S. K. Saha

Toward a revival of stellar intensity interferometry

Building on technological developments over the last 35 years, intensity interferometry now appears a feasible option by which to achieve diffraction-limited imaging over a square-kilometer synthetic aperture. Upcoming Atmospheric Cherenkov Telescope projects will consist of up to 100 telescopes, each with ~100m2 of light gathering area, and distributed over ~1km2. These large facilities will offer thousands of baselines from 50m to more than 1km and an unprecedented (u,v) plane coverage. The revival of interest in Intensity Interferometry has recently led to the formation of a IAU working group. Here we report on various ongoing efforts towards implementing modern Stellar Intensity Interferometry.

SS 433: results of a recent multiwavelength campaign

We conducted a multiwavelength campaign in 2002 September-October, to observe SS 433. We used the Giant Meter Radio Telescope for radio observations, the Physical Research Laboratory Infrared Telescope at Mt Abu for infrared (IR), the ARIES telescope at Nainital for optical photometry, the telescope at the Vainu Bappu observatory for spectral measurements and the Rossi X-ray Timing Explorer for X-ray observations. We find sharp variations in intensity on time-scales of a few minutes in the X-ray, IR and radio wavelengths. Differential photometry in the IR observations clearly indicates significant intrinsic variations on short time-scales of minutes throughout the campaign. Combining the results for these wavelengths, we find a signature of delay of about two days between the IR and radio signals. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broad-band spectrum averaged over the campaign duration.

Development of a Speckle Interferometer and the Measurement of Fried's Parameter (ro) at the Telescope Site

A new optical speckle interferometer for use at the 2.34 meter Vainu Bappu Telescope (VBT), at Vainu Bappu Observatory (VBO), Kavalur, India, has been designed and developed. Provisions have been made for observation both at the prime focus (f/3,25), as well as at the Cassegrain focus (f/13) of the said telescope. The technical details of this sensitive instrument and the design features are described. An interface between the telescope and the afore-mentioned interferometer is made based on a concept of eliminating the formation of eddies due to the hot air entrapment. The performances of this instrument has been tested both at the laboratory, as well as at the Cassegrain end of the telescope. It is being used routinely to observe the speckle-grams of close-binary (separation < 1 arc second) stars. The size of the Frieds parameter, ro, is also measured.

Computationally efficient method for retrieval of atmospherically distorted astronomical images

Speckle Imaging based on triple correlation is a very efficient image reconstruction technique which is used to retrieve Fourier phase information of the object in presence of atmospheric turbulence. We have developed both Direct Bispectrum and Radon transform based Tomographic speckle masking algorithms to retrieve atmospherically distorted astronomical images. The latter is a much computationally efficient technique because it works with one dimensional image projections. Tomographic speckle imaging provides good image recovery like direct bispectrum but with a large improvement in computational time and memory requirements. The algorithms were compared with speckle simulations of aperture masking interferometry with 17 sub-apertures using different objects. The results of the computationally efficient tomographic technique with laboratory and real astronomical speckle images are also discussed.

Single-dish Diffraction-limited Imaging

Turbulence is a state of the flow of a fluid in which apparently random irregularities occur in the instantaneous velocities, often producing major deformations of the flow. In the terrestrial atmosphere, large-scale temperature inhomogeneities caused by non-uniform heating of different portions of the Earth’s surface produce random micro-structures in the spatial distribution of temperature causing the fluctuations in the refractive index of air. The large-scale refractive index inhomogeneities are broken up by non-uniform winds and convection, spreading the scale of the inho-mogeneities to smaller sizes, until thermal molecular dissipation dominates. The disturbance takes the form of distortion of shape of the incoming wavefront and affects the intensity distribution, thereby introducing phase fluctuations. This leads to blurring of the image.

Principles of Interference

The word ‘coherence’ describes the ability of radiation to produce interference phenomena and the notion of coherence is defined by the correlation properties between the various quantities of an optical field. The optical coherence is related to the various forms of the correlations of the random processes (Born and Wolf 1984; Mandel and Wolf and Wolf 1995). The interference phenomena stems from the principle of superposition, which states that the resultant displacement (at a particular point) produced by two or more waves is the vector sum of the displacements produced by each one of the disturbances. It reveals the correlations between light waves. The degree of correlation that exists between the fluctuations in two light waves determines the interference effects arising when the beams are superposed. The correlated fluctuation can be partially or completely coherent

Diluted-aperture Stellar Interferometry

Success in radio interferometry caught the attention of astronomers working at optical and infrared (IR) wavelengths, spanning the whole range from 0.35 to 20 μm. Thus, a long baseline optical interferometer (LBOI) came into existence offering unprecedented resolution. As stated earlier in Sect. 3.2, stellar interfero- metry can be achieved with a single telescope into which light from a distant object can pass through two (or more) apertures in a mask covering the telescope and then combine the two light beams to produce fringes. Another way to achieve stellar interferometry is to use two or more telescopes looking at the same star and reflect-ing light beams into a single receiver.

Introduction to Wave Optics

Light is an electromagnetic wave propagating as a disturbance in the electric and magnetic fields. These fields continually generate each other, as the wave propagates through space and oscillates in time. The Maxwell equations give rise to the wave equation that enumerates the propagation of electromagnetic waves.

Basic Tools and Technical Challenges

Interferometric imaging requires detection of very faint signals and reproduction of interferometric visibilities to high precision; the time resolution of the detectors should reach 1 ms. Developing such an interferometer using Michelson technique is very challenging, which is linked with the advancement of required technology in the areas of opto-mechanics, opto-electronics, and computing. Apart from the design and construction of such an instrument, the physical stability, equalization of pathlength, fringe-tracking, vibration control, adaptive optics, dispersion, and calibration problems are necessary to look into. As stated earlier (see Sect. 5.1) the fringes can be obtained if the optical path difference (OPD) between the light from a stellar source through the two telescopes to the point of interference is smaller than the coherence length, ≤lc. Over an extended field-of-view(FOV), this OPD needs to be continuously measured and corrected before the beam combination takes place. The contrast of the fringes tones down due to:

Applications of Interferometry

In astronomy, interferometry can be traced back to 1868 when Fizeau (1867) proposed to the Academie des Sciences that this technique could be used to measure stellar diameters. This scheme consists of installing a mask with two small openings (apertures) separated in distance at the entrance of a telescope. When pointing at a star, the superposition of the two images in the focal plane, where both beams intersect, creates interference fringe (classical Young’s fringes) across the combined image, and thereby measuring the fringe visibility as a function of aperture separation. He realized that a relationship exists between the aspect of interference fringes and the angular diameter of the light source, in this case a star.