Aperture Fever

Slaking the Thirst for Bigger Mirrors

Sometimes astronomers come down with a peculiar condition called Aperture Fever. In short, no matter what size their telescope mirror is, they always want a bigger one.  But, there’s a limit to the size of mirror (or radio dish) you can build and still have it be useful.  You could probably pour a piece of glass 100 meters across, if you wanted to. You could build a huge radio dish, if you wanted to.  But, getting a big piece of glass or a monstrous dish on a support and keeping them from breaking under the pull of gravity (or at the very least, keeping them from distorting and bending due to gravity’s tug on it) would render them useless.

Light collected by three VLT Auxiliary Telescopes, and combined using the technique of interferometry, provides astronomers with vision as sharp as that from a giant telescope with a diameter equal to the largest separation between the telescopes used. To obtain the image of T Leporis using data from the Very Large Telescope Interferometer, astronomers used the four 1.8-metre Auxiliary Telescopes in different configurations to mimic a telescope almost 100 metres in diameter, as represented schematically on this artist’s impression of the Paranal platform
Light collected by three VLT Auxiliary Telescopes, and combined using the technique of interferometry, provides astronomers with vision as sharp as that from a giant telescope with a diameter equal to the largest separation between the telescopes used. To obtain the image of T Leporis using data from the Very Large Telescope Interferometer, astronomers used the four 1.8-metre Auxiliary Telescopes in different configurations to mimic a telescope almost 100 metres in diameter, as represented schematically on this artist’s impression of the Paranal platform

There are some cures for aperture fever, however. The latest was demonstrated in Chile by a group of French astronomers who ganged together all the telescopes at the European Southern Observatory.

Essentially, what the astronomers did was create a 100-meter-wide interferometer — a sort of “virtual” telescope consisting of several smaller (1.8-meter) VLT Auxiliary telescopes. The result was an aperture the size of a much larger telescope. They made several observing runs with this special set-up to collect the light  streaming from their target, and then combined that light into one very fine image.

What makes this use doubly cool is that they used it to create one of the first infrared interferometry observations.  That’s quite a feat.

Their target was the star T Leporis, a type of pulsating star called a Mira variable (named after the star Mira, which is the “prototype” for these kinds of stars).

Mira stars are among the biggest factories of molecules and dust in the universe. T Leporis is a fine example of this activity. It pulsates with a period of 380 days and loses the equivalent of the Earth’s mass in dust and gas every year. Since the molecules and dust get created in the layers of atmosphere surrounding the central star,  astronomers would like to be able to look at these layers in great detail to see how it all happens. But this is no easy task, given that the stars themselves are so far away. Even though we’re talking about a huge star, from a distance of 500 light-years T Leporis appears quite small — about half a millionth of the size of the Sun. This is where interferometry and repeated observing runs can make a huge difference.

The reconstructed image shows this star up-close. It’s 100 times larger than the Sun, and is surrounded by a sphere of gas about three times larger than the star itself. That we can even see this level of detail in a star that lies 500 light-years away shows that aperture fever can be slaked with a virtual telescope and the right amount of observing time.

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