Smear correction of highly variable, frame-transfer CCD images with application to polarimetry

Iglesias, F. A.; Feller, A.; Nagaraju, K.

doi:10.1364/ao.54.005970

Cited by 12 publications

(8 citation statements)

References 13 publications

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“…The common mode signal is estimated from shielded pixels and subtracted from each semi-row. The image smearing, caused by shutter-free operation of frame-transfer CCD detectors, is corrected by using a modified standard model developed and implemented by Iglesias et al (2015). After the basic reduction, 800 frames within each modulation state and wavelength position are averaged to a single image.…”

Section: Data Reductionmentioning

confidence: 99%

Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter

Zeuner

Feller

Iglesias

et al. 2018

A&A

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Context. Scattering polarization in the Sr I 4607.3 Å line observed with high resolution is an important diagnostic of the Sun's atmosphere and magnetism at small spatial scales. Investigating the scattering polarization altered by the Hanle effect is key to constraining the role of small-scale magnetic activity in solar atmospheric activity and energy balance. At present, spatially resolved observations of this diagnostic are rare and have not been reported as close to the disk center as for µ = 0.6. Aims. Our aim is to measure the scattering polarization in the Sr I line at µ = 0.6 and to identify the spatial fluctuations with a statistical approach. Methods. Using the Fast Solar Polarimeter (FSP) mounted on the TESOS filtergraph at the German Vacuum Tower Telescope (VTT) in Tenerife, Spain, we measured both the spatially resolved full Stokes parameters of the Sr I line at µ = 0.6 and the center-to-limb variation of the spatially averaged Stokes parameters.Results. We find that the center-to-limb variation of the scattering polarization in the Sr I line measured with FSP is consistent with previous measurements. A statistical analysis of Stokes Q/I (i.e., the linear polarization component parallel to the solar limb), sampled with 0.16 pixel −1 in the line core of Sr I reveals that the signal strength is inversely correlated with the intensity in the continuum. We find stronger linear polarimetric signals corresponding to dark areas (intergranular lanes) in the Stokes I continuum image. In contrast, independent measurements at µ = 0.3 show a positive correlation of Q/I with respect to the continuum intensity. We estimate that the patch diameter responsible for the excess Q/I signal is on the order of 0.5 -1 . Conclusions. The presented observations and the statistical analysis of Q/I signals at µ = 0.6 complement reported scattering polarization observations as well as simulations. The FSP has proven to be a suitable instrument to measure spatially resolved scattering polarization signals. In the future, a systematic center-to-limb series of observations with subgranular spatial resolution and increased polarimetric sensitivity (<10 −3 ) compared to that in the present study is needed in order to investigate the change in trend with µ that the comparison of our results with the literature suggests.

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Section: Data Reductionmentioning

confidence: 99%

Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter

Zeuner

Feller

Iglesias

et al. 2018

A&A

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“…The smear problem is thus to estimate the contributions of the various photo-sites crossed by a given pixel of the image. The basic approach would be to derive the contributions s(i) from the image itself as proposed by Powell et al (1999) who subtracts the summed column of the image properly scaled by the transfer time, or Iglesias et al (2015) which adapts that principle to varying illumination during the exposure. But, this approach does not apply to CHEOPS since only a part of the CCD is downloaded and because the image continuously varies over time: between two consecutive 1 min exposures the image is rotated by 3.6 • and undergoes a different pointing jitter.…”

Section: Smear Correctionmentioning

confidence: 99%

Expected performances of the Characterising Exoplanet Satellite (CHEOPS)

Hoyer

Guterman

Demangeon

et al. 2020

A&A

100

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The CHaracterizing ExOPlanet Satellite (CHEOPS) is set to be launched in December 2019 and will detect and characterize small size exoplanets via ultra high precision photometry during transits. CHEOPS is designed as a follow-up telescope and therefore it will monitor a single target at a time. The scientific users will retrieve science-ready light curves of the target that will be automatically generated by the CHEOPS data reduction pipeline of the Science Operations Centre. This paper describes how the pipeline processes the series of raw images and, in particular, how it handles the specificities of CHEOPS data, such as the rotating field of view, the extended irregular point spread function, and the data temporal gaps in the context of the strict photometric requirements of the mission. The current status and performance of the main processing stages of the pipeline, that is the calibration, correction, and photometry, are presented to allow the users to understand how the science-ready data have been derived. Finally, the general performance of the pipeline is illustrated via the processing of representative scientific cases generated by the mission simulator.

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“…Relevant calibration issues when applying them to polarimetry are common-mode noise 236 and frame-to-frame variable offsets, which can be reduced by subtracting the signal of specially shielded pixels; 39 response non-linearities, 240 that can be calibrated down to below ∼ 1% using look-up tables; 69 and, for shutter-less designs, frame-transfer artifacts that can be numerically corrected. 241 The need for large sensors areas, to properly sample the focal plane of bigger telescopes; for higher frame rates; and the great performance improvements and cost reductions, have motivated the usage of CMOS solutions in many recent instruments, i.e., all polarimeters dated 2017 or beyond in Fig. 2 employ CMOS detectors.…”

Section: Ccd Detectorsmentioning

confidence: 99%

Instrumentation for solar spectropolarimetry: state of the art and prospects

Iglesias

Feller

2019

Opt. Eng.

Self Cite

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Given its unchallenged capabilities in terms of sensitivity and spatial resolution, the combination of imaging spectropolarimetry and numeric Stokes inversion represent the dominant technique currently used to remotely sense the physical properties of the solar atmosphere, and in particular, its important driving magnetic field. Solar magnetism manifests itself in a wide range of spatial, temporal and energetic scales. The ubiquitous but relatively small and weak fields of the so called quiet Sun are believed today to be crucial for answering many open questions in solar physics, some of which have substantial practical relevance due to the strong Sun-Earth connection. However, such fields are very challenging to detect because they require spectropolarimetric measurements with high spatial (subarcsec), spectral (< 100 m) and temporal (< 10 s) resolution along with high polarimetric sensitivity (< 0.1% of the intensity). In this review we collect and discuss both well-established and upcoming instrumental solutions developed during the last decades to push solar observations towards the above-mentioned parameter regime. This typically involves design trade-offs due to the high dimensionality of the data and signal-to-noise-ratio considerations, among others. We focus on the main three components that form a spectropolarimeter, namely, wavelength discriminators, the devices employed to encode the incoming polarization state into intensity images (polarization modulators), and the sensor technologies used to register them. We consider the instrumental solutions introduced to perform this kind of measurements at different optical wavelengths and from various observing locations, i.e., ground-based, from the stratosphere or near space.

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Smear correction of highly variable, frame-transfer CCD images with application to polarimetry

Cited by 12 publications

References 13 publications

Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter

Detection of spatially structured scattering polarization of Sr I 4607.3 Å with the Fast Solar Polarimeter

Expected performances of the Characterising Exoplanet Satellite (CHEOPS)

Instrumentation for solar spectropolarimetry: state of the art and prospects

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