How they do it:
The processing of Hubble images
from B&W into stunning full-color
By Mike Barrett
Everyone has seen the dramatic images produced by
the Hubble Telescope from the iconic Pillars of Creation
to the hundreds of galaxies in one shot, but how are
these images created?
The Hubble Telescope captures images in monochrome, just like the old black and white photographs
with no color, but the images we see are stunning vibrant
color pictures. How are these created and what elements determine the color mapping?
To understand the elements that create the images
we need to understand what we see as an image. A typical image is created from a grid of pixels or dots. Each
pixel represents a colour and intensity in the image. These
are grouped together so tightly that the eye cannot see
the individual pixel, but sees a smooth transition from one
pixel to the next.
Basically a color image is created from three separate
components: Red; Green; and Blue. From these prime
colours any other color can be represented by mixing
them in varying proportions. Each pixel of the image has
3 components Red, Green and Blue with a scale of 0 to
32,768 representing the intensity of the color for that pixel.
In this case 0 represents no colour and 32,768 is full color.
So a pixel having an RGB value of 0:0:0 is black and that
having 32,768:32,768:32,768 will be white.
HST WFC3/UVIS images of the galaxy group Stephan’s Quintet in three broad-band
visible-light filters; left: F439W (B), center: F555W (V) and right: F814W (I).
Credit: STScI, OPO, Zolt Levay
Knowing how an image is constructed allows us to
start to understand how a Hubble image is put together.
Hubble has a number of instruments on the telescope,
but the one we shall examine in this article is what is
known as the Wide Field Camera 3 or WFC3. The camera
of WFC3 is capable of recording a much larger spectrum
than the human eye can see. This ranges from ultraviolet
through visible light to near-infrared. The WFC3 camera is
a 16 mega-pixel monochrome camera which produces
a greyscale image. If an unfiltered image is taken it will
include all the spectrum from ultraviolet to near infrared
with each pixel having an intensity value of 0 to 32,768
representing the sum of all the light entering the camera.
As this is greyscale there is just a single value for each
pixel representing the intensity of the data recorded.
Screen image of the FITS Liberator GUI specially developed by the ESA and NASA
for processing Hubble’s images. Credit: STScI, OPO, Zolt Levay
Capturing the entire spectrum is not particularly useful
and the image needs to be restricted to certain wavelengths of light. To restrict the type of data recorded by
the camera there are a series of filters that can be placed
in front of the camera’s sensor to restrict the data recorded to particular wavelengths. To create a true colour
image the camera must take 3 images: one only allowing
red to pass through; one recording only the green light;
and one the blue. These will all be monochrome images,
but can be reconstructed into a normal color image.
Allowing a wide range of the spectrum to pass through
a filter is known as Broadband filtering. A process that
amateur astronomers use to eliminate the effects of light
pollution is known as narrowband filtering. This allows astrophotography to be carried out even from city centres
where the effects of the sodium lighting are filtered out
allowing other light to pass through.
This is a fundamental principle for narrowband imaging
and is based around spectrometry. Spectrometry is the
ability to determine the composition of certain elements
based on the light that is produced. This is of particular
use to astronomy as it allows the study of particular gasses in the universe. Filters have been developed to isolate
the more useful elements and can be used for imaging.
In particular most emission nebula mainly consist of hydrogen and a hydrogen alpha can be used to restrict
only light of the 656 nm wavelength to pass through.
This principle of only letting certain wavelengths of the
spectrum to reach the camera also applies to non-visible
light such as ultraviolet at the blue end of the spectrum
and infrared at the red end. The camera is able to see
and record these wavelengths as differing intensities in