SkyandTelescope.com March 2014 27Finally, a fourth test involves projecting a simple pat-
tern of lines at the mirror and comparing the refl ected
result with a reference pattern.Tracking Down Mysterious Errors
The polishing of the fi rst GMT mirror did not go as
smoothly as anyone would have liked. The fi rst two
devices, the asymmetric null corrector and the scanningpentaprism, test for optical defects such as spherical aber-
ration. And to the scientists’ chagrin, the results from
these two tests did not agree. Something was wrong with
at least one of the tests, and the mirror might have had
more optical defects than anticipated.
“We looked for a lot of potential sources of error,” Mar-
tin explains. “We kept fi nding that one error after another
was not there.”
After months of tests and experimentation, they fi nally
found it: a small error in the pentaprism test. The camera
used in the device is no ordinary camera — it contains
an array of microscopic lenses, one for each of its detec-
tor’s pixels. Unbeknownst to the scientists, the lenslets
had displaced the pentaprism’s focused beam by a tiny
amount that depended on the beam’s direction. A small-
scale lab test showed that the displacement caused by the
lenslets mimicked spherical aberration almost exactly.
Correcting for this eff ect brought the two tests into
much better agreement. Now the scientists knew that the
mirror’s spherical aberration was small enough that they
could easily correct for it using the 165 active supports
that control the mirror’s large-scale shape. These supports
bend the mirror based on feedback from sensors within
the telescope because, as with all large mirrors, the GMT
mirrors will fl ex under their own sizable weight.Telescope GMT TMT E-ELT
Location Las Campanas, Chile Mauna Kea, Hawaii Cerro Armazones, Chile
Segments in primary mirror 7 circles 492 hexagons 798 hexagons
Segment size 8.4 meters 1.44 meters (corner to corner) 1.44 meters
Eff ective diameter of primary 24.5 meters 30 meters 39.3 meters
Construction start April 2014 April 2014 March 2014
First science 2020* 2022 2024
Projected cost $800 million $1.2 billion $1.5 billion
Funds raised to date More than half About 80% About two-thirds* Using four of seven mirrors.
Installation of remaining mirrors
scheduled to complete by 2022.POLISHING An engineer monitors the Large Polishing Machine
as it shapes the fi rst GMT mirror. Tests are performed throughout
polishing to ensure the mirror’s face has the correct shape.Visit skypub.com/
megatelescopes
for more information on
the three mammoth tele-
scopes’ progress.The Megatelescopes Are Coming
The Giant Magellan Telescope isn’t the only behemoth planned for the next decade. The Thirty Meter Telescope (TMT) and the European
Extremely Large Telescope (E-ELT) are competing with the GMT for the largest-in-the-world title. The table below summarizes the plan, prog-
ress, and schedule for the three megatelescopes.R. BERTRAM / STEWARD OBS. MIRROR LAB / UNIV. OF ARIZONAGIANT MAGELLAN TELESCOPE / GMTO CORPORATION TMT OBSERVATORY CORPORATION ESO / LUÍS CALÇADAGMT Mirror.indd 27 12/23/13 11:34 AM