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Optical Interferometry in Space
Gerard van Belle, Lowell Observatory
Space Astrophysics Landscape for the 2020s and Beyond
2019 April 2G van Belle, Lowell - Optical Interferometry2
The Need for
Angular
Resolution
Paul Signac, “Antibes, die Türme”, 1911
2019 April 2G van Belle, Lowell - Optical Interferometry3
Spatial Resolution Advances Science
Example: Planetary science
Is the surface old or new?
Implications for population &
dynamics of Kuiper Belt
Variations in surface morphology
Chemical composition
Seasonal variations in the surface?
Evidence for plate tectonics?
2019 April 2G van Belle, Lowell - Optical Interferometry4
Heliophysics Best example: observations of the sun
Roughly 1,000,000 closer than any other star
SOHO observations of the Sun
Interesting structure Sun spots Phlages Prominences Mass ejections
Interactions with the surrounding environment
Wish to extend these observations to other stars Conversely, other stars will inform us about
the sun
2019 April 2G van Belle, Lowell - Optical Interferometry5
(12 m)(2.4 m)
Extragalactic
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Our Science Menu
From the near to the
distant
Blue = bright
Red = faint
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Our Science Menu
From the near to the
distant
Blue = bright
Red = faint
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Current State of
the Art
2019 April 2G van Belle, Lowell - Optical Interferometry9
2019 April 2G van Belle, Lowell - Optical Interferometry10
Under
construction
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Under
construction
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Under
construction
Current resolution
leader: CHARA Array
(330m resolution in
visible, NIR; 6x1m)
Current sensitivity
leader: VLTI (130m
resolution in
NIR, MIR; 4x8.4m)
2019 April 2G van Belle, Lowell - Optical Interferometry13
Science Enabled by Extreme Resolution
Still interesting things to learn
about bright objects
Stellar surface imaging
Limb darkening: upper stellar
structure
Spot mapping: convection
physics, magnetic field strength
and persistence
2.5 mas
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Imaging: Stars are Photogenic The past 10 years
Parametric modeling at first, and nowadays Direct imaging
Already starting to see some surprises Stellar structure not as expected from simple
models, particularly gravity darkening Nearly 1/6 of all Astro2020 Science WPs
concern stars
CHARA-MIRC Images of Rapid Rotators: Monnier+ 2007, Zhao+ 2009, Che+ 2011
2019 April 2G van Belle, Lowell - Optical Interferometry15
VLTI-GRAVITY
Observations of Sgr A* at
the Milky Way center
S2 passes within 1,400
Schwarzschild radii
Mass: ~10 M
Speed: 2.5% c
Acceleration: 1/6 gee
A unique strong-gravity
laboratory
1515
Gravity Collaboration,
Abuter+ 2018a
15
JWST
Diffraction
Limit
(77mas)
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Departure from Newtonian Dynamics
Gravity Collaboration, Abuter+ 2018a
Gravitational
Redshift
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Exoplanets
HR8799e spectrum from VLTI-
GRAVITY
Lacour et al. 2019 A&A 623 L11
Ja
so
n W
ang
2019 April 2G van Belle, Lowell - Optical Interferometry20
Imaging Exoplanet Transits
NPOI & CHARA, other facilities
can observe exoplanet transits
Planet’s shadow is ‘perfect’ star
spot
λ-specific observations
atmospheric composition
Extreme challenge: needs very
high signal-to-noise
HD189733 predict:
van Belle 2008
Venus
Dr. van Belle’s Patented Six-Slide
Crash Course in Interferometry
You too will be an expert in 180 seconds
2019 April 2G van Belle, Lowell - Optical Interferometry22
The Telescope: What’s Happening Inside?
Our parallel rays enter and bounce around – in a very special way
Every path of every ray from
the star traces the same
pathlength through the
telescope
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The Telescope: What’s Happening Inside?
When light rays from a source satisfy
this pathlength condition, the can
form an image
This is an ‘interference
phenomenon’
Special secret: all telescopes
are interferometersInterference
is why ‘point-like’stars appear as
Airy disks
(though thiseffect is usuallywashed out bythe atmosphere)
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The Telescope: What’s Happening Inside?
This pathlength condition is
true for other nearby stars in the
field of view of the telescope, at
slightly different angles
This dictates the very special
shape of the mirrors
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The Telescope: What’s Happening Inside?
Screw this up?
