How
Heritage Images are
made from HST Data
The Hubble
Heritage Project team sees the Hubble Space
Telescope, in addition to being a research instrument,
as a tool for extending human vision. The detectors
on this space observatory are much more sensitive
than the human eye. For example, they capture radiation
with energies in the electromagnetic
(EM) spectrum beyond the human visual range,
as well as light that is fainter than we can see.
The challenge is to convert these remarkably extended
data into visual images that convey the knowledge
they've captured.
The detectors measure how the radiation from the
sky varies in lightness and darkness and so render
only black and white images. However particular
energies of radiation can be selected, before it
reaches the detector, by inserting filters which
pass only specific
wavelengths of light; these filters work like
colored glass. Sometimes a set of 3 filters are
used which approximate the same wavelength range
of the EM spectrum as covered by the human eye.
Combining black and white images from these filters
generate natural
color images. The structures in this kind of
image are similar to those we would see with our
eyes if we could traveled to the object of study.
Since there is a lot more than meets the eye in
our astronomical subjects, data are usually collected
using filters which isolate radiation emitted during
astrophysical processes. Exposures made through
these filters cannot be made into natural color
images and they need to be assigned to the human
visual range. If we retain the wavelength direction,
as in this figure, then the ordering of filters
is referred to as chromatic. If we select
other orderings, also in the figure, the order is
referred to as composite. Since this color
assignment emphasizes subtle structures, as well
as delicate light effects, it often takes precedence
in Heritage images.
In the final production of these images we also
attempt to reveal detail usually hidden to the human
eye. To do so we use methods that are either similar
to astronomical techniques for studying observations
or those that are standard in photography (for example,
the digital equivalent of dodging and burning).
When using these techniques we do our best to avoid
generating features in the image that were not originally
in the data.
Each image is a vision produced by the collaboration
of the Hubble Heritage team rather than the expression
of a single individual. Our approach is described
in a bit more detail at the Hubble
Heritage Project page.
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