The sixth monolithic mirror for the GMT has been cast and is now cooling

Cal Jeffrey

Posts: 3,491   +1,040
Staff member
In context: The Giant Magellan Telescope is set to be the world's most powerful telescope when engineers complete its construction by the end of this decade. It will have 10 times the Hubble Space Telescope's viewing power and four times the strength of the James Webb Space Telescope.

Even though the casting of the sixth massive mirror for the Giant Magellan Telescope (GMT) commenced about a week ago, it will still be another two months before it cools to room temperature. After that, it will take two years to polish and another two years to apply the finishing touches, including the 1,000-atom-thick reflective aluminum layer coating the front surface.

It is an excruciatingly long process carried out by the University of Arizona's Richard F. Caris Mirror Lab. Even though the project started in 2005, only two mirrors have been completed and are waiting in storage. Mirror six and three others are in various stages of the production process. Although the GMT only requires seven mirrors for viewing, the lab will build an eighth as a spare to be swapped into the telescope whenever one of the others needs maintenance.

Part of the reason the production process is so long is that these are the biggest mirrors ever created. Each one is over 27 feet in diameter and more than two-feet thick. Despite their large size, the mirrors have a honeycomb-like structure that makes them 80-percent hollow and much lighter than solid mirrors. That said, each of the monolithic disks requires 20 tons of high-purity, low-expansion, borosilicate glass (called E6 glass). The honeycombing also provides the stiffness the mirrors need to resist flex as the telescope moves and to resist the effects of gravity.

Another factor that contributes to the long fabrication time is the polishing required. Both the front and the back of the mirror need polishing, but the reflective side has to be painstakingly accurate.

"Once cooled, the mirror will be polished for two years before reaching an optical surface precision of less than one-thousandth of the width of a human hair or five times smaller than a single coronavirus particle," said the GMTO Corporation in a press release.

Even though two years seems like a long time to polish a mirror, it would take even longer if engineers had to grind the curvature out of a flat surface. To shave years off construction, the Richard F. Caris Mirror Lab built the world's only spinning furnace. The glass is heated to 2,129 F while the mold rotates at five revolutions per minute. Centrifugal force pushes the glass to the edges of the mold and creates the rough parabolic curvature the mirror needs saving a lot of time (below).

The seventh mirror is scheduled to be cast sometime in 2023. In the late 2020s, all the mirrors will be transported to a port city, probably Houston. From there, they will be shipped through the Panama Canal, down the South Pacific Coast to Chile. By that time, the observatory, which is already under construction, will be ready to have the mirrors mounted. Scientists will not have to wait for all seven mirrors to be hooked up to start making observations. The GMT only needs four mirrors in place to function.

In addition to the giant telescope, the observatory will have dormitories, and full facilities once completed so scientists can live on-site. However, astronomers will have remote access to the location as well. The entire project should reach completion around 2030.

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yRaz

Posts: 4,398   +5,118
Someone correct me if I'm wrong, but doesn't aluminum oxidize and turn into aluminum oxide giving it a matte finish?

I'm sure the people building this are smarter than me but I do have to wonder
 

wiyosaya

Posts: 7,693   +6,626
Someone correct me if I'm wrong, but doesn't aluminum oxidize and turn into aluminum oxide giving it a matte finish?

I'm sure the people building this are smarter than me but I do have to wonder
Much like amateur telescopes, I bet this mirror will have an anti-reflection coating applied over the aluminum layer. AR coatings are typically some sort of oxide that will protect the aluminum from oxidizing. It might say more about it here - https://www.gmto.org/

@Cal Jeffrey WRT this telescope being the world's most powerful, it is likely more accurate to say that this will be the most powerful American/US telescope. With a filled aperture diameter of 39m as opposed to the GMT's 24.5m that is comprised of several smaller yet separate mirrors in its design, the European Extremely Large Telescope will easily exceed the capabilities of the GMT. The E-ELT is expected to be operational in 2025. See https://www.eso.org/sci/facilities/eelt/

This is going to be an amazing decade for Astronomy.
 

umbala

Posts: 603   +1,007
Someone correct me if I'm wrong, but doesn't aluminum oxidize and turn into aluminum oxide giving it a matte finish?

I'm sure the people building this are smarter than me but I do have to wonder
I'm not sure I understand your question. I assume they sandwich this ultra thin layer of aluminum between the glass and some other backing material that protects it from oxidization. That's how it works with regular mirrors too.
 

captaincranky

Posts: 18,722   +7,665
I'm not sure I understand your question. I assume they sandwich this ultra thin layer of aluminum between the glass and some other backing material that protects it from oxidization. That's how it works with regular mirrors too.
Not really. Home mirrors are rear surface, which means the silver is applied to the back of the glass, and then covered over, (as you say)..

Due to their massive thickness,, these mirrors mus be, "first surface mirrors". So, the silver coat can't be "backed", as you describe, but rather covered with some sort of clear coat.

Were you to coat the rear of the glass, the only light that would reflect back in exactly the same plar it struck the mirror, would be dead center, and at a 90 degree angle. Any angular light striking it, would have its angle doubled in the reflection, creating what is know as parallax error. Which would be greatly increased by the extreme thickness of the glass. Different thickness and different curvatures would aggravate the effect.

I know it seems incongruous to cast a piece of glass that size and thickness just to put the silver on top. But in reality, glass is one of the few materials able to be polished to the degree necessary.

I'm assuming the glass being used, due to its thickness would tolerate changes in temperature without expanding and contracting. Any shift in the air temperature, wouldn't register in the glass for days, thereby eliminating any change in size. Which would in turn, fracture the silver coating.

