February 29, 2020 1

2.3 Observing Solutions [Astronomy: State of the Art]

2.3 Observing Solutions [Astronomy: State of the Art]


So how are we overcoming these limitations
on telescopes? The biggest limitation, historically, was the size of the mirror. In 1948 the Palomar
200 inch telescope was commissioned in Southern California. It was the largest mirror ever
built and for decades it would be the largest mirror in the United States until, the University
of Arizona, about 10 years ago, built a bigger mirror. When the Palomar 200 inch was
built nobody thought there would be a larger telescope. If you ever been there, the dome
is the size of a cathedral, has ten thousand tons of moving glass and steel. It’s a huge
and fantastic device. But our bigger telescopes are actually in smaller buildings because
we have compact telescope designs. So the first progress has been made in building bigger
mirrors in 5 meters or 200 inches. The mirror for the Large Synoptic Survey telescope or
LSST was the largest mirror ever made at the time we cast it under the stands of our
football stadium at the University of Arizona. It’s a phenomenally accurate, as well as large
mirror, if you imagine this mirror which is larger than most people living rooms, expanded
to the size of the continental United States the biggest imperfections would be bumps less
than 1 inch high. It’s the most accurate surface ever made. Telescopes with mirrors this large
also have physically large detectors. They’re still charge coupled devices like used in
your phone or camera but they are much bigger. The CCD in your camera or cell phone might
be a size of your little finger nail but you can see the size of the detector in the Large
Synoptic Survey telescope. It’s a huge data rate. Every time you push the button and take
a picture gigabytes pour in and the data rate in 1 night will be 20 terabytes when this
telescope is commissioned in a couple of years. That’s a firehose worth of data that
astronomers will struggle to keep up with. As I mentioned, we started making the biggest
mirrors in the world under the stands of our football stadium. Our football team is kind
of mediocre, but the mirrors we make in the football stadium are exceptional. We use them for own projects, and we also sell them to other consortia for their own telescopes. And occasionally, we make mirrors for people like the Air Force, but that’s so secret I can’t even talk about
it. The first time we eclipsed the Palomar 200 inch mirror, was with a 6 and a half meter mirror for the multiple mirror telescope. With this 1 monolithic mirror replaced the 6 1.8 meter mirrors that
had been inside previously. This telescope is located on Mount Hopkins in Southern Arizona.
having cut our teeth on our first 6 1/2 meter mirror,we started to make more, 1 about every
18 months. The guru of this work is a man called Roger Angel, a pioneer who first worked out
how to make large and light mirrors; he is a professor in the department here. We made
two 6.5 meter mirrors for the twin magellan telescopes located in Chile, at the Las Campanas
Observatory. I’ve used both of them a number of times; they’re magnificent telescopes on
one of the darkest sights on earth. Then we stepped up to a 8.4 meters which is the largest
mirrors that can be made with the technique we’re currently using. Two of these
8.4 meter mirrors were installed in the last couple of years in the Large Binocular Telescope
located on Mount Graham in Southern Arizona. These two mirrors work together to make the
effective of aperture of over 11 meters; Making this the largest telescope on earth. Seeing
the twin mirrors in their cells inside the LBT dome is as awesome sight. These are enormous
pieces of hardware, and they’re all so exquisitely precise. The object of our current dreams
and aspirations is a telescope called the Giant Magellan Telescope, also to be located
in Chile, on an adjacent mountaintop to the twin 6.5 meter mirrors we already have. This
one will take 7 of the 8.4 meter mirrors and put them together like flower petals around a
central mirror. Creating an equivalent of a 22 or 23 meter mirror telescope, which would
be by far the largest telescope in the world. We hope to finish this project within 6 years
and get first light in about 7 years. What’s the trick by which we make such large mirrors
when it seemed impossible, and for 30 or 40 years no larger mirrors were made in the United
States or anywhere in the world. These tricks all come from Roger Angel, and the very clever
scientists and engineers who work in the mirror lab at Steward Observatory. The mirror is
essentially a honeycomb, which means it’s not a solid block of glass The glass used is
accidentally borosilicate, which is just a variant of common sand, so a very simple
ingredient. The honeycomb mirror has a smooth and continuous face plate of glass which is
only 1 inch thick but most of the volume of the mirror is a honeycomb where its mostly
air. The a reason for a honey comb mirror is twofold. The first is to make the mirror
light much lighter than it would be if it was a solid block of glass that thick. The
second is that a light mirror can have its temperature equalized with the surroundings.
