Friday, May 19, 2017

Lies, Damned Lies, and Astronomy Photos

I'm sure you've all see beautiful images like this NASA photo of the Crab Nebula:

Who you gonna believe, me, or your own eyes?

Just imagine if you looked at the Crab Nebula through a powerful telescope -- you'd see something that looked... nothing at all like this. That's because the colors in astronomy photos are almost never the "true" colors that you would see with the naked eye. They're "false color" photographs, with different colors assigned to different wavelengths of light or (in this case) to images derived from different telescopes.

When I first realized that false color photos were the standard in astronomy, I was dismayed. What else had NASA been fibbing about? The Moon landings? But NASA hasn't been lying at all, and there's no reason to get upset about false color photos. And here's the reason why: your own eyes give you a false-color picture of reality.

Take a step back and think about your sense of hearing.  Sound consists of vibrations of air at a bunch of different frequencies, and your ears are sensitive to each individual frequency. When you listen to music, your ears can distinguish each note as it's combined with the others.

But your eyes work in a completely different way. If you have normal color vision, you've got three different color receptors in your retina, and each one is activated to a greater or lesser extent by every frequency of light. Your brain then combines the "signal strength" from each of the three receptors into a single color. So your eyes grossly simplify the true information contained in the light.  Here's the spectrum of light reflected from a leaf:
If your eyes worked like your ears, you'd be able to pick out all of the details in this spectrum.  But your eyes don't work that way -- all you see is "green"! This is the basis of all of the color mixing rules we learned in school. Every kindergarten student knows that yellow and red paint mixed together make orange. But if your eyes were capable of perfect frequency detection, a mixture of red and yellow would look completely different from orange.

Which leads to an interesting question: why haven't any animals developed the ability to detect the frequencies in light directly? I'm no biologist, but it seems difficult to come up with a biological system that would do the job. Also, there probably isn't much survival value in it -- most mammals don't even have color vision. But I suppose it would open up whole new vistas in art -- or in astronomy.


Kathy said...

Back in high school, I had an acquaintance who owned a small telescope. If you saw a nebula through it, it was pretty colorless. But he could take extended time exposures and some of the colors did appear in the developed negatives (No way that can work with standard digital cameras today).

I also around that time read an explanation from Arthur C. Clarke saying the colors are real, but too faint for the human eye to make out. Ergo the extended exposure on film.

Robert Scherrer said...

And these days, all professional optical astronomy is done with CCDs -- so no one ever looks through a telescope. I wonder if some astronomers regret losing the romance of the old days. Maybe not -- it must have been tedious and uncomfortable.