While many pictures from the Hubble Space Telescope are familiar, less common is an understanding that the telescope does not work exactly like a snapshot camera or a video camera. (There are similarities, however, particularly with a digital still camera.)
Do these pictures look like what we could see with our eyes? The (somewhat glib) answer is, yes and no. The pictures are usually quite different from what we would see, mostly because the telescope and its cameras work quite differently from our eyes.
We cannot see very faint light, though we can distinguish a very wide range of brightnesses in the same scene. We cannot see very dim light in color. HST, by comparison, can detect very faint light, and some light (infrared and ultraviolet) that our eyes cannot see at all.
We reconstruct color photos from black and white digital images returned from the telescope.
"Raw" data.
Two exposures with the telescope pointed at precisely the same spot in
the sky. All HST exposures include "cosmic rays" (the result
of energetic particles in space causing an exposure recorded by the cameras).
CRs are distributed randomly but the target image does not change.
This object here is a planetary nebula called NGC 6369.
The individual exposures
are combined to remove the cosmic rays by comparing what changed and what
did not between the exposures.
(If something did move in the target between exposures, that would be
removed too; in that case we would need to be a bit more sophisticated.)
Hubble's cameras produce
grayscale (black & white) images. The cameras include color filters,
each of which transmits a selected range of color (blocking the other
colors). Separate exposures are made through different color filters.
These images are each combined from multiple exposures, as above.
(These particular images were made using "narrow-band" filters,
which transmit a very small range of wavelengths, essentially a single
color, matched to the color of emission from particular elements: hydrogen,
nitrogen, and oxygen, in this case.)
Color (hue) is applied separately to each gray image, as appropriate (red for longer wavelength, blue for shorter).
The assigned colors do not necessarily match the colors transmitted by the filters. Using the additive primaries (red, green and blue) results in the broadest range of displayed colors.
All of the exposures and filters are combined to produce the color composite.
Minor image/detector defects (artifacts) are removed.
Below are three versions of the image that have been combined to produce the above color composite. These were processed differently to preserve image detail and improve the contrast separately in the brightest and the faintest parts of the image. These separately processed datasets were combined. This is very similar to how a photographer would "dodge" and "burn" in producing a print from a negative to preserve details in highlights and shadows. (Similar to the examples above, but these are the color composites rather than just a single fileter exposure.)
