M.Sc Thesis

M.Sc StudentShoulga Georgiy
SubjectTowards Spectral Intensity Interferometry in Space
DepartmentDepartment of Physics
Supervisors DR. Erez Ribak
PROF. Pinchas Gurfil
Full Thesis textFull thesis text - English Version


The highly detailed pictures of galaxies, nebulas and interstellar gas clouds taken by space telescopes like Hubble Space Telescope, Swift, Spitzer and others are not only beautiful, but also provide scientists with a huge amount of information about the detailed structure of such objects. However, these objects are very large, compared to the size of stars, and it is not surprising that we have a very short list of detailed images of stars nowadays. Detailed stellar images can provide information about their structure and above all, their physics: stellar winds, accretion discs (in case of binary stars), photospheres, spin velocities (hence elliptical shape), convection zones and flares.

During the1930s only about six stellar diameters were measured in the visible band by a conventional technique - amplitude interferometry. This method uses at least two telescopes and has many disadvantages: high precision, in the order of the wavelength, on the distance between the telescopes as well as a strong dependence on the phase disturbances - atmospheric turbulence, non-ideal optical components, gravity and winds. Despite these severe limitations, recent interferometers with more telescopes and can reach finer details, mostly of brighter objects. Efforts to construct a space interferometer were stymied because of the accuracy requirements.

In the late 1950s Hanbury Brown and Twiss suggested a new technique for measuring and imaging stellar objects - intensity interferometry. This method is almost independent of phase disturbance effects and allows much lower accuracy on the distance between the telescopes and their optical quality, and is immune to atmospheric disturbances. On the other hand, it requires longer observations and faster data processing, not available at the time.

This research deals with a first step of constructing a first space-based intensity interferometer, which should be much easier to deploy. We constructed in the lab an interferometer system, holding three telescopes, as well as artificial stars. We measured successfully diameters of single stars and binary stars’ spacing using two telescopes. In addition, we recovered the phase of a triple star asterism using all three telescopes and obtained a complete image of the source.

Since intensity interferometry measures a correlation of photons’ arrival times at each of the telescopes, we also performed a study of photon statistics. In the case where photons’ coherence time was shorter than the time interval bin, we obtained a Bose-Einstein distribution. In the opposite case, where coherence time was longer than the bin interval, we obtained a Poisson distribution, and in the case where the two times were on the same order - a mixture of these two distributions.

In addition, we proposed an improvement to the intensity interferometry technique, which was modeled and simulated in an optical design software. The improvement arises from dispersion of the incoming light into multiple spectral bands, allowing us to correlate only between photons of the same wavelength, improving signal-to-noise ratio, and hence the observation and processing times.