Search

The development of a spectrometer is being carried out in the Near-Infrared (NIR) wavelength range of 1.0-1.7 μm using an Acousto-Optic Tunable Filter (AOTF). The AOTF is based on Tellurium-di-oxide (TeO) crystal that acts as a dispersing medium and is driven by an external Radio Frequency (RF) signal. The characterization of AOTF is based on the wavelength-tuning frequency relation, spectral bandwidth as a function of wavelength through AOTF Transfer Function (Sinc), their dependence on the angular aperture and the peak diffraction efficiency of the AOTF. To demonstrate the full scanning capability of the AOTF in the wavelength range of 1.0-1.7 μm, Argon (Ar) and Krypton (Kr) line sources are used. The overall functionality of the spectrometer using the wavelength selection capability of AOTF is demonstrated by scanning the RF in the NIR band to detect water vapour absorption signatures in the Earth's atmosphere. This spectrometer is being designed for future interplanetary missions of ISRO with an aim to study the atmospheres of planets such as Mars or Venus. [ABSTRACT FROM AUTHOR]

This paper addresses some of the issues related to externally occulted solar coronagraph; vignetting and achievable resolution due to an external occulter. The analytical expression by Evans (J Opt Soc Am 38:1083-1085, ) is used to perform the initial calculations. An expression for the vignetting for a given external occulter and field angle is derived. The values obtained with the derived expression are verified with those obtained by ZEMAX an Optical design software. The degradation in angular resolution of the system due to vignetting is also presented and an empirical relation to calculate the normalized resolution for a given amount of vignetting is obtained. [ABSTRACT FROM AUTHOR]

Snapshot spectroscopic imagers/instruments (SSI) are a class of spectroscopic instruments that are capable of acquiring spectral information of a given field of view in a single frame. Standard spectroscopic instruments like a grating-based spectrograph or a Fabry-Pérot-based spectrograph obtain two dimensional data of 2D space or 1D space and 1D wavelength. But SSIs have three dimensional data of 2D space and wavelength embedded in two dimensional detector/image plane. So standard data reduction techniques are not applicable. Lenslet array spectroscope is a novel SSI which images the object on to a hybrid spatio-spectral image plane. A procedure to extract the spatial and spectral information of the field of view from this hyperplane is presented. We demonstrate the snapshot capabilities of this instrument to study dynamic activities of the Sun as inferred from two measurements: (i) Evershed flow in a sunspot in NOAA 12526 at Fe I 6301.5 Å and (ii) oscillations in a quiescent prominence at Hα 6562.8 Å. This instrument can be used for large or small scale structures, making it efficient for studying a wide range of dynamic activities like helioseismology, Moreton waves, prominence oscillation etc. [ABSTRACT FROM AUTHOR]

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 defendant 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 &asymph; 1.8% uncertainty in the determination of its response matrix. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The outer atmosphere of the sun - called the corona - has been observed during total solar eclipse for short periods (typically < 6 min), from as early as the eighteenth century. In the recent past, space-based instruments have permitted us to study the corona uninterruptedly. In spite of these developments, the dynamic corona and its high temperature (1-2 million K) are yet to be fully understood. It is conjectured that their dynamic nature and associated energetic events are possible reasons behind the high temperature. In order to study these in detail, a visible emission line space solar coronagraph is being proposed as a payload under the small-satellite programme of the Indian Space Research Organisation. The satellite is named as Aditya-1 and the scientific objectives of this payload are to study: (i) the existence of intensity oscillations for the study of wavedriven coronal heating; (ii) the dynamics and formation of coronal loops and temperature structure of the coronal features; (iii) the origin, cause and acceleration of coronal mass ejections (CMEs) and other solar active features, and (iv) coronal magnetic field topology and three-dimensional structures of CMEs using polarization information. The uniqueness of this payload compared to previously flown space instruments is as follows: (a) observations in the visible wavelength closer to the disk (down to 1.05 solar radii); (b) high time cadence capability (better than two-images per second), and (c) simultaneous observations of at least two spectral windows all the time and three spectral windows for short durations. [ABSTRACT FROM AUTHOR]