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Off-Axis Telescopes for Dark Energy Investigations

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. Our observed universe: expanding, accelerating, lumpy Hubble: and many many others: expanding! H(0)

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Off-Axis Telescopes for Dark Energy Investigations

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  1. 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)

  2. 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… Ωm+ Ωk + ΩΛ = 1 • Empirically today… 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? SIX PARAMETER FLAT ΛCDM Physical baryon density Ωb Physical CDM density Ωc Physical DE density ΩΛ Scalar curvature Δ2R Spectral index ns Reionization optical depth τ Lampton Sholl & Levi 2010

  3. 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. Lampton Sholl & Levi 2010

  4. JDEMInterim Science Working Group http://jdem.lbl.gov (2010) Lampton Sholl & Levi 2010

  5. JDEMInterim Science Working Group http://jdem.lbl.gov (2010) Lampton Sholl & Levi 2010

  6. Baryon Acoustic Oscillations: what are they? Komatsu et al arXiv 1001.4538 • The very early universe had broadband small amplitude thermoacoustic waves • At decoupling (z=1100, t=0.4My) this wave structure froze out and is still visible today in CMB • Subsequently in the expanding universe these waves grew in amplitude due to gravity • Matter waves are visible today in 3-D galaxy correlations, e.g. the 2dF Galaxy Redshift Survey • BAO can be used to test theories about the growth of structure in the universe Lampton Sholl & Levi 2010

  7. BAO: Requirements & Implementation • 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” Lampton Sholl & Levi 2010

  8. Type Ia Supernovae: What are they? Kowalski et al arXiv 0804.4142 (2008) • “SD” model: Whelan & Iben (1973) • Carbon or oxygen white dwarf star; no H or He • Accrete matter to 1.38 Msun = • Radius begins shrinking rapidly • Gravitational energy = -1E44 joule • It will heat and collapse. Fusion ensues… • 12C→24Mg →56Ni →56Co →56Fe + 0.12% Mc2 • If 67% efficient: 2E44 joule • Annihilates the WD star! • Roughly 1E44 joules remain for KE & light • Good uniformity: calibrated standard candles • Measure each peakbrightness and redshift • Fit a SN population to a distance modulus curve • Each DE model predicts a distance modulus curve • So… compare these to constrain models. Lampton Sholl & Levi 2010

  9. 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 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 accurate 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 explosion Reference spectrum Off-peak spectra Figure courtesy A.G.Kim 2010 Lampton Sholl & Levi 2010

  10. Supernova Program Implementation Top curve: deep spectrum SNR taken near peak light, z=1.2 • Discovery Phase: repeatedly visit tiered survey fields with a two-filter imager • 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 K-correction required for fixed filter edges & responses. Figure courtesy A.G.Kim 2010. Lampton Sholl & Levi 2010

  11. Weak Lensing: what is it? • Dark matter is invisible yet is by far the largest source of gravitation in the universe • Dark matter can be mapped by its deflection of light from background galaxies • Strong lensing is already a well established tool for mapping individual massive clusters (A2218) • Weak lensing is a statistical buildup of ellipticity (shear) as light paths traverse volumes of space containing irregularly distributed matter • The measurement of shear of 1E9 galaxies, with a wide range of redshifts, could yield a useful measure of the growth in structure over cosmic time. http://www.cita.utoronto.ca/~hoekstra/lensing.html Lampton Sholl & Levi 2010

  12. WL: Requirements & Implementation • 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 Rhalf, arcseconds Jouvel et al., “Designing Future Dark Energy Missions” A&A 504, 359 (2009) Lampton Sholl & Levi 2010

  13. Supernovae, BAO, and CMB constrain the equation of state of the Universecurrent (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. Lampton Sholl & Levi 2010

  14. Survey Rate for simplest caseContinuum target, Diffuse background 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 This talk To maximize survey rate: maximize that last group of factors, and of course minimize the half light radius of the faintest images. Lampton Sholl & Levi 2010

  15. 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 • 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 Lampton Sholl & Levi 2010

  16. 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 Lampton Sholl & Levi 2010

