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In Defense of Mapped Color Imaging

Bob Berman recently wrote a column in Astronomy (October 2012) where he says of mapped color imaging, “It’s Disneyland meets Sagittarius.  But is it ethical?”  He admits that “Emission nebulae are always the same repetitious shade of red,” but describes (accurately, I’d allow) Hubble palette images as “fake colors” and “lovely but unreal.”  All fair points.  And it seems that the Astronomy editorial board generally agrees–in the same issue, they show 100 of the “greatest” pictures of the universe.  Less than 20% are mapped color images by my estimate, and nearly all of those are from either ESO or NASA.  Images of emission nebulae from amateurs, with only a couple of exceptions, are limited to true-ish colors.  I take the message to be, “we like our nebulae red, unless you are a professional.  (It’s still okay to turn the saturation knob to 11 for galaxy image, though.)”

The question of ethics in image processing goes back to the beginning of photography, though electronic imaging certainly brings the issue to the fore.  Should we expect an image to exactly represent reality as we would perceive it?  The question comes up most frequently with fashion photography:  when a model’s skin is digitally airbrushed, is that an acceptable improvement or has a line been crossed?  But images have been enhanced for nearly as long as they have existed.  Ansel Adams extensively burned and dodged his prints, not only to increase the perceived dynamic range, but also for purely aesthetic reasons.  These were subjective alterations designed to emphasize the more interesting objects by creating artificial contrast.

The problem with such questions is that they are by definition subjective:  an image is never an accurate rendition of reality.  Cameras and lenses are nothing like our visual system, so photography’s relationship with reality can only be described on a spectrum (pardon the pun) that starts with “slightly inaccurate” and goes to “completely fabricated.”  If we truly limited ourselves to staying close to the way our eye-brain system “sees” things, then nearly everything would be out of bounds.  No wide-angle or telephoto lenses. No exposures longer than a second, so no  images where motion is blurred.  The very fact that we use long exposures in astronomical imaging should disqualify every image, since the human eye can integrate less than a second of photons in total.  Certainly any radio frequency, x-ray, or ultraviolet spectrum images could not be allowed, and at best you could argue that they should be monochromatic.

The point is that images are not reality, but aesthetic representations of reality.

It seems that Mr. Berman’s primary issue is with mapped colors in narrowband images.  I see the point.  Not only are we assigning light from specific emission lines to different colors in the final image, but we also equalize these spectral lines so they are approximately equally bright, when the reality is that one of them (H-alpha) is typically far brighter than the others.  H-alpha, N-II, and S-II are all red.  O-III is blue-green.  That is the extent of our palette for the vast majority of emission objects in the sky, which when rendered as the eye would see it amounts to red.  And visually, they are all too dim to trigger cone cells anyway, so they are all grey unless you can put an eyepiece in the Keck telescope (which would have too narrow of a field, even if you could!).  The diagram below shows how the Hubble palette maps the narrowband emission lines to RGB color.

For me, mapped color narrowband images reflect reality even better than a “true-color” image.  Nearly all of the visual spectrum energy from these objects is emitted in these three or four narrow wavelength slices, each one emitted by a different type of ionized gas.  Emission nebulae are not broad spectrum objects, so to render them the same way we would a galaxy, or for that matter a family photo, seems to fall short of their potential.  We have readily available filters to separate these emission lines, so why would we choose to render them in a way that doesn’t distinguish them in the final image?

I avoid any arbitrary adjustments, additions, or deletions to my images, other than fixing optical defects like dust motes.  (I think Apple’s deletion of M110 from an image of the Andromeda Galaxy for the OS X Lion wallpaper was clearly beyond the pale, but then again, commercial images have different goals.)  I don’t believe that mapping emission lines from ionized gases in a nebula is augmenting reality; it reveals structure that is quite real, but would otherwise be unresolved.  I think it is perfectly acceptable to create contrast in luminance or color that reveals and emphasizes what is actually there.  Just because it’s not visible at the eyepiece doesn’t make it a fake.

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