The Trials of G-CLEF (or So You Want to Build a Giant Observatory)

We are mere days away from first light on the Vera C. Rubin Observatory, the next great Earth-based telescope! It’s happening on June 23! It’s been a long time coming and it could open up entire new realms for astronomy. But have you ever wondered what goes into making a facility of this sort?

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The answer is a whole heckuva lot. To give you an idea, I’m going to dive into some of the things that need to happen to design, build, and install a single instrument on such an observatory, quite aside from the construction of the telescope itself. In this case I will be focusing on G-CLEF, the GMT-Consortium Large Earth Finder which will be installed on the Giant Magellan Telescope (GMT) scheduled for first light in the early 2030s.

I recently got to take a tour of the facility at the Smithsonian Astrophysical Observatory where this instrument is being put together, and the things that Dr. Andrew Szentgyorgyi and his team, who are building G-CLEF, had to think about that while designing it absolutely blew my mind (a big thanks to Dr. Szentgyorgyi for his help in writing this post!).

Design Your Instrument

Okay, so you want to put an instrument onto a large observatory. Congratulations! First you have to design the instrument. G-CLEF, specifically, is going to use the spectra of stars to hunt for Earth-sized planets, to search for signs of bioactivity outside the solar system, and to look for evidence of some of the earliest structures in the universe (no big deal).

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Having figured out what your science goals are, you next figure out what physical evidence you will need to achieve your science goals. For instance, to find an Earth-sized planet you will want to be able to do mass measurements of exoplanets. Then you figure out what parameters you need to measure (mass, in this case) to reach that objective. Then you figure out what kind of instrument is needed to measure that parameter, and then you do all of that many more times to figure out what your full instrument needs to look like (I’m telling on myself a little here, but I found the detailed spreadsheet that Dr. Szentgyorgyi showed me to outline this step-by-step process extremely satisfying).

Then, of course, you have to build the instrument, often to very exacting parameters. For instance, the lenses that were ground to be installed in G-CLEF’s two cameras required nanometer-level precision. So just building the instrument requires exquisite attention to detail. 

In this process you also have to consider all the things that could happen that would prevent your instrument from being able to do its job. Let’s take a close look at a single factor that could impact G-CLEF’s performance: temperature.

Hot and Cold

G-CLEF is so precise that even small temperature fluctuations can impinge on its ability to do its job as things flex and contract due to thermal variations. So in the design, Dr. Szentgyorgyi and his team had to go to extreme lengths to make sure that, whatever is happening to the temperature outside of G-CLEF’s shell, the inside stays as unaffected as possible.

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Those precisely-ground lenses are made from a special type of glass that does not expand or contract with temperature changes. Metal parts were avoided anywhere it was possible to do so, given metal’s tendency to flex with thermal changes. Those pieces that have to be made from metal were made from a special alloy that has minimal reactions to temperature. The optical bench, a critical component holding some of the most sensitive pieces of the instrument, was built from carbon fiber reinforced polymer (or, to quote Dr. Szentgyorgyi, “very expensive fiberglass”), so it can essentially ignore any thermal fluctuations. The entire inside of G-CLEF’s shell is a vacuum chamber, to limit the ability of the outside temperature to affect things on the inside, while the shell itself is wrapped in thermal tiles. To top it off, the whole shebang will live in a temperature-controlled room.

And all that’s just to control a single external factor. There are many more that had to be considered when designing G-CLEF—some of which are more predictable than others.

Rocking and Rolling

Remember that G-CLEF is going to be installed on the Giant Magellan Telescope and, like most telescopes are designed to do, GMT is going to be moving around. That means G-CLEF will be moving as well, and its design needs to take mitigating the effect of that movement on the instrument into account.

For instance, as GMT swivels itself around, it can actually cause the whole structure to tilt slightly. G-CLEF doesn’t like that, so Dr. Szentgyorgyi and his team are currently working on testing a self-leveling system to install on G-CLEF to make sure that it is unaffected by even the slightest pitching caused by the telescope’s pointing.

Then there’s the fact that nothing the size of GMT, which is going to be enormous, can move completely smoothly. There’s going to be vibrations, and G-CLEF doesn’t like that either—vibration can affect the instrument’s resolution. So when it’s installed, G-CLEF will be the proud owner of its own anti-vibration system, which Dr. Szentgyorgyi describes as being like noise-cancelling headphones, only for ground vibrations instead of sound.

