Transcript Off-Axis Telescopes for Dark Energy Investigations SPIE 7731-52, 30 June 2010 M.Lampton (UC Berkeley) M.

Off-Axis Telescopes for Dark
Energy Investigations
SPIE 7731-52, 30 June 2010
M.Lampton (UC Berkeley)
M. Sholl (UC Berkeley)
M. Levi (LBNL Berkeley)
Dark Energy?
• A name we give to describe the observed acceleration
of the expansion of the universe
• Could be the “cosmological constant” in GR
– Very hard to explain why that isn’t huge, or zero
• Could be something else!
– Varying over time; maybe even over space!
• Different theories predict how DE evolves
• Test: BAO – a standard ruler, shows expansion history
• Test: SNe – a standard candle, shows expansion history
• Test: WL – shows growth of structure over history
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Baryon Acoustic Oscillations: standard rulers
http://mwhite.berkeley.edu/BAO/bao_iucca.pdf
How to measure redshifts of 30 million
galaxies per year, with σz = 0.001/(1+z)?
Use slitless spectroscopy!
Komatsu et al arXiv 1001.4538
Then: z=1100
Ruler = 400kly
Now: z=0
Ruler = 400Mly
Tighter PSF => smaller σz => Bigger Survey
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Type Ia Supernovae: standard candles
Red: SNR at peak. Others: earlier and later times
Kowalski et al., ApJ 686, 749 (2008)
Figures courtesy A.G.Kim 2010
Peak spectrum
explosion
Reference spectrum
Tighter PSF => Less Texp > Bigger Survey
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Weak Lensing: probe growth of structure
Space based WL program seeks 30
galaxies/sqarcmin, 0.2 arcsec, 25th mag
galaxies; needs good PSF and stability
Strong
lensing
A2218
http://www.cita.utoronto.ca/~hoekstra/lensing.html
Weak
lensing
statistical
concept
Rhalf, arcseconds
Jouvel et al., “Designing Future Dark
Energy Missions” A&A 504, 359 (2009)
Tight PSF and small pixels are
mandatory to get these galaxies
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JDEM
Interim Science Working Group Report http://jdem.lbl.gov (2010)
Science Objective
Supernova Redshift
Survey
BAO Galaxy Survey
Weak Lensing Survey
Design A
1500 supernovae
Redshifts 0.2<z<1.5
Tiered survey areas for discovery
Halpha flux 2e-16 erg/cm2sec
Spectroscopic redshifts 1.3<z<2.0
RMS z < 0.001·(1+z)
16000 square degrees in 1.5 years
none
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Design B
Same as Design A
Same as Design A
10000 square degrees
30 galaxies per square arcmin
Redshifts from Photo-Z
1e5 spectro calibration galaxies
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JDEM
Interim Science Working Group Report http://jdem.lbl.gov (2010)
Element
Telescope
Wide field imager
For BAO centroids
For SN discovery searches
In Design B, for cosmic shear
Design A
1.1m unobscured aperture TMA
0.5 square degree FoV
Two bands: 0.7-1um, 1-1.5um
32 Mpixels, each 0.45arcsec
HgCdTe 2Kx2K
Design B
Similar to A
Similar to A
Similar to A
More & finer pixels
HgCdTe and/or Si CCD
Slitless prism spectrometer
For BAO galaxy redshifts
0.5 square degree FoV
One waveband 1.5 – 2.0 um
32 Mpixels, each 0.45arcsec
Similar to A
Similar to A
Similar to A
Supernova Slit or IFU spectrometer
Light curves, spectra, host redshifts
Narrow field (a few arcseconds)
One waveband 0.4 – 2.0um
Similar to A
Similar to A
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Key Mission Requirement: Survey Rate
• Simple formulas like…
N 2min  Δλ FoV AEF
Survey Rate  0.25

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SNR  B π R 2half
• JSIM: a public web tool created by M.Levi
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http://jdem.lbl.gov/ “Exposure Time Calculator”
• JSIM inputs are high-level mission parameters
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This talk
Telescope Aperture, central obstruction size, WFE…
Field of view on sky, pixel scale, focal length, number of sensor chips
Detector Technology: pixel size, pixels per chip, waveband, QE curve
Fraction of time allocated to BAO, SNe, WL, calibration, downlink, …
Mission duration
• JSIM outputs are “high level” mission yield & FOMs
• JSIM outputs also available at “low level” individual FO’s.
