Tuesday, September 24, 2013

9/18 Colloquium: Thirty Meter Telescope

Today's Astronomy and Astrophysics colloquium was from Dr. Warren Skidmore of the Thirty Meter Telescope (TMT) Observatory Corporation. Long story short, Dr. Skidmore was mostly trying to sell us on the idea that the Thirty Meter Telescope project is actually going to happen. Much of astronomy thinks of the TMT as a far-off pipe dream, something too big or too expensive or too likely to be plagued by persistent mechanical failures.  Obviously the issue couldn't be discussed in too much detail in a one-hour talk, but the TMT Corp says the design is doable and that the TMT could be online and gathering scientific data in as little as 10 years.

So... why do we want to build a telescope that big in the first place?

To start, I should talk about exactly what I'm talking about when I refer to a telescope's size. The size of a telescope is usually given as the full diameter of the telescope's aperture, the opening through which it gathers light. In most basic telescopes, this will be the same as the diameter of its mirror, but not always There are designs where the mirror is bigger than the aperture, like the design used on the Kepler spacecraft that has its own special benefits (turns out that telescope design is complicated as hell!). So the Thirty Meter Telescope is the project to built a telescope whose mirror is thirty meters across. That's one third of a football field, or more than the distance between home plate and first base. That's also three times the diameter of the largest optical telescope in the world, the Canaries Great Telescope.

"Size matters not. Look at me. Judge me by my size do you?"
From this quote alone, we can tell that Yoda was not an astronomer. For telescopes, size matters a lot. For a telescope, actually for any light-collecting instrument like binoculars or a camera, aperture size gains you two things. First, bigger mirrors can gather more light simply by having a larger surface area, so you can see fainter objects more easily. Second, a larger telescope allows a higher angular resolution. A telescope's angular resolution is just a fancy way of saying how well it can tell that two individual, but close-together, objects are actually two distinct objects or how well it can distinguish the small features of an object.

While these sound great all-around, bigger isn't always better. For one, as telescopes get bigger, they get very expensive to build (cost estimates of the TMT are roughly 1 billion dollars). Second, you don't always need to gather as much light as physically possible. If you're studying nearby stars that are relatively bright, for instance, you can get away with using a smaller telescope so you don't overload your detectors.

Big telescopes getting better images isn't automatically the case though. Large telescopes still have to deal with the #1 source of bad images in all Earth-based telescopes: the atmosphere! In fact, for big telescopes the problem becomes even worse because of the the telescope is looking through a larger column of atmosphere. This wouldn't be a problem if the atmosphere was perfectly still. So naturally, just to spite us, it isn't. We have constant air currents caused by temperature differences between the air and the ground or between different parts of the atmosphere. Figure 1 below compares an image of a galaxy in the Hickson Compact Group 87 taken with the Hubble Space Telescope (2.4 meters) to the same galaxy as imaged by Gemini South, an 8-meter telescope in the mountains of Chile.
Image courtesy AURA: http://www.aura-astronomy.org/news/archive/hst_vs_ao_2.pdf
So why bother with large ground based telescopes at all if they're just going to be trumped by those in space? Because we can, in fact, beat the atmosphere.

Well, more accurately said, we can sometimes correct out the flaws in our images caused by turbulence in the atmosphere if we do it very carefully. The general name for the systems that perform these operations is "adaptive optics". There are a number of different ways such a system can work. At its essence though, AO systems use a series of small re-positionable or flexible mirrors to undo all of the light-bending that comes from our atmosphere. In practice, real AO systems are VERY complicated (as I'm learning right now in Larry Ramsey's seminar), so I'll spare you the gory details. If you want to read about AO in some detail, I highly recommend Claire Max's ASTR 289 class page from UC Santa Cruz.

What can adaptive optics do for us? The following figure shows two images of the IW Tau system taken with the 200-inch Hale Telescope on Mount Palomar with and without their adaptive optics system.
IW Tau imaged with and without adaptive optics. Image courtesy http://www.astro.caltech.edu/palomar/AO/
Correcting for the atmosphere allows you to approach what we call the "diffraction limit", the telescope's natural resolution based on its size. I say "approach" because you never quite get to the diffraction limit itself (yet?), but you can get pretty close with the right systems. Suddenly space-based telescopes look pretty obsolete huh?

Not so fast. As good as adaptive optics can get these days, there's one part of the atmosphere that we can't correct for no matter how hard we try: absorption. Our atmosphere is made up of a number of molecules that like to do annoying things such as absorbing radiation that we astronomers would really like to see. Figure 3 below shows how atmospheric transmission varies with wavelength from the part of the spectrum we see to the far infrared. You'll probably want to click on the image to see the full version. Anything beyond visible tends not to be that interesting as the transmissivity just drops off quickly as you get to higher and higher energies, with basically no transmission past mid-ultraviolet.
While we do okay in the visible, anything in infrared pretty much sucks. There are clearly some parts of IR where we can observe from the ground and do a decent job of it. But for major infrared observations, particularly mid- or far-infrared, we really want to go to space. The catch, of course: putting telescopes in space is way more expensive.

Let's bring this back to the topic originally at hand. We've established that for large telescopes, many of the problems that result from Earth's atmosphere can be fixed with adaptive optics, which makes large optical ground-based telescopes scientifically useful. The amount of science we could get from such a massive telescope is pretty exhaustive, and I recommend checking out some of the documents available at http://tmt.org/documents. I'm also not going to get too involved in the engineering aspect of such a telescope, because I'm an astronomer. I hope I've done a decent job outlining some of the benefits and challenges faced by large telescopes. It's also good to finally finish this post, because I've clearly been working on it for nearly a week, and I have at least 2 other drafts and one idea waiting to be finished, including student questions Round 2. Thanks for reading!

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