TheGiant Magellan Telescopeis easy the most challenging terrestrial astronomy program human race ’s ever devised . It has — quite literally — been built from the earth up by leveraging a splendid , alone off - axis invention and haemorrhage - edge fabrication techniques . And nowhere is that more apparent than in the telescope ’s enormous mirror system .
These mirror , which are still in the physical process of being cast , ground , and polished to vastly tight optical tolerances , are the effect of a radical yield process acquire by the squad at University of Arizona ’s Steward Observatory Mirror Lab . We sit down down with Buddy Martin , an astronomer and U of A Project Scientist who has head up the university ’s optical shining program since the 1980s , for more on how these incredible mirror are being made .
Gizmodo : Can you tell me a mo more about the mirror themselves — where is the glass sourced from , what does the production process entail ?
We buy the glass from Ohara , a Japanese company . It ’s a high - lineament borosilicate glass very like to Pyrex — at Ohara , they call it E-6 glass .
Some canonical facts about the mirrors :
They ’re the large mirrors ever made
They ’re the only gravid lightwave mirror ever made
They ’re arranged in a honeycomb structure that take in them 5x scant than a hearty mirror of the same dimension .
This imparts a number of advantages , which is why the GMTO determine to use these mirror . They rest stiff thanks to the honeycomb social organization and wo n’t twist under its own weight well-nigh as much as other mirror designs would . And because all the deoxyephedrine section are very thin — nothing is more than 28 mm thick — the glass follows the temperature of the zephyr around it so as the mirror cools through the Nox , the deoxyephedrine cools with it . And that avoids create turbulence that could spoil the images .
For the GMT , that means you’re able to make a 25 m telescope with very few act contribution , or parting that you have to control — it ’s only got the seven primary mirror section and the seven subaltern mirror segments .
And among the competing undertaking , the next simple had more than 500 master mirror segment ( that ’s the 30 MB scope ) and the European undertaking had even more than that : 700 – 800 segment . It makes it so there are few pieces that you have to control at a high accuracy .
The honeycomb - shaped substratum of the mirrors . Image : University of Arizona
And how are these mirror coat and dressed ?
The polishing is an interesting problem because of the figure of these mirrors — they’re very very dissimilar from a spherical surface that has a perpetual curve . And that ’s kind of been the chronicle of oculus fabrication — how to make more and more aspheric curvature .
The first telescope that Isaac Newton made had to have a spheric mirror because that was the only affair they could produce at the time . And he could get away with it because it was a very belittled telescope . But with larger scope you have to make the mirror a more parabolical form and when you get up to 25 meters there ’s a big difference between that parabolic cast and a spherical shape .
With the GMT , we ’re dealing with 14 mm of deviation from a spherical shape — it looks like a saddle , or a Pringles cow chip . It ’s really challenging to polish that shape and even more challenging to valuate it accurately . Almost everything we do has a natural disposition to work with a spherical surface where the curvature is undifferentiated and we have to essentially force it to work with this strange parabolic shape — every tool has to accommodate this change curve .
effigy : Ray Bertram , Steward Observatory , University of Arizona
We utilise a variety of shaft , mainly of two types : we have an active tool called a stress lap — a circle is just a ecumenical name for a polishing tool that we stretch , or “ stress ” across the mirror surface to tally its curvature at any given instant . That ’s a technology that we break here so as to make more aspherical surface , which is what the scope designs were calling for . It ’s a comparatively bombastic tool — about a meter in diam — and it ’s very steadfast so that if you rub it on a mirror aerofoil , it ’s stiff enough to naturally “ pick out ” the high spots , the parts that need to be assume down , and makes it fluent .
The other type of instrument we use is a little , more pliant pecker — it ’s flexile enough to follow the changing curvature of the mirror — those are between about 10 and 35 cm in diam . We use those , not for smooth — they’re not stiff enough for that — but rather for “ direct disfiguring ” where we describe the in high spirits pip and drop more metre rubbing on those spots to lend them down to mate the low-toned places .
