High-resolution, high-sensitivity, ground-based solar spectropolarimetry with a new fast imaging polarimeter: I .Prototype characterization

Abstract

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 e ff ects of the modulation frequency on the levels of seeing-induced spurious signals, and the e ff ect 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 e ffi ciency 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 narrow- band, 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 multi- object 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.