We present an innovative concept for a new full-disk solar spectropolarimeter. The instrument is part of the new container-based Tautenburg Solar Laboratory (TauSoL), which is currently under construction and commissioning on site of the Th¨uringer Landessternwarte Tautenburg (TLS), Germany. The initial design approach is to use a single Fabry-P´erot interferometer with a 150mm clear aperture that is placed at the aperture of the solar telescope inside the container lab. In this paper, we present the current status of TauSoL and the detailed design of the light feed and the initial multi-line solar spectropolarimeter.
Vacuum Tower Telescope is a 70 cm off-axis solar telescope situated at Teide Observatory, Tenerife. It hosts a suit of instruments including HELioseismic Large Region Interferometric DEvice (HELLRIDE). HELLRIDE is a dual Fabry-P´erot etalon-based imaging spectroscopic instrument. It records solar activity at multiple heights by means of multi-line spectroscopy with a cadence of less than a minute. Polarimeter development for the aforementioned instrument is discussed, towards producing the magnetograms at high cadence. Liquid crystal variable retarders and Polarizing beam-splitter are chosen as polarimetric modulators and analyzer respectively. A broadband imager is designed to acquire continuum images at the same time as polarized images. Two spectral lines are selected for the polarimetric observations: Fe i 6302.5 Å and Ca ii 8542 Å, with a scope for adding more lines.
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.
For moderate resolution spectroscopy of the Sun, an imaging spectrograph is being developed at Indian Institute of Astrophysics. With this instrument images of the region of interest of the Sun can be obtained with low spatial and moderate spectral resolution. Dopplergrams can also be obtained with the acquired data to get line of sight velocity maps. The instrument is a back-end for a telescope with tracking system i.e. stable image of the Sun is projected onto the focal plane at all times. Modular approach is followed in the design, keeping sections of the instruments fairly independent. Scanning-slit assembly is a module that can linearly move in one direction to sweep the region of interest in the image. Spectrograph assembly consists of another slit, optics and dispersing element along with the detector so that spectral information about spatial locations on the slit can be obtained. This module is designed to obtain Intensity vs. (x,λ) (x is along the slit) and as the scanning-slit is swept along the y-direction, Intensity vs. (x,y,λ) information is built. The spatial resolution will be seeing limited as there's no correction system. Field of view is 6 arc minute along the slit direction, as the features of interest include sunspots and surrounding region. For testing, a front end system of 100mm clear aperture with f/22.5 is being used. The dispersing element is a reflecting grating with 1200 grooves/mm. For 6563 Å(H-alpha line) spectral resolution is 35 mÅ in second order. Linear dispersion is about 38 mÅ /pixel for the pixel size of 7.5μm, indicating that slit-width limited spectral resolution can be obtained.
One of the three planned back-end systems for the proposed National Large Solar Telescope (NLST) is the Solar Dynamics Imaging System (SDIS) which is intended to obtain near simultaneous images in multiple wavelengths. As a first step, a prototype system with two channel imaging has been developed and installed at the back-end of the White light Active Region Monitor (WARM) telescope at Kodaikanal Observatory. A two-mirror Coelostat serves as a light feeding system to a refracting objective while an optical breadboard serves as a platform for the back-end instruments. A re-imaging system is used before the prime focus to get two light channels for the observations in two wavelengths. The re-imaging system is designed using ZEMAX and the alignment of the system is done using a laser. Full disk images are obtained using a red filter (674.2nm/10nm) and a G-band filter (430.5nm/0.84nm). Design aspects of the re-imaging system, preliminary observations and image reduction methods are described in this paper.
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