For 25 years, the Hubble Space Telescope has provided the raw material for some of the most iconic images in the history of astronomy.
And for 22 years, Zoltan Levay has led the effort to convert Hubble’s exquisitely detailed data – which reach Earth as black-and-white images – into those stunningly colored icons. Mr. Levay and colleagues take their color cues from Hubble’s suite of onboard filters, which emphasize different wavelengths, or colors, of light – from near-infrared, through visible, and into the ultraviolet range. Scientists apply these filters either individually or in combinations in order to highlight cosmic features for further study. The intent is not to make an image that would accurately represent what the human eye would see looking at the same object. The limits of human vision would hardly capture the amount of scientific information to be gleaned.
Instead, “I think of it as making visible the features that are inherent in the data.Those structures are there in the galaxy,” he says, pointing to a wispy dust lane running up one of the galaxy’s spiral arms as an example. “They’ve been recorded by the cameras, and we want to see them.” Hubble’s cameras essentially are large digital imagers that use arrays of tiny light sensors, or charge-coupled devices (CCDs), that record only variations in the intensity of light.
By the time Levay’s group gets an image, scientists already have corrected it for imperfections that the cameras themselves introduce or that cosmic rays inject, thereby producing a point of light that doesn’t correspond to anything in the telescope’s field of view. For its color palette, Levay’s team relies on the same primary colors – red, green, and blue – long familiar to photographers who work with color images. When Hubble delivers images taken with filters covering visible wavelengths, the image-processing software team assigns colors that map closely to the colors the human eye can see, although contrast
or color intensity may need tweaking to bring out details that might otherwise remain hidden.
If the filters used to make the image fall within ultraviolet or near-infrared wavelengths – in other words, outside the range of human vision – the team maps the spectrum humans can see to the range of wavelengths covered by the filters. For instance, red is assigned to the longest wavelengths while blues and violets are assigned to the shortest. Colleagues at the European Space Agency, NASA’s international partner on Hubble, perform similar roles for images that European scientists gather. The approach has become increasingly common in amateur astronomy, too, as CCD cameras and processing software have become more affordable. Amateurs can now replace their telescope’s eyepiece with either dedicated astronomical CCD cameras or off-the-shelf digital cameras.
Such advances in astronomical imaging have resulted in a “humongous paradigm shift,” Levay explains. An observation process that once involved looking through an eyepiece and hand-sketching what the viewer sees has evolved to taking photographs on specially formulated glass plates, which essentially “allows you to make a record of the brightness of light,” he says. In some ways digital technology is not conceptually different from its
photographic predecessors, Levay adds, although it has specific advantages. “But you’re still recording light,” he says, a technological shift so significant it is on the scale of Galileo first “turning a telescope onto the night sky.”