E Pluribus

Mr Feder’s a determined fault‑finder. “That gold on James Webb Space Telescope‘s mirror — it’s gonna make all its pictures look funny, yellow‑like instead of whatever the real colors are.”

Cathleen bristles a little. “We astronomers have built our science on recognizing. accounting for and overcoming instrument limitations. Hubble, for instance, went up with a mirror that had been misground so its resolution was a factor of 10 worse than it was supposed to be. It took three years for NASA to install corrective optics. In the meantime we devised a whole catalog of math and computer techniques for pulling usable data out of the mess. Anyway, JWST‘s not designed to make pretty pictures.”

“I thought it was gonna replace Hubble. If it can’t take pictures, what’re we putting it up there for?”

“It’s a successor, not a replacement. JWST is designed to answer a completely different set of questions from the ones that Hubble has been used for. I’m sure we’ll keep using Hubble for as long as it continues to operate. By the way, the Hubble pictures you’ve seen aren’t what Hubble took.”

“Bunk! I’ve seen Hubble shots of the Moon and they look just like what I see through my binocs. Same colors and everything.”

“Not much color in the Moon, Mr Feder. Just different grays except for during a lunar eclipse.”

“That’s true, Al, but the resemblance is no accident. All major telescopes including Hubble, gray‑scale is all they do. Professional and amateur scientists help out by combining and coloring those gray‑scale images.”

“Wait, how do they combine images? Back in the film days I’d forget to wind forward after taking a picture and the double exposures were always a mess.”

“Film and digital are very different technologies, Mr Feder. The sensors in your camera’s film were microscopic silver halide crystals embedded in the coating. Each photon that reached a crystal transformed one silver ion to elemental silver and darkened the image there just a bit. More photons in a particular area, more darkening. There’s no reset, so when you clicked twice on a frame the new darkening supplemented what was already there. Those silver atoms and their location on the film encoded the photos you took.”

<with a sneer> “Wooo — encoded! What’d the processing labs do, count the atoms?”

“In an analog sort of way. Your lab made positive prints by shining light through your negatives onto photosensitive paper that worked the same way as the film. Shadow from the negative’s dark silver atoms prevented silver ion darkening in the corresponding part of the paper. What was bright in the original scene came out bright in the print. And viceversa.”

“But I was taking color photos.”

“Same analog scheme but with fancier chemistry. Your color film had three photosensitive layers. Each layer was designed to record a different set of wavelengths, red, green or blue. Blue photons would darken the bluesensitive layer and so on. From then on the encoding and decoding logic worked the same, color by separate color. Your eyes combine the colors. JWST‘s cameras don’t do any of that.”

“I guess not, it being a million miles away from processing labs.”

“Right, we can only work with numbers that can be transmitted back to Earth. Modern telescopes use digital sensors, dense grids of transistorsize devices that literally count the photons that strike them. Graph how many photons hit each part of the grid during an interval and you’ve got a picture. Better yet, you can do arithmetic on the counts. That opens up a world of analytical and pictorial opportunities that were tedious or impossible with photographic data.” <opens laptop, taps keys> “Here’s a lovely example I recently received from NASA’s Astronomy Picture of the Day service. Gorgeous, hm?”

Symbiotic R Aquarii” — Image Credit: Optical (red, blue): NASA/ESA/STScI;
X-ray (purple): NASA/CXC/SAO/R. Montez et al.;
Processing: Judy Schmidt (CC BY-NC-SA)


“Image arithmetic in action. That’s two stars in weird orbits around each other. Ms Schmidt combined two Hubble images with one from Chandra, a separate telescope looking at a different part of the spectrum. Old‑style astrophotography couldn’t do that.”

~~ Rich Olcott

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