How to identify light of particular long distance object

In our atmosphere, we receive the combined light of all stars, How can we measure light of a particular star or galaxy or quasars? 

Your own eyes can tell one star from another because the light from star A comes to your eye from a different direction than the light from star B. Similarly, a telescope is able to separate the light from star A from that from star B because the light enters the telescope traveling in somewhat different directions.

Since you can distinguish individual stars with a telescope, clearly the atmosphere is not mixing all the light together. So if you can focus light from one star, you can use a spectrometer to measure the spectrum or a photometer to measure the brightness.
If you can't resolve a galaxy into stars, then you can only measure the combined light, true.

Although we receive light from many sources, the light comes from different DIRECTIONS.
If all you had was a flat plate then it would simply receive a uniform illumination from all sources.

But the idea of a LENS and an IMAGE is that light from specific directions is placed at a specific point within the image and that point can be measured.

Whether in a telescope, radio telescope, camera, microscope or even the eye, the same principle applies.
Light entering from different directions is focussed onto different positions on a receiving surface.

by distinguishing it
when you look at a yellow and red object u can distinguish between it
u dont see it as an orange object ...
similarly u can distinguish for such light!