Browsing by Author "Das, H.K."
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Item Design and development of an optical-fibre-based Integral Field Unit (IFU) on the IUCAA 2-m telescope(Mon. Not. R. Astron. Soc., 2011-07-31) Srivastava, Mudit K.; Ramaprakash, A.N.; Das, H.K.An optical-fibre-based Integral Field Unit (IFU) has been developed for the Inter-University Centre for Astronomy and Astrophysics (IUCAA) Faint Object Spectrometer and Camera (IFOSC), the main back-end instrument on the IUCAA 2-m telescope at Girawali, Pune, India. This IFU enables IFOSC to perform two-dimensional spectroscopy of extended astronomical objects and is being used as one of the modes of IFOSC. Based on the concept of coupling the telescope focal plane with the spectrograph slit using a fibre bundle, the IFU (named the Fibre-based Integral Field Unit for IFOSC, hereafter FIFUI) uses 100 optical fibres, each associated with a tiny lenslet on its tip, to sample the incoming field of view spatially. In addition, FIFUI uses some coupling optics to realize this two-dimensional interface. FIFUI offers three different spatial sampling scales of 0.8, 1.0 and 1.2 arcsec fibre−1. It is optimized for the visible spectrum and for a field of view of ∼13 × 6 arcsec2 on the sky for the nominal 1-arcsec sampling mode. FIFUI was commissioned on the IUCAA 2-m telescope during 2010 February–March after a series of sky tests and science-verification observations and a dataanalysis pipeline was developed to extract the spectra and reconstruct the sky maps. Here we report on the development of FIFUI, including its opto-mechanical design and commissioning observations.Item Exposure time calculator for IFOSC and sky background estimation(Bull. Astr. Soc. India, 2005-06-22) Chakraborty, Pavan; Das, H.K.; Tandon, S. N.This paper describes a spectroscopy exposure time calculator for the faint object spectrograph, the \IUCAA Faint Object Spectrograph" (IFOSC). It is intended to provide reasonable estimates of exposure times for observations. The background sky brightness is modeled for different phases and angular distance from the Moon. The code automatically calculates the Lunar Ephemerides from the date and time of observations and uses it for sky brightness computations. We adopt a new technique of scaling the Rayleigh and the Mie scattering functions, according to the corresponding extinction terms. This estimation of the sky brightness with moonlight ¯ts reasonably well with the observed sky brightness measured at the IUCAA telescope site. A simple component model is used for the optical elements. Each compo- nent is described by simple wavelength dependent extinction tables. At present, a blackbody/power-law spectrum is chosen as the input spectrum. Model or actual spectrum can also be used. The input blackbody spectrum is computed using the stellar \Spectral type" and scaled by the apparent V magnitude of the star. The programme is easily con¯gurable for di®erent sites and instruments. The code is written in ANSI C and thus portable to any system. A graphically interactive interface using HTML-cgi script has also been developed.Item Implementation of transfer-pupil in a spectrograph on optical telescopes(2011-07-06) Das, H.K.; Kohok, Abhay; Tandon, S. N.Based on the transfer pupil technique we develop a method to measure the lower order aberration coe±cients for optical telescopes; the method makes use of a Hartmann mask in a spectrograph for sampling the wavefront at di®erent locations.Item Interstellar extinction and polarization - A spheroidal dust grain approach perspective(2010-01-10) Das, H.K.; Voshchinnikov, N. V.; Il'in, V.B.We extend and investigate the spheroidal model of interstellar dust grains used to simultaneously interpret the observed interstellar extinction and polarization curves. We compare our model with similar models recently suggested by other authors, study its properties and apply it to fit the normalized extinction A(λ)/AV and the polarizing efficiency P(λ)/A(λ) measured in the near IR to far UV region for several stars seen through one large cloud. We conclude that the model parameter Ω being the angle between the line of sight and the magnetic field direction can be more or less reliably determined from comparison of the theory and observations. This opens a way to study the spatial structure of interstellar magnetic fields by using multi-wavelength photometric and polarimetric observations.Item Interstellar extinction and polarization – a spheroidal dust grain approach perspective(2010-01-15) Das, H.K.; Voshchinnikov, N.V.; Il’in, V. B.We extend and investigate the spheroidal model of interstellar dust grains used to simultaneously interpret the observed interstellar extinction and polarization curves. We compare our model with similar models recently suggested by other authors, study its properties and apply it to fit the normalized extinction A(λ)/AV and the polarizing efficiency P(λ)/A(λ) measured in the near-infrared to far-ultraviolet region for several stars seen through one large cloud. We conclude that the model parameter being the angle between the line of sight and the magnetic field direction can be more or less reliably determined from comparison of the theory and observations. This opens a way to study the spatial structure of interstellar magnetic fields by using multiwavelength photometric and polarimetric observations.Item IUCAA 2 meter telescope and its first light instrument IFOSC(Bull. Astr. Soc. India, 2002-05-05) Gupta, Ranjan; Burse, Mahesh P.; Das, H.K.; et.alThe various features of the IUCAA 2-meter telescope, its site parameters and considerations made for preserving the local seeing in terms of ventilation, thermal emissivity of the dome etc are described. The first light back-end instrument for this telescope, i.e. Iucaa Faint Object Spectrometer and Camera (IFOSC), too is described in some detail.Item IUCAA 2 meter telescope and its first light instrument IFOSC(Bull. Astr. Soc. India, 2002-06-05) Gupta, Ranjan; Burse, Mahesh P.; Das, H.K.; Kohok, A.; Ramaprakash, A.N; Engineer, S.; Tandon, S. N.The various features of the IUCAA 2-meter telescope, its site parameters and considerations made for preserving the local seeing in terms of ventilation, thermal emissivity of the dome etc are described. The first light back-end instrument for this telescope, i.e. Iucaa Faint Object Spectrometer and Camera (IFOSC), too is described in some detaiItem Modelling interstellar extinction and polarization with spheroidal grains(2008-01) Voshchinnikov, N. V.; Das, H.K.We calculate the wavelength dependence of the ratio of the linear polarization degree to extinction (polarizing efficiency) P(λ)/A(λ) from the ultraviolet to ear-infrared. The prolate and oblate particles with aspect ratios from a/b = 1.1 up to 10 are assumed to be rotating and partially aligned with the mechanism of paramagnetic relaxation (Davis-Greenstein). Size/shape/orientation effects are analyzed. It is found that the wavelength dependence of P(λ)/A(λ) is mainly determined by the particle composition and size whereas the values of P(λ)/A(λ) depend on the particle shape, degree and direction of alignment.Item Site characterization for the IUCAA telescope(2015-03-11) Das, H.K.; Menon, S.M.; Paranjpye, A.; Tandon, S.N.We present the report on characterisation of the site for IUCAA telescope, which was conducted during October '96 to April '97. The main points highlighted are the results on: seeing, contribution of atmospheric turbulence neai• the surface to seeing , extinction and sky brightness, cloud coverage and surface weather data. In addition the key design features of the instruments developed for these observations are presented.Item Systematic Variations of Interstellar linear polarization and growth of dust graint(IUCAA, 2015-02) Voshchinnikov, N.V.; Das, H.K.; Yakovlev, I. S.