# Optical Very Large Array - Possible? **URL:** \ **Category:** Factual Questions\ **Created:** [April 28, 2003, 5:42pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727 "2003-04-28T17:42:31Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![tanstaafl](https://avatars.discourse-cdn.com/v4/letter/t/ba8739/32.png) [@tanstaafl](https://boards.straightdope.com/u/tanstaafl)\ **Post date:** [April 28, 2003, 5:42pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/1 "2003-04-28T17:42:31Z")
I know that out in New Mexico there is the Very Large Array radio telescope. This is a bunch of radio telescopes spread out across the desert that are connected via computer to act like a single telescope. This gives them the resolving power of a single, much larger telescope. Could this same principle be applied to optical telescopes? In other words, could we link a bunch of optical telescopes together to let us see finer details, like surface details on Mars or maybe some of these extra-solar planets we keep hearing about? Would it work? If not, is it impossible or simply impractible? TIA ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [April 28, 2003, 6:01pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/2 "2003-04-28T18:01:43Z")
Well, there are multiple mirror optical scopes. And at least one where two phyicaly distant mirrors are compined electonically. The main issue is that optical wavelengths are MUCH (several orders of magnitude) shorter than radio wavelengths. Brian ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [April 28, 2003, 6:07pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/3 "2003-04-28T18:07:19Z")
Yes. [http://www.eso.org/projects/vlt/](http://www.eso.org/projects/vlt/) > [@](#): > > The Very Large Telescope Project > > ESO is building what will be the World’s largest optical telescope array, The Very Large Telescope (VLT)… > > The VLT Project > The ESO Very Large Telescope will consist of four 8-meter telescopes which can work independently or in combined mode. In this latter mode the VLT provides the total light collecting power of a 16 meter single telescope, making it the largest optical telescope in the world. The four 8-m telescopes supplemented with 3 auxilliary 1 m telescopes may also be used in interferometric mode providing high angular resolution imaging. I believe others have also been planning space-based optical arrays, positioned thousands of miles apart, so that you could resolve earth-sized planets orbiting other stars. Also, there are a number of optical telescopes that make use of segmented mirrors, rather than large single mirrors. [http://www.astro.psu.edu/het/](http://www.astro.psu.edu/het/) (note that this scope is geared toward spectroscopy) ---
**Author:** ![Phage](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@Phage](https://boards.straightdope.com/u/Phage)\ **Post date:** [April 28, 2003, 6:08pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/4 "2003-04-28T18:08:18Z")
Yes! People are doing it… a google might turn something up, but it is practical. ---
**Author:** ![Linus\_Van\_Pelt](https://avatars.discourse-cdn.com/v4/letter/l/ebca7d/32.png) [@Linus\_Van\_Pelt](https://boards.straightdope.com/u/Linus_Van_Pelt)\ **Post date:** [April 28, 2003, 6:08pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/5 "2003-04-28T18:08:22Z")
It’s being done - [The VLT](http://www.eso.org/projects/vlt/) (I just love the names that scientists come up with for things). It’s being constructed on a very high plateau in Chille and will consist of four 8 meter telescopes that can work independently or together. ---
**Author:** ![Linus\_Van\_Pelt](https://avatars.discourse-cdn.com/v4/letter/l/ebca7d/32.png) [@Linus\_Van\_Pelt](https://boards.straightdope.com/u/Linus_Van_Pelt)\ **Post date:** [April 28, 2003, 6:11pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/6 "2003-04-28T18:11:00Z")
Darn it, **toadspittle**! Beat me by a minute. I blame the hamsters. ---
**Author:** ![Sock\_Munkey](https://avatars.discourse-cdn.com/v4/letter/s/aeb1de/32.png) [@Sock\_Munkey](https://boards.straightdope.com/u/Sock_Munkey)\ **Post date:** [April 28, 2003, 6:21pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/7 "2003-04-28T18:21:08Z")
I have heard about plans to put several other Hubble-type telescopes in orbit to create an array larger than the diameter of the earth. YMMV ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 28, 2003, 7:06pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/8 "2003-04-28T19:06:43Z")
But there are two reasons why they’re not as common as radio arrays, both linked to the wavelength. First, interferometry is much easier for long wavelengths (short frequencies). If a wave has a frequency of 1 Hz (one cycle per second), then you need to have timing precise to a fraction of a second on your measurements. If it’s a million Hz, then you need your timing to be precise to a fraction of a microsecond, which is much harder. And for light, with a frequency of about 10[sup]15[/sup] Hz, you need timing measurements precise to 10[sup]-15[/sup] seconds. That usually means that you need your optical scopes to be fairly close together, so you can have a direct optical link between them (literally a pipe that you shine the light through), whereas with radio measurements, you can just record your data on tape using a good clock and FedEx them across the country. The other reason is that interferometry isn’t as necessary for optical scopes. The angular resolution you can get (smaller is better) is proportional to the wavelength of light you’re using, divided by the aperature of the telescope you’re using. So if you’re looking in 1 m radio waves, you would need a 100 km telescope, to get the same kind of resolution you could get for visible light with a 6 cm scope. Of course, more resolution is always better, but for a telescope on the ground, you’re limited by the atmosphere, anyway. Until adaptive optics were devoloped, there was just no point in doing optical interferometry, since you couldn’t do any better with two telescopes than with one, anyway. And even adaptive optics systems aren’t perfect, which is why we want to put the really big interferometers in space. ---
**Author:** ![tanstaafl](https://avatars.discourse-cdn.com/v4/letter/t/ba8739/32.png) [@tanstaafl](https://boards.straightdope.com/u/tanstaafl)\ **Post date:** [April 29, 2003, 1:57pm UTC](https://boards.straightdope.com/t/optical-very-large-array-possible/171727/9 "2003-04-29T13:57:25Z")
Thanks everyone. I had tried Googling but kept coming up with the Very Large Array out in New Mexico; I didn’t know the one in Chile was called the VLT. So it sounds like it can be done but is much more difficult and doesn’t buy you as much anyway. I hadn’t realized that the wavelength made that big of a difference. Thanks again.