Analysis of spectropolarimetric observations of two circular sunspots located close to disk centre in H$${\alpha }$$ (6562.8 Å) and Fe i (6569.22 Å) is presented in this paper. The corresponding active region numbers are NOAA 10940 and NOAA 10941 referred to as AR1 and AR2, respectively. The vector magnetic field at the photosphere is derived through inversion of Stokes profiles of Fe i under Milne–Eddington atmospheric model. The chromospheric vector magnetic field is derived from H$${\alpha }$$ Stokes profiles under weak-field approximation. Azimuthally averaged magnetic field as a function of radial distance from the centre of sunspot at the photosphere and chromosphere are studied. At the photosphere, the radial variation shows a well known behaviour that the total field and the line-of-sight (LOS) component monotonically decrease from centre to the edge of the sunspot and the transverse component initially increases, reaches a maximum close to half the sunspot radius and then decreases. LOS and the transverse components become equal close to half the sunspot radius consistent with the earlier findings. At the chromosphere, all the components of the magnetic field decrease with the sunspot radius. However, the LOS component decreases monotonically whereas the transverse component decreases monotonically up to about 0.6 times the sunspot radius after which it reaches a constant value. Azimuthally averaged magnetic field gradient from photosphere to chromosphere is also presented here.

AbstractMeasurement of magnetic field in this layer is challenging both from point of view of observations and interpretation of the data. We present in this work about spectropolarimetric observations of a pore, simultaneously in Ca ii (CaIR) at 854.2 nm (CaIR) and H α (656.28 nm). The observed region includes a small scale energetic event (SSEE) taking place in the region between the pore and the region which show opposite polarity to that of pore at the photosphere. The energetic event appears to be a progressive reconnection event as shown by the time evolution of the intensity profiles. Closer examination of the intensity profiles from the downflow regions suggest that the height of formation of CaIR is higher than that of Hi α, contrary to the current understanding about their height of formation. Preliminary results on the inversion of Stokes-I and V profiles of CaIR are also presented.

The National Large Solar Telescope (NLST) aims primarily to carry out observations of the solar atmosphere with high spatial, spectral and temporal resolution. A comprehensive site characterization programme, that commenced in 2007, has identified an excellent site in the Ladakh region at the Pangong lake, India. With an innovative optical design, NLST is an on-axis Gregorian telescope with a low number of optical elements to reduce the number of reflections and yield a high throughput with low polarization. In addition, it uses high-order adaptive optics to produce close to diffraction limited performance. To control atmospheric and thermal perturbations of the observations, the telescope will function with a fully open dome, to achieve its full potential atop a 25 m tower. The post-focus instruments include broadband and tuneable Fabry-Perot narrow band imaging instruments and a high-resolution spectropolarimeter.

The National Large Solar Telescope (NLST) aims primarily to carry out observations of the solar atmosphere with high spatial, spectral and temporal resolution. A comprehensive site characterization programme, that commenced in 2007, has identified an excellent site in the Ladakh region at the Pangong lake, India. With an innovative optical design, NLST is an on-axis Gregorian telescope with a low number of optical elements to reduce the number of reflections and yield a high throughput with low polarization. In addition, it uses high-order adaptive optics to produce close to diffraction limited performance. To control atmospheric and thermal perturbations of the observations, the telescope will function with a fully open dome, to achieve its full potential atop a 25 m tower. The post-focus instruments include broadband and tuneable Fabry–Perot narrow band imaging instruments and a high-resolution spectropolarimeter.

The present work discusses astronomical site survey reports on dust content, vertical distribution of atmospheric turbulence, precipitable water vapor (PWV), surface and upper-air data, and their effects on seeing over the Indian Astronomical Observatory (IAO) Hanle. Using Laser Particulate Counter, ambient dust measurements at various sizes (0.3 μ m to 25 μ m) were performed at various locations at the site during November 2015. Estimated volume concentration for the particle size at 0.5 μ m was around 10,000 per cubic foot, which is equivalent to ten thousand class of clean room standard protocol. During the measurement, surface wind speed varied from 0-20 m s −1 , while estimated aerosol optical depth (AOD) using Sky radiometer (Prede) varied from 0.02-0.04 at 500 nm, which indicates the site is fairly clean. The two independent measurements of dust content and aerosol concentrations at the site agreed well. The turbulence or wind gust at the site was studied with wind profiles at three different heights above the ground. The strength of the wind gust varies with time and altitude. Nocturnal temperature across seasons varied with a moderate at summer (6−8 ∘ C) and lower in winter (4−5 ∘ C). However, the contrast between the two is significantly small due to cold and extremely dry typical climatic conditions of the site. The present study also examined the effects of surface and upper-air data along with Planetary Boundary Layer (PBL) dynamics with seeing measurement over the site. Further, a comparative study of such observed parameters was conducted with other high altitude astronomical observatories across the globe.

A major project called the National Lar ge Solar Telescope (NLST) has been proposed for pursuing Solar Astronomy at High Altitude. The project envisages the development of a state-of-the-art 2-m class telescope to carry out high-resolution studies of the solar atmosphere. This project is led by the Indian Institute of Astrophysics and has national and international partners. Its geographical location will fill the longitudinal gap between Japan and Europe and is expected to be the lar gest solar telescope with an aperture lar ger than 1.5 m till ATST and EST come into operation. NLST is an on-axis alt-azimuth Gregorian multi-purpose open telescope with the provision of carrying out nighttime stellar observations using a spectrograph at the Nasmyth focus. The telescope utilizes an innovative design with low number of reflections to achieve a high throughput and low polarization. High order adaptive optics is integrated into the design that works with a modest Frieds parameter of 7-cm to give diffraction limited performance. The telescope will be equipped with a suite of post-focus instruments including broad and narrow band imagers, a high-resolution spectrograph and a polarimeter . A comprehensive site characterization programme has demonstrated the presence of at least two excellent sites for setting up observational facilities for solar astronomy at high altitude in India.

Automation of the spectropolarimetric observations at Kodaikanal tunnel telescope needs placement of the Sun's image on the spectrograph slit and scanning it across the region of interest. This requirement can be achieved by controlling the tilt of the secondary mirror. Hence a fine image motion controller for secondary mirror in Kodaikanal Tunnel Telescope was developed and tested with telescope. A slo-syn stepper motor drive model SS2000D6 was used for driving the Steper Motor HS50. A multi function PCI board from measurement computing model PCI 2517 was used in giving signals to the drive. Control software and user Interface were developed. The system was calibrated to facilitate regular observations.

Observations of the transit of Venus were made from the Kodaikanal Observatory on 2012 June 06 in Ca-K wavelengths. Only half of the event was visible from India. We utilized this unique opportunity to compute the contribution of the scattered light within the Twin Telescope optics at Kodaikanal. The instrumental and atmospheric scattered light are derived from the model point spread function which is a combination of four Gaussians with different widths and weights. The restoration procedure has significantly improved the rms contrast of the images. The rms contrast of the stray light corrected images are almost two fold larger than that those of the uncorrected images. The derived network element sizes matches well with previous observations.

Spectroscopic magnetic field measurements in sunspots have been carried out at Kodaikanal since such research was started by John Evershed (Evershed 1944). Subsequently, a Stokes polarimeter was built by Balasubramaniam and collaborators for the spectrograph at the Kodaikanal Tower Telescope (KTT), with the goal of measuring vector magnetic fields. Although the achieved accuracy is limited, the telescope model developed by them is very accurate (Balasubramaniam et al. 1985) and is still used even today to correct for instrumental polarization. A similar polarimeter was developed by Sankarasubramanian and collaborators, which is more modern in having motorized rotation of the polarization optics and in using CCD detectors instead of photographic registration on film (Sankarasubramanian et al. 2002). The accuracy in magnetic field measurement achieved with this instrument was quite high. Very recently, a dual-beam polarimeter has been installed, which uses an efficient and well-balanced modulation scheme. The calibration and characterization of this instrument are presented in Nagaraju et al. (2008b); results on magnetic structuring from the photosphere to the chromosphere are presented in Nagaraju et al. (2008a, 2009).

Physical properties of G-band bright; points (GBPs) surrounding a pore are studied using observations spanning from photosphere to chromosphere. The magnetic field strengths obtained in these bright points are of the order of 1.2 kG. These GBPs show very little linear polarization and the magnetic field lines are vertical. The observed GBPs show a fill-fraction between 20 to 40%, revealing that they are smaller than the achieved spatial resolution of 0.2 arcsec. The observed GBPs harbor accelerated downflows.

India’s National Large Solar Telescope (NLST) will provide opportunities to observe the Sun with high spatial, spectral, and polarimetric resolution. The large aperture also enables high-cadence spectropolarimetry with moderate spatial resolution. A multi-slit spectropolarimeter is planned as one of the back-end instruments for this powerful telescope, primarily to measure vector magnetic fields in both active and quiet regions. An integral-field unit added with the multi-slit spectropolarimeter will enable fast-cadence observation. Here we discuss the scientific requirements for such an instrument, along with advantages and limitations of the concept and preliminary design details.

Initial results on the simultaneous spectropolarimetric observations of an active region at the photosphere and chromosphere are presented. For this purpose, the Fe I line at lambda 6569 and the H I at lambda 6563 (H alpha) are used. Stratification of the line-of-sight (LOS) velocity and magnetic fields above an active region are discussed. The LOS magnetic field strengths are derived using the center-of-gravity (COG) method and the LOS velocity gradients are derived using the bisector technique. From this analysis it is found that both the velocity and magnetic gradients are larger in the umbral region above the sunspot compared to the penumbral region. And the magnetic field strength decreases much faster with height in the umbral region compared to the penumbral region. Upflows with larger LOS velocity gradients are located in the regions where stronger photospheric fields are observed.

Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. The Fe I line at λ6569 and Hα at λ6563 are used for deriving the physical parameters at photospheric and chromospheric heights, respectively. The LOS magnetic field obtained through the center-of-gravity method shows a linear relation between the photospheric and chromospheric fields for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from Hα are much weaker than what one gets from the linear relationship, and also from those expected from the extrapolation of the photospheric magnetic field. We discuss in detail the properties of the magnetic field observed in Hα from the point of view of observed velocity gradients. The bisector analysis of Hα Stokes I profiles shows larger velocity gradients in those places where strong photospheric magnetic fields are observed. These observations may support the view that the stronger fields diverge faster with height compared to weaker fields.

Calibration and characterization of a dual-beam polarimeter installed at Kodaikanal Tower Telescope (KTT) for spectropolarimetric observations are presented in this paper. It was found that a slit width of 48 mu is optimum for the spectrograph setup at KTT and corresponding spectral resolution is 32.47 m angstrom. It was demonstrated that the precision in polarization measurement can be achieved better than 0.1% by increasing the exposure time. However, the polarimetric calibration accuracy is limited to 0.35% for Stokes Q and U parameters and 0.2% for Stokes V parameter, mainly due to the uncertainty in the retardance of the calibration retarder. A comparison of the magnetic field measurement between the Solar Optical Telescope (SOT) onboard Hinode and KTT for an active region was made and a good match was found after spatial smearing of Hinode/SOT measurements by 5.12".

An eight stage balanced modulation scheme for dual beam polarimetry is presented in this paper. The four Stokes parameters are weighted equally in all the eight stages of modulation resulting in total polarimetric efficiency of unity. The gain table error inherent in dual beam system is reduced by using the well known beam swapping technique. The wavelength dependent polarimetric efficiencies of Stokes parameters due to the chromatic nature of the waveplates are presented. The proposed modulation scheme produces better Stokes $Q$ and $V$ efficiencies for wavelengths larger than the design wavelength whereas Stokes $U$ has better efficiency in the shorter wavelength region. Calibration of the polarimeter installed as a backend instrument of the Kodaikanal Tower Telescope is presented. It is found through computer simulation that a 14% sky transparency variation during calibration of the polarimeter can introduce $\approx 1.8%$ uncertainty in the determination of its response matrix.