarimeter will furnish a myriad of science cases: from measuring stellar magnetic fields (e.g., Ap stars, white dwarfs, massive stars) to determining asymmetric structures around young stars and in supernova explosions., Comment: Proc. SPIE 8446-76

Published
2012

16. Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters

Author

Snik, F. (author), Rietjens, J.H.H. (author), Apituley, A. (author), Volten, H. (author), Mijling, B. (author), Di Noia, A. (author), Heikamp, S. (author), Heinsbroek, R.C. (author), Hasekamp, O.P. (author), Smit, J.M. (author), Vonk, J. (author), Stam, D.M. (author), Van Harten, G. (author), De Boer, J. (author), Keller, C.U. (author), Snik, F. (author), Rietjens, J.H.H. (author), Apituley, A. (author), Volten, H. (author), Mijling, B. (author), Di Noia, A. (author), Heikamp, S. (author), Heinsbroek, R.C. (author), Hasekamp, O.P. (author), Smit, J.M. (author), Vonk, J. (author), Stam, D.M. (author), Van Harten, G. (author), De Boer, J. (author), and Keller, C.U. (author)

Abstract

To assess the impact of atmospheric aerosols on health, climate, and air traffic, aerosol properties must be measured with fine spatial and temporal sampling. This can be achieved by actively involving citizens and the technology they own to form an atmospheric measurement network. We establish this new measurement strategy by developing and deploying iSPEX, a low-cost, mass-producible optical add-on for smartphones with a corresponding app. The aerosol optical thickness (AOT) maps derived from iSPEX spectropolarimetric measurements of the daytime cloud-free sky by thousands of citizen scientists throughout the Netherlands are in good agreement with the spatial AOT structure derived from satellite imagery and temporal AOT variations derived from ground-based precision photometry. These maps show structures at scales of kilometers that are typical for urban air pollution, indicating the potential of iSPEX to provide information about aerosol properties at locations and at times that are not covered by current monitoring efforts., Space Engineering, Aerospace Engineering

Published
2014
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28. Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations.

Author

Di Noia, A., Hasekamp, O. P., van Harten, G., Rietjens, J. H. H., Smit, J. M., Snik, F., Henzing, J. S., de Boer, J., Keller, C. U., and Volten, H.

Subjects
AEROSOLS, ARTIFICIAL neural networks, SPECTROPOLARIMETERS, REFRACTIVE index, ITERATIVE methods (Mathematics)
Abstract

In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from ground-based spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as a first guess of the iterative retrieval scheme, the accuracy of the fine and coarse mode optical thickness are further improved while for the other parameters the improvement is small or absent. The resulting scheme (neural network + iterative retrieval) is compared to the original one (look-up table + iterative retrieval) on a set of simulated ground-based measurements, and on a small set of real observations carried out by an accurate ground-based spectropolarimeter. The results show that the use of a neural network based first guess leads to an increase in the number of converging retrievals, and possibly to more accurate estimates of the aerosol effective radius and complex refractive index. [ABSTRACT FROM AUTHOR]

Published
2014
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29. Mirror contamination in space I: approach.

Author

Krijger, J. M., Snel, R., van Harten, G., Rietjens, J. H. H., and Aben, I.

Subjects
MIRRORS, DIFFUSERS (Fluid dynamics), ARTIFICIAL satellites, WAVELENGTHS, EQUATIONS
Abstract

We present a comprehensive model that can be employed to describe and correct for degradation of (scan) mirrors and diffusers in satellite instruments that suffer from changing optical UV-VIS properties during their operational lifetime. As trend studies become more important, so does the importance to understand and correct for this degradation. This is the case not only with respect to the transmission of the optical components, but also with respect to wavelength, polarisation or scan-angle-effects. Our hypothesis is that mirrors in-flight suffer from the deposition of a thin absorbing layer of contaminant, which slowly builds up over time. We describe this with the Mueller matrix formalism and Fresnel equations for thin multi-layer contamination films. Special care is taken to avoid the confusion often present in earlier publications concerning Mueller matrix calculus with out of plane reflections. The method can be applied to any UV-VIS satellite instrument, and in any orbit, both low and geostationary. We illustrate and verify our approach on the optical behaviour of the multiple scan mirrors of SCIAMACHY (on board of ENVISAT). [ABSTRACT FROM AUTHOR]

Published
2014
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33. SPEX airborne spectropolarimeter calibration and performance.

Author

Smit JM, Rietjens JHH, van Harten G, Di Noia A, Laauwen W, Rheingans BE, Diner DJ, Cairns B, Wasilewski A, Knobelspiesse KD, Ferrare R, and Hasekamp OP

