BTW, dpn't you think your blog needs a better wordpress template?

Good luck to the Chinese. I like their food, but my favorite Chinese restaurant in Dublin Georgia got shut down for serving dog and cat.

I was bitterly disappointed.

Yep – There are many reasons such a project would be impractical if not impossible at the moment, and you've ably highlighted a couple more…

It would be an impressive building though, eh…

Flaming pope the hubble mirror is 2.4 meters.

I asked why not make a 500 meter (that is to say, 200 times larger diameter) optical telescope?

>>> Conic – a telescope's capability is limited by the accuracy of it's primary optic's figure. The accuracy required of the figure is determined by the wavelength of the radiation you wish to reflect and bring to a focus – the accuracy required is crucial to allow the optics to be able to form an image with the incoming electromagnetic radiation of the object under study. Typically, you need the surface of the mirror (or other reflecting surface) to deviate from the perfect mathematical shape by no more than 1/8 of the wavelength of the radiation to be focused.

Now, radio waves have wavelengths ranging from cm to km's in length. So a radio reflecting dish would have to be accurately figured to within a couple of mm – i.e. over it's entire surface, no part of it deviates from a perfect paraboloid by more than 1 or 2 mm. This is within reach of today's engineering capability for a 500 m radio dish.

Optical radiation has a far shorter wavelength – about 400 – 800 nm, or 4 – 8 ten-millionths of a meter. Hence, to get a surface that will act properly as a reflector for a telescope, it needs to be perfectly figured to at least 1/8th of that number or about 50 billionths of a meter, over it's entire surface. So if we were to try to fashion a 500 m primary mirror, it would have to be so accurate that if we were to blow it up in our minds to the width of Australia, the largest surface deviation would be on the order of a millimeter or so from perfect. Also note that the '1/8 of the wavelength' criterion is the bare minimum – professional telescopes should be (and are) figured to much higher accuracy and precision than that. This is extremely difficult to do, and to date, such accuracies have been achieved with mirrors up to about 10 m in diameter.

But that's not the real killer – large mirrors can be made to be that accurate by using multiple segments, and there is no reason that such technology can't continue to be pushed out. The thing is, the rest of the structure must be proportionately accurate too – and remain so under the stresses and strains of gravity etc. Radio telescopes can be built so big because with clever engineering, the structures that hold the detectors over the dish can be made to flex only minutely – small enough not to stuff up the observations. Even a tiny tiny tiny amount of flexure in an optical telescope' structure though will ruin observations – defocus images, warp the image planes etc etc..

Essentially, such telescopes are beyond our engineering capability because we can't work out how to make their support structures rigid enough. For now…

Oh, and there's little matter that such beyond-the-frontiers engineering would cost a bomb of cash.

http://www.eso.org/sci/facilities/eelt/owl/index_3.html

If this was built (and it has been on the drawing board for some time, but it isn't silly engineering) it would probably stay the largest telescope on earth, because larger ones (500m) would probably have to be made in space.

Have some patience

Wait & see what really takes place

That is what will really matters

I asked why not make a 500 meter (that is to say, 200 times larger diameter) optical telescope?

But yea Woo Hoo! Go China, go China yeaaaah, yeaaahhh!

What an awesome mass scale government experiment of communism. This is communism at its best (as apposed to USSR). I guess the leader of China is a good guy at heart after all- at least towards science.

Now then… lets do it better in America. Lets show them America is better at science than any communist state corrupted or its peak. They have radio tele and fusion parts, Europe has the collider, Singapore with bio tech, Japan is machine tech – What have we got to show these days? – dusty fission, oil companies>>failed solar/hydrogen, inefficient biofuels(wastes energy to make), material science (minor plus), points is, SHOW US THE SCIENCE!

I'd like to go there when it's finished, and shoot a movie of it.

Sili says:
"But … just what would a small 500-meter telescope look like?"
Yours truly wonders: Do they actually have small meters (or metres) in China, for building such a project?