Are Astronomical Images All Faked?

Saturn’s moon Tethys, as seen by the Cassini space probe. Images like this are usually composites, created using light passed through two or three filters. This particular image was created (by Emily Lakdawalla) from the raw data from Cassini’s infrared, green, and ultraviolet filters, so it’s not what your eyes would see if you were in orbit around Saturn.

(I am writing this post in the terminal of the Charlotte, NC airport. We live in The Future, people.)

Images from the Hubble Space Telescope, the Cassini probe in orbit around Saturn, the MESSENGER probe circling Mercury, and from so many other telescopes and spacecraft are some of the most evocative pictures of our time. They reveal a Universe we can’t see without the aid of advanced technology and scientific knowledge. In the era before telescopes, we couldn’t know about the moons of other worlds, about the distances to stars, the existence and properties of other galaxies, or the age of our Universe.

However, these images aren’t what your eyes would see. Most images you see are at the very least “enhanced color” (meaning color contrast is increased to bring out features), but more to the point, the very way modern instruments create pictures is different than the way the human eye does. Cameras aboard spacecraft and on telescopes are based on charge-coupled devices (CCDs), which trigger a small electric current when a photon strikes them. Different colors of course correspond to different wavelengths of light, but the CCD doesn’t distinguish, as long as the light corresponds to the general range of wavelengths where it is sensitive. In other words, pure raw CCD images don’t look like much, so to recreate the colors actually present, multiple images of the same object are taken through filters that select several narrow wavelength ranges. These individual images are still just electric currents; astronomers reconstruct something resembling full color by electronically coloring and combining the images. Pictures from infrared, ultraviolet, and other non-visible wavelengths are also made using analogous processes, though CCDs may or may not be involved (as with X-rays and gamma rays, which would burn right through a CCD!).

The Eagle Nebula, from Kitt Peak Observatory. The beautiful colors are indeed present in the real nebula (though perhaps not quite as strongly!), even though the image here is reconstructed from separate monochromatic images.

While this is a completely standard process, and one which Phil Plait (AKA The Bad Astronomer) has described many times, including this excellent recent post about Titan, some people are a bit disillusioned. From a certain point of view, it may seem like every astronomical image is faked, Photoshopped, deceptive in some way. In fact, I’ve seen people argue that these images are not representative of reality, and must be regarded as artistic at best.

I don’t buy that argument, as you might have guessed already (following the “headlines asking yes-0r-no questions are always answered in the negative” rule). Think of it this way: the human eye contains cells called cones, which trigger nerve impulses when light strikes them. In most people, cones come in three varieties, roughly corresponding to red, green, and blue wavelengths of light; other colors are constructed by the brain by combining signals from multiple cones. Sound familiar?

While cones aren’t CCDs, and the brain isn’t a computer, we should note that perception is relative to ourselves—our physiology and our mental makeup. In a true sense, we are limited in our perception by our cones, and a “true” image must include all wavelengths together, which we simply can’t do. Astronomy makes visible the invisible, and if the process doesn’t replicate what our eyes would see, that doesn’t make its images any less real—and may reveal beauty ordinarily imperceptible to us.

9 responses to “Are Astronomical Images All Faked?”

  1. I think it’s just a part of selling astronomy to the public, who, afterall, pay for it. Compare with the images found in journals: zzzzzzzzzzzzz. Boring! (I actually find them very cool, but I’m sure many don’t). I think the last image I truly loved (and felt the non-astronomical community would too) is an image of the jetted outbursts of SS433 affecting the W50 SNR nebula, I believe it was from an article written by Blundell et al., a group from Oxford University, I think! Nice article tho!

  2. great article. the existence of photomanipulation decreases the authority of ALL printed images, not just astronomical ones. that’s not a problem with the image or even the method of manipulation – as you suggest, it’s a problem with what people consider fake or real. in an age where we understand some things to be plainly invisible to the naked eye and some things to be misleading, we put a suspicious amount of weight in ‘seeing is believing’. I suppose ‘observing is believing’ just isn’t catchy.

  3. The purpose of enhanced-color images can be scientific: you can spot features that might not be visible in less aesthetically pleasing versions. However, raw images can also be striking, like the Hubble Ultra Deep Field image I use for my blog background.

  4. […] Merchandise « Are Astronomical Images All Faked? […]

  5. […] we’re on the topic of out-of-this-world images, Matthew Francis has a thoughtful post that asks “Are Astronomical Images All Faked?” In keeping with Daily Mail tradition of posing questions-to-which-the-answer-is-no in headlines, he […]

  6. […] think so: most astronomical images we see are processed to some degree…and that’s fine! They are not “fakes” because we need technologyto create […]

  7. […] with most astronomical photos, the XDF survey draws together images using filters (in this case 8 separate but overlapping […]

  8. […] this is not a real photo, since the colors in the image aren’t what we would perceive, but I disagree strongly. We use technology as extensions to our senses all the time: eyeglasses, microscopes, magnifying […]

  9. […] the colors of astronomical images are metaphors, since humans can’t perceive X-rays — and telescope cameras don’t “see” light the same way we do either. If we looked through a telescope, we wouldn’t see NGC 2392 as it is in the image, but […]

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