They Really Do Fire Lasers Into The Sky. It Makes One Dot.

CERRO PARANAL, Chile — There is a layer of sodium atoms about ninety kilometres up, left behind by meteors. Fire a laser tuned to exactly the right yellow at it and the atoms glow.

You get a dot. An artificial star, on demand, wherever you point.

Observatories do this nightly, on a published schedule, and the beam is visible from a public road. We drove up and stood under it.

What it is for

Not projection. Correction.

Air moves, and moving air smears a star into a blob. Adaptive optics measures the smear hundreds of times a second and bends a mirror to cancel it — but you need a bright star near whatever you are looking at to measure against, and often there isn't one.

So they make one.

Dr. Consuelo Ferreira-Amankwah, who runs it, described the moment she first understood the trick as "the single most outrageous thing I have been told in a lecture and it turned out to be true and my job."

From a dot to a picture

We asked her what it would take to make an image, and she worked the arithmetic on a whiteboard over two nights. It is published as PX-2053.

The chain breaks in four places, and only the first two are engineering.

Brightness. The guide star is roughly as bright as a faint star — visible to a telescope, not to you. Making it obviously visible to the naked eye is a large power problem and power scales badly.

Spot count. A crude image needs thousands of spots. Each spot is a beam, or a beam steered fast enough to persist. Both routes multiply the power problem by the number of spots.

Daylight. The sky is already brighter than anything you can put on it. This is not a technology limit; it is a contrast limit, and no amount of laser fixes a background.

The one that is not an engineering problem

The fourth is the one our correspondent went to Chile for and had not properly understood until she was standing under it.

The dot is at a fixed point in space, ninety kilometres up. Two people in two cities two hundred kilometres apart are looking at the same object from different angles — so they see it in different places against the stars, at different elevations, and one of them may not see it at all.

For an image to appear identically to two separated observers, it has to be at each of them. That is what a screen does and what the sky cannot.

"You would need the picture to be near each person," she said. "At which point you have not built a sky projector. You have built a very expensive television, one per customer, and we already have those."

The people in the car park

Two visitors on the public tour were asked why they had come. Both had read about this facility on sites that describe it as concealed.

One had driven eleven hours.

"It is on a map," he said, cheerfully, and without any apparent sense that this undercut anything. "They give you a badge."

He then asked our correspondent the best question anybody asked her all week, which she has not stopped thinking about: if the thing itself is public, the schedule is public, the tours are free and the beam is visible from the road — what exactly is the secret supposed to consist of?

She did not have an answer and does not have one now.