Then there’s the kind of tilting and vibrating that we puny humans have no control over whatsoever.

Worst Case Scenarios

GMT is being built in the high desert of Chile, one of the best observing sites in the world. But it comes with a catch, naturally. Chile marks the edge of the South American tectonic plate, and the very mountains where GMT is being built are themselves being pushed up due to the fact that the Nazca plate is diving under the South American plate. In other words, Chile gets earthquakes. 

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G-CLEF needs to be able to survive a certain amount of tectonic activity. Data similar to actuarial tables were pored over in the design phase to figure out how often earthquakes of varying strength occur at the GMT site, and G-CLEF is being built with an earthquake protection system that will keep it safe—up to a certain point.

I actually got to see part of this system, a series of clamps designed to lock down and prevent G-CLEF from knocking itself loose, being tested. As a fun fact the anti-vibration system that helps protect G-CLEF from the everyday shuddering of the telescope can actually interfere with the earthquake system. The instrument needs to be able to sense when the shakes are above and beyond the normal everyday stuff to shut down the anti-vibration system and engage the earthquake clamps.

I naturally asked Dr. Szentgyorgyi how big a shake G-CLEF could survive. Turns out it has been designed to withstand 5gs (so 5 times the normal force of Earth’s gravity) in any direction. In fact, the design had to take into account what he called a “survival level earthquake”, only in this case the survival isn’t actually G-CLEF’s. This is the earthquake level where G-CLEF itself gets destroyed but manages not to break lose and wreck the rest of GMT.

Travel Plans

Okay, so you’ve designed your instrument to do its job perfectly. You’ve used the experience of decades of previous instruments and observatories to figure out all the things that could go wrong, and have done your best to (within reason) account for all those factors and keep your instrument going. Now you’ve got to actually get it to the telescope site.

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For one thing, do you want to ship it whole, or in pieces? Shipping it whole has the benefit of only having a single large package to worry about, but it has several disadvantages as well. For instance, if G-CLEF were shipped fully assembled, that would mean the very delicate optical bench would be sitting right next to the 400+ lb diffraction element for the whole trip. If a big enough jolt occurred to knock the diffraction element around, it could destroy the optical bench through sheer mass. Much safer, then, to disassemble the instrument and ship the components in their own carefully padded cases, and reassemble it onsite. Now to get those pieces to Chile!

You could, of course, just put it on an airplane. That would get it there nice and quick, certainly. But, to quote Dr. Szentgyorgyi again, “airplanes are kind of scary”. Things tend to get tossed around when shipped by airplane, and if there’s one thing you don’t want happening to your super-precisely built and calibrated instrument parts it’s getting tossed around.

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Studies that used accelerometers to measure how much force packages undergo when shipped via different methods show that going by boat is safer. Of course, you’d rather things not have to get repeatedly loaded and unloaded along the way (one loading crane operator having a bad day could really wreck your scientific ambitions, after all), so that means avoiding going through the Panama Canal where such repeated loads and unloads are normal.

Fortunately a ship going out of California doesn’t need to go through the Canal! Unfortunately, California is on the opposite side of the country from Massachusetts, where G-CLEF is being built. That means loading everything onto an air rider truck, driving it to California, loading it onto a ship that will sail to a port south of Valparaiso, where it will all get unloaded and put on another truck that will get it up the mountain to the GMT construction site, where it will be reassembled and installed. Whew!

Eyes on the Skies

All of that is a fraction of what needs to happen for a single instrument on a single telescope. So why do we do it? Because when the Giant Magellan Telescope finally opens its eyes, it’s going to be one of the largest optical observatories ever built.

Its ability to reveal the mysteries of the universe to us (as well as, inevitably, uncovering new ones) will be unprecedented—as long as it has the right instruments to help it do so. It may take an incredible amount of time and effort to create an instrument like G-CLEF, but that’s what it takes to support an observatory of GMT’s magnitude.

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And it’s the same process that went into making the Vera C. Rubin Observatory a reality. So, as we approach the moment of Rubin’s first light we can look forward to the wonders it will reveal to us, and anticipate the day that the Giant Magellan Telescope (with G-CLEF’s help!) will join it in our never-ending quest for knowledge of the heavens. Welcome to the world, Rubin! We’ve been waiting for you.