the clear night sky offers one of the
most beautiful views in nature the I
adapts to the dark and the pupil widens
to collect more light and thus allow
fainter stars to become visible but the
light-collecting area of the human eye
is tiny to appear much deeper into the
night sky astronomers need telescopes
with enormous primary mirrors to do a
much better job
this is the esocast cutting-edge science
and life behind the scenes of ESO the
European Southern Observatory exploring
the ultimate frontier with our host dr.
J aka dr. Joe Liske why do astronomers
want to have bigger and bigger
telescopes
well it's pretty simple actually there's
only two reasons number one reason is
that the bigger the primary mirror of
your telescope the more like you can
collect $MONEY per unit time and that
means you can observe fainter and
fainter objects number two reason is
that the resolution of your telescope
that is the sharpness of the images that
you can make with your telescope depends
on the size of the primary mirror the
bigger your telescope the sharper the
images you can make but one of the
limits how big can you make a telescope
and what are the challenges encounter by
telescope builders in making bigger and
bigger mirrors
since the invention of the reflecting
telescope mirrors have become larger and
larger when he sews 3.6 meter telescope
at last year started operations in 1977
it was a typical example of the
classical design of the largest
telescope that . the primary mirror
consisted of a single glass dish with a
diameter of 3.6 meters in order to make
such a big mirror stiff and solid it has
to be relatively think which makes it
very heavy the 3.6 meter mirror is about
half a meter thick and weighs some 11
tons to allow this very weighty mirror
to be pointed precisely a massive yet
precisely balanced telescope structure
has to be built around it telescopes
with even larger thicker and hence
heavier mirrors have been constructed
but eventually it became obvious that
the limit of the classical design had
been reached did telescope engineers
have to give up at this point and stop
dreaming of even bigger telescopes
well of course not but painting the way
to larger and lighter mirrors required
some innovative thinking the result was
easels new technology telescope or
entity for short
the entity was a truly revolutionary
telescope at the time it was built
because it featured a system called
active optics now before the invention
of active optics telescope mirrors had
to be thick and therefore heavy in order
to be stiff but with active optics
telescope mirrors could be allowed to be
flexible and therefore relatively light
and thing the thin mirror of the NTT is
even more likely to bend due to gravity
with active optics the flexible mirror
is placed on a complex support system
with computer-controlled actuators that
adjust the shape of the mirror and
compensate for the bending of the mirror
during observations this way the best
possible image quality is preserved at
all times the NTT was a tremendous
success although its main mirror is 3.6
meters in diameter
it is only 24 centimeters thick the new
mirror design made it possible to break
the six meter barrier of classical
telescopes and strife for larger mirrors
in the eight metre class
telescopes like esos Very Large
Telescope or VLT the VLT consists of
four unit telescopes with primary
mirrors of 8.2 meters diameter each
mirror blank is only 17 and a half
centimeters thick and weighs only some
23 tons naturally active optics plays a
vital role here the shape of the mirror
is actively controlled by means of a
hundred and fifty axial force actuators
based on the latest available technology
the VLT delivers images of outstanding
optical quality but a solid single piece
8-meter mirror is pretty much at the
limit of what can be handled transported
and maintained to be able to construct
telescopes with even larger
light-collecting areas you really have
no choice but to split up the primary
mirror into individual pieces called
segments it was the concept of a
segmented primary mirror that allowed
astronomers and engineers to conceive of
truly gigantic telescopes telescopes
such as the future
european extremely large telescope which
is currently in its early stages of
construction
the e-elt will have a gigantic main
mirror 39 meters in diameter
it will be made up of 798 individual
hexagonal segments about 1.4 meters
white and just five centimeters think
each segment and its position with
respect to the neighboring segments are
computer controlled by the active optics
system to maintain the perfect overall
shape of the main mirror and its
outstanding surface precision all in all
the main mirror offers unprecedented
light-collecting area of 978 square
meters which will collect about 15 times
more light than any other existing
telescope with 39 meter primary mirror
the European extremely large telescope
will be by far the largest optical and
infrared telescope in the world and not
just at the time of its completion but
for decades to come
however I'm sure that won't stop the
engineers from conceiving of ways to
build even larger telescopes who knows
what size barrier will be cracked in the
distant future
this is dr j signing off for the esocast
join me again next time for another
cosmic adventure
I can't hear about the only person
it doesn't have TV coverage of the
things and it man they got the black up
now you're gonna be around getting a TV
picture now happen
bill yet we are getting a
baby pictures turn out
quantity
ok