An eye to the sky

The Postdoc Perspective was a blog for the Physics and Astronomy Department at McMaster University in Canada that I kept while I was a postdoctoral researcher. Many of the topics were talks presented at the McMaster Origins Institute seminar series.  

Altitude sickness, the safety liability waver form told us, was unlikely to be severe below 3000 m. At 2715 m, the location of the Gemini South telescope in the Chilean Andes should be fine for most visitors but if it wasn't, we were commanded to mention it to observatory personnel. The two hour drive down the steeply descending narrow dirt track from the mountain top was not the place to make mistakes.

When the observatory heard our plans to accompany Gemini Fellow, Dr Michelle Edwards, on a tour of the telescope, they suggested this could be counted as an official visit since many of our group were astronomers. Such listing would enable us to stay on the mountain for longer if we wished. Michelle explained that three non-astronomers were also attached to our party but it wasn't until she added that a theorist was also present did Gemini write the whole idea off and give us tourist passes.

Set on the foothills of the Andes and backing onto one of the driest deserts on Earth, Gemini's position in Chile is an ideal location for astronomers to study the southern sky. The domes of three telescopes could be seen as we ascended the mountain. The 4.1m Southern Astrophysical Research Telescope (SOAR), the Blanco 4m telescope and our destination, the 8m Gemini South.

Half-way up the mountain is a look out point with three slender metal tubes mounted on the stone wall. These resembled smaller versions of the instruments we were going to see but in fact proved to contain no magnifying lenses and were just used to guide your eye to the appropriate glittering silver hemisphere. Strangely, one was pointing at an entirely empty space which turned out to be the planned site for a new telescope, the Large Synoptic Survey Telescope (LSST).

Gemini, as its name suggests, is one of two identical telescopes both with primary mirrors that are 8m in diameter. Its twin sits on the dormant volcano, Mauna Kea, in Hawaii where it points towards the northern sky. Currently, the governments of the USA, Canada, UK, Chile, Argentina, Brazil and Australia share Gemini's operational costs to enable their astronomers to observe at the two sites.

It was swelteringly hot when we reach the mountain top, but the inside of the Gemini dome is cool. It is important, Michelle explains, to keep the inside temperature to approximately what it will be at night when the dome is rolled back to expose the telescope to the night sky. If this isn't done, the hot air rises out of the dome and over the aperture to create a phenomenon known as 'dome seeing'; the distortion of an image from turbulence due to the hot air rising. This turns the telescope's sharp view of the stars into fuzzy blobs, a waste of the excellent view the observatory has from the mountain top.

The huge mirrored dish of the telescope is supported high above us as we stand at the telescope's base. Its silvered surface is refreshed every five years by a large saucer-shaped device that is stored in the observatory's basement. The story goes that when this equipment was delivered to a port in Chile, the locals believed it to be a discovered UFO.

Climbing up the blue frame work, it was possible to peek under the mirror cover and see the surface that would, in another 8 hours, be pointed at the heavens. This giant star-studying cyclops made my eyes seem weedy by comparison.

Over to the far right was Gemini's newest tool; GeMS, a five beam laser guide star system. Stars 'twinkle' because the Earth's atmosphere is distorting the star light, an effect that is much worse for an 8m telescope than for your 1inch eyeball. To compensate for this, a system called 'adaptive optics' was developed that allows the telescope's mirror to deform depending on the atmospheric conditions to produce the best possible image. These adjustments can occur at a rate of 100s of times per minute. To measure what changes the mirror should make, most telescopes use 'guide stars'. These are stars of a known brightness that show minimal variations and can be used to calibrate the system. Gemini's new instrument goes one better than this; by firing lasers up at the sky, they can build an extremely accurate picture of how the atmosphere is distorting the light over a very wide area. While four guide stars are still needed, they can be much fainter objects and an area of the sky 2-3 times that of traditional adaptive optics systems can be 'corrected' for in a single measurement. This allows for far sharper images, especially in crowded areas of the sky --for instance near the galactic centre-- where locating a reliable, bright guide stars is difficult.

Of course, shooting light up into the sky does come with some associated risk to other, non-celestial, objects that might be in its path. If care is not taken, the laser beam could blind either an aircraft or a spy satellite.

The latter, ladies and gentlemen, is an act of war.

The average observing run tries to avoid such inconveniences by co-ordinating the times they wish to use the laser system with US space command (totally didn't make that name up). They then receive a list of times they may not use the laser system in particular directions.

Avoiding aircraft is a less sophisticated business. Although in theory, the flight paths and times of planes are known, this information is not accurate enough to be used. Instead, 'spotters' are employed to stand in regions close to the telescopes with a device like a hula hoop with which they can estimate the distance to the plane and whether the laser system will pose a risk. When an aircraft approaches, the spotters can contact the observatory and ensure the lasers are not being used.

This ... advanced ... aircraft detection technique that is apparently used by million dollar telescopes all over the world seemed so completely implausible that I demanded its verification from both Michelle and two other observers who were in our group. Their stories were consistent. I decided no observer was ever in a position to criticize an approximation I made in my simulations ever again.


(Left image is the Gemini dome, top right is SOAR and the bottom right is the view over the mountains. Many thanks to Michelle for taking the time to proof-read this piece!)