Laser beams and the cosmos

Sometimes what can be done with technology just blows you away and this is perpetually the case with the technologies developed and used in Astronomy.

The GALACSI sodium laser guide stars in operation at the VLT on Cerro Paranal (Image Credit: Ruari Mackenzie).

I was recently working at the Very Large Telescope (the VLT for short — and always to be followed with some comment on the lack of imagination/sense of humour of astronomers — the next one’s going to be the Extremely Large Telescope) using a new-ish instrument called MUSE and a brand new add-on called GALACSI.

The four VLT Unit Telescopes: Untu, Kueyen, Yepen and Melipal (L-R).

The VLT is actually four separate telescopes: Untu (UT1), Kueyen (UT2), Melipal (UT3), and Yepun (UT4). MUSE is mounted on Yepun, which translates as Evening Star or Venus in the language of the Mapuche (an indigenous people of Chile). MUSE in and of itself is pretty amazing. An Integral Field Spectrometer (IFS), it simultaneously takes about 6000 images across a range of wavelengths of light from the optical into the infrared. This gives an vast amount of information and data on 100s of galaxies at a time over a patch of sky slightly bigger than Jupiter’s size on the sky.

Clipped on to this recently, like some sort of death ray add-on, has been the laser-based Adaptive Optics module, GALACSI. GALACSI fires out four bright orange-hued laser beams (sodium Laser Guide-Stars) from within the VLT enclosure towards the night sky. The clever bits follow, with cameras (or more accurately Wave Front Sensors consisting of 1600 individual apertures each) tracking the tiny variations in the beams on the sky caused by turbulence in the atmosphere.

To put it bluntly, the atmosphere is a complete pain in the arse for astronomers across a range of wavelengths as it disrupts, disturbs and absorbs astronomical light. What the lasers plus wave-front sensors do, is to measure this disturbance and send the information to the next fancy bit of tech, a fast computational computer named SPARTA hooked up to a deformable mirror (known as the DSM). SPARTA and the DSM effectively invert the signal they receive from the wave-front sensors and transform the astronomical light coming into the detector to ‘undo’ at least some of the mess the atmosphere made of it.

And all this happens at 1000 Hz. That’s 1000 times per second.

And that blows my mind. This is a 1 metre diameter mirror, with over a thousand mini-actuators, each positioning itself on scales of a thousandth of a millimetre every thousandth of a second. The effort and financial risks to get to the point of this technology working and operational on one of the leading telescope facilities in the world are phenomenal and quite breathtaking.

And I’m very much looking forward to getting to use it as much as I can get away with.

Sunset from Cerro Paranal.
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