Sridharan Rengaswamy | Indian Institute of Astrophysics (original) (raw)

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Papers by Sridharan Rengaswamy

Women in Optics and Photonics in India 2022

Solar Physics

Modern large ground-based solar telescopes are invariably equipped with adaptive optics systems t... more Modern large ground-based solar telescopes are invariably equipped with adaptive optics systems to enhance the high angular resolution imaging and spectroscopic capabilities in the presence of the Earth's atmospheric turbulence. The quality of the images obtained from these telescopes can not be quantified with the Strehl ratio or other metrics that are used for nighttime astronomical telescopes directly. In this paper, we propose to use the root mean square (rms) granulation contrast as a metric to quantify the image quality of ground-based solar telescopes. We obtain semilogarithmic plots indicating the correspondence between the Strehl ratio and the rms granulation contrast for most practical values of the telescope diameters (D) and the atmospheric coherence diameters (r 0), for various levels of adaptive optics compensation. We estimate the efficiency of a few working solar adaptive optics systems by comparing the results of our simulations with the Strehl ratio and rms granulation contrast published by these systems. Our results can be used in conjunction with a plausible 50% system efficiency to predict the lower bound on the rms granulation contrast expected from ground-based solar telescopes.

Journal of Astronomical Telescopes, Instruments, and Systems, May 17, 2018

Spatial Heterodyne Spectroscopy (SHS) is a relatively novel interferometric technique similar to ... more Spatial Heterodyne Spectroscopy (SHS) is a relatively novel interferometric technique similar to the Fourier transform spectroscopy with heritage from the Michelson Interferometer. An imaging detector is used at the output of a SHS to record the spatially-heterodyned interference pattern. The spectrum of the source is obtained by Fourier transforming the recorded interferogram. The merits of the SHS-its design, including the absence of moving parts, compactness, high throughput, high SNR and instantaneous spectral measurements-make it suitable for space as well as for ground observatories. The small bandwidth limitation of the SHS can be overcome by building it in tunable configuration (Tunable Spatial Heterodyne Spectrometer, TSHS). In this paper, we describe the design, development and simulation of a TSHS in refractive configuration suitable for optical wavelength regime. Here we use a beam splitter to split the incoming light compared with all-reflective SHS where a reflective grating does the beam splitting. Hence the alignment of this instrument is simple compared with all-reflective SHS where a fold mirror and a roof mirror are used to combine the beam. This instrument is intended to study faint diffuse extended celestial objects with a resolving power above 20000, and can cover a wavelength range from 350 nm to 700 nm by tuning. It is compact and rugged compared with other instruments having similar configurations.

Astronomy & Astrophysics, 2016

VLTI-AMBER velocity-resolved aperture-synthesis imaging of η Carinae with a spectral resolution o... more VLTI-AMBER velocity-resolved aperture-synthesis imaging of η Carinae with a spectral resolution of 12 000 Studies of the primary star wind and innermost wind-wind collision zone ,

Advances in Stellar Interferometry, 2006

We present a formal comparison of the performance of algorithms used for synthesis imaging with o... more We present a formal comparison of the performance of algorithms used for synthesis imaging with optical/infrared long-baseline interferometers. Five different algorithms are evaluated based on their performance with simulated test data. Each set of test data is formatted in the OI-FITS format. The data are calibrated power spectra and bispectra measured with an array intended to be typical of existing imaging interferometers. The strengths and limitations of each algorithm are discussed.

Optical and Infrared Interferometry IV, 2014

We present the latest update of the European Southern Observatory's Very Large Telescope interfer... more We present the latest update of the European Southern Observatory's Very Large Telescope interferometer (VLTI). The operations of VLTI have greatly improved in the past years: reduction of the execution time; better offering of telescopes configurations; improvements on AMBER limiting magnitudes; study of polarization effects and control for single mode fibres; fringe tracking real time data, etc. We present some of these improvements and also quantify the operational improvements using a performance metric. We take the opportunity of the first decade of operations to reflect on the VLTI community which is analyzed quantitatively and qualitatively. Finally, we present briefly the preparatory work for the arrival of the second generation instruments GRAVITY and MATISSE.

SPIE Proceedings, 2008

We present the results of the third Optical/IR Interferometry Imaging Beauty Contest. A formal co... more We present the results of the third Optical/IR Interferometry Imaging Beauty Contest. A formal comparison is presented of the performance of algorithms used for imaging data from optical/infrared long-baseline interferometers. The contest consists of blind imaging of test data sets derived from model sources and distributed in the OI-FITS format. The test data consisted of datasets on two objects each

Optical and Infrared Interferometry III, 2012

Bulletin of the Astronomical Society of India, 2003

Adaptive Optics systems have now become an integral part of any large ground based telescope oper... more Adaptive Optics systems have now become an integral part of any large ground based telescope operating at optical and infra-red wavelengths. They enhance the performance of the ground based imaging system by sensing and correcting a distorted wave-front in real time using ...

Astronomical Adaptive Optics (AO) technology enables real time diffraction-limited imaging and sp... more Astronomical Adaptive Optics (AO) technology enables real time diffraction-limited imaging and spectroscopy with high sensitivity from groundbased telescopes. It is achieved by using corrective optical elements in the path of the light beam before the final image is formed. The Indian Institute of Astrophysics is deliberating on building new large solar and stellar telescopes, namely the National Large Solar Telescope (NLST) and the National Large Optical Telescope (NLOT) equipped with AO. In view of this, an AO program has been initiated at IIA, with the primary objective of generating expertise in the field of astronomical adaptive optics by developing and demonstrating AO systems in existing small telescopes. In this paper, we start with an overview of astronomical adaptive optics and elaborate on the AO related activities carried out at IIA. Mr. Hemanth Pruthvi was affiliated with IIA till April 2019.

