Solar polarimetry aims at measuring the full set of Stokes vectors (I, Q, U, V) to extract the magnetic field information of the solar atmosphere. During the polarimetric observations, the oblique reflections from the telescope mirrors modify (crosstalk) or even produce polarization (instrumental polarization). For accurate polarimetric measurements of the source, it is important to correctly model and remove the instrumental polarization and crosstalk introduced by the telescope. The Multi-Application Solar Telescope (MAST) at the Udaipur Solar Observatory is a Gregorian-Coude telescope with a 50 cm off-axis parabolic primary mirror. It consists of nine mirrors that rotate as the telescope tracks the object and delivers a constant beam at the observing floor. Here, we present a formalism for an analytical estimation of the telescope's Mueller matrix using a polarization ray tracing algorithm. The model was experimentally verified at 6173 Å using the observations taken from the facility imaging spectro-polarimeter. The observations were split into two sets, during January and May 2018, to verify the model's consistency. The instrumental polarization was found to vary between 1.3% to 5.54%, and 3.5% to 4.3% throughout the observation from 9 AM to 4 PM during summer and winter respectively. The maximum value of the crosstalk (Q to V and U to V) was found to be 29.08% and 39.46% respectively. We obtained a reasonable match between the model and the observations with some offsets. We also discuss the possible reasons for the observed deviations and their effects.
The multislit spectro-polarimeter (MSSP) is a grating-based littrow spectrograph with five slits at the entrance aperture. The polarimeter consists of a nematic liquid crystal variable retarder as the modulator and a Savart plate as an analyzer. It is one of the facility instruments on the Multiapplication Solar Telescope at the Udaipur Solar Observatory, developed to measure the magnetic fields of the Sun in the photosphere and chromosphere. MSSP currently operates only at 630.2 nm (FeI), but will be upgraded to cover CaII at 854.2 nm, HeI at 1083.2 nm, and FeI at 1565.3 nm. The spectrograph has a spectral dispersion of 15.8 mÅ ± 1.2 mÅ at 630.2 nm. The polarimeter has a sensitivity of the order of 10 − 2 and the root mean squared noise in the Stokes spectrum (continuum wavelength points of Stokes Q, U, and V) is 0.015I. To obtain an estimate of the instrument induced polarization, an analytical model is developed to determine the polarization introduced by the telescope. A polarimetric calibration (PolCal) unit is used to calibrate the downstream optical path from the telescope exit pupil up to the detector in MSSP. A residual polarization cross talk of 10% is measured in the data after applying PolCal corrections. The polarimetric data obtained from the engineering run (first-light) are inverted using NICOLE, to extract the magnetic field parameters. The field strength derived from MSSP observations is compared with the data obtained from helioseismic and magnetic imager and is found to lie within ±70 G in the umbral region and ±200 G in the penumbral region.
An image stabilization system has been developed and demonstrated for solar observations in the visible wave-length at Udaipur Solar Observatory (USO) with a 15 cm Coudé-refractor. The softwa4re and hardware components of the system are similar to that of the low cost solar adaptive optics system developed for the 1.5 m McMath-Pierce solar telescope at Kitt Peak observatory for solar observations in the infrared. The first results presented. The system has a closed loop correction bandwidth in the range of 70 to 100 Hz. The root mean by a factor of 10 to 20. The software developes and key issues concerning optimum system performance have been addressed.
A Multi-Application Solar Telescope (MAST) is proposed to be installed at the Udaipur Solar Observatory (USO)
in India to monitor the Sun in optical and near infra-red wavelengths. The median value of the Fried's parameter
at this site is 4 cm. USO is in the process of building an Adaptive optics (AO) system in order to have diffraction
limited performance of the MAST under this moderate seeing condition. AO helps in achieving high-resolution
imaging by compensating the atmospheric turbulence in real-time. We have performed simulations to evaluate
the performance of AO for various seeing conditions. It was concluded that with the present availability of AO
system components, a 55 cm aperture telescope would yield optimum performance with AO, in combination with
post-processing techniques like speckle imaging and phase diversity. At present, we are developing a proto-type
AO system at USO to demonstrate its performance with a 15 cm Coude´ refracting telescope as a preparation for
the main AO system to be deployed on the MAST. The prototype AO system is being realized in two phases.
In the first phase, we have developed an image stabilization system to compensate the global tilt of the wave-front.
The second phase consists of sensing and correcting the local tilts of the wave-front by integrating a
micro-machined membrane deformable mirror with the image stabilization system and is currently in progress.
Here, we present the details of our proto-type AO system. We also present preliminary results obtained from
simulations using Phase Diversity as a post processing technique.
Meter Aperture Solar Telescope (MAST) is a proposed modern solar telescope equipped with Adaptive Optics (AO) facility for observing the Sun in Optical and infra-red wavelengths. It is planned to develop a low-order AO system at the re-imaged pupil plane of the MAST. Before developing such an AO system, one would like to answer a few questions like what is the size of the sub-apertures required to achieve optimum performance under typical seeing conditions? What is the required bandwidth? Is it possible to operate the system with a narrow bandwidth of 0.1 nm? Is it possible to achieve diffraction limited imaging by using speckle imaging on the low-order AO corrected images? In this paper, we try to answer these questions through extensive computer simulations and arrive at a final
optimal specification ot the low-order AO system of the MAST. We simulate distorted wave-fronts for various seeing conditions (for both Kolmogorov and von Karman spectrum) using large phase screens generated using Fourier transfrom method. We find the local slopes of the distorted wave-front over the sub-apertures of different lenslet array geometries using a least square modal recontruction method. Then we estimate the structure functions, optical transfer functions, Strehl resolution of the corrected wave-front and evaluate the performance. We have developed a speckle-masking code and parallelised it using a 16-processor IBM-SP machine. We use a series of AO corrected images to obtain a speckle reconstruction of the object. We evaluate the performance of this hybrid imaging system in achieving diffraction limited imaging of small-scale solar features.