I was under the impression that SALT (Southern African Large Telescope) is currently the biggest optical telescope in the world, at least it has the largest single primary mirror:

http://en.wikipedia.org/wiki/List_of_largest_optical_reflecting_telescopes

An amateur astronomer myself, I have the privilege of working at the VLT (you can guees by my website). There seems to be some confusion regarding the ELT and the OWL. The thing is that ESO dropped the idea of the OWL, and replaced it by the ELT. Preliminary studies resulted in: The OWL is sooooo expensive, that it may never be built before 2070, so we will concentrate on building something that we will see on our lifespan, so we make it smaller, and we rename it as the ELT, saving the OWL name for some telescope in the future. This does not come free anyway. They (ESO) are talking about shutting down the operations on La Silla, an ESO owned observatorie, one of the biggest in Chile in domes count:

http://www.ls.eso.org

Hope this helps. Cheers from Paranal (Home to the VLT)

Alex.

Now, for the electromagnetic spectrum-impaired (that would be me), what do you consider as the largest optical telescope in the world?

I hear Keck and VLT routinely cited as the world’s largest telescopes. But are they? The Large Binocular Telescope in Arizona has two 8.4 meter mirrors on a single alt-azimuth mount. (That counts as one telescope in my book.) The mirrors themselves are the largest-diameter monolithic telescope mirrors ever made…and given the direction technology is going (Keck-style with multiple segments), they are likely to hold that record for a long time.

Phil, can you help…what is currently the definitive largest telescope in the world? (Yeah, I work for Steward Observatory/U of A…just a little bit of pride on the line here.) Whenever people ask me the question I always have to quantify it as I’ve done above. Is it not just a simple answer anymore?

Wonder how they’ll test the design of the 42 meter telescope? That will be interesting to see!

“The Perfect Machine” by Ronald Florence, at some public libraries and, of course, available online.

Kevin

In order to solve it, they are going to outfit the the Kecks with FIVE lasers each so they can do corrections for atmospheric distortions at different parts of the field! Wonder what the system would be like on a 42 meter telescope?

At least they’re still reserving the name for something truly Overwhelmingly Large!

Telescopes of Unusual Size? I don’t think they exist…

So here you go: http://www.spacedaily.com/telescopes.html#17
The answer is that you would need a 200 meter mirror to see the flag and a 75 meter to even make out the lunar rover. It’s been awhile since I took Optics. Hope this link helps out.

Astrogirl

Interferometers have high resolution, but relatively small collecting area. For an optical telescope, this would make them less useful if you just wanted to take spectra, for instance (which is what a lot of the large telescopes spend most of their time doing). They also tend to have problems with extended objects – they cannot see details larger than a certain size. If all that was wanted was resolution, then an optical interferometer might well do better than a single dish (they’re going to need some fancy AO to get anywhere near their diffraction limit – thus the five mirrors in the optical path), but if you want a huge light-bucket then a single dish is the thing to go for.

I still things “more than one hundred times more sensitive than the present-day largest optical telescopes, such as the 10-m Keck telescope” (the press release again) is pushing it somewhat though! It will have about 18 times the collecting area – where’s the other factor of 6 coming from?

It is interesting the way these things keep shrinking. OWL was 100-m, EURO-50 was 50-m, the E-ELT is now 42-m. One joke doing the rounds at the AAS in Calgary was that the acronym ‘TMT’ was chosen (for one of the US ELT projects) so that it could be scaled down from Thirty Meter Telescope to Twenty Meter Telescope without having to change the logo…

I recall some time back a new adaptive optic scope talking about imaging the moon to try out the adaptive optics. There was some possibility that it would see something and the best resolution.

Getting it all up there will be the hard part.

and damn thatsa purdy telescope.

As for earth-based vs space-based ‘scopes, I think that one of the biggest advantages to earth-based is that you can swap out the instrument packages easily. That can be something of a problem for the space-based scopes.

(Frakking Enormous Telescope)

And Nigel Depledge, you are right about the moon being too bright. The moon (and most of the planets too bright for astrophotography) are too bright through even a 10-20″ Dobsonian.

No matter what it costs, or how long it takes to build, I hope that they do build this wonderful telescope. Also hope we can get some on this order into orbit!

Astrogirl

Have I got my maths wrong, or will this thing be able to resolve some of the junk left on the Moon from the Apollo programme?

Space-based ‘scopes have some big advantages in non-visible chunks of the spectrum. No amount of optical compensation can fix the fact that some parts of the spectrum are totally absorbed by the atmosphere. And spce-based ‘scopes can be kept much cooler, making them more sensitive to e.g. infra-red.

But orbital observatories are not cheaper than land ‘scopes: Launching, maintaining and upgrading the little floating buggers gets quite pricey.

So there’s pros and cons to both approaches. It seems that astronomers are making the most of these different approaches by using each type where it’s stronger.

Why build a multi-zillion dollar, ground-based ‘scope when a space-based scope can see better, farther, faster, etc.?

It seems to me that the same money could buy a smaller space scope that can do far more. Why when we can put these into space are we still building them on the ground?

I understand from an amateur point of view (it’s cool to look at space from your backyard) but in the science community it makes no sense to me.

Anyway, they’re doing it so there must be a good reason. What is it?

Giant gradient index optics might be easier to manufacture in deep space as well- if the focal ratio is long and the primary aperture very large the gradient can be so gradual as to be almost imperceptible, and so easy to make. Its the small ones with small f ratios that are hard.

What has me bugged is whether in deep space low temperatures mean that impacts might propogate cracks through the optic, resulting in its destruction.

Personally though, it might be interesting to have myriad scopes out about where Sedna is and the foci accessible from earth.

Or we could use the money on projects that need doing now and can’t wait- like saving endangered languages and lifeforms. But heck, who needs apes and whales, or Yahgan…… Stars are just so pwetty….

Jess Tauber

I can’t remember the details, but I remember seeing a documentary about a group of telescopes, linked together, providing a much larger “virtual” aperture. I know they’ve been doing this since forever with radio-telescopes, but this plan was for optical ‘scopes. At the time the documentary I saw was shot, only the first of the ‘scopes was up, and already producing good results, but the astronomers were buzzing with excitement at the thought of having an aperture of ~100 meters once everything was in place.

Did this idea not pan out in practice? Because if it did, it would surely eclipse even this monster’s resolving power.

I was sure there was a recent article in Scientific American about this, but I can’t seem to find it. If anyone knows which issue it was, I’d be much obliged.