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Comment by tpmx | Hacker News Reader

https://en.wikipedia.org/wiki/Extremely_Large_Telescope

It has around 250 times the light gathering area of the Hubble Space Telescope and, according to the ELT's specifications, would provide images 16 times sharper than those from Hubble.

Edit: Wow, this construction image from August:

https://upload.wikimedia.org/wikipedia/commons/thumb/7/75/Al...

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Extremely Large Telescope is the budget version.

The original plan was Overwhelmingly Large Telescope https://www.eso.org/sci/facilities/eelt/owl/OWL_design.html

God I still hope that becomes a thing.

Somewhat unlikely: there is a race on between arrays of cheaper telescopes linked by software and ever larger single telescopes and optical bags of tricks to combine the images from multiple smaller telescopes. So an even larger instrument might well be obsolete before it could be constructed and that's before we get into the cost of such a project.

Yeah, and on top of that, making your telescope bigger does not have a linear cost increase. I know it's unlikely, but I can dream!

There are some interesting parametric cost models for telescopes floating around and various papers describing the models (and the data that went into the models).

From that page: “Keeping OWL 3264 segments aligned to a few millionths of a millimeter is a daunting but not impossible task”

And that’s for a 100m diameter main mirror.

"The ESO specialists expect resolutions from OWL that are up to 40 times higher than those of the Hubble Space Telescope. If the 100-meter mirror cannot be financed, a 60-meter variant is being planned. The name OWL would remain the same. Because then the project is jokingly called »Once was larger«."

https://www.itespresso.de/2006/04/17/groesstes-observatorium...

--> Google or other translate

Question from a total noob: How modular are these segmented mirrors? Ie, if the budget runs to 'only' 39m now, is it possible to upgrade it to 42m later by adding segments to what's already there?

I have no special expertise on the subject but by reading and watching about it I think that’s more complex than you might think because there’s a lot a extra systems to make the mirror work with that set up. So it’s not a matter of just bolting in a new ring of hexagonal mirrors and I suspect the majority of the cost comes not from the actual mirrors themselves.

The segments themselves are 100% modular and you could tile an US state with them. The main problem is engineering: the whole thing needs to behave like a single object, turning 360 degrees and moving between horizon and zenith (horizontal and vertical), with a minimum of mechanical flexure. And also resisting wind load. In essence, it's a lightweight building that has to move freely, and this one won't get bigger.

They are modular but not very interchangable (there are 133 different designs of the segments in the primary mirror).

Making a much larger mirror would also face another obstacle: the focal length is designed to be practical for a ~40m mirror.

Despite public perception to the contrary, the Hubble Space Telescope is not an especially large telescope. Telescopes on the ground of a similar size are a dime a dozen. What made HST special is that it was in space and could get much higher resolution than ground based telescopes at the time. With modern advances in adaptive optics that advantage has generally been negated, so a telescope needs to have some other reason to be put in space.

Wasn't it being cold in space so there's more infra red coming in also a factor?

More than temperature, the main problem is the atmosphere. Hubble can see a lot of infrared, and also ultraviolet, because it is in space. Ground-based IR telescopes are somewhat limited, and UV is non-existent.

For ground-based telescopes, I am continually impressed with what amateur astronomers construct in their back gardens, e.g., https://youtu.be/tFZVmVRItbg

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