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Curiosity about the Extremely Large Telescope

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#1 SillySMS

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Posted 10 February 2020 - 09:59 PM

Out of pure curiosity, I'm wondering a few things about the Extremely Large Telescope, the 39.3m reflector that the Europeans are building, and a little bit more generally about the new wave of, well, extremely large telescopes (e.g. the Thirty Meter Telescope in Hawaii).

 

First: there seems to be a pretty decent amount of scaffolding in the light path, but part of that may just be visual bias. I wonder roughly what fraction of the mirror becomes obstructed, and how that would compare to amateur reflectors.

 

Second: at this scale, do you think the mirrors would be aluminized, silvered, or given some more exotic coating? I'd imagine they'd want to capture as much light as possible, even given a stable mount permitting long exposures.

 

Finally: the published resolution of 0.005 arcseconds is quite close to the visual-spectrum Rayleigh resolution of 0.0026-0.0044 arcseconds (depending on wavelength). Does that mostly just keep on going with adaptive optics, or do new issues crop up for these superscaled telescopes?



#2 ButterFly

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Posted 10 February 2020 - 11:19 PM

The f/0.93 elliptical segmented primary mirror has a diameter of approximately 39 m and a 11.1 m central obstruction.

 

That's about 8% by area - fairly normal.  Of course, "central obstruction" is a bit misleading with a cassegrain that has a hole in the middle.  A 4.2 meter active secondary is quite heavy, so scaffolding is just a part of life.  Most of these big observatories have aluminizers on site.  Uncoated aluminum has good reflectivity in the visible band and it comes off easily without damaging the substrate surface.  It's just cheaper to recoat it every few months than to overcoat it a risk damaging the substrate.  The Webb, on the other hand, is built for infrared, so it gets a gold coating instead.


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#3 seryddwr

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Posted 13 February 2020 - 10:13 PM


 

Finally: the published resolution of 0.005 arcseconds is quite close to the visual-spectrum Rayleigh resolution of 0.0026-0.0044 arcseconds (depending on wavelength). Does that mostly just keep on going with adaptive optics, or do new issues crop up for these superscaled telescopes?

Yes, these telescopes are designed to use AO from the get-go. They are designed to get diffraction limited observations in the near infrared, as that requires much less in terms of computational power as well as being mechanically easier (the shorter the wavelength, the faster things need to run and the more finely you need to sample the telescope pupil. We are already talking about 2000+ frames per second for proper wavefront correction in the near IR, for this class of telescope).


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