Chandra Sketches: The Nuts and Bolts of Astronomy -
Coloring the Universe - Part 2

Narrator (April Hobart, CXC): Last time, we looked at the electromagnetic spectrum. This time we'll look at how astronomical images are made.  

Many images of space don’t start off looking like this: 
They look a bit more like this:  
Or really, more like this:

It's hard to make sense of that (unless you're a computer)!

When a satellite observes an object in space, its camera records photons – packets of energy that make up electromagnetic radiation –  which then come down to Earth from the spacecraft via NASA’s Deep Space network coded in the form of 1's and 0's.  

Scientific software translates that data into a table showing time, energy and position of each photon that struck the detector during the observation.  

The data is further processed with software to form the visual representation of the object.  
(Notice it's still black and white.)

We assign a color to a slice of the data based on energy level, elements present, or topographical feature.

For example, making a 3-color image: the Pillars of Creation (M16)

Red: light from sulfur atoms
Green: light from hydrogen atoms
Blue: light from oxygen atoms

RED + GREEN + BLUE = Composite Image

You can do the same with more colors, and with many kinds of light.   

In this image of a supernova remnant, G292, an exploded star about 20,000 light years away, 5 colors were chosen to emphasize emission from particular elements:
oxygen (yellow and orange)
neon (red)
magnesium (green)
silicon and sulfur (blue)

So, how a telescope "sees" is very different from how our eyes see.  Telescopes give us super-human vision.  In most cases they make the invisible visible.