IOTA

I am back from my meeting. Back to reality after a six hour flight, from sunny and warm southern California to freezing Massachusetts (I couldn’t believe it, but I was greeted by a few inches of fresh snow). I didn’t have much time to wonder around with my camera, so no photos from Pasadena this time.

Today’s photo is from the Fred Whipple Lawrence Observatory, in Arizona. I took it from the road going to the MMT telescope and shows the IOTA interferometer. I have actually been at the interferometer trying to observe Cepheid stars, but unfortunately the instrument was not working at the time, and we didn’t get much done (in fact, we had to hurry back to Tucson one day before the end of our run, because of a snow storm).

I posted the photo to answer a question that was raised a couple of days ago. Do we see anything special when observing the nearest stars? My answer was that no, for ordinary telescopes even the closest stars are too far to see anything, they just appears as dots in the sky. The problem is that, even ignoring the blurring effect of the atmosphere, telescopes are limited in their visual acuity by the size of their mirrors. Larger the aperture of the primary mirror, smaller is the magnification that a telescope can achieve without blurring the image. The current limit in the size of a telescope is 10m, with the record held by the Keck Observatory in Hawaii. The smallest detail that such telescope can potentially achieve (again, ignoring the effects of the atmospheric turbulence which blurs the image and can only be partially corrected by advanced techniques called “adaptive optics”) is a little more than 0.1 second of arc (1 second of arc is about the size of a coin two miles away). Stars are much smaller than that.

Alpha Centauri, which is one of the nearest stars and very similar to the Sun, has a diameter of about 0.006 seconds of arc, and thus appears as a featureless point even with the largest available telescopes. Even nearby giant stars, like Betelgeuse, are too small for Keck or the other large telescopes.

There is however a solution to this problem, which is called interferometry. An interferometer works by combining the light of pairs of telescopes. When the matching is done properly, the visual acuity of the pair is equivalent to the one of a single mirror having the size of the separation of the two telescopes. If the two telescopes are put far enough, a resolving power below the 1/1000 of seconds of arc can be achieved. It is very difficult to make actual images with this techniques, but a lot of progress have been made in recent years, which have lead to a very accurate measurement of the diameter and the variation of the surface brightness of nearby stars.

The IOTA is one of such interferometers, and works by combining the light of three small telescopes that can be places at different distances by moving them on a rail. The interferometric measurement is performed by accurately measuring the different path traversed by the light entering the three telescopes, and by correcting at the same time for the distorsions caused by the atmospheric turbulence.

The two Keck telescopes themselves are being retrofitted to work as a single interferometer, and the European Southern Observatory has recently completed a giant interferometer by connecting four 8m telescopes, plus three smaller ones into the most advanced interferometer available, called VLTI.