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SPACE CHRONICLEST
his is the first segment blank for
mirror 1 (M 1 ) of the ELT. Manu-
factured by SCHOTT, it was com-
pleted at the end of August 2018.
M 1 is the primary mirror of the ELT
and, when completed, will be 39
metres in diameter and consist of
798 segments. The ELT is a revolu-
tionary project that will allow as-
tronomers to obtain images of
larger planets and characterise their
atmospheres, possibly enabling de-
tection of biomarkers that could in-
dicate life on other planets. [ESO]of the low-expansion glass-ceramic
Zerodur©and are circular, measuring
1. 5 metres across and about 5 cen-
timetres thick. The back of each
blank is flat and the front surface is
concave. The blanks come in three
types with slightly differentlyshaped
front surfaces, depending on the
planned location of the segment in
the primary mirror. The first primary
mirror segments were cast by
SCHOTT in 2018. SCHOTT manufac-
tures the blanks to approximate
shape so that Safran Reosc does not
need to remove too much material
during the polishing process. Safran
Reosc will polish the blanks before
they are cut into hexagons and re-
ceive a final precise polishing using
Ion Beam Figuring. When it is com-
pleted, the primary mirror will com-
prise 798 hexagonal segments andwill have a total light-collecting area
of 978 m^2. In total, more than 900
segments will need to be cast and
polished (including a spare set of
133 segments). Once built the ELT
will be the largest ground-based tele-
scope in operation. The telescope is
being built at Cerro Armazones in
the Atacama Desert of northern
Chile. With a primary mirror almost
40 metres in diameter, the ELT will
gather almost 100 million times
more light than the human eye,
which will allow scientists to tackle
some of the biggest challenges in
current astrophysics, such as the for-
mation of the first stars, the assem-
bly of the first galaxies, the char-
acterisation of the atmospheres of
Earth-like exoplanets, or the nature
of dark matter and dark energy,
among many other questions.shock sensors travel with the blanks
to measure any sudden acceleration
or shock that could affect the blanks.
The shipping container with the first
18 blanks was sent from SCHOTT in
Mainz, Germany, on 23 July and ar-
rived at Safran Reosc the following
day. The ELT mirror blanks are made!chronicles EN_l'Astrofilo 29/08/2019 15:56 Page 23