Ravinder Kumar Banyal

Opto-thermal analysis of a lightweighted mirror for solar telescope

In this paper, an opto-thermal analysis of a moderately heated lightweighted solar telescope mirror is carried out using 3D finite element analysis (FEA). A physically realistic heat transfer model is developed to account for the radiative heating and energy exchange of the mirror with surroundings. The numerical simulations show the non-uniform temperature distribution and associated thermo-elastic distortions of the mirror blank clearly mimicking the underlying discrete geometry of the lightweighted substrate. The computed mechanical deformation data is analyzed with surface polynomials and the optical quality of the mirror is evaluated with the help of a ray-tracing software. The thermal print-through distortions are further shown to contribute to optical figure changes and mid-spatial frequency errors of the mirror surface. A comparative study presented for three commonly used substrate materials, namely, Zerodur, Pyrex and Silicon Carbide (SiC) is relevant to vast area of large optics requirements in ground and space applications.

High-contrast, all-optical switching in bacteriorhodopsin films

We report experiments with nonlinear-absorption-based, high-contrast, all-optical switching in photochromic bacteriorhodopsin (BR) films. The switching action is accomplished by control of the transmission of a weak probe beam through a BR sample with the help of strong pump beam illumination at 532 nm wavelength. We found that the switching properties of BR films depend on several experimentally controllable parameters such as probe wavelength, pump beam intensity, and excitation rate. A comparative study of the switching behavior and other parameters of practical use was carried out at three probe wavelengths (543, 594, and 633 nm) and various beam powers and pump excitation rates. The results are presented for commercially available wild-type and D96N variant BR films.

Fabry-Pérot based narrow band imager for solar filament observations

We have recently developed a narrow band imager (NBI) using an air gap based Fabry-P e rot (FP) interferometer at the Indian Institute of Astrophysics, Bangalore. Narrow band imaging is achieved by using an FP interferometer working in combination with an order sorting pre-filter. The NBI can be tuned to a different wavelength position on the line profile by changing the plate separation of the FP. The interferometer has a 50 mm clear aperture with a bandpass of ∼247.8 m A and a free spectral range of ∼5.3 A at λ = 656.3 nm. The developed NBI is used to observe the solar filament in the Hα wavelength. The instrument is being used to image the Sun at chromospheric height and it is also able to scan the Hα spectral line profile at different wavelength positions. We have also made Doppler velocity maps at chromospheric height by taking the blue and red wing images at ±176 m A wavelength positions separately away from the line center of the spectral line. In this paper, we present a description of the NBI including lab test results of individual components and some initial observations carried out with this instrument.

Pixel size and pitch measurements of liquid crystal spatial light modulator by optical diffraction

We present a simple technique for the determination of pixel size and pitch of liquid crystal (LC) based spatial light modulator (SLM). The proposed method is based on optical diffraction from pixelated LC panel that has been modeled as a two-dimensional array of rectangular apertures. A novel yet simple, two-plane measurement technique is implemented to circumvent the difficulty in absolute distance measurement. Experimental results are presented for electrically addressed twisted nematic LC-SLM removed from the display projector.

Light-induced absorption in photorefractive BaTiO3 crystals

We present the experimental results on the measurement of temporal and steady-state light-induced absorption change in undoped and rhodium-doped barium titanate (Rh:BaTiO3) crystals at different wavelengths and intensities. Theoretical calculations based on a two-center charge transport model agree well with the experimental results which supplement the earlier studies carried out using photorefractive BaTiO3 crystals. We also report a wavelength-specific light-induced effect that modifies the absorption dynamics in a peculiar manner. This can be attributed to light-induced thermal effects. However, the possibility of additional photorefractive centers becoming active cannot be ruled out.

Alfred Wegener—From continental drift to plate tectonics

What is the nature of the force or mechanism that moves massive continents thousands of miles across? What causes violent earthquakes to displace huge landmasses abruptly? How could great mountain ranges like Himalayas and Alps rise to such incredible heights? What makes earths interior so restless? Answers to some of these questions may lie in understanding the Earths interior itself.

Estimation of asymmetries in point spread function for the echelle spectrograph operating at Vainu Bappu Telescope for high precision radial velocity studies

Precision in the Radial Velocity (RV) measurements depends upon the efficiency of the technique to remove instrumental artifacts from stellar measurements. Iodine absorption cell technique is being implemented for high precision studies with the Echelle spectrograph operating at Vainu Bappu Telescope (VBT), Kavalur, India. Since the star spectrum is convolved with the PSF of the spectrograph, the asymmetries in the PSF are imposed on the stellar spectral lines. The fiber fed Echelle spectrograph is a general purpose instrument, designed for high resolution (R = 60,000) spectroscopic observations. The asymmetries in the Point Spread Function (PSF) arise due to the off-axis launching of the stellar beam into the collimator and vignetting across the field. Apart from this, due to usage constraints, the grating of the spectrograph is a movable component. The impact on the Doppler shift calculations due to the movable components in the spectrograph is to be estimated. For upgrading the spectrograph for precision studies, the component level sensitivity for RV is to be analyzed. Thus, instrument design asymmetries and component induced PSF variations are analyzed to estimate the limitations of the spectrograph for precision studies. We have developed Zemax based optical design of the spectrograph to estimate the PSF variations and design limitations on the RV studies. Here, we present a model developed in Zemax and a preliminary analysis on RV sensitivity and the PSF asymmetries of the spectrograph. These instrument variations are to be taken as input during RV data reduction for precision measurements.

Development of a Lunar Scintillometer as part of the national large optical telescope site survey

Ground layer turbulence is a very important site characterization parameter used to assess the quality of an astronomical site. The Lunar Scintillometer is a simple and effective site-testing device for measuring the ground layer turbulence. It consists of a linear array of photodiodes which are sensitive to the slight variations in the moon’s brightness due to scintillation by the lower layers of the Earth’s atmosphere. The covariance of intensity values between the non-redundant photodiode baselines can be used to measure the turbulence profile from the ground up to a height determined by the furthest pair of detectors. The six channel lunar scintillometer that has been developed at the Indian Institute of Astrophysics is based closely on an instrument built by the team led by Andrei Tokovinin of Cerro Tololo Inter-American Observatory (CTIO), Chile (Tokovinin et al., Mon. Not. R. Astron. Soc. 404(3), 1186–1196 2010). We have fabricated the instrument based on the existing electronic design, and have worked on the noise analysis, an EMI (Electromagnetic Induction) resistant PCB design and the software pipeline for analyzing the data from the same. The results from the instrument’s multi-year campaign at Mount Saraswati, Hanle is also presented.

Infrared Scanning Cloud Monitor for the Indian Astronomical Observatory, Hanle

A large number of clear nights at a given location is one of the key prerequisites for establishing an astronomical observatory. This parameter needs to be reliably determined before the site location is finalized. For an already exiting observatory site, monitoring of clouds on real-time basis helps automating many observing procedures, thus optimizing the scientific returns from the facility. We have developed an infrared (IR) scanning cloud monitor which generates a local sky-map and provides reliable information about the changing sky conditions. Our device measures the IR sky brightness temperature using a circular array of thermopile sensors mounted on a rotating system. In this paper, we give a detailed description of the instrument hardware, laboratory and on-site calibration procedures and further tests carried out to check the reliability of the device. The cloud monitor has been in regular use since its installation at the Indian Astronomical Observation (IAO), Hanle, Ladakh in 2015 December. W...