• Bottom line: smaller point spread function boosts yields
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JSIM Internal Databases & Models
http://jdem.lbl.gov/ “Exposure Time Calculator”
• BAO emission line galaxy Hα flux, size, and redshift distribution
– Ilbert et al 2005
• WL galaxy magnitude, size, and redshift distribution
– Leauthaud et al 2008 zCOSMOS; Jouvel et al 2009
• Supernova occurrence rate vs redshift
– Lesser of published curves by Sullivan et al 2006 and Dahlen et al 2008
• Zodiacal light vs wavelength and ecliptic latitude
– Leinert et al 1998; Aldering 2001
• Optical point spread function
– MTF contributions from pupil diffraction and WFE via Fischer’s Hopkins Ratio
– Gaussian two dimensional random attitude control errors
– Sensor pixel size; interpixel diffusion
• Sensor contributions (dark current, read noise, QE)
• Signal-to-noise ratio estimation
– Optimal extraction, convolving galaxy exponential with system PSF
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Obscured vs Unobscured Focal TMAs
These historical examples are both focal but afocal configurations are equally good
Obscured, here with 1.2m aperture
f/11; 13mEFL 18um = 0.285”
FoV = 0.73x1.46deg =166 x 330mm
Easy fit to 4x8 sensors.
< 3umRMS theoretical PSF
Real Cassegrain image: control stray light
Real exit pupil: control of stray heat
Best with auxiliary optics behind PM;
Easy heat path for one focal plane.
Unobscured, also with 1.2m aperture
f/11, 13mEFL, 18um=0.285”
FOV = 0.73 x1.46deg = 166x330mm
Easy fit to 4x8 sensors.
< 3umRMS theoretical PSF
Real Cassegrain image: control stray light
Real exit pupil: control of stray heat
Easy heat path to cold side of payload for
entire SM-TM-FP assembly; can
accommodate several focal planes.
Korsch,D., A.O. 16 #8, 2074 (1977)
Cook,L.G., Proc.SPIE v.183 (1979)
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PSFs For Unaberrated Pupils
Scaled for equal incident flux and equal PM diameter
Shows both obstructed light loss and diffraction loss
Fresnel-Kirchoff diffraction integral
Unobstructed
Obstructed: 50% linear, 25% area
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Encircled Energy as a Fraction of the Total
Transmitted Light with no aberrations
Fresnel-Kirchoff diffraction integral: Schroeder 10.2
Linear obstruction = 0%, 10%, 20%, 30%, 40%, 50%
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EE50 Radius (arcsec) Comparison
Held constant: f/11, WFE=0.1µm rms, pixel =18µm, blur= 1µm, ACS blur=0.02 arcsec.
1.1m obscured
1.3m obscured
1.1m unobscured
1.3m unobscured
• Results show little
difference in the visible
since we are not
diffraction limited there
• However longward of one
micron, diffraction
dominates the PSF, and
the unobscured looks
attractive.
• On the faintest targets,
Rhalf hurts you like its
square (ouch)
Wavelength microns
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Eliminating the SM support spider legs
HST file image courtesy STScI
At Galactic midlatitudes, diffraction rings and spikes bring the focal plane irradiance
to twice Zodi over 1% of random locations. Elimination: slightly improved survey
efficiency; eases background subtraction, reduced “coverage gap” correlation .
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Some Unobscured Concepts
Mountaintop
Solar
Mountaintop
General Astron
Spaceborne
Remote Sensing
Spaceborne
Stellar
Spaceborne
Planet Search
McMath: Pierce11
NST: Denker et al.12
ATST: Rimmele13
LAPCAT (proposed): Storey et al14
NPT (proposed): Moretto & Kuhn15
4m DFL (proposed): Moretto & Kuhn16
MTI: Kay et al.17
TopSat: Price18
QuickBird: Figoski19
EO-1 ALI: Lencione et al20
CartoSat: Subrahmanyam et al21
GAIA: Perryman22
DIVA (proposed): Graue et al23
JPF (proposed): Krist et al24
TPF (proposed): Noecker25
ECLIPSE (proposed): Trauger et al26, Hull et al27
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Manufacturing & Testing Challenges?
