3. Spectro-polarimeter

The spectro-polarimeter (SP) design for the NLST will be based on the Spectropolarimetry for the Infra-red and Optical Region (SPINOR) design. It would consist of two units, (i) Polarimetric Modulation unit and
(ii) Flexible Spectrograph unit.

(i) Polarimetric Modulation Unit: The polarimetric modulation unit will be placed close to the secondary focus (F2) of the NLST (see the optical layout of the NLST for the location of F2). The modulator will consist of an achromatic 0.375 waves rotating waveplate. The achromatic retarder can either be made using the Pancharatnam technique or using the bi-crystalline achromatic retarder (Guimond and Elmore, 2004). Both the achromatic retardershave been in operation for several years in Astronomical observations. The bi-crystalline modulator used in the SPINOR covers a wide wavelength range from 430nm to 1600nm. The details of the SPINOR instrument can be seen in Socas-Navarro et al. (2006). The modulator optics will be Anti-reflection (AR) coated in order to reduce any reflection loss. The wavelength range covered by this modulator can accommodate most of the spectral lines of interest to the scientific community.

Table 3 SP instrument specifications

Just before the modulator optics, a calibration unit will be placed. The calibration unit is a must in order to calibrate any residual polarization from the telescope. Due to this calibration mechanism, the achieved accuracies can be as high as 10^{-4}. The calibration unit will consist of an achromatic quarter-wave plate followed by an achromatic linear polarizer. When the quarter wave plate and the polarizer are in the optical beam, then it produces circularly polarized beam to the modulator. For the generation of linear polarization, the quarter wave plate is taken out of the beam. During the regular observations of the solar features, both the quarter wave plate and the polarizer will be out of the beam. The rotation modulator uses the principle similar to that of the Advanced Stokes Polarimeter (Elmore et al., 1992).

(ii) Flexible Spectrograph unit: A spectrograph at the focal plane of the telescope is used to capture the spectral line profiles. A 40-60micron slit at the focal plane will be the slit of the spectrograph. The required instrument specifications are listed in Talbe 3.
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