High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter

Abstract

Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance when attempting to respond to a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (∼10 −1 arcsec) and low noise (10 −3 to 10 −5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC). Aims. We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC. Methods. We describe the instrument in depth, including the fast pnCCD camera employed, the achromatic modulator package, the main calibration steps, the effects of the modulation frequency on the levels of seeing-induced spurious signals, and the effect of the camera properties on the image restoration quality. Results. The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6%) and very low noise (4.94 e − rms). The modulator is optimized to have high (>80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals that are present in narrowband, non-modulated, quiet-sun measurements are (a) lower than the noise (7 × 10 −5 ) after integrating 7.66 min, (b) lower than the noise (2.3 × 10 −4 ) after integrating 1.16 min and (c) slightly above the noise (4 × 10 −3 ) after restoring case (b) by means of a multiobject multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low S/N of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s.

Instrumentation for solar spectropolarimetry: state of the art and prospects

“…37,38 This translates in a requirement of sub-arcsec 4 The density scale height that is a fundamental length for many basic physical processes in the Sun is ∼ 150km at the photosphere. angular resolution, few times 0.01% polarimetric sensitivity 39,40 and mid-high cadence (see Fig. 1), which is challenging from an instrumental point of view.…”

“…Due to the differential nature of the polarization measurements and low photo-electron counts obtained in high-resolution solar applications, there are specific aspects of the detectors that gain relevance compared to spectroscopic or broadband imaging applications, see e.g., Ref. 39. Some of these are described in the following subsections, within the context of each detector technology.…”

“…However, the ZIMPOL approach also presents drawbacks, in particular when high spatial resolution is required. 39 These are the non-square pixels resulting from the usage of a micro-lens array, required to maximize filling factor, which produce two different spatial sampling frequencies that complicates image restoration; up to date the ZIMPOL concept has been implemented in slow readout sensors (∼2 fps) limiting the study of fast events and the application of image restoration techniques in combination with high duty cycle. The FSP, employs a frame-transfer, fully-depleted CCD detector 253,254 that was custom-made with column parallel readout.…”

“…The 400 fps, almost 100% duty cycle and low noise (4.9 e − RMS) of the detector allowed FSP to avoid seeing effects and reach the ∼0.01% polarimetric sensitivity level using FLC-based temporal modulation only. 39 However, the sensor was small (264x264 pixel 2 ) and the development of a full-sized version (1024x1024 pixel 2 ) proved difficult and expensive when compared with CMOS competitors. The prototype of another very promising custom detector type for polarimetry has developed very recently: the Quadropix DePFET.…”

Numeric Study of Novel Compact Polarimeter for Full-Stokes Polarization Imaging Without Micro-Polarizer Array

“…Polarization imaging technologies are capable of recording light polarization of a scene, which implies certain characteristics of the objects, such as surface features, shapes, shadows, and roughness. It improves the target recognition in complicate environment and provides valuable information for target detection [1], biomedical imaging [2], [3], material characterization [4], environmental monitoring [5], astronomy observation [6] and remote sensing [7]. The Stokes vector is generally used to describe the state of polarization (SOP) of incident light, and it can provide more information about reflecting objects than traditional intensity-based cameras.…”