Hot Stuff! Huge Secondary Mirror Cast for Extremely Large Telescope

[Judi Lynn]

By Sarah Lewin, Staff Writer | May 24, 2017 07:00am ET


An enormous piece of glass-ceramic has been cast for the secondary mirror of the European Southern Observatory's (ESO) Extremely Large Telescope, entering a year-long cooldown and heat-treatment period until it's ready to be ground and polished for use with the megatelescope.

The completed mirror will measure nearly 14 feet across (4.2 meters) and weigh 3.9 tons (3.5 metric tons) — larger than the primary mirror for many current research telescopes, ESO officials noted in a statement. For the Extremely Large Telescope, though, the mirror will be suspended upside down over the 128-foot (39 meter) primary mirror, which is made of 798 hexagonal mirror segments. The telescope is scheduled to reach first light in 2024 on a mountain in Chile's Atacama Desert. ESO documents the casting process in a new video.

- click for photo -

https://img.purch.com/h/1400/aHR0cDovL3d3dy5zcGFjZS5jb20vaW1hZ2VzL2kvMDAwLzA2Ni8xNTAvb3JpZ2luYWwvZWx0LW1pcnJvci1tb2xkLmpwZz8xNDk1NDg3NTIz

After the year of cooling, the mirror base will be transported from Germany to France, where it will be ground into a precise convex shape and then polished precisely to within 15 nanometers to meet the giant telescope's needs. According to the ESO, the piece will be the largest convex mirror ever produced and the largest secondary mirror to be used in a telescope.

More:
http://www.space.com/36952-extremely-large-telescope-secondary-mirror-cast.html

[Victor_c3]

1. 14' accross and ground with a tollerance 15 nanometers?!?!?!

That is a crazy level of precision.

If I recall correctly, I believe the approximate distance between a Si-Si bond is about .25 nanometers, so we are talking a precision down to within the thickness of 60 Si atoms!!

For some perspective, the head of a pin is about 1,000,000 nanometers across and a human hair is between 50,000 and 100,000 nanometers thick.

[eppur_se_muova]

2. For decades, the largest reflector in the world was the Hale 200" (5.08 m) ...

This thing's 80% of that diameter, and it's the *secondary*.

(The Soviets built a 6 meter (236&quot reflector to outclass the Americans, but it never worked as well, and few photos taken with it were ever published.)

If you ever get the chance, be sure to read The Perfect Machine or Explorer of the Universe for some really fascinating background details. My favorite: When the project for the 200" was first initiated, there was also a proposal for a 300" floating around (plans are shown in TPM). The final decision to forego the 300 in favor of the 200 turned out to be a very fortunate one, as the challenge of building such a big mirror very nearly exceeded the technology (and funding) available. A failed attempt at a 300 might very well have resulted in further such projects being postponed for decades.