Observing the Sun with micro-interferometric devices: a didactic experiment

Measuring the angular diameter of celestial bodies has long been the main purpose of stellar interferometry and was its historical motivation. Nowadays, stellar interferometry is widely used for various other scientific purposes that require very high angular resolution measurements. In terms of spatial scales probed, observing distant stars located at 10 to 100 pc away with a large hectometric interferometer is equivalent to observing our Sun with a micrometric baseline. Based on this idea, we have manufactured a set of micro-interferometric devices and tested them on the sky. The micro-interferometers consist in a chrome layer deposited on a glass plate that has been drilled by laser lithography to produce micron-sized holes with configurations corresponding to proposed interferometer projects such as CARLINA, ELSA, KEOPS, and OVLA. In this paper, we describe the interferometric devices and present interferometric observations of the Sun made in the framework of Astrophysics lectures being taught at the Liege University. By means of a simple photographic camera placed behind a micro-interferometric device, we observed the Sun and derived its angular size. This experiment provides a very didactic way to easily obtain fringe patterns similar to those that will be obtained with future large imaging arrays. A program written in c++ also allows to reproduce the various point spread functions and fringe patterns observed with the micro-interferometric devices for different types of sources, including the Sun. The principle The Sun has an angular diameter ranging between 0.524 and 0.542 degree, depending on the Sun-Earth distance. In order to resolve it in the middle of the visible wavelength range (i.e., 555.5 nm), a single monolithic pupil should have a linear diameter of approximately 70 μm.