This topic came up on another thread (CoRoT Planets) and I thought that it deserved a thread of its own. Here is a list of ground-based optical telescopes with primary mirrors of at least 6.5 metres equivalent diameter. I include telescopes that are currently operational, under construction or in serious planning with significant funding support.

42m -- EELT (European Extremely Large Telescope) -- Planning -- ESO Council -- site to be selected by late 2009 -- based upon the Euro50 and OWL studies
30m -- TMT (Thirty Metre Telescope) -- Planning -- USA / Canada -- Mauna Kea, Hawaii, USA -- has received over 400 million dollars in donations and pledges to date
21.4m -- GMT (Giant Magellan Telescope) -- Construction -- USA / Australia -- Las Campanas, Chile -- primary mirror fabrication in progress
16.4m -- VLT (Very Large Telescope) -- Operational -- ESO Council - Cerra Paranal, Chile -- four monolithic-mirror 8.2-metre telescopes in combined interferometric mode
11.8m -- LBT (Large Binocular Telescope) -- Operational -- USA / Italy / Germany -- Mt. Graham, Arizona, USA -- two monolithic 8.4-metre primary mirrors
11.0m -- SALT (Southern African Large Telescope) -- Operational -- South Africa / USA / Germany / Poland / New Zealand / India -- Sutherland, South Africa -- segmented-mirror design
10.4m -- GTC (Gran Telescopio Canarias) -- Operational -- Spain / Mexico / USA -- La Palma, Canary Islands, Spain -- segmented-mirror design
10.0m -- Keck I -- Operational -- USA -- Mauna Kea, Hawaii, USA -- segmented-mirror design
10.0m -- Keck II -- Operational -- USA -- Mauna Kea, Hawaii, USA -- twin to Keck I
9.2m -- HET (Hobby-Eberly Telescope) -- Operational -- USA -- Mt. Foulkes, Texas, USA -- segmented-mirror design
8.4m -- LSST (Large Synoptic Survey Telescope) -- Planning -- USA -- Cerro Pachon, Chile -- this is the telescope that Google is helping to fund
8.3m -- Subaru -- Operational -- Japan -- Mauna Kea, Hawaii, USA -- monolithic mirror
8.1m -- Gemini North (Gillett) -- Operational -- USA / UK / Canada / Australia / Argentina / Brazil / Chile -- Mauna Kea, Hawaii, USA -- monolithic mirror
8.1m -- Gemini South -- Operational -- USA / UK / Canada / Australia / Argentina / Brazil / Chile -- Cerro Pachon, Chile -- twin to Gemini North
6.5m -- MMT -- Operational -- USA -- Mt. Hopkins, Arizona, USA -- converted from old MMT (Multiple Mirror Telescope) into a single monolithic mirror
6.5m -- Magellan I (Walter Baade) -- Operational -- USA -- La Serina, Chile -- monolithic mirror
6.5m -- Magellan II (Landan Clay) -- Operational -- USA -- La Serina, Chile -- twin of Magellan I

Bill

As a sign of how fast technology has developed, none of those mirrors dates back farther than 1990. (Note careful wording - the MMT's mounting and some of its structure go back to its multimirror days). To go earlier, if I count right, the next one on the list would be the 6-m Bol'shoi Teleskop Azimutal'nyi (BTA) in the Caucasus, and only at number 18 do we find the Glass Giant of Palomar. Who'd have thought? I can remember when a three-meter telescope was "large" and now in NSF planning, anything 6 meters and down is "small".

Definitely a Renaissance for ground-based astronomy is in progress (and thanks for the list, Bill! )

Sad fact of the matter is that HST is great, but maintaining it is difficult & expensive. We probably won't see very large space-based scopes until there is a robust & economically sustainable human presence in space.

