Palomar Testbed Interferometer (PTI). The PTI is used to measure the diameters of stars, the size of the emissions surrounding young stars, and binary star orbits. A network of beam pipes are seen between instrument houses, each containing a small telescope, of which there are three in total. The resolving power of all three telescopes are combined to produce a higher resolution that is the equivalent of a 110-metre telescope. The PTI is located at the Palomar Mountain Observatory, which sits at an elevation of 1867 metres within the Palomar mountain range in California, USA.