North Hawaii News Articles from CFHT

Is bigger really better?

or `why astronomers always want larger telescopes'

The size of a telescope is, in the simplest sense, what determines its power; bigger is better. This is due to two main reasons. The first one is the "light bucket" factor: if you want to collect rain with a bucket, you should make the opening of the bucket as large as possible. The amount of light emitted by a faint and remote galaxy is constant per unit area as it reaches the earth, so making the collecting area of a telescope as large as possible helps to collect those precious photons so that they all contribute to the final image. Of course leaving your bucket out in the rain longer also has the desired effect, similar to taking long exposure with your camera. The second reason is a little more complex, as it implies an optical concept known as diffraction, which relates to the wave-like nature of light. The idea is that in theory, the larger the diameter of the telescope, the smaller details can be seen on the object that is being observed, in other words, the better its resolution.

In practice, however, atmospheric turbulence blurs images seen by telescopes to such an extent that diffraction effects are rarely seen, unless the telescope used is approximately 10 centimeters or less (4 inches) in diameter. What this implies is that telescopes larger than this size do not get a better resolution than these small telescopes! One might then rightfully ask why it is necessary to build such giant behemoths, because if the only advantage were to collect more light, to see fainter and more distant objects, one could simply take longer exposures.

The answer comes from a technique called adaptive optics which aims at removing the blurring effect of atmospheric turbulence by cancelling it out in real time by using small deformable mirrors. A sensor allows to rapidly measure the atmospheric turbulence and a powerful computer tracks and optimizes the mirror shape to compensate for it. This technique works best in the infrared domain of the light (where the wavelength is longer), and the resolution can increase by a factor 5 on telescopes such as CFHT to a factor 12 on larger telescopes such as Keck, allowing to see unprecedented details on a variety on astronomical objects!

This technique is so powerful that nowadays, no telescope is being designed without its adaptive optical system: the Institute for Astronomy at UH-Manoa developed such systems that were used on the Canada-France-Hawaii Telescope and are now being used on the Gemini Telescope. The W.M. Keck observatory has its own adaptive optics program, in association with the Lawrence Livermore National Laboratories, and the SUBARU telescope developed its own instrument. And this is only on top of Mauna Kea, as everywhere else in the world has similar efforts!

So with the help of adaptive optics systems, the size of a telescope does indeed matter, because it helps to recover the "diffraction limit", i.e. the theoretical limit of the smallest details that can be seen by a telescope. But the story doesn't end there! With a technique called interferometry, you can take the light from two independent telescopes and combine it to make images with even higher resolutions. In effect, in the technique of interferometry we are fooling the light into believing that it is going through one very, very large telescope, when really they are two separate ones. This is what the Keck Interferometer aims to do, with its twin telescopes 85 meters apart. The resolution of this interferometric telescope will be equivalent to that of an 85 meter telescope. That is one hundred times larger than when by limited by the atmosphere alone!

But if we can combine the light of two telescopes to make one giant telescope, couldn't it be possible to build one giant telescope by linking up all the telescopes on top of Mauna Kea? They are, after all, spread over a one kilometer line which would yield resolutions up to a thousand times that of the atmospheric turbulence limit, 10 times that of the Keck Interferometer alone. The problem is that using "conventional" methods, this would imply digging tunnels all across the summit, to bring the light of each telescope to one common focus. Of course, this not desirable. However, recent breakthroughs in optical fiber technology may allow to create such a giant interferometer: The OHANA project (OHAHA is an acronym for Optical Hawaiian Array for Nanoradian Astronomy) is raising interest amongst many astronomical communities that were previously in competition, and in true ohana spirit, would now have to collaborate and work together. The stakes are so high (nothing less than the largest optical interferometric telescope in the world's best astronomical site) that such cooperations and collaborations are not only possible, they are actually starting.

Of course, such a technique does not allow us to collect more light, thereby seeing fainter and more distant objects, but only to increase the resolution on objects that we already know, seeing smaller and smaller details in the hearts of galaxies, with the hope of being able to detect black holes and other exotic phenomena in their core. Therefore, in an interferometer, it is not the size of the telescopes that counts, but how far apart they stand. But for single telescopes, yes, bigger really is better because, on the one hand, we can study more distant, fainter objects, and on the other, with the help of adaptive optics, we can produce images with exquisite details.

Olivier Lai