Here's how the various best sites around the world compare in terms of seeing:

Median Seeing

0.07 Dome C in Antarctica (no telescopes yet)
0.45 Mauna Kea Observatory
0.55 Mt. Graham (Emerald Peak sites)
0.60 MMT (Mt. Hopkins/summit)
0.66 VLT (Paranal)
0.76 Magellan (Las Campanas) & ESO (La Silla)

It is quite remarkable how much the seeing varies, even at the same site. For example, at Mauna Kea, Subaru's average is 0.6 arcsec, versus 0.43 for CFHT.

The individual telescopes also produce "wakes" that interfere with other telescopes' seeing -- thus you don't want to populate the summit dome-to-dome.

It's amazing how good the seeing is on Dome C, compared to the other sites on the list. In addition, the IR seeing is unmatched, due to the very low levels of atmospheric water vapor above it (it's actually one of the driest sites on the planet), and the low temperatures. Not to mention the over three month long dark night, and the fact that almost the entire visible sky is circumpolar.

Here is a useful site about Dome C. The following Questions and Answers are from the site:

How would a telescope at Dome C compare with the Hubble Space Telescope?

The Hubble Space Telescope (HST) has a 2.4 m mirror, and delivers 0.05 arcsecond resolution at visible wavelengths. The best seeing we measured at Dome C was 0.07 arcseconds, however, this figure becomes lower when corrected for the finite size of the outer scale of turbulence (see Tokovinin, PASP, 114, 1156-1166). We don't have enough information yet to accurately determine the correction. While a 2.4 m telescope on the ground can never equal HST's performance, a somewhat larger telescope, say 4m, at Dome C could well produce images of equivalent resolution to HST for about 10% of the time. And in the near-infrared (e.g., the K band at 2.4 microns), the percentage should go up to 50%.

Would an Antarctic telescope be just for optical wavelengths, or would it take in the infrared?

There are compelling scientific reasons to build Antarctic telescopes from optical through the infrared and out to sub-millimetre wavelengths. In the infrared you gain factors of 10 to 100 reduction in the sky brightness, due to the cold atmosphere and cold telescope. A US group is building a 10 m sub-millimetre telescope called SPT at the South Pole (at these wavelengths the improved seeing at Dome C relative to the South Pole is not such an issue).

What are the main ways that an Antarctic telescope could benefit astronomy?

The main benefits are much greater performance at much lower cost. While there is an additional cost to "winterize" a telescope design, this is offset by far superior performance and savings in other areas (e.g., there is no need for a traditional dome structure). See Ashley et al 2004 "Robotic telescopes on the Antarctic plateau" for a comprehensive discussion of the advantages and disadvantages of Antarctica.

Another aspect is that Antarctica may be the only viable site for the next generation of Extremely Large Telescopes. The reasoning is that the low winds (yes! on the plateau it is very calm) and lack of seismic activity in Antarctica greatly eases the engineering challenges in constructing huge (30 m or greater) telescopes.

Are there any astronomical problems you could tackle from Dome C that you couldn't do elsewhere?

Yes. Wide-field, high resolution surveys in the optical and infrared would be a natural fit to the superb seeing conditions. However, interferometry is the field that has the largest potential gain. An interferometer at Dome C could be designed to directly image planets around other stars. Also, an interferometer designed for narrow-angle differential astrometry could directly determine the nature of the planetary systems around nearby stars; this is currently our best hope for detecting earth-like planets. The nature of the atmospheric turbulence at Dome C makes such interferometer projects many hundreds of times more sensitive than similar instruments at mid-latitude sites, which effectively makes the science feasible.

If we consider telescopes that operate at longer wavelengths, e.g., the sub-millimetre, there are whole new regions of parameter space that open up due to the increased transparency of the cold Antarctic atmosphere.

edit -- After looking further into the matter, it seems clear that the absolute best optical / IR astronomical site on Earth is 'Dome A', also in Antarctica, which is about 800m higher than Dome C, with resulting lower temperatures (important for reducing IR skyglow), lower atmospheric water vapor, and better seeing. It is also about half the distance to the South Pole as Dome C (meaning four months of uninterrupted dark night, instead of the three months at Dome C). Right now, a Chinese expedition is setting up an automated observatory which will, among other things, conduct tests of the seeing over the 2008 winter. But from what we already know or can predict, at many IR wavelengths this site approaches outer space in seeing and transmissibility. (Plus the winter cloud cover is expected to be very low, with no clouds or haze whatsoever at least 95% of the time.)

