Chromospheric magnetic fields are of paramount importance in understanding the dynamics of energetic events in the solar atmosphere. At the Kodaikanal Solar Observatory, several polarimeters were developed in the past to study the active region magnetic fields. A polarimeter has been developed and installed at Kodaikanal Tower-Tunnel Telescope to study the active regions at the chromospheric level, in Ca ii 8542 Å spectral line. Design aspects of the instrument and polarimetry strategy are discussed. Telescope instrumental polarization has been revisited, and possible ways to reduce it have been proposed. The telescope polarization model developed in Zemax to examine the analytical instrumental polarization model is discussed. The polarimeter control unit, and the software developed to operate the polarimeter are briefly described. Polarimetric calibration of the instrument, observations, corrections for instrumental polarization, and the sample Stokes profiles are presented. Polarimetric accuracy and sensitivity are estimated to be better than 3 × 10 − 2 and 3 × 10 − 3, respectively.
In the context of the conceptual design study for the European Solar Telescope (EST) we have investigated
different metallic mirror coatings in terms of reflectivity, polarization properties and durability. Samples of the
following coating types have been studied: bare aluminum, silver with different dielectric layers for protection
and UV enhancement, and an aluminum-silver combination. From 2009 to 2011 we have carried out a long-term
durability test under realistic observing conditions at the VTT solar telescope of the Observatorio del
Teide (Tenerife, Spain), accompanied by repeated reflectivity measurements in the EST spectral working range
(0.3 - 20 μm), and by polarization measurements in the visible range. The test results allow us to find the
optimum coatings for the different mirrors in the EST beampath and to eventually assess aging effects and
re-coating cycles. The results of the polarization measurements are a valuable input for an EST telescope
polarization model, helping to meet the stringent requirements on polarimetric accuracy.
High-precision full-Stokes polarimetry at near diffraction limited spatial resolution is important to understand
numerous physical processes on the Sun. In view of the next generation of ground based solar telescopes, we have
explored, through numerical simulation, how polarimetric accuracy is affected by atmospheric seeing, especially
in the case of large aperture telescopes with increasing ratio between mirror diameter and Fried parameter. In this
work we focus on higher-order wavefront aberrations. The numerical generation of time-dependent turbulence
phase screens is based on the well-known power spectral method and on the assumption that the temporal
evolution is mainly caused by wind driven propagation of frozen-in turbulence across the telescope. To analyze
the seeing induced cross-talk between the Stokes parameters we consider polarization modulation scheme based
on a continuously rotating waveplate with rotation frequencies between 1 Hz and several 100 Hz.
Further, we have started the development of a new fast solar imaging polarimeter, based on pnCCD detector
technology from PNSensor. The first detector will have a size of 264 x 264 pixels and will work at frame rates of
up to 1kHz, combined with a very low readout noise of 2-3 e- ENC. The camera readout electronics will allow for
buffering and accumulation of images corresponding to the different phases of the fast polarization modulation.
A high write-out rate (about 30 to 50 frames/s) will allow for post-facto image reconstruction. We will present
the concept and the expected performance of the new polarimeter, based on the above-mentioned simulations of