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.

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    $\begingroup$ Are you asking how we distinguish objects along the same line of sight, or how telescopes have directional selectivity? $\endgroup$ – Mike G Jun 15 '19 at 19:05
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    $\begingroup$ An example: light pours into your eyes from all directions in front of you. How do you identify the light from individual objects within that 180° visual field? Or: If it's night and a car shines its headlights towards you from a half-mile away, you can see two points of light - but why doesn't the light get interfered with by the street lights? $\endgroup$ – Chappo Hasn't Forgotten Monica Jun 16 '19 at 0:57
  • $\begingroup$ If you make a photo from your home, how do you see on the photo, where the photons arrived from? $\endgroup$ – peterh - Reinstate Monica Jun 16 '19 at 19:31
  • $\begingroup$ Convention requires that someone mention Olber's Paradox $\endgroup$ – user21 Jun 24 '19 at 2:48
  • $\begingroup$ Could you please clarify what you are asking. Do you want to know how telescopes work? Do you want to know how to find certain stars/galaxies? Or do you want to know how the optics of telescopes works? $\endgroup$ – jan.sende Jun 28 '19 at 7:24

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,& 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 & very luminous happens within them,such as a supernova or gamma ray burst.

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