How to find the source of the light? There are millions of galaxies out there in the entire universe. When a telescope is set to a direction to to identify the stellar objects, it can capture all lights from all the objects. How do we identity that the particular light is emitted from one star/galaxy and not the other one?

Even if we precisely point to one star, there are chances that other luminous objects interfere with the light ray.
The positions of many galaxies are recorded, so astronomers can point their telescopes to the exact point in the sky that they wish to observe. In addition,the relatively nearby galaxies can be recognised by their appearance,&amp; these are the galaxies most intensively studied. Professional astronomers have memorised most of the stars you will see in the night sky, and the constellations (Sagittarius, Ursa Major, Orion etc) guide them to their target. Those galaxies near the limit of observation are of lttle interest except when something unusual &amp; very luminous happens within them,such as a supernova or gamma ray burst.