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Off-axis: more material removal and greater aspheric departure
Off-axis: non axisymmetric test setups need more time & care
Existence proof: Giant Magellan Telescope segments: 8.4m!
Today’s laser trackers can deliver submicron surface metrology
Vendors caution us that going off-axis is do-able but not “free”
Aspheric Departure of 1.1m f/11 On-axis and Off-axis TMA Primary Mirror
Aspheric Departure (mm)
0.5
0.4
On-axis Telescope
Off-axis Telescope
0.3
0.2
0.1
0
-500
0
500
1000
Radius from optical axis (mm)
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1500
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Payload Packaging Challenges?
• Traditional space telescope payloads are on-axis cylinders
• Traditionally the launch fairing is a cylinder plus an ogive
extension
• Good match!
• Off-axis telescope is not a good match
• However … going to Earth-Sun L2 Lagrange point requires an
EELV whose launch fairing is 4m diameter. Plenty of room for
any layout of a 1.1 m aperture telescope
• As presently envisioned, JDEM packaging is not constrained by
EELV fairing, even though off axis
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Many JDEM Trade Studies Remain
Content et al.; Sholl et al.; Lieber et al.; Noecker; Edelstein et al.; Besuner et al.; Reil et al.
• Focal vs Afocal rear-end architecture
• Imager requirements and design
– Field of view; plate scale; pixel size; waveband(s)…
– How to calibrate it: flats, darks, wavelength, linearity…
• Wide field spectrometer requirements
– Field of view; plate scale; pixel size; waveband…
– Resolving power; issue of redshift accuracy.
– How to calibrate it: flats, darks, wavelength, linearity…
• Supernova spectrometer requirements
– Single slit vs integral field slicer architecture
– Field of view; plate scale; pixel size; waveband
– How to calibrate it: flats, darks, wavelength, linearity…
• The overall mission design: how to best integrate objectives
• And then… of course … there’s all the engineering!
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Obscured
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Unobscured
Traditional in space astronomy
Axisymmetric PM has lower
manufacture & test cost for given
aperture because total departure
from sphere is less
If Wide field: SM baffle is large then
there is appreciable light loss from
SM blockage of the pupil
Diffraction by SM: a concern
Scattering by SM support spiderlegs:
an annoyance, esp for WL
Spider leg flex can contribute to
resonances that influence PSF
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Unobscured space telescopes are
employed for terrestrial remote
sensing (DoE M.T.I.) with severe
requirements on stray light
Superior MTF, PSF, and EE nearly equal
to ideal Airy pattern
Industry lacks flight mirror experience
in sizes above 0.6m => higher risk and
potentially higher fab cost
Potentially reduced stray light, stray
heat => tiny risk reduction and possibly
more thorough testing
Potentially a stiffer, stronger structure:
no spider legs
Decision: to be based on benefits, cost, and risk assessment
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Conclusions
• At λ>1µm, pupil obstruction is a concern
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Diffraction dominates the PSF and EE
PSF and EE influence science return
S/N ratio is major driver on Texp, aperture, FoV.
BAO team seeks a high survey rate in the NIR
WL team seeks a high survey rate and a high density of resolved
galaxies, which is very sensitive to PSF growth
– SN team seeks high S/N spectroscopy at highest redshifts
• Unobstructed pupil improves performance in all these areas
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Backups
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Dark Energy
• Our observed universe: expanding, accelerating, lumpy
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Hubble: and many many others: expanding! H(0)
COBE , WMAP: warm, isotropic, shows primordial structure
Perlmutter et al; Riess et al.: SNe, standard candles: accelerating! H(z)
Eisenstein et al; Cole et al.; structure; standard rulers: BAO => H(z)
• Explanations
– Einstein (1917) General Relativity: geometry; many tests tried and passed
– Many alternative theories are out there
SIX PARAMETER FLAT ΛCDM
• If GR is correct…
– Empirically today…
Ωm + Ωk + ΩΛ = 1
0.27 + 0 + 0.73 ≈ 1
• …But there are puzzling aspects of this!
