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What would be possible if the Overwhelmingly Large Telescope was built?


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Was researching what the biggest telescope in world is and discovered this image:

Comparison_optical_telescope_primary_mirrors.s.thumb.png.6d0731ff6e1f39fbb452b584dac2c5a9.png

At the bottom the Overwhelmingly Large Telescope which Europe cancelled sadly (we are good at this, I know) instead went on to built a miserly 39m telescope.

With this giant circle and light sensitivity of ~70 (hubble: 61) its interesting to know what we would've seen?

Appreciate every answer, though hyperthetical images are much more appreciated •√•

Edited by StarFiveSky
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While a 39m telescope is by no means miserly, I have been wondering what the necessary apperture would be required to view the nearest planet. It turns out that the necessary apperture would be 200 Km, which is veeeeeery big compared to what we can build today. So I would not expect any ground-breaking results, unless a major step up in optics and computing occurs, like the array used to image the black hole, I think for now there won't be any visually striking new images. Also with such a large telescope, I think skyglow would be a limiting factor.

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4 minutes ago, Alexandros said:

It turns out that the necessary apperture would be 200 Km

We're getting there. The Event Horizon Telescope is the size on the Earth, although it only operates in infrared.

Also not sure what you mean by imaging exoplanets - several such images including timelapses have been taken.

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1 hour ago, Ags said:

We're getting there. The Event Horizon Telescope is the size on the Earth, although it only operates in infrared.

Also not sure what you mean by imaging exoplanets - several such images including timelapses have been taken.

EHT operates far from infrared (or to be precise - all IR apart from FIR) - it is actually radio telescope. We yet don't have technology to do aperture synthesis for shorter wavelengths (at least I think so - speckle interferometry is similar approach but uses single telescope with multiple apertures).

Image of black hole was done in 1.3mm band (and for comparison, IR is 2-3 orders of magnitude shorter - about 1-25um). Technically IR goes up to 1mm, but atmosphere is opaque to anything above about 25um so usable part of FIR for ground based observations is overlapping with radio and is generally considered radio (above 1mm). Doing IR image with baseline of entire earth would yield serious resolving power - probably enough to resolve features on exo planet - and that is what I suspect Alexandros meant by imaging exo planets - rather than just spotting their position / orbit.

On the other hand optical telescope on earth will always suffer from atmospheric seeing, although I wonder at those sizes if it behaves differently than with smaller apertures. After all, atmospheric disturbance is limited in size and if aperture is bigger than that - it will have less impact. Then there is adaptive optics.

Main advantage of having large aperture is of course light gathering capability. Scope of 39m diameter will have x38025 gathering power over common 8" amateur scope :D

That means that with such telescope in 1 minute exposure (on particular resolution) you can get image "matching" ~26.4 full days of stacking on 8" telescope (on same resolution), or if you are limited to 6 hours a night, you would need more than three months of constantly clear weather to do the same (about 106 six hour sessions). This is highly hypothetical since 39m diameter scope is going to have huge focal length, and resolution is going to be extreme because of that (and probably optics needed to reduce it down). But in principle that explains one reason for larger telescopes - you reduce measurement time because you gather so much more light.

With reduced measurement time you can get much more measurements done (be that imaging, spectroscopy or whatever) ...

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To be honest I saw millimeter wavelengths and thought infrared. Plus their picture was orange 😃 Thanks for correcting me.

On a sub-planet scale I think the ELT integrates several optical / NIR dishes via interferometry, and successfully measured the angular diameter of nearby red dwarfs (which are nearly planet sized).

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1 minute ago, Ags said:

To be honest I saw millimeter wavelengths and thought infrared. Plus their picture was orange 😃 Thanks for correcting me.

On a sub-planet scale I think the ELT integrates several optical / NIR dishes via interferometry, and successfully measured the angular diameter of nearby red dwarfs (which are nearly planet sized).

Well, technically IR goes up to 1mm (FIR) but atmosphere does not allow for that part of spectrum to be used:

image.png.2cafffe57fecfd8cf810a858ab40dd20.png

I wonder how they do interferometry with such small wavelengths?

As far as I can tell, there are two different approaches to it - one is physical by using wave guides and doing physical interference of waves, and another is synthetic (I believe that is one EHT uses since it is not feasible to run wave guides all over the world :D ) - but for that one you need to be able to properly sample and record wanted frequency - which happens to be in GHz range for such short wavelengths. Imagine that sampling at 800Ghz (400GHz base frequency) with two bytes per sample - that would produce something like 1.6TB of data every second for every point in the image sampled per dish! And then correlating all that data - no wonder it took them years of work to get the image done.

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https://www.eso.org/public/teles-instr/technology/interferometry/?lang

https://www.eso.org/sci/facilities/paranal/telescopes/vlti.html

"

The four 8.2-m Unit Telescopes (UTs) and the four 1.8-m Auxiliary Telescopes (ATs) are the light collecting elements of the VLTI. The UTs are set on fixed locations while the ATs can be relocated on 30 different stations. The light can be re-combined for either three (triplet) or four (quadruplet) telescopes depending on the beam-combining instrument. After the light beams have passed through a complex system of mirrors and the light paths have been equalized by the delay line system, the light re-combination is performed by the near infrared instruments AMBER or PIONIER."

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16 hours ago, vlaiv said:

Doing IR image with baseline of entire earth would yield serious resolving power - probably enough to resolve features on exo planet - and that is what I suspect Alexandros meant by imaging exo planets - rather than just spotting their position / orbit.

Yes, that was exactly what I meant, thanks for clearing that up. I realise from your posts that I am slightly behind on our capabilities with respect to what we can image, as I thought that we could not even see the other planets, just infer their existence from dips in light curves. I have some studying to do! 

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7 minutes ago, Alexandros said:

Yes, that was exactly what I meant, thanks for clearing that up. I realise from your posts that I am slightly behind on our capabilities with respect to what we can image, as I thought that we could not even see the other planets, just infer their existence from dips in light curves. I have some studying to do! 

Have a look here for a (short) list and further reference:

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

There is very nice animation of one planetary system imaged on that page

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