It ’s a very iterative process . we got through 50 to 80 cps of polishing and measuring . When we assess , we return a contour single-valued function of the errors in the mirror surface . We feed those mapping into a computer that hold the smooth tools so that it either spends more time or exerts more pressure on the high spots .
Wait , so the process is automated ?
It ’s largely automated yes . At least the shining itself is . When we ’re running these tools around on the methamphetamine , it ’s entirely calculator controlled . The human element is deciding what kind of tactic to habituate free-base on the error map .
prototype : Carnegie Institution for Science
And what do you use to generate that contour map ?
There are a number of unlike measuring devices . The main and most important one , and this is pretty much standard throughout the manufacture , is ring an interferometer . It illuminates the mirror surface with laser illumination , the light reverberate back to the instrument which collect that light .
The optics in the gimmick are set up so that with a perfect mirror , the igniter will return with the same wave front that initially give the instrument which is spot as a perfect surface .
If the mirror has a bump on it , then that gets impress onto the return wavefront and the tool can analyze that visible radiation coming back , identifying the high and first to a resolution of about 1/100th of a wavelength — about 5 millimicron .
And what sort of truth tolerances is the team aim for ?
Roughly speaking , we can have an modal error of about 25 nm . That is , the open has to be within 25 nm — one one-millionth of an inch — of a “ unadulterated ” surface form . The measure ( which , again , resolve down to 5 nm ) are good enough , the bigger challenge is polishing it and controlling the control surface at a 25 New Mexico accuracy .
How long does it take to fully grind and polish a mirror segment ?
It ’s rough a year of polishing per mirror . We ’ve only polish off one , and that was the first . It was the first mirror of this kind that had ever been built anywhere so it took us a little longer with that one as we had to develop and build all of the equipment and technique for it . We ’re already beginning work on the second mirror and we ’re expecting that , once we hit our stride , it will be a yr of round per segment .
There are other level of the fabrication too . First we have to evaporate the glass into its signifier , then we have to do some grinding . Those are more or less the three stage : casting the mirrors , dig them into pattern , then polishing them . fortuitously , we have three station so we can be work on three mirrors at the same time .
So how much deviance do the mirrors decease the casting physical process with ? How much travail is required before the mirror goes on to the polishing phase ?
It ’s worth noting that we do spin casting so we ’re rotating the glass as it ’s melted which will eventually organize a parabolical Earth’s surface but not one that ’s terribly precise . We ’re not assay to achieve optical calibre here . For a more traditional , symmetric mirror , we can get it within about a millimeter of the right shape out of cast . But because of uncertainties in the casting physical process we also have to sum some extra glass so it end up being a 6 to 8 mm deviation that we have to grind out .
Now GMT mirror are dissimilar in that they have these 14 mm of saddle cast departure from a perfectly symmetric surface so our casting physical process — if you spin a liquid state , you get a parabola and that ’s what our casing produces . We have a symmetrical parabola which is the best estimate of the bicycle seat - shaped , off - axis parabola that we involve so we always have to crunch in that 14 millimeter of saddleback shape .
The mirror has that shape is because the entire 25 m telescope is a symmetrical parabolical mirror — but the segments , which are not in the center , hence “ off - axis”—are asymmetrical bit of that larger parabola and when you take these offset firearm of it , you see the 14 millimeter of saddleback - shaped departure . That ’s what we need to put into the outer segments of the GMT mirror . During grinding , we get down to an truth of about 100 micron . Then the polishing will reduce that gradually down to the 25 nm tolerance .
Do you anticipate any other applications for these polishing tool and methodological analysis outside of the GMT project ?
It ’s a somewhat specialized process but is upright for more than just big astronomy telescope mirrors . Really any sort of exact visual system that uses lenses or mirror — especially aspheric variety — can do good from this . The technology is being used in other place to make mirror for telescopes , imaging system , and light - concenter optics for fusion energy programs .
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