– What is Λ? Physics offers no answer.
– Why is Ωm ~ ΩΛ today, i.e. why now?
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Physical baryon density Ωb
Physical CDM density Ωc
Physical DE density ΩΛ
Scalar curvature Δ2R
Spectral index ns
Reionization optical depth τ
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Committees & Reports
• Dark Energy Task Force
– Albrecht et al., Sept 2006
– http://www.nsf.gov/mps/ast/aaac/dark_energy_task_force/
• Figure Of Merit Science Working Group
– Albrecht et al., Dec 2008
– http://jdem.gsfc.nasa.gov/
• JDEM Science Coordination Group
– Gehrels, April 2009
– http://jdem.gsfc.nasa.gov/
• Interim Science Working Group
– Moos & Baltay (co-chairs) May 2010
– http://jdem.lbl.gov/
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DETF Recommendations
http://www.NSF.gov/mps/ast/detf.jsp (2006)
“… For these reasons, the nature of dark energy ranks among the very
most compelling of all outstanding problems in physical science. These
circumstances demand an ambitious observational program to determine
the dark energy properties as well as possible.”
• Recommended that multiple techniques be pursued
• Baryon Acoustic Oscillations: less affected by astrophysical
uncertainties than other methods, but presently less proven
• Supernovae: presently is most powerful & best proven; but
systematics will depend on astronomical flux calibration
• Weak Lensing: emerging technique; may become the most
powerful technique in constraining dark energy.
• Clusters: good statistical potential; but presently has largest
systematic errors.
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BAO: Requirements & Implementation
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Require: redshift range 1.3<z<2.0
Survey 16000 sq degrees of sky
Identify emission line galaxies by the Hα
line feature, and/or other lines
Sample faint enough to reach ~2E-16
erg/cm2sec line flux
Yields about 1 galaxy /sq arcmin
Yields about 50 million galaxies
Required accuracy σz = 0.001/(1+z)
Plan: slitless spectrometer with a wide
FoV ~ 0.5 square degree
Span wavelengths 1.5µm<λ< 2.0µm
Exposure time ~ 1ksec/field
32000 spectro fields + cal fields
http://jdem.lbl.gov/ “Rolling Disperser”
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Type Ia Supernovae: standard candles
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“SD” model: Whelan & Iben (1973)
WD accretes matter from a binary companion
Carbon or oxygen white dwarf star; no H or He
WD mass reaches 1.38 Msun =
– Radius begins shrinking rapidly
– Gravitational energy = -1E44 joule = -1 “foe”
It will heat and collapse. Fusion ensues…
12C→24Mg →56Ni →56Co →56Fe + 0.12% Mc2
– If 67% efficient: 2E44 joule = +2 foe
Annihilates the WD star!
Roughly 1E44 joules remain for KE & light
Good uniformity: calibrated standard candles
Measure each peak brightness and redshift
Fit the observed SNe to a distance modulus curve
Each DE model predicts a distance modulus curve
So… compare these to constrain models.
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Kowalski et al., ApJ 686, 749 (2008)
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Supernova Program Requirements
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Quantity of Supernovae for statistics
– Span the redshift range 0.2<z<1.5
– Discover and analyze about 100 SNe per redshift bin Δz=0.1
– Use ~ four day cadence revisiting discovery fields, two wavebands
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Diagnostic spectra and fluxes throughout light curve for systematics
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“Onion peeling” to detect unusual changes in colors for subclassification
Approx 12 lightcurve spectra on a four day cadence in SN restframe
Near peak, one deep precise spectrum with R1pixel = 100, SNR/pix = 17 @ Si II
Accuracy: error of a few percent per supernova is OK…..
But relative systematic flux error over redshift should be less than 1%
One or more reference spectra post-supernova for subtraction
Peak spectrum
Off-peak spectra
explosion
Figure courtesy A.G.Kim 2010
Reference spectrum
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Supernova Program Implementation
• Discovery Phase: repeatedly visit tiered
survey fields with a two-filter imager
Top curve: deep spectrum SNR taken
near peak light, z=1.2; nominal Texp.
– Nearby SNe: short exposures, broad field
~ 10 sqdeg, large A∙W
– Distant SNe: long exposures, smaller
field ~ 1.6 sqdeg, small A∙W
– Efficient! <10% of SN program time
– Can reject some Type II supernovae
• Spectroscopy Phase: revisit with
dedicated spectrometer, R>100
– Early rejection of Type II SNe from first
few spectra: presence of hydrogen
– Subclassification of Type Ia’s using
synthetic photometry lightcurve
– Detailed subclassification near peak
– Also gives host galaxy redshift
Lower curves: short exposure SNRs
before and after peak; sufficient SNR
for broad “UBVRI” colors, and no Kcorrection required for fixed filter edges
& responses. Nominal Texp.
Figure courtesy A.G.Kim 2010.
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Supernova Redshift Range & Model Constraints
Figures 1, 2 from Kent et al. arXiv 0903.2799 (2009)
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WL: Requirements & Implementation
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Requires a dense survey: 30 galaxies per
square arcminute
Translates to ABmag ~ 25
Requires a wide survey: > 10000 square
degrees
Requires good PSF: e.g. 0.2 arcsec pixels
Requires Photo-Z grade redshifts
That in turn means an associated redshift
calibration program
Plan: Wide Field Imager, ~ 0.5 sqdeg
Texposure ~ few kiloseconds
20000 frames, with 4x dithering
Use stars in each frame for instrumental
PSF map and shear calibration
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Rhalf, arcseconds
Jouvel et al., “Designing Future Dark
Energy Missions” A&A 504, 359 (2009)
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Supernovae, BAO, and CMB constrain the
equation of state of the Universe
current (2010) data constraints
Equation of state w = p/ρ
For a cold gas or nonrelativistic fluid,
w=0
For a DE dominated Λ universe,
w = -1
Then … w is a key diagnostic of the
universe and the prevalence of dark
energy, including its evolution over
cosmic time.
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BAO Emission-Line Galaxy Sizes
Schlegel & Mostek “Exposure Time Requirements for JDEM BAO Measurements” 3 Dec 2008
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Survey Rate for simplest case
Continuum target, Diffuse background
N 2min  Δλ FoV AEF
Survey Rate  0.25

2
SNR  B π R 2half
Nmin = minimum needed continuum photon flux
SNR = required signal to noise ratio
B = diffuse sky continuum level
FoV = imager survey area on sky
A = telescope light gathering area
E = system throughput efficiency
This talk
F = fraction of time allocated
Δλ = wavelength bandpass
Rhalf = half light radius of target image
To maximize survey rate: maximize that last group of factors, and
of course minimize the half light radius of the faintest images.
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JSIM
http://jdem.lbl.gov/ “Exposure Time Calculator”
• Public web-based tool created by M.Levi with Project Office inputs
• Inputs are high-level mission parameters
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Telescope Aperture, central obstruction size, WFE…
Field of view on sky, pixel scale, focal length, number of sensor chips
Detector Technology: pixel size, pixels per chip, waveband, QE curve
Fraction of time allocated to BAO, SNe, WL, calibration, downlink, …
Mission duration
• Also low-level inputs for sensors, filter bandwidths, etc
• Outputs are available at “high level” i.e. productivity yield measures
per year of operations for a given objective and figures-of-merit
scaled from comparisons with DETF estimates
• Also “low level” outputs, decomposing yield into redshift bins, for
estimating individual cosmological parameter constraints
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JSIM Primary Mission Input Parameters
http://jdem.lbl.gov/ “Exposure Time Calculator”
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JSIM Summary Output Results
http://jdem.lbl.gov/ “Exposure Time Calculator”
• Gives both broad & detailed predictions of a JDEM design
• Confirms the notion that shrinking Rhalf boosts performance
• Roughly, 1.1m unobscured aperture ≈ 1.4m 50% obscured
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