Bill

Exciting stuff. One downer (no pun intended) re: Antarctica is that half the sky is never visible, and the ecliptic is always close to the horizon. But for surveys of patches of sky, that matters fairly little, especially with the galactic center down south.

This is true. On the other hand, sites on the Greenland ice cap (and the Ellesmere Island ice sheet) are also being investigated. While not as good as the Antarctic sites, they are most likely still greatly superior to the 'traditional' mountain tops -- Mauna Kea, Cerro Pachon, etc. -- currently being used. From this pdf:



More information about Arctic astronomy may be found here. From the web page:



Bill

I can testify as to the quality of seeing conditions in cold, arid areas during temperature inversions. In Montana at an elevation of 5000 ft. during subzero conditions (-30, -40), the stars are almost steady points; there's very, very little scintillation. I swear I got better views of Saturn with my teeny little Sears refractor as a child from there than I can now achieve with my Celestron 8 out in the California desert.

That bit about "winds never being very strong" is true for the summits of icecaps, but definitely not for mountains near the periphery. There are fierce katabatic winds coming off the icecaps. Adelie land in Antarctica isn't known as "the home of the blizzard" for nothing.
If it wasn't for the winds I would suggest the Transarctic Mountains or Pensacola Mountains might be even better than the Ice Domes. Higher altitudes, more stable ground and perhaps even drier air. The dry valleys of the Transantarctic Mountains are generally considered to be the best match for Martians conditions on this planet.

PLATO has now been installed at 'Dome A' in Antarctica. Look forward to learning how the telescopes cope with the harsh conditions up there on Dome Argus.

Wow this is a really nice and important tip for everybody, thanx buddy.

An update on the TMT (second largest of those currently under development / construction)

Thirty Meter Telescope Selects Mauna Kea

The development phase has been completed ($77 million), and the project has raised an additional $300 million (plus Canada has agreed to supply the enclosure, telescope structure and adaptive optics system -- not sure what the financial value is).

This project is similar to the Keck I and Keck II telescopes, only with nine times the light gathering power.

Very useful list!
Small updates: ESO VLT is operational in interferometric mode also, two interferometric instruments (MIDI and AMBER) are operating, http://www.eso.org/sci/facilities/paranal/
LBT is now operational with both "eyes" (http://medusa.as.arizona.edu/lbto/firstbinocularlight_press_release.htm)


While the excellent quality of the antarctic sites is out of question, I seem to remember that the actual numbers are somewhat controversial. Unfortunately I cannot quickly find a reference. I think it had to do with the altitude above the surface where the seeing data were taken.

Thanks for the update -- have edited the original post.

Bill

With the exception of South-Pole & South-America, I've visited most of those observatories. Getting to the LBT site is quiet an "adventure".
TMT will be placed on Hawaii...
ESO's VLT in Interferometry only uses 3 of the 4 telescopes...

We shouldn't forget the "smaller" telescopes such as the 1m93 at the OHP in Southern France, which was used to find/confirm the first exo-planet 51 Peg b using the ELODIE spectrograph. The instrument was replaced by SOPHIE but OHP considers to re-use ELODIE for follow-on projects...

I have created a spreadsheet of all ground-based astronomical telescopes (that I know of) of at least 200cm aperture that are currently operational, under construction or in serious planning, with 82 entries:

Large Telescope List

Excel version


Bill

Two shots of "Hawaiian" telescopes both on the same commercial flight from Hilo to Honolulu on August 16 2009 at ~10 am local time
Both have been Photoshoped to enhance details.

On the left end side Mauna Kea:
Click to view attachment

On the right end side Haleakala on Mauai Island:
Well, next post...

Here it is:
Click to view attachment