Off-Axis Telescopes for Dark Energy Investigations SPIE , 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 Lampton Sholl & Levi 20102

Baryon Acoustic Oscillations: standard rulers Lampton Sholl & Levi Komatsu et al arXiv Then: z=1100 Ruler = 400kly Now: z=0 Ruler = 400Mly How to measure redshifts of 30 million galaxies per year, with σ z = 0.001/(1+z)? Use slitless spectroscopy! Tighter PSF => smaller σ z => Bigger Survey

Type Ia Supernovae: standard candles 4 Lampton Sholl & Levi 2010 Kowalski et al., ApJ 686, 749 (2008) explosion Peak spectrum Reference spectrum Figures courtesy A.G.Kim 2010 Red: SNR at peak. Others: earlier and later times Tighter PSF => Less Texp > Bigger Survey

Weak Lensing: probe growth of structure Lampton Sholl & Levi Jouvel et al., “Designing Future Dark Energy Missions” A&A 504, 359 (2009) Rhalf, arcseconds Strong lensing A2218 Weak lensing statistical concept Space based WL program seeks 30 galaxies/sqarcmin, 0.2 arcsec, 25 th mag galaxies; needs good PSF and stability Tight PSF and small pixels are mandatory to get these galaxies

JDEM Interim Science Working Group Report (2010) Science ObjectiveDesign ADesign B Supernova Redshift Survey 1500 supernovae Redshifts 0.2<z<1.5 Tiered survey areas for discovery Same as Design A BAO Galaxy Survey Halpha flux 2e-16 erg/cm2sec Spectroscopic redshifts 1.3<z<2.0 RMS z < 0.001·(1+z) square degrees in 1.5 years Same as Design A Weak Lensing Surveynone square degrees 30 galaxies per square arcmin Redshifts from Photo-Z 1e5 spectro calibration galaxies Lampton Sholl & Levi 20106

JDEM Interim Science Working Group Report (2010) Lampton Sholl & Levi ElementDesign ADesign B Telescope1.1m unobscured aperture TMASimilar to A Wide field imager For BAO centroids For SN discovery searches In Design B, for cosmic shear 0.5 square degree FoV Two bands: 0.7-1um, 1-1.5um 32 Mpixels, each 0.45arcsec HgCdTe 2Kx2K 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 Supernova Slit or IFU spectrometer Light curves, spectra, host redshifts Narrow field (a few arcseconds) One waveband 0.4 – 2.0um Similar to A

Key Mission Requirement: Survey Rate Lampton Sholl & Levi This talk Simple formulas like… JSIM: a public web tool created by M.Levi “Exposure Time Calculator” JSIM inputs are high-level mission parameters – 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

JSIM Internal Databases & Models “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 9Lampton Sholl & Levi 2010

Obscured vs Unobscured Focal TMAs These historical examples are both focal but afocal configurations are equally good Lampton Sholl & Levi 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. Korsch,D., A.O. 16 #8, 2074 (1977) Cook,L.G., Proc.SPIE v.183 (1979) 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.

11Lampton Sholl & Levi 2010 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 UnobstructedObstructed: 50% linear, 25% area

Encircled Energy as a Fraction of the Total Transmitted Light with no aberrations Fresnel-Kirchoff diffraction integral: Schroeder Lampton Sholl & Levi 2010 Linear obstruction = 0%, 10%, 20%, 30%, 40%, 50%

EE50 Radius (arcsec) Comparison Held constant: f/11, WFE=0.1µm rms, pixel =18µm, blur= 1µm, ACS blur=0.02 arcsec. 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) Lampton Sholl & Levi m obscured 1.3m obscured 1.1m unobscured 1.3m unobscured Wavelength microns

14Lampton Sholl & Levi 2010 Eliminating the SM support spider legs 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. HST file image courtesy STScI

Some Unobscured Concepts Lampton Sholl & Levi MountaintopSolarMcMath: Pierce 11 NST: Denker et al. 12 ATST: Rimmele 13 MountaintopGeneral AstronLAPCAT (proposed): Storey et al 14 NPT (proposed): Moretto & Kuhn 15 4m DFL (proposed): Moretto & Kuhn 16 SpaceborneRemote SensingMTI: Kay et al. 17 TopSat: Price 18 QuickBird: Figoski 19 EO-1 ALI: Lencione et al 20 CartoSat: Subrahmanyam et al 21 SpaceborneStellarGAIA: Perryman 22 DIVA (proposed): Graue et al 23 SpacebornePlanet SearchJPF (proposed): Krist et al 24 TPF (proposed): Noecker 25 ECLIPSE (proposed): Trauger et al 26, Hull et al 27

