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Posted by: JRehling Aug 27 2021, 02:30 PM

The thread name here seems to be the going classification (which I find needlessly vague and sensational) for telescopes of a distinctly larger size than any that now exist. By a quirk of funding, strategy, and even rivalry, the size of the world's largest telescope is in the process of undergoing a revolution, rather than an evolution. Rather than seeing the record broken many times, year by year or even decade by decade, we're now in the midst of an long run where the largest telescope has plateaued at about 10 meters aperture and nothing new is going to come along for quite a while.

This is, in part, because rather than the world's resources focusing on one new ground-based telescope, then moving on to top that feat, three different efforts are all working simultaneously on three different projects, and as a result, each of them is moving comparatively slowly, and one fine day, possibly at the end of this decade, we will have three new telescopes, all much bigger than anything we have now.

I am posting now just to note the state of various delays that have afflicted each of the three efforts; naturally, COVID has introduced delays into each. The ELT (Extremely Large Telescope, co-opting the general term that people are using for all telescopes in this class) has an expected first light of 2027, and it will shatter the record for largest telescope, offering an aperture of 39.3 m, about 9 times the light gathering area of the largest existing telescope. Then the runners-up, the Gran Magellan Telescope and Thirty Meter Telescope, are expected to have first light in 2029. The GMT will come online incrementally, with some of its component mirrors put into place before it is complete. Meanwhile, the TMT is facing issues in approval and acceptance that make any deadline that is quoted dubious.

ELT and GMT will be located in northern Chile, while TMT will be in Hawaii, which will give it a distinctive exclusivity in seeing the far northern sky while the other two will share access to the far south.

It's tantalizing to have all of these undergoing delays; as of 2014, hopes were for first light in 2024. When they come online, we may get a sudden rush of discoveries, and the comparative advantages of each of them will not mean that the ELT alone will notch all of the future breakthroughs. Instruments on the GMT may end up making it the best for certain exoplanet science, even though it will be the smallest of the three.

In the meantime, sit back and be patient. The 2030s should be quite a decade for astronomy, with all three (or even two) of these both at work.

Posted by: PDP8E Oct 15 2021, 11:30 AM

didn't know quite where to put this -- I saw that JPL has a Sherlock image down from Mars on OCT 14, near the end of Solar conjunction

... a Large DSN scope
Let's say pointing at Mars --- at or near conjunction, it must be pointing at or near the Sun.

Pointing anything at the Sun requires precautions -- the parabolic dish must concentrate some solar energy at the 'horn' (!?)
I would be interested to hear what operations does about this.

Posted by: mcaplinger Oct 15 2021, 12:51 PM

The DSN antennas are painted white and are diffuse in the visible, so it would surprise me if there was significant heating at the focus. I've never heard anything about this being an issue.

Posted by: JRehling Oct 15 2021, 02:36 PM

Mars was about 3° away from the Sun, as seen from Earth, at that time. I don't know how wide the passive FOV of a DSN radio dish is, but I have personally photographed Mercury with a telescope when it was 5° from the Sun, and there was no hazard.

Presumably the paint on a radio dish is as close as possible to Lambertian so if you were near the focal point looking back at the scope's collecting area, it wouldn't look much brighter when aimed at the Sun than when aimed several degrees away.