  17. JSIM Primary Mission Input Parameters http://jdem.lbl.gov/ “Exposure Time Calculator” Lampton Sholl & Levi 2010

  18. 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 Lampton Sholl & Levi 2010

  19. Obscured vs Unobscured Focal TMAsThese 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. Korsch,D., A.O. 16 #8, 2074 (1977) 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. Cook,L.G., Proc.SPIE v.183 (1979) Lampton Sholl & Levi 2010

  20. PSFs For Unaberrated Pupils Scaled to include both obstructed light loss and diffraction Fresnel-Kirchoff diffraction integral Unobstructed Obstructed: 50% linear, 25% area Lampton Sholl & Levi 2010

  21. 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% Lampton Sholl & Levi 2010

  22. Eliminating the SM support spider legs HST file image courtesy STScI For a Galactic Midlatitude distribution of stars, diffraction rings and spikes bring the focal plane irradiance to twice or more times Zodi over 3% of random locations. Elimination: slightly improved survey efficiency; eases background subtraction. Lampton Sholl & Levi 2010

  23. EE50 Radius (arcsec) ComparisonHeld 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. 1.1m obscured 1.3m obscured 1.1m unobscured 1.3m unobscured Wavelength microns Lampton Sholl & Levi 2010

  24. Some Unobscured Concepts Lampton Sholl & Levi 2010

  25. Manufacturing & Testing Challenges? • Off-axis: more material removal and greater aspheric departure • Off-axis: non axisymmetric test setups need more time & care • Vendors caution us that going off-axis is do-able but not “free” Lampton Sholl & Levi 2010

  26. Many JDEM Trade Studies RemainContent 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 2010

  27. 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: a minor annoyance, even 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 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 Lampton Sholl & Levi 2010

  28. 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 can help achieve all these results Lampton Sholl & Levi 2010

  29. Backups Lampton Sholl & Levi 2010

  30. Supernova Redshift RangeFigures 1, 2 from Kent et al. arXiv 0903.2799 (2009) Lampton Sholl & Levi 2010

  31. Jouvel et al “Designing Future Dark Energy Missions” A&A 504, 359 (2009) HST ACS PSF 0.07 arcsec from Koekemoer et al ApJS 172 196 (2007) half light radius Lampton Sholl & Levi 2010

  32. JSIM Secondary Input Fields http://jdem.lbl.gov/ “Exposure Time Calculator” Lampton Sholl & Levi 2010

  33. JSIM Secondary Results: WL and BAO http://jdem.lbl.gov/ “Exposure Time Calculator” Lampton Sholl & Levi 2010

  34. JSIM Secondary Results: SN Spectroscopy http://jdem.lbl.gov/ “Exposure Time Calculator” Lampton Sholl & Levi 2010

  35. WL-Specific Assumptions • Aperture size (1.1m unobscured, 1.3m obscured) • Jitter: 0.025 arcsec, rms/axis • Detector diffusion = 1.9mm NIR, 3.8mm CCD • WFE for imaging: 70 nm • 4 Dithers • NIR: 1.7um and Tsca=130K, Idark=0.01 e-/pix-s • NIR: Read Noise per Exposure: 7e- (conservative) • Assumed 40s repointing time per exposure. • Assumed 22 hours/day for science. Lampton Sholl & Levi 2010

  36. Weak Lensing Assumptions • Require photometric measurement of 5% in NIR band. • Eg. filter 1040nm-1410nm (30%) • S/N=20 • Require ellipticity measurement se<0.2. • if r1/2 > 1.5*ee50, then S/N>14.4 to achieve requirement • if r1/2 > 1.25*ee50, then S/N>16 • ee50 is the 50% encircled energy radius • The latter specification has 20% better FoM, but the former size cut has COSMOS heritage. Lampton Sholl & Levi 2010

  37. Limiting Magnitude • At 24.0thmag: >19 resolved gal/sq.amin (@ l=0.8mm) • At 24.5thmag: >28 resolved gal/sq.amin • At 25.0thmag: >40 resolved gal/sq.amin Euclid Lampton Sholl & Levi 2010

  38. Weak Lensing Assumptions Lampton Sholl & Levi 2010

  39. 1.1m Obstructedl=1.7mm: 0.402” Lampton Sholl & Levi 2010

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