Manufacturing & Testing Challenges? 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” Lampton Sholl & Levi

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 Lampton Sholl & Levi

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! Lampton Sholl & Levi

Obscured 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 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 19Lampton Sholl & Levi 2010

Conclusions At λ>1µm, pupil obstruction is a concern – 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 Lampton Sholl & Levi

Backups Lampton Sholl & Levi

Dark Energy Our observed universe: expanding, accelerating, lumpy – 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 If GR is correct… Ω m + Ω k + Ω Λ = 1 – Empirically today… ≈ 1 …But there are puzzling aspects of this! – What is Λ? Physics offers no answer. – Why is Ω m ~ Ω Λ today, i.e. why now? 22Lampton Sholl & Levi 2010 Physical baryon density Ω b Physical CDM density Ω c Physical DE density Ω Λ Scalar curvature Δ 2 R Spectral index n s Reionization optical depth τ SIX PARAMETER FLAT ΛCDM

Committees & Reports Dark Energy Task Force – Albrecht et al., Sept 2006 – Figure Of Merit Science Working Group – Albrecht et al., Dec 2008 – JDEM Science Coordination Group – Gehrels, April 2009 – Interim Science Working Group – Moos & Baltay (co-chairs) May 2010 – Lampton Sholl & Levi

DETF Recommendations (2006) 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. Lampton Sholl & Levi “… 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.”

BAO: Requirements & Implementation Require: redshift range 1.3<z<2.0 Survey 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 σ z = 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 spectro fields + cal fields Lampton Sholl & Levi “Rolling Disperser”

Type Ia Supernovae: standard candles “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… 12 C→ 24 Mg → 56 Ni → 56 Co → 56 Fe % Mc 2 – 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. 26 Lampton Sholl & Levi 2010 Kowalski et al., ApJ 686, 749 (2008)

Supernova Program Requirements 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 Diagnostic spectra and fluxes throughout light curve for systematics – “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 = 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 Lampton Sholl & Levi explosion Peak spectrum Reference spectrum Figure courtesy A.G.Kim 2010 Off-peak spectra

Discovery Phase: repeatedly visit tiered survey fields with a two-filter imager – Nearby SNe: short exposures, broad field ~ 10 sqdeg, large A∙  – Distant SNe: long exposures, smaller field ~ 1.6 sqdeg, small A∙  – 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 Supernova Program Implementation 28Lampton Sholl & Levi 2010 Top curve: deep spectrum SNR taken near peak light, z=1.2; nominal Texp. Lower curves: short exposure SNRs before and after peak; sufficient SNR for broad “UBVRI” colors, and no K- correction required for fixed filter edges & responses. Nominal Texp. Figure courtesy A.G.Kim 2010.

Supernova Redshift Range & Model Constraints Figures 1, 2 from Kent et al. arXiv (2009) Lampton Sholl & Levi

WL: Requirements & Implementation Requires a dense survey: 30 galaxies per square arcminute Translates to ABmag ~ 25 Requires a wide survey: > 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 frames, with 4x dithering Use stars in each frame for instrumental PSF map and shear calibration Lampton Sholl & Levi Jouvel et al., “Designing Future Dark Energy Missions” A&A 504, 359 (2009) Rhalf, arcseconds

Supernovae, BAO, and CMB constrain the equation of state of the Universe current (2010) data constraints Lampton Sholl & Levi 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.

BAO Emission-Line Galaxy Sizes Schlegel & Mostek “Exposure Time Requirements for JDEM BAO Measurements” 3 Dec 2008 Lampton Sholl & Levi

Survey Rate for simplest case Continuum target, Diffuse background Lampton Sholl & Levi 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 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. This talk

JSIM “Exposure Time Calculator” Public web-based tool created by M.Levi with Project Office inputs Inputs are high-level mission parameters – 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 34Lampton Sholl & Levi 2010

JSIM Primary Mission Input Parameters “Exposure Time Calculator” 35Lampton Sholl & Levi 2010

JSIM Summary Output Results “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 36Lampton Sholl & Levi 2010