Long-exposure stellar images recorded with large ground-based telescopes are blurred due to the t... more Long-exposure stellar images recorded with large ground-based telescopes are blurred due to the turbulent nature of the atmosphere. The VLT employs active and adaptive optics (AO) systems to compensate for the deleterious effects of the atmosphere in real time. The speckle imaging technique provides an alternative way to achieve diffraction-limited imaging by post-processing a series of short-exposure images. The use of speckle imaging with the no-AO mode of NACO at the VLT is demonstrated. Application of this technique is particularly suited to the J-band and it provides versatile high angular resolution imaging under mediocre conditions and/or in imaging extended objects. The implementation of this mode underlines the continuing attractiveness of NACO at the VLT.

Monthly Notices of the Royal Astronomical Society

The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in th... more The intermediate-mass Herbig Ae star V1787 Ori is a member of the L1641 star-forming region in the Orion A molecular cloud. We report the detection of an M-type companion to V1787 Ori at a projected separation of 6.66 arcsec (corresponding to 2577 au), from the analysis of VLT/NACO adaptive optics Ks-band image. Using astrometric data from Gaia DR2, we show that V1787 Ori A and B share similar distance (d ∼ 387 pc) and proper motion, indicating that they are physically associated. We estimate the spectral type of V1787 Ori B to be M5 ± 2 from colour–spectral type calibration tables and template matching using SpeX spectral library. By fitting PARSEC models in the Pan-STARRS colour–magnitude diagram, we find that V1787 Ori B has an age of 8.1$^{+1.7}_{-1.5}$ Myr and a mass of 0.39$^{+0.02}_{-0.05}$ M⊙. We show that V1787 Ori is a pre-main-sequence wide binary system with a mass ratio of 0.23. Such a low-mass ratio system is rarely identified in Herbig Ae/Be binary systems. We conclud...

$Research paper thumbnail of Seeing Measurements with the Merak H$\upalpha$ Telescope$

Research in Astronomy and Astrophysics

We developed a tip-tilt system to compensate the turbulence induced image motion for 1.3 m telesc... more We developed a tip-tilt system to compensate the turbulence induced image motion for 1.3 m telescope at Vainu Bappu Observatory, Kavalur. The instrument is designed to operate at visible wavelength band (480-700 nm) with a field of view 1 ′ × 1 ′. The tilt corrected images have shown up to ≈ 57% improvement in image resolution and a corresponding peak intensity increase by a factor of ≈ 2.8. A closed-loop correction bandwidth of ≈ 26 Hz has been achieved with on-sky tests and the root mean square motion of the star image has been reduced by a factor of ∼ 14. These results are consistent with theoretical and numerical predictions of wave-front aberrations caused by atmospheric turbulence and image quality improvement expected from a real-time control system. In this paper, we present the details of the instrument design, laboratory calibration studies and quantify its performance on the telescope.

ABSTRACT Long-exposure stellar images recorded with large ground-based telescopes are blurred due... more ABSTRACT Long-exposure stellar images recorded with large ground-based telescopes are blurred due to the turbulent nature of the atmosphere. The VLT employs active and adaptive optics (AO) systems to compensate for the deleterious effects of the atmosphere in real time. The speckle imaging technique provides an alternative way to achieve diffraction-limited imaging by post-processing a series of short-exposure images. The use of speckle imaging with the no-AO mode of NACO at the VLT is demonstrated. Application of this technique is particularly suited to the J-band and it provides versatile high angular resolution imaging under mediocre conditions and/or in imaging extended objects. The implementation of this mode underlines the continuing attractiveness of NACO at the VLT.

The Astrophysical Journal, 2015

ABSTRACT Hydrocarbon organic material, as found in the interstellar medium, exists in complex mix... more ABSTRACT Hydrocarbon organic material, as found in the interstellar medium, exists in complex mixtures of aromatic and aliphatic forms. It is considered to be originated from carbon enriched giant stars during their final stages of evolution, when very strong mass loss occurs in a few thousand years on their way to become planetary nebulae. We show here that the same organic compounds appear to be formed in previous stages of the evolution of giant stars. More specifically, during the first ascending giant branch K-type stars. According to our model this happens only when these stars are being abruptly enriched with lithium together with the formation of a circumstellar shell with a strong mass loss during just a few thousand years. This sudden mass loss is, on an average, a thousand times larger than that of normal Li-poor K giant stars. This shell would later be detached, specially when the star stops its Li enrichment and a rapid photospheric Li depletion occurs. In order to gain extra carbon-based material to form the organic hydrocarbons, and also to explain the presence of complex inorganic compounds in these stars, we propose an interaction of these strong winds with remaining asteroidal/cometary disks that already existed around these stars since they were dwarf A-type stars. The mechanism of interaction presented here is successful to explain the presence of inorganic compounds, however it is unable to produce new carbon free atoms to form the organic hydrocarbon compounds. Finally, we discuss some suggestions and speculations that can eventually help solving the long-standing puzzle of Li-rich giants.