One of the things that degrades images from a large telescope is when the temperature
of the mirror is different from the air surrounding it, or the structure surrounding it.these
light honeycomb mirrors can be kept in close harness with the temperature of the surrounding
structure to within a tenth of a degree C. We can walk through the steps involved in making
1 of these huge mirrors. All of this taking place invisibly to the students and faculty
of the University of Arizona, under the stadium stand. A large tub has ceramic fiber boxes
installed, each one in the shape of a hexagon. There are 1681 of them; each one is unique,
and together they form the contour of a parabolic surface. So the mirror can be formed by liquid glass in more less the shape it needs to be. Next, 18 tons of borosilicate
glass are loaded on top of the pre-formed hexagons. borosilicate is close to common
sand, but it needs to be a of unbelievable purity to make a very accurate mirror surface.
We get this glass from a small foundry, on the northern island of Hokkaido. In fact, it’s
a family run business and it’s amazing that such an operation can make glass with the
purity we need. We need so much of their glass that I think we have the order book filled
a decade. After carefully loading the top of the preformed shapes with the blocks of
glass, each one about 10 pounds, the lid of the oven is closed and sealed. The oven starts
to spin, reaching a peak speed about 5 revolutions per minute. Within it, a carefully controlled
temperature cycle begins, which will reach a peak temperature of 1160 degrees Celsius
hot enough to melt the glass. As the glass melts, it liquefies and falls between the
shapes of the hexagons filling the space between them and leaving a 1 inch faceplate that’s
in the perfect parabolic shape as the oven spins. Then the temperature is slowly reduced.
Glass can fracture if its cooled too quickly so the mirrors go through a 3 month slow cooling
called a kneeling, which prevents bubbles from forming and the glass from fracturing
due to stresses. Its a nerve racking time because you’re not allowed to look at the
mirror or even crack the lid for a few weeks and you really don’t know until the end of
the 3 months whether you’ve made a good mirror or a bad mirror. All this work is just the
first step. By spinning the mirror you’ve approximated a parabolic shape which is what
we need for the final mirror, but it’s not yet with the precision needed. The mirror
is then moved on to what’s called a stress lab polishing machine where using something
like jewelers rouge machines grind away the surface down to the millionth of a meter level
to produce a perfect optical shape. This process also takes months sometime 6 or 8 months to
produce the spec on the mirror. When the mirror is finished if you could imagine in the mirror
doing this, it could read a newspaper at a distance of 5 miles. There are still challenges
ahead. Once you’ve made your mirror and polished it to the required spec, you have to get it
on top of a remote mountain top. these are huge pieces of glass that need to be encased
and protected. It turns out that even the best military lift helicopter cannot get a
mirror of this mass on to a high mountain top where the air is thin. They must be taken
up by road. Often the road has to be remade to get the huge mirror on it’s articulated
vehicle around the hairpin turns. The vehicle itself has huge cantilever weights that swing
in and out to stop the tractor trailer from toppling off the mountain side. It’s a hair raising procedure and it takes a week to 10 days to get one of these mirrors from
Tucson to one of our nearby mountain tops. Once the mirror is located in the telescope
cell, it never moves. Older style telescopes like the Palomar 200 inch used to have the
mirrors taken out of their cells every year so, moved across the floor and re-aluminized
in a special chamber.These modern telescopes are far too fragile to ever risk them by moving
them out of the telescope so they’re re-aluminized every couple of years, in situ, in place,
very thin coats aluminum are sputtered onto the surface to make it clean. Remember that
telescopes sit in a natural environment that involves dust, moisture, wind, hopefully no
rain, but they degrade over time and so every couple of years all large telescopes need
their surfaces recoated. let’s meet some of the people who make these large mirrors here
at the University of Arizona. We’ve seen that the historical impasse on the largest size
of an optical mirror has been broken by making lightweight mirrors. The University of California
and Keck observatory has pioneered the use of thin, light weights segments with are mosaic
to make a single large surface. Here at the University of Arizona we make large monolithic
mirrors where they are made light by having a honeycomb structure where most of the volume
is in fact air yet, they’re very stiff and light and can move easily around the sky.
Mirrors 8 .4 meters in size have been made this way, and were planning to make 6 or more
as part of a huge telescope called the Giant Magellan Telescope.

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