The following equipment were developed for studies on the ionosphere radio wave propogation characteristics of standard frequency signals in the HF Band at Calcutta; observations were carried out to study (1) regular behaviour of HF standar signals, (2) effect of solar flares on the standard frequency signals in the HF band, (3) effect of solar eclipse on the field strength of HF signals, (4) propagation delay in ionospheric and transionospheric signals, and (5) frequency shift for ionospheric and transionospheric propagation.
An elaborate programme for observing the effects of the total solar eclipse of February 16, 1980, on the field intensity of radio signals and noise was made (Sen et al., 1980, 1981, 1982). Long wave, medium wave, short wave and sweeper were received from Puri, Varanasi and Calcutta. The propagation path mid-point for the transmissions employed were in and around the path of totality of the eclipse.
A ground based filter photometer was developed to measure (i) the density of atomic oxygen at the lower thermosphere, as well as to measure (ii) the temperature of dominant OH level at the mesopause.
An intereferometer has been developed to record the speckles of the bright stars (sim 3rd magnitude without using any intensifier) using a 16 mm movie camera as a detector at the Cassegrain end of the 102 cm telescope, Vainu Bappu Observatory (VBO), Kavalur (Saha et al., 1987). Fringes were also recorded for alpha-Boo and alpha-Lyrae by placing the Fizeau mask (with 2 aperture) over the said telescope at VBO, Kavalur, using the same camera at the Cassegrain focal plane (Saha et al., 1987). By replacing the movie camera with an uncooled intensified CCD camera as detector, a series of specklegrams of several close binaries (< 1 arc sec.) were obtained at the Cassegrain end of the 2.34 meter Vainu Bappu Telescope (VBT), VBO, kavalur (Saha & Venkatakrishnan, 1997; LANL-9910381)
Participated in the developmental programmes of the Grand Interferometric a deux Telescope (GI2T), an optical interferometer with a pair of 1.5 meter spherical telescopes on a North-South baseline (extendable upto 70 meter), which d'Azur, C E R G A., Caussols, France (Mourard et al., 1989); observations carried out with this instrument had resolved the rotating emission envelope of hot star gamma-Cassiopeia.
Developed the new speckle interferometer for the 2.34 meter VBT, Kavalur, (Saha et al., 1999; LANL-9910282); a few optical components were developed at the laboratory as well.
The figure depicts the speckle interferometer at the Cassegrain end of the 2.34 meter VBT, VBO, Kavalur, India (Saha, 1999; LANL=9910506).
Diffraction-limited imaging with large and moderate telescopes.
The figure depicts speckles and fringes obtained in the laboratory. The image was digitized with the PDS 1010M micro-densitometer and processed using the COMTAL image processing system of the VAX 11/780. The clipping technique was used to enhance the contrast in grey levels. The clipped image is superposed on the histogram-equalized original image. These results were corroborated by computer simulations as well (Saha et al., 1988).
Necessary softwares for the 2-D auto-correlation technique, associated softwares to feed the PC based data into IRAF format for analyzing the speckle-grams of the objects using blind iterative de-convolution technique (BID) etc. have been developed.
The figure depicts the 2-dimensional maps of the simulated image of (a) binary star, (b) simulated atmospheric PSF, (c) the convolved functions of these two (bpsf), (d) the retrieved image of the binary star, (e) reconstructed atmospheric PSF; blind iterative deconvolution algorithm has been used to retrieve the computer simulated convolved functions of binary star and the PSF caused by the atmosphere and the telescope (Saha, 1999; LANL-9910506).
The figure depicts the finite element analysis that is used to carry out for arriving at the final configuration and sizes of the speckle interferometer for 2.34 meter VBT, as well as to have prior information about the deflections, stress, flexure etc. of the enclosure; left window shows the deformation pattern for load 1 and right window shows deformation pattern for load 2. The grey-scale blocks in the picture indicate the extent of deformations at individual places in the model of the interferometer (Saha et al., 1999; LANL-9910282).
The figure depicts the autocorrelation of alpha~Andromeda that comprises the width of the seeing disk, as well as the width of the speckle component (Saha et al., 1999; LANL-9910282).
The figure depicts the night time variation of the Fried's parameter, ro during 28-29 March 1991, at the Cassegrain focus of 2.34 m Vainu Bappu Telescope (VBT), at Vainu Bappu Observatory (VBO), Kavalur, India.
The figure depicts the specklegram of a binary star, HR4689; observations were carried out at 2.34 meter VBT, Kavalur, India with the speckle interferometer (Saha, 2000: LANL-0003125).
The figure depicts the result of summing 128 specklegrams of a binary star, HR4689; observations were carried out at 2.34 meter VBT, Kavalur, India with the speckle interferometer (Saha, 2000: LANL-0003125).
The figure depicts the auto-correlation of a close binary star HR5747. The specklegrams were obtained on 16/17 March, 1990. The numbers on the axes denote pixel numbers with pixel being equal to 0.026 arcsec. (Saha, 1999: LANL-9910506).
The figure depicts the deconvolved image of a close binary star HR5138; the reconstructions were carried out with the blind iterative deconvolution algorithm. The specklegrams were obtained on 16/17 March, 1990. The numbers on the axes denote pixel numbers with pixel being equal to 0.026 arcsec. (Saha and Venkatakrishnan, 1997: LANL-9910381).
A speckle interferometer was developed to record the images of the entire planetary disk of Jupiter in the optical band, during the period of 17-22th. July 1994, of when the fragments of comet Shoemaker - Levy 9 (SL-9) slammed into Jupiter, using 1.2 meter telescope at Japal-Rangapur Observatory of Osmania University.
The figure depicts the Jupiter image recorded on 24th. July, 1994 (Saha et al., 1997; LANL-9910510).
The figure depicts the reconstructed image of the Jupiter using blind iterative deconvolution technique July, 1994 (Saha et al., 1997; LANL-9910510).
A solar speckle interferometer was developed to conduct the experiment for observing the solar eclipse of 24 October, 1995 visible as a partial eclipse from Bangalore using Carl-Zeiss 15 cm Cassegrain Schmidt telescope. The figure (fig2c.jpg) depicts schematic diagram of the instrument developed for observing the partial eclipse of the sun (Saha et al., 1997; LANL-9910304).
Participated in the installation, initial testing and observational programmes of the Indo-French project "Fibre-Fed SILFID Spectrograph" at the Cassegrain end of the 2.34 meter VBT, Kavalur as an active member of the said project.