Whenever light passes through a piece of (seemingly) clear glass, there is still a light loss associated with it. This manifests itself severely in multi element lenses. This is why all sorts of high tech lens coatings have been developed to combat light loss.

As an example, I have a 70--200 mm zoom lens. Although the lens is physically an F 2.8 aperture, the camera's light meter metadata, perceives it as an F 3.0, or slightly "slower", (presenting less light to the sensor),than its actual physical dimensions.

(I think I have that right, does anybody else want to ring in on this)?
 
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quadibloc

Posts: 357   +239
Someone correct me if I'm wrong, but doesn't aluminum oxidize and turn into aluminum oxide giving it a matte finish?

I'm sure the people building this are smarter than me but I do have to wonder
Certainly, aluminum does oxidize. Silver, which reflects light better than aluminum, also oxidizes, though. Other metals or alloys that stay shiny permanently are considerably less reflective than those choices. The aluminum can be protected with a coating that will also improve how well it reflects light.
 

Tempus

Posts: 27   +18
Someone correct me if I'm wrong, but doesn't aluminum oxidize and turn into aluminum oxide giving it a matte finish?

I'm sure the people building this are smarter than me but I do have to wonder
The first surface aluminum is finished with a Silicon Monoxide overcoat to prevent exposure to oxygen as well as offer additional protection to the aluminum.
 

wiyosaya

Posts: 7,693   +6,626
Not really. Home mirrors are rear surface, which means the silver is applied to the back of the glass, and then covered over, (as you say)..

Due to their massive thickness,, these mirrors mus be, "first surface mirrors". So, the silver coat can't be "backed", as you describe, but rather covered with some sort of clear coat.

Were you to coat the rear of the glass, the only light that would reflect back in exactly the same plar it struck the mirror, would be dead center, and at a 90 degree angle. Any angular light striking it, would have its angle doubled in the reflection, creating what is know as parallax error. Which would be greatly increased by the extreme thickness of the glass. Different thickness and different curvatures would aggravate the effect.

I know it seems incongruous to cast a piece of glass that size and thickness just to put the silver on top. But in reality, glass is one of the few materials able to be polished to the degree necessary.

I'm assuming the glass being used, due to its thickness would tolerate changes in temperature without expanding and contracting. Any shift in the air temperature, wouldn't register in the glass for days, thereby eliminating any change in size. Which would in turn, fracture the silver coating.

Whenever light passes through a piece of (seemingly) clear glass, there is still a light loss associated with it. This manifests itself severely in multi element lenses. This is why all sorts of high tech lens coatings have been developed to combat light loss.

As an example, I have a 70--200 mm zoom lens. Although the lens is physically an F 2.8 aperture, the camera's light meter metadata, perceives it as an F 3.0, or slightly "slower", (presenting less light to the sensor),than its actual physical dimensions.

(I think I have that right, does anybody else want to ring in on this)?
I would be surprised if there were a telescope with a "second surface" mirror in it. The image would be degraded over a first-surface mirror. Even commercial scopes for those of us that cannot afford to buy as big of a scope that is the subject of this article, :laughing: are all first-surface mirrors (Meade, Celestron, et. al).

Perhaps, there is a manufacturer out there that makes a second-surface mirror reflector, however, by amateur astronomers in the know, they would be quickly classified as a manufacturer to stay away from - just like those refractor manufacturers that make 2" scopes that claim 360X magnification - just to sell telescopes to those who know no better. Years ago, the highest recommended usable power in a telescope is 60X per inch of aperture whether reflector or refractor. I do not think that recommended value has changed.
 

captaincranky

Posts: 18,722   +7,665
I would be surprised if there were a telescope with a "second surface" mirror in it. The image would be degraded over a first-surface mirror. Even commercial scopes for those of us that cannot afford to buy as big of a scope that is the subject of this article, :laughing: are all first-surface mirrors (Meade, Celestron, et. al).
I knew that, but felt I had rambled on long enough. Even my long windedness, has some semblance of standards. Besides, you know I type way slower than you.
 

captaincranky

Posts: 18,722   +7,665
Not to mention that this scope could probably see them in your neighborhood from San Francisco. ;)
Oh hell, even the average run of the mill spy satellite could do that. I will say that if you pointed that scope at one of those big ol' butts at sundown, you would have yourself an artificial solar eclipse disc. You know a "total solar eclipse" is when the moon covers the sun entirely. :rolleyes:

Not that that was on topic, but I'm going to stray a bit further..,....

Way back when the Philadelphia Zoo had elephants. I was standing way back with a telephoto, trying to line up the backsides of the female visitors with the rears of the beasts. You know morbid obesity in the female Homo sapiens has reached its zenith, when you can use a 200 mm, tele to do a "spatial relations" shot. :scream:

(In fairness, I did use a couple of astronomical terms, (in bold), to try to keep this on topic).
 
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wiyosaya

Posts: 7,693   +6,626
I knew that, but felt I had rambled on long enough. Even my long windedness, has some semblance of standards. Besides, you know I type way slower than you.
Rambling? Since when is being coherent rambling?

On that note, and to keep this somewhat on topic, The Canon EOS Ra is a mirrorless EOS camera that has a built-in IR filter that admits more of the Hydrogen Alpha line for astronomical imaging (emission nebulae) - however - it also, according to my information, works well for daylight photography, too. https://skyandtelescope.org/astronomy-resources/astrophotography-tips/astro-dslr-cameras/ Unlike previous Canon EOS astropotography suitable DSLR entries.

I know people do not like the Ra, but I might consider buying a new piggy-bank to save up my spare pennies for it. :laughing: