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New Worlds Imager
Webster Cash, University of Colorado
New Worlds Imager
W. Cash – University of Colorado 1
New Worlds Imager
An Alternative to TPF
Webster Cash
Jim Green
Eric Schindhelm
Nishanth Rajan
Jeremy Kasdin
Bob Vanderbei
David Spergel
Ed Turner
Sara Seager
Alan Stern
Steve Kilston
Erik Wilkinson
Mike Leiber
Jim Leitch
Jon Arenberg
Ron Polidan
Chuck Lillie
Willard Simmons
University of Colorado
Princeton University
Carnegie Institution – Washington
Southwest Research Institute – Boulder
Ball Aerospace
Northrop Grumman
MIT
and growing…
New Worlds Imager
W. Cash – University of Colorado 2
New Worlds Imager
W. Cash – University of Colorado 3
Exo-Planets
CExo-planets are the planets that circle stars other than our Sun.
CThere are probably 10,000 exo-planets within 10pc (30 light years) of
the Earth.
CPlanets are lost in the glare of parent star.
CThe Earth as viewed from light years is 10 billion times fainter than
the Sun.
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Planet Finding: Extinguish the Star
Courtesy of N-G
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Terrestrial Planet Finder
CTelescopes must be PERFECT to suppress scatter:
λ/5000 surface,
99.999% reflection uniformity
CTPF is very difficult
CIs there any easier way?
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New Worlds Imager vs. New Worlds Observer
CTwo Levels of Difficulty
CNew Worlds Observer
– Two Spacecraft
– Goal is Finding Planets
– Science from Photometry and Spectroscopy
– Technology is In-Hand Today
CNew Worlds Imager
– Five Spacecraft
– Goal is True Imaging of Earth-like Planets
– MUCH Tougher – Technology 10-15 years out
New Worlds Imager
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Initially New Worlds was a Pinhole Camera
Perfect Transmission
No Phase Errors
Scatter only from edges – can be very low
Large Distance Set by 0.01 arcsec requirement
diffraction: λ/D = .01” à D = 10m @500nm
geometric: F = D/tan(.01”) = 180,000km
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Diffraction Still a Major Problem for Pinhole
Answer: Shape the Aperture
(Binary Apodization)
Developed by Princeton
Group for Apertures
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The Occulter Option
CSmaller Starshade
– Create null zone, image around occulter
CObserve entire planetary system at once
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The Diffraction Problem Returns
CSeveral previous programs have looked at occulters
CUsed simple geometric shapes
– Achieved only 10-2 suppression across a broad spectral band
CWith transmissive shades
– Achieved only 10-4 suppression despite scatter problem
http://umbras.org/
New Worlds Imager
BOSS
Starkman (TRW ca 2000)
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Extinguishing Poisson’s Spot
COcculters Have Very Poor Diffraction Performance
– The 1818 Prediction of Fresnel led to the famous episode of:
– Poisson’s Spot (variously Arago’s Spot)
– Occulters Often Concentrate Light!
CMust satisfy Fresnel Equation, Not Just the Fraunhoffer Equation
CMust Create a Zone That Is:
– Deep
– Wide
– Broad
Below 10-10 diffraction
A couple meters minimum
Suppress across at least one octave of spectrum
CMust Be Practical
– Binary
– Size
– Tolerance
New Worlds Imager
Non-transmitting to avoid scatter
Below 150m Diameter
Insensitive to microscopic errors
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The Vanderbei Flower
CDeveloped for Aperture in TPF focal plane
CWas to be only 25µ across
CVanderbei had determined it would work for the
pinhole camera but did not work for occulter.
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The Apodization Function
Found this in April. Extended in June.
This Function Extinguishes Poisson’s Spot to High Precision
A( ρ ) = 0
for
ρ < r1
and
A( ρ ) = 1 − e
New Worlds Imager
 ρ − r1 
−

r
 2 
2n
for
ρ > r1
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Suppression of Edge Diffraction
Can Be Understood
Using Fresnel Zones and Geometry
CThe occulter is a true binary optic
– Transmission is unity or nil
CEdge diffraction from solid disk is
suppressed by cancellation
– The power in the even zones cancels the
power in the odd zones
r2
r1
ØNeed enough zones to give good deep cancellation
• Sets the length of the petals
– Petal shape is exponential ~exp(-((r-r1)/r2)2n)
Ør2 is scale of petal shape
Øn is an index of petal shape
Ør1 is the diameter of the central circle
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Doing the Math
(Cash, 2005)
Is = E
CThe Residual Intensity in the Shadow is
CBy Babinet’s Principle
Es = 1 − E A
2
s
where EA is field over Aperture
CSo We Must Show
k
2π d
C
2π r1
∫ ∫e
ik ρ 2
2d
e
ik ρ s cosθ
−
d
ρ d ρ dθ +
0 0
2π ∞
∫ ∫e
ik ρ 2
2d
e
ik ρ s cosθ
−
d
2n
e
 ρ −r1 
−

 r2 
ρ d ρ dθ = 1
0 r1
d is distance to starshade, s is radius of hole, k is 2π/λ
CTo one part in
New Worlds Imager
C ≈ 10−5
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Contrast Ratio
CPreceding integral shows the contrast ratio is
–
 ( 2n ) !  d λ 
R =  2n 2 n 

 r1 r2  2π 
2n



2
– n is an integer parameter, currently n=4
CTo keep R small r1~r2
– this is the reason the occulter has that symmetric look
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Off Axis Performance
CThe off axis performance shows a rapid rise to unit transmission
for the radii greater than the inner edge of the habitable zone
Shadow Profile
0
-2
n=2
Log10 Contrast
-4
-6
-8
-10
-12
n=6
-14
n=4
-16
-18
0
5
10
15
20
25
30
35
Radius (meters)
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Modified Rendering
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“Standard” Observatory Views the Starshade
~0.1” resolution is needed (just to separate planets)
High efficiency, low noise spectrograph (e.g. COS)
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Count rate estimation
CAssuming visible solar flux and a half-earth viewed at 10
pc,
2 2
FS rE DT
C∝
2
εγ dS
CCan achieve 5 counts per second with 80% efficient 10
meter telescope Telescope Time required for
1 meter
2 meter
4 meter
8 meter
New Worlds Imager
S/N=10 detection
33.3 minutes
8.3 minutes
2.1 minutes
31 seconds
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Another Issue:
Scattered Light
CSunlight Scatters Off
Starshade
CCan be Controlled in Multiple
Sun
Ways
Target
–Look at right angles to sun
ØImposes restrictions on revisit times
–Operate in shadow
ØEarth’s umbra
ØWith additional shade
• Likely hard at L2
• Easier in heliocentric orbit
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Starshade Tolerances
CPosition
ØLateral
ØDistance
Several Meters
Many Kilometers
CAngle
ØRotational
ØPitch/Yaw
None
Many Degrees
CShape
ØTruncation
ØScale
ØBlob
1mm
10%
3cm2 or greater
CHoles
ØSingle Hole
ØPinholes
New Worlds Imager
3cm2
3cm2 total
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Fly the Telescope into the Shadow
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Dropping It In
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Typical Observing Timeline
CAlignment
3 days
Travel
– Other astrophysics
CDeep Photometry
1 day
CPreliminary Spectroscopy 1 day
CDetailed Studies
3 days
– Deep Spectroscopy
– Extended Photometry
–
CReturn After Months
–
New Worlds Imager
Find Planets
Classify Planets
Search for
Water
Surface Features
Life ?!