You get Hubble:
Mirror missed spec by 2000×
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In the Pursuit of Clever (at the risk of Stupid)
Here’s a neat trick: satisfy the pathlength
condition with separate pieces of glass for your
primary mirror
Examples: Keck, GTC, E-ELT, TMT, GMT
Keck ‘hex’ PSF
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Cracking the Resolution Problem
Taking the neat trick even further:
really chop up your telescope into a
long baseline interferometer
This works as long as some light is
getting to the back end, and if the
pathlength condition is met
Can make the ‘diameter’ very big
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Cracking the Resolution Problem
Taking the neat trick even further: really chop
up your telescope by making it many
telescopes
(Still have to satisfy the pathlength condition)
Viola! High spatial resolution
NB. for greatest sensitivity in the optical, one must
mix-then-detect; for radio, detect-then-mix is OK
~ Thus Concludes the Lesson ~
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Interferometry
from Space
Paul Signac, “La Corne D'or, Les Minarets”, 1907
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SIM: The Elephant in the Room?
Space Interferometry Mission
Specifically endorsed in 1990, 2000
decadals
Specifically un-endorsed in 2010
decadals
An astrometric mission that
happened to use interferometry
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Mechanical Tolerances
Simple imaging far easier than astrometry, GWs
Even easier than single-aperture work in certain regards
Mission
Physical
scale units
Mechanical
Precision units
Dynamic
range ratio task
Optical Precision during an observation
Imaging
Interferometer100 m 30 nm 3.33E+09 1 imaging / spectroscopy
SIM 10 m 50 pm 2.00E+11 60 astrometry
LISA 2500000 km 20 pm 1.25E+20 3.75E+10 grav. waves
Mechanical stability during an observation
Imaging
Interferometer100 m 1 cm 1.00E+04 1 imaging / spectroscopy
JWST 6.5 m 32 nm 2.03E+08 20313 imaging / spectroscopy
2019 April 2G van Belle, Lowell - Optical Interferometry33
Space >> Ground
Effects of atmospheric
turbulence
Coherence time: 1ms versus
10-1,000sec
Atmospheric vs. instrumental
Chaotic vs. predictable
Small space-based apertures
quickly win over larger
ground-based ones
Object:
constant
Atmospheric
turbulence:
1-10ms time
scales
Ground-based telescopes
stuck in the soup
Untrackable
spacecraft
disturbance:
10-1,000s
time scales
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Baby Steps
Simple space interferometer
Based on 2×10m manufactured booms, visible operations (non-cryogenic)
Small apertures (2”) easily more sensitive than CHARA, NPOI (1 meter!)
Made In Space
Optimast-SCI
SBIR Phase I study
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Interstellar Asteroid Oumuamua
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ISS Additive
Manufacturing First, second generation of
additive manufacturing printers
are aboard ISS
Commercial fiber
manufacturing experiment also
on-board
Further developments
‘Extended structure’
manufacturing
Thermal/vac demonstrated
2019 April 2G van Belle, Lowell - Optical Interferometry37
0.1
1.0
10.0
100.0
1,000.0
10,000.0
0.1 1.0 10.0 100.0 1,000.0 10,000.0
Size
(m
)
Wavelength (microns)
Mid-IR: Achieving 0.1” Resolution
telescope diameter (m) max interferometer baseline (m)
5 m
UV/Vis/NIR
Mid-IR/Far-IR/Sub-mm mm/radio SPIRIT
Herschel
Breaking the Confusion Limit
2019 April 2G van Belle, Lowell - Optical Interferometry38
ISO
HERSCHEL
SPIRIT
HH30
Above: Observable water that forms in regions of
protoplanetary disks, where temperatures are well below
1000 K. SOFIA/HIRMES will have access to all the water
lines shown, including the 25.9 μm line (yellow), the only
one accessible to JWST. However, without corresponding
spatial resolution, the origin of these lines can only be
inferred from disk modeling.
Left: Hubble image of HH30 compared to 100 μm beam
sizes
Water in Protostellar Disks
2019 April 2G van Belle, Lowell - Optical Interferometry39
After the Easy Things
Experience base built up with
imaging missions can lead to
Example: nulling, for planet
spectroscopy
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Why Does NASA Care?
Virtually all missions discussed in the 2015 ‘Enduring Quests Daring Visions’ report are interferometric in nature
These tools are needed to establish the fundamental nature of the cosmos
NASA will need a workforce that can plan, design, implement, and use these facilities
2019 April 2G van Belle, Lowell - Optical Interferometry41
Going from Science Fiction to Science
Image credit:
Made In Space
2019 April 2G van Belle, Lowell - Optical Interferometry42
Carpe Posterum: Exo-Earth Mapper1 pixel 10×10
100km3×325km baseline
30×30250km
100×1001000km
300×300
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Recommendations
Challenge to interferometry community
Propose Explorer and Probe-class missions in the 2020s,
motivated by the science
Demonstrate the slam-dunk science from long-baseline
interferometry
Challenge to NASA
Based on their merits, select those missions!
Support and (re-)build the interferometry community in
anticipation of 2030 Flagships – including ExoEarth Mapper
2019 April 2G van Belle, Lowell - Optical Interferometry44
Questions?
Paul Signac, “Port St. Tropez”, 1899