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.
