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Hubble Heritage Information Center

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.

Mapping Selected Radiation Onto the Visual Range

Illustration Credit: Stuart Robbins (CWRU)
Illustration Concept Jayanne English (U. Manitoba)

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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