Sky & Telescope - December 2018 - 16

Extremely Large Telescopes, Part II

On the exoplanet front, instruments such as the GMTConsortium Large Earth Finder (G-CLEF) and the MidInfrared ELT Imager and Spectrograph (METIS) may be able to
detect so-called biomarkers - chemicals that would suggest
the presence of extraterrestrial life - in the atmospheres of
nearby Earth-like exoplanets. "METIS is going to be a killer
instrument for exoplanets," says principal investigator
Bernhard Brandl (Leiden University, The Netherlands). That's
partly because of its high spectral resolution of 100,000 in
q A GIANT AMONG MEN This preliminary ELT schematic shows
the scales involved in the three telescopes' instruments (representative structures shown on platforms). Notice the people included for
scale on the platform and ground.

Instrument
platforms
sit on either
side of the
rotatable
telescope

Lasers
to create
artiﬁcial
stars for
adaptive
optics

the mid-infrared: At a wavelength of 3 microns, for example,
the instrument will be able to resolve spectral features as
small as 0.03 nanometer.
But it's not going to be easy (or cheap) to build and operate this new generation of astronomical detectors. Current instruments are already huge. The European Southern
Observatory's Multi Unit Spectroscopic Explorer (MUSE)
instrument at the Very Large Telescope measures a few meters
across. The 570-megapixel Dark Energy Camera (DECam),
which sits at the 4-meter Blanco Telescope at the Cerro Tololo
Inter-American Observatory in Chile, weighs in at 4 tons. But
a much larger telescope aperture also means a much larger
focal plane, explains Bundy. "In the case of the TMT, the focal
plane is about 2 meters wide - it's as large as a barn door." As
a result, the instruments for the new megascopes also need to
be humongous. "WFOS is the size of a large cargo truck and
will weigh 40 tons or so."
Moreover, explains G-CLEF principal investigator Andrew
Szentgyorgyi (Harvard-Smithsonian Center for Astrophysics),
the huge size and weight of the new instruments will make it
hard to retain the necessary optical stability. During observations, major parts of any astronomical instrument need to
rotate to compensate for the diurnal motion of the sky, and
even the slightest gravitational ﬂexure would compromise the
measurements. To cope, instruments need to be constructed
of light materials with high tensile strength. "That's why the
optical bench of G-CLEF will be made of carbon ﬁber epoxy
instead of steel," says Szentgyorgyi. According to Brandl, the
European METIS instrument will even have its own internal
version of adaptive optics to meet the stability requirements.

Cosmological Challenges

Seismic isolators

DECE MBE R 2 018 * SK Y & TELESCOPE

Work with Hubble has laid the foundation for what the
coming megascopes will discover about the early universe.
Thanks to the bonus magniﬁcation of gravitational lensing by
foreground clusters, Hubble has shown us galaxies so remote
that the light we see today was emitted when the universe

COSMIC TIMELINE: G REGG DINDER M A N / S&T; ELT: ESO

p EARLY UNIVERSE By building larger telescopes and looking at longer wavelengths, astronomers are seeing earlier and earlier in cosmic time. Currently, the earliest infant galaxies are those seen as they were less than a billion years after the Big Bang. But the GMT, TMT, ELT, and JWST will push
even further, to when the first stars and galaxies formed.

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018