The improvements of the optical delay lines in stellar interferometry

After being collected by the telescopes,the light must be directed to a central facility for beam combination.While it may seem trivial to set up a series of mirrors for this purpose,there are many subtle issues that must be addressed.Because of the Earth's rotation,the apparent position of an astronomical object is constantly changing.In order to track this sidereal motion,a movable delay line is needed to compensate for changing geometrical delay between wavefronts reaching any two telescopes.