This is a question about something I read in a book first written back in the 50's and revised since, about telescopes Standard Handbook for Telescope Making, on page 12 it said; “It seems practically impossible to cast a glass disk of over 200 inches that will not crack or become otherwise distorted during the cooling period. But the advance of scientific knowledge may come up with an answer to this problem, as it has to others where the problem can be recognized and identified. If this is to happen, the world may one day see a 30-foot reflector. Such a day will be an exiting one in astronomical circles, for a telescope of this size is potentially capable of revealing the disk of a star other than the sun, something man has never seen. All telescopes now in use can do no more than record a star as a point of light, even though there are other means of determining it's size and distance.” To me, this was the most important comment in the whole book and I've always remembered it almost word for word for over the more than 20 years ago when I first read it. My question is, “Is this true, a 30-foot reflector should reveal the actual optical disk of another star than our sun's? This seems like a big deal, and should be one of those great, scientific milestones if accomplished. If so, why haven't we seen this accomplished yet, seems like an important precursor to photographing actual planetary bodies around foreign stars? Segmented, computer driven telescope technology has brought us telescopes larger than 30 feet, haven't they?


How interesting that you've remembered this prediction for so long! The answers are yes – yes, we have telescopes that large; yes, we've seen the disks of other stars, and yes, we're even directly imaging planets now.

First, as you've remarked, we have several large segmented-mirror telescopes that are on the order of 10 meters (~33 feet) in diameter. Interestingly, these were not the first telescopes to actually image a star. In '95-6, astronomers from Harvard used the Hubble Space Telescope (a diameter of only ~2.4 meters, ~8 feet) imaged Betelgeuse, a red supergiant in the constellation of Orion. (See the press release at http://www.cfa.harvard.edu/press/archive/betel97.html).

Why can Hubble succeed with such a small mirror? Well, it turns out that the real killer in the past has not been the size of the mirror but the blurring effects of the atmosphere. (Think about looking at a penny lying at the bottom of a pool on a windy day: as the water moves, the image of the penny gets distorted and moves around.) By going above Earth's atmosphere, Hubble can avoid that problem, and get very high-resolution images – such high resolution images, in fact, that it has even recently directly imaged a large planet around Formalhaut, the brightest star in the southern constellation of Piscis. (See http://scienceblogs.com/catdynamics/2008/11/direct_imaging_of_extrasolar_p.php for more information.) Of course, the planet shows up only as a tiny point of light.

I'll also note for clarity that we can't by any means image the optical disks of *all* stars – only those that are large enough and close enough. There are a variety of other techniques to measure the sizes of stars; a brief discussion can be found here: http://www.physics.uc.edu/~sitko/Astrophysics-II/2-StellarMasses/2-StellarMasses.htm

As mentioned on that site, today ground-based telescopes are competing with Hubble by using Adaptive Optics, a complicated engineering feat that allows us to correct for most of the distortions from the atmosphere. Using this, large telescopes like Keck Observatory are able to take high-resolution images as well. In case 10-meter class telescopes don't sound big enough, there are currently three 30-meter class telescopes in development for the next decade. These will also be outfitted with Adaptive Optics systems.

Cheers, Anne Medling

Image Credit: Andrea Dupree (Harvard-Smithsonian CfA), Ronald Gilliland (STScI), NASA and ESA

 
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