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The ESO New Technology Telescope NTT is a 3'58m telescope which pioneered the use of active optics'

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Title: The ESO New Technology Telescope NTT is a 3'58m telescope which pioneered the use of active optics'


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The ESO New Technology Telescope (NTT) is a 3.58m
telescope which pioneered the use of active
optics. The telescope and its enclosure had a
revolutionary design for optimal image quality.
The NTT first light happened in 1989.
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  • Vatican Observatory, Mt Graham, Arizona.
  • a 1.8-m f/1.0 honeycombed construction,
    borosilicate primary mirror, manufactured at the
    University of Arizona Mirror Laboratory.
  • Pioneered both the spin-casting techniques and
    the stressed-lap polishing techniques now being
    used for telescope mirrors up to 8.4-m in
    diameter.
  • The primary mirror is so deeply-dished that f/D
    1.
  • allows a structure that is about three times as
    compact as the previous generation of telescope
    designs.

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Advantages of Spin-Casting
Rotating the furnace--a technique called
spin-casting--saves time and money in the
production of large telescope mirrors. It allows
centrifugal forces to shape a natural curve in
the surface of the molten glass. (Swirling a
liquid in a glass creates the same effect.) The
curvature created this way is close to the
mirror's final parabolic figure. If the mirror
were cast in the traditional stationary furnace,
its surface would be flat, and huge quantities of
glass would have to be ground out and discarded
to create the curvature needed to focus
starlight. For the first 6.5 meter mirror, that
would have equaled 12 tons of wasted glass and
one year of extra work. The rotation speed is
determined by the desired mirror curvature. To
create the f/1.25 focal ratio for the 6.5 meter
mirror, the furnace was spun at 7.4 rpm. Steward
Observatory's honeycomb mirrors are made of
borosilicate glass that is similar to that used
in commercial glass ovenware.
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From VLT webpage
The 8.2-m Zerodur primary mirrors of the ESO Very
Large Telescope are 175 mm thick and their shape
is actively controlled (active optics) by means
of 150 axial force actuators, the necessary
active corrections being obtained from wavefront
sensors located off-axis on the image surface.
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The mirror blanks are produced by spin-casting.
The process (figure 3) starts with the casting of
approximately 45 tons of glassy Zerodur into a
concave mold. Thereafter the mold is transported
onto a rotating platform where it is spun until
solidification. When the temperature has
decreased to about 800 ºC and the viscosity is
such that the blank will retain its meniscus
shape, it is brought into an annealing furnace
where it is cooled down to room temperature in
about 3 months.
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From Gemini Webpage
For many decades it was thought impossible to
build a telescope as large as Gemini because the
main (primary) mirror would have to be
excessively thick and heavy to maintain its
precise shape. Now, thanks to new technology,
Gemini uses a relatively thin primary mirror that
is able to hold its precise shape with a little
help. Mounted behind the mirror are 120
"actuators" that constantly nudge the mirror back
into perfect form. These adjustments are
typically only about 1/10,000 the thickness of a
human hair and are enough to keep starlight
precisely focused so astronomers can study the
universe. Using this technology Gemini should
lead the way for a whole new generation of
telescopes that will be larger than anything ever
imagined by astronomers even a decade ago.
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Gemini 1 mirror being moved from the grinding
table to the polishing table at REOSC, July
1997.
After fusing into flat monolithic piece
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From Gemini Webpage Whenever starlight passes
through our atmosphere, it's distorted by
turbulence. The effects of this turbulence on
star light is called twinkling. Because twinkling
blurs images made through a telescope, scientists
go to great lengths (and heights) to reduce its
effects. One of the reasons that the Hubble
Space Telescope was put high above the earth's
atmosphere was to escape its adverse effects.
Since earth orbit is not an option for Gemini, a
relatively new technology called Adaptive Optics
will be used. Adaptive Optics simply takes a
sample of starlight, determines how the
atmosphere bent it, and then uses a deformable
mirror to "straighten" the starlight out again.
Because stars are so far away, starlight passing
through our atmosphere consists of parallel rays
of light that are bent and diverted by air of
different temperatures and therefore different
densities. Of course our atmosphere is constantly
changing and mixing together, so the effect is
very random and quite dynamic. When starlight
enters a telescope like Gemini, if nothing is
done, the distortions caused by the atmosphere
are magnified and stars often look more like
shimmering blobs than the pin-points of light
they would be if viewed from space. However,
before starlight passes into many of the
instruments or cameras on Gemini, a
representative column of starlight is diverted
into what is called a "Wavefront Sensor." The
column of light entering the wavefront sensor is
a representative sample of the light that is
being collected across the entire main mirror of
the telescope. In other words, any distortions
that are visible to the wavefront sensor
correspond directly to distortions somewhere in
the atmosphere above the telescope. In order to
use this information, the wavefront sensor
separates the column of light into many areas or
zones and samples each zone to determine how the
light was altered by our atmosphere. By taking
samples many times per second, the information
from the wavefront sensor is fed back to a
"flexible" mirror that can be adjusted to
counteract for the distortions caused by the
atmosphere. These adjustments are very small, and
can't even be seen if you were to watch the
mirror. AO systems work best with longer
wavelength light, which means that Gemini will
see the most dramatic results with infrared
observations. Using this system, it is expected
that Gemini will produce the sharpest images yet
of the infrared sky and dramatically improve many
other types of observations as well.
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The Sodium laser launching from the side of the
Lick Observatory 120" Shane Telescope. This is a
10 minute time exposure - note the star trails.
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What is Speckle Imaging? Speckle imaging is a
technique that allows astronomers obtain very
high-resolution information about astronomical
objects. It works by taking many short exposure
images in rapid succession. In other words, a
speckle observation is simply a movie. In each
frame, the blurring effects of the atmosphere
spread the light out over a fuzzy patch, but with
in that patch there are bright points, called
speckles. From frame to frame, the position and
intensity of the speckles changes, and it turns
out that by studying the speckles in each short
exposure, you can get the information you need to
"reconstruct" a high resolution image. If you
just take a long exposure image, the speckle
character will wash out and you'll just be left
with a fuzzy patch. One use of the much higher
resolution that speckle imaging gives you is to
resolve very close binary stars that would
otherwise be blurred together in normal CCD
images taken from the ground. We can also
determine the mass of such systems and compare
them to theoretical predictions. This process
represents one of the most basic checks on what
we think we know about how stars really work.
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