Measure Orbits
New Planets from Glare
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The First Image of Solar System
Uranus
Galaxies
Zodiacal Light
Jupiter
Saturn
Neptune
10 arcseconds
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Great Science with Small Telescopes
CLower limit on telescope size set by need to acquire adequate
signal and resolve planets from one another
– 1 m diameter telescope needed to see 30M object in minutes
ØResolution of 0.1 arcsec
– 2 m diameter gives count rate 0.2 sec-1 for Earth at 10 pc at half illumination
Uranus
Earth
Jupiter
Mars
50,000 seconds
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Saturn
Neptune
400,000 seconds
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Zodiacal light
CPlanet detectability depends on system inclination and telescope
resolution
– Face-on 0.3 AU2 patch of zodi equal to Earth’s brightness
CZodiacal light can wash out planets at low inclinations
60°
30°
10°
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Zodiacal Light – 0.05” IWA
Pole-on
New Worlds Imager
Edge-on
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Spectroscopy
CR > 100 spectroscopy will distinguish terrestrial
atmospheres from Jovian with modeling
H2O
O2
CH4
NH3
S. Seager
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Photometry
Calculated Photometry of
Cloudless Earth as it
Rotates
It Should Be Possible to Detect Oceans and Continents!
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Alternate Operations Concepts
CGround based telescope
ØRelay mirror at GEO
ØSouth Pole
CSpace based telescope
ØAs JWST instrument
ØDedicated telescope and mission
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Occulter and Detector Craft
Functions
CPropulsion
CStation keeping
CAlignment establishment and maintenance
– Measurement and reporting of relative location
CData transfers
CPointing requirements dependent on tolerancing of
occulter
– Pointing error results in an error in the occulter shape by projection
CWhat is the role of the ground in directing the two SC?
– Cost trade?
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Formation Flying Simulation
CLargest problem is solar radiation pressure
– Pinhole craft’s cross sectional area: 7150 m²
– Craft will be thrown out of libration point orbit
after several days
Total stationkeeping ? V [m/s]
L2
L5
20,000 km
10.2
20.3
200,000 km
9.8
20.7
L4
L1
L2
Number of burns during exposure
L2
L5
20,000 km
6700
3740
200,000 km
6700
3810
New Worlds Imager
L5
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Formation Flying Simulation
CStationkeeping ?V estimated in STK/Astrogator
– Detector craft assumed active; pinhole craft assumed passive
– Control box of 10 cm half-width defined
– Active S/C thrusts when box boundaries reached
– Gravity of Earth, Sun, Moon included, plus solar radiation pressure
– Separations of 20,000 km and 200,000 km considered at Earth-Sun L2 and L5
h
Detector
New Worlds Imager
Detector craft
center of mass
Pinhole
Control box
[inertially fixed with respect to
optics craft]
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EELV 5
meter
heavy
Up tp 150 m New Worlds Observer
Will Fit in an ELV Heavy
Generic L2
Bus
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New Worlds Deploys Like Solar
Arrays
CSimple,
robust,
proven
deployment
scheme
New Worlds Imager
Simple low cost solar
array style deployment
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TRUE PLANET IMAGING
3000 km
1000 km
300 km
100 km
Earth Viewed at Improving Resolution
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Solar System Survey at 300km Resolution
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starshades
50,000km
NWI Concept
combiner
collector craft
1500km
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Holding the Array
field star beams
Delay Lines – Mixers - Detectors
primary collector
planet beams
field star collimator
planet collimator
to combiner
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Planet Q-Ball
Imperfection visible
with adequate signal
One Imperfection
Fringes as Telescopes
Separate
Information is there:
We will study the realistic limits of two element interferometers
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Resolution Limitation Set By Signal
CAt 10 km resolution the interferometer is photon-limited
CNeed Much Bigger Telescopes – Too Expensive
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The Phase II Study
CTwo Year Study Began on September 1
CObserver Mode Well Understood
ØComplete Architecture Study Completed in First Year
ØLaboratory Demonstration of Diffraction Suppression
CImager Mode More Difficult
ØWill Study Requirements in Detail
ØWill Look for Ways to Make the Mission More Affordable
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Conclusion
By 2011
By 2018
H2O
O2
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