Dodging powerful laser beams, a drone captured this stunning aerial view. The confrontation took place above the 8.2 meter diameter Very Large Telescopes of the Paranal Observatory on planet Earth. Firing during a test of the observatory’s 4 Laser Guide Star Facility, the lasers are ultimately battling against the blurring effect of atmospheric turbulence by creating artificial guide stars. The guide stars are actually emission from laser excited sodium atoms at high altitudes within the telescopic field of view. Guide star image fluctuations are used in real-time to correct for atmospheric blurring by controlling a deformable mirror in the telescope’s optical path. Known as adaptive optics, the technique can produce images at the diffraction limit of the telescope. That’s the same sharpness you would get if the telescope were in space.

Had the drone been hit by the laser, the worst that would happen is the camera would have been blinded…

One of the things that made the use of Laser guide stars so challenging was the lack of lasers that could reach the exact wavelength needed- in the early 2000′s there simply was no easy way to generate 10′s of Watts of laser power at 589nm.

The first observatory systems had to use dye lasers – dye lasers are very ugly beasts.

A dye molecule was synthesized that – when ‘pumped’ by intense laser light at 532nm – would fluoresce over a broad range of wavelengths around 589nm. The dye in our case was dissolved into ~50 gallons of pure ethanol.

This flammable mixture went to 3 laser subsystems: Master oscillator, Pre-amplifier, and Amplifier.

The master oscillator generates a very low power of light locked to the sodium transition at 589nm (this is the seed light that gets amplified)- the dye is happy to lase at any of many wavelengths around this wavelength, so there is a complex setup of etalons, sodium vapor cell, and a feedback loop to ensure it only lased at the exact wavelength we needed. The dye was excited by a Neodymium YAG laser, that was frequency doubled by a non-linear crystal to output ~30 Watts of green 532nm light.

The seed light was transported to the laser room bolted to the side of the telescope where the pre-amplifier and amplifier were housed.

The pre-amplifier is a glass cell, the dye is flowing through this glass cell at a high rate. The pulses of seed light from the master oscillator pass through this cell of flowing dye. Additionally,intense pulses of green light from more Neodymium YAG lasers- precisely timed to overlap the pulse of seed light- were focused into the same volume of dye that the seed is passing through. The green light excites the dye, and the seed light stimulates the dye molecules to emit at the same wavelength of 589nm.

The pre-amp boosted the power of the 589nm light to ~100mW – this light is then sent to the final amplifier where the same process was repeated to boost it to ~10 Watts.

this light was then transmitted out to create the laser guide star.

Technology has advanced greatly since then, and the dye laser is no longer used. Its now much easier to achieve lasers at 589nm… The first Dye laser based sodium guide star was fielded at the Lick observatory, and the second was at Keck- both were built by a top-notch team of physicists and engineers at Lawrence Livermore National Lab.

Now, you can buy one that uses far friendlier and efficient technology from commercial vendors (IF you have the money)

As the beam goes up through the atmosphere the air becomes thinner, and at 25km or so is not visible… (as seen by the telescope)

At around 85km there is a layer of sodium atoms about 10km thick… these sodium atoms are deposited constantly by meteors burning up in the upper atmosphere. And they are removed when they combine with the rarified water molecules up there. At any time there are a few kilograms of sodium atoms in this layer around the earth.

The laser is precisely tuned to the sodium D2 Transition at 589nm (yellowish orange). The sodium atoms absorb these photons and then re-emit them. This creates a column of glowing atoms where the beam intersects the sodium layer. When viewed end on by the telescope this column appears as a 10th magnitude yellow star. By looking at how the light from this ‘star’ is distorted by the atmosphere, you can use deformable mirrors to reverse the effect of turbulence… this is called Adaptive optics. You can use this to achieve near the diffraction limit…

You need a bright star to do Adaptive optics, but there is usually not one near what you want to view. The laser lets you create one exactly where you need it.

I will try to post more later…

My old laser has long since been replaced by a far more efficient solid state laser as technology has rapidly advanced.

The beam would be aimed at a point somewhere in the field of view of the telescope, close to the region of interest, since the amount of correction falls off as you look further away from the guide star. For this reason some observatories transmit a ‘constellation’ of guide stars, 4 beams that provide references across the field of view.