Abstract

To improve our understanding of the complex role of aerosols in the climate system and on air quality, measurements are needed of optical and microphysical aerosol. From many studies, it has become evident that a satellite-based multiangle, multiwavelength polarimeter will be essential to provide such measurements. Here, high accuracy (∼0.003) on the degree of linear polarization (DoLP) measurements is important to retrieve aerosol properties with an accuracy needed to advance our understanding of the aerosol effect on climate. SPEX airborne, a multiangle hyperspectral polarimeter, has been developed for observing and characterizing aerosols from NASA's high-altitude research aircraft ER-2. It delivers measurements of radiance and DoLP at visual wavelengths with a spectral resolution of 3 and 7-30 nm, respectively, for radiance and polarization, at nine fixed equidistant viewing angles from -56° to +56° oriented along the ground track, and a swath of 7° oriented across-track. SPEX airborne uses spectral polarization modulation to determine the state of linear polarization of scattered sunlight. This technique has been developed in the Netherlands and has been demonstrated with ground-based instruments. SPEX airborne serves as a demonstrator for a family of space-based SPEX instruments that have the ability to measure and characterize atmospheric aerosol by multiangle hyperspectral polarimetric imaging remotely from a satellite platform. SPEX airborne was calibrated radiometrically and polarimetrically using Jet Propulsion Laboratory (JPL) facilities including the Polarization Stage Generator-2 (PSG-2), which is designed for polarimetric calibration and validation of the Airborne Multiangle SpectroPolarimetric Imager (AirMSPI). Using the PSG-2, the accuracy of the SPEX airborne DoLP measurements in the laboratory setup is found to be 0.002-0.004. Radiometric calibration is realized with an estimated accuracy of 4%. In 2017, SPEX airborne took part in the "Aerosol Characterization from Polarimeters and Lidar" campaign on the ER-2 that included four polarimeters and two lidars. Polarization measurements of SPEX airborne and the coflying Research Scanning Polarimeter (RSP), recorded during the campaign, were compared and display root-mean-square (RMS) differences ranging from 0.004 (at 555 nm) up to 0.02 (at 410 nm). For radiance measurements, excellent agreement between SPEX airborne and RSP is obtained with an RMS difference of ∼4%. The lab- and flight-performance values for polarization are similar to those recently published for AirMSPI, where also an intercomparison with RSP was made using data from field campaigns in 2013. The intercomparison of radiometric and polarimetric data both display negligible bias. The in-flight comparison results provide verification of SPEX airborne's capability to deliver high-quality data.

Published
2019
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34. Intercomparison of airborne multi-angle polarimeter observations from the Polarimeter Definition Experiment.

Author

Knobelspiesse K, Tan Q, Bruegge C, Cairns B, Chowdhary J, van Diedenhoven B, Diner D, Ferrare R, van Harten G, Jovanovic V, Ottaviani M, Redemann J, Seidel F, and Sinclair K

Abstract

In early 2013, three airborne polarimeters were flown on the high altitude NASA ER-2 aircraft in California for the Polarimeter Definition Experiment (PODEX). PODEX supported the pre-formulation NASA Aerosol-Cloud-Ecosystem (ACE) mission, which calls for an imaging polarimeter in polar orbit (among other instruments) for the remote sensing of aerosols, oceans, and clouds. Several polarimeter concepts exist as airborne prototypes, some of which were deployed during PODEX as a capabilities test. Two of those instruments to date have successfully produced Level 1 (georegistered, calibrated radiance and polarization) data from that campaign: the Airborne Multiangle Spectropolarimetric Imager (AirMSPI) and the Research Scanning Polarimeter (RSP). We compared georegistered observations of a variety of scene types by these instruments to test whether Level 1 products agreed within stated uncertainties. Initial comparisons found radiometric agreement, but polarimetric biases beyond measurement uncertainties. After subsequent updates to calibration, georegistration, and the measurement uncertainty models, observations from the instruments now largely agree within stated uncertainties. However, the 470 nm reflectance channels have a roughly +6% bias of AirMSPI relative to RSP, beyond expected measurement uncertainties. We also find that observations of dark (ocean) scenes, where polarimetric uncertainty is expected to be largest, do not agree within stated polarimetric uncertainties. Otherwise, AirMSPI and RSP observations are consistent within measurement uncertainty expectations, providing credibility for the subsequent creation of Level 2 (geophysical product) data from these instruments, and comparison thereof. The techniques used in this work can also form a methodological basis for other intercomparisons, for example, of the data gathered during the recent Aerosol Characterization from Polarimeter and Lidar (ACEPOL) field campaign, carried out in October and November of 2017 with four polarimeters (including AirMSPI and RSP).

Published
2019
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35. Spectral line polarimetry with a channeled polarimeter.

Author

van Harten G, Snik F, Rietjens JH, Martijn Smit J, and Keller CU

Abstract

Channeled spectropolarimetry or spectral polarization modulation is an accurate technique for measuring the continuum polarization in one shot with no moving parts. We show how a dual-beam implementation also enables spectral line polarimetry at the intrinsic resolution, as in a classic beam-splitting polarimeter. Recording redundant polarization information in the two spectrally modulated beams of a polarizing beam-splitter even provides the possibility to perform a postfacto differential transmission correction that improves the accuracy of the spectral line polarimetry. We perform an error analysis to compare the accuracy of spectral line polarimetry to continuum polarimetry, degraded by a residual dark signal and differential transmission, as well as to quantify the impact of the transmission correction. We demonstrate the new techniques with a blue sky polarization measurement around the oxygen A absorption band using the groundSPEX instrument, yielding a polarization in the deepest part of the band of 0.160±0.010, significantly different from the polarization in the continuum of 0.2284±0.0004. The presented methods are applicable to any dual-beam channeled polarimeter, including implementations for snapshot imaging polarimetry.

Published
2014
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