Sky & Telescope - December 2018 - 18

NGC 4650A: ESO / M USE CONSORTIU M / R. BACON;
WAVELENGTHS: TERRI DUBÉ / S&T

Extremely Large Telescopes, Part II

Indirectly, the new instruments will also help to solve the
nagging problem of the composition of the universe. Various
lines of evidence indicate that baryonic matter (essentially,
atoms) constitute at most 5% of the matter/energy content
of the universe. The remainder is in the form of dark matter and dark energy, the true nature of which is frustratingly
unknown. Thanks to their exquisite resolution and sensitivity, extremely large telescopes will be able to study the distribution of dark matter in tiny dwarf galaxies and to reveal
small-scale substructure in the distribution of dark matter in
the vast cosmic web through gravitational lensing.
As for dark energy: Observations of supernova explosions
in the most distant galaxies will help establish the expansion history of the universe, which is ultimately dictated
by the subtle balance between the attractive gravity of dark
matter and the repulsive "anti-gravity" of dark energy (S&T:
May 2018, p. 14). Moreover, ELT scientists hope to be able to
measure the resulting accelerated expansion of the universe
directly, independent of assumptions about the large-scale
curvature of spacetime and cosmological models. Thanks to
the accelerated expansion, the apparent recession velocity of
distant quasars (which is a substantial fraction of the speed
of light) should increase with a few centimeters per second
over a decade, slightly increasing the object's redshift. The
huge light-collecting power of the ELT, together with new
developments in quantum optics, should enable astronomers
to actually measure this tiny redshift drift. Such a unique
measurement would constitute the first direct evidence of
accelerated expansion and would certainly help in solving the
mystery of dark energy.
High-resolution spectra of remote quasars may also
provide an answer to yet another nagging question: Are the
constants of nature really constant, or do they slowly change
with time? In particular, the fine structure constant (Į) - a
measure of the strength of the electromagnetic interaction
q COLOR-CODED GALAXY Using the MUSE integral-field spectrograph on the Very Large Telescope, astronomers created this color
composite of the polar ring galaxy NGC 4650A. The colors represent the
velocities of star-forming regions due to the rotation of the galaxy's disk:
Blue regions are approaching, red regions receding.

First-generation Eyes
Listed are instruments
currently slated to be part
of the first-light package.
Astronomers are already
discussing second-generation instruments. Wavelength ranges and fields
of view are not final, as
some teams are still considering design options.

THIRTY METER
TELESCOPE (TMT)
1. WFOS (Wide Field
Optical Spectrometer)
Near-ultraviolet and optical widefield multi-object imager and
spectrograph
Wavelength range:
0.31 - 1 μm
Field of view:
~10 arcmin diameter
2. IRIS (Infrared Imaging
Spectrometer)
Near-infrared adaptive opticsfed integral field imager and
spectrograph
Wavelength range:
0.8 - 2.5 μm
Field of view:
32 × 32 arcsec (imager)

EXTREMELY LARGE
TELESCOPE (ELT)
1. MICADO (Multi-AO
Imaging Camera for Deep
Observations)
Near-infrared adaptive opticsfed imaging camera and
spectrograph
Wavelength range:
0.8 - 2.4 μm
Field of view:
50.5 arcsec × 50.5 arcsec
2. HARMONI (High Angular
Resolution Monolithic
Optical and Near-infrared
Integral Field Spectrograph)
Near-infrared integral-field
spectrograph
Wavelength range:
0.47 - 2.45 μm
3. METIS (Mid-Infrared ELT
Imager and Spectrograph)
Mid-infrared imager (with
coronagraphic capability)
and medium-resolution
spectrograph
Wavelength range:
3 - 20 μm
Field of view:
17.6 × 17.6 arcsec

q WAVELENGTH RANGES Astronomers are pushing into the
infrared (about 1 to 1,000 microns, or 1,000 to 1 million nanometers)
with both current and future telescopes. In part, that's because light
reaching us from the first galaxies has redshifted to this range. But
infrared wavelengths are also good for peering through dust; detecting cool, red stars; studying planet-forming disks; and observing bodies in our solar system. Blue indicates future telescopes.

Hubble Space Telescope 0.115 - 2.5
TMT 0.31 - 2.5
Keck 0.3 - 5
GMT 0.34 - 5.4

0

18

DECE MBE R 2 018 * SK Y & TELESCOPE

5

10

15
Microns



Sky & Telescope - December 2018

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

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