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SuperNova Acceleration ProbeResearch and Development Efforts Michael Lampton UCBerkeley Space Sciences Laboratory Chris Bebek UCBerkeley Lawrence Berkeley National Laboratory 7 May 2002
SNAP Collaboration G. Aldering, C. Bebek, W. Carithers, S. Deustua, W. Edwards, J. Frogel, D. Groom, S. Holland, D. Huterer*, D. Kasen, R. Knop, R. Lafever, M. Levi, S. Loken, P. Nugent, S. Perlmutter, K. Robinson (Lawrence Berkeley National Laboratory) E. Commins, D. Curtis, G. Goldhaber, J. R. Graham, S. Harris, P. Harvey, H. Heetderks, A. Kim, M. Lampton, R. Lin, D. Pankow, C. Pennypacker, A. Spadafora, G. F. Smoot (UC Berkeley) C. Akerlof, D. Amidei, G. Bernstein, M. Campbell, D. Levin, T. McKay, S. McKee, M. Schubnell, G. Tarle , A. Tomasch (U. Michigan) P. Astier, J.F. Genat, D. Hardin, J.- M. Levy, R. Pain, K. Schamahneche (IN2P3) A. Baden, J. Goodman, G. Sullivan (U.Maryland) R. Ellis, A. Refregier* (CalTech) J. Musser, S. Mufson (Indiana) A. Fruchter (STScI) L. Bergstrom, A. Goobar (U. Stockholm) C. Lidman (ESO) J. Rich (CEA/DAPNIA) A. Mourao (Inst. Superior Tecnico,Lisbon)
Overview • What is SNAP? • SNAP Reviews & Milestones • What are our current R&D efforts? • Mission Development & Optimization • Optical performance trades • Attitude Control System issues • Shutter technology • Bandpass filter technology • Calibration plan • IFU/Spectrometer technology • Si CCD’s • HgCdTe’s • Detector Electronics
SNAP Introduction • Supernova data shows an acceleration of the expansion, implying that the universe is dominated by a new Dark Energy! • Remarkable agreement between Supernovae & recent CMB. Credit STScI
Mission Design SNAP a simple dedicated experiment to study the dark energy • Dedicated instrument, essentially no moving parts • Telescope: 2 meter aperture, diffraction limited beyond 1 micron • Photometry: with 1deg FOV half-billion pixel mosaic camera, high-resistivity, rad-tolerant p-type CCDs and HgCdTe arrays. (0.4-1.7 mm) • Integral field optical and IR spectroscopy: 0.4-1.7 mm, 2”x2” FOV
SNAP Motivation • Precision cosmology to distinguish models • There are a LOT of models • Dark energy is not understood • Early universe was dominated by gravitation, hence deceleration • Only more recently could dark energy have become dominant • Need an accurate redshift-magnitude diagram • must extend a large range of redshifts • small z: recent epoch with acceleration • z>1.5 to probe possible early deceleration epoch • must minimize systematics < few percent • must minimize statistics < few percent
Standard Candles • Cosmic accelerometer: need lookback time and expansion for each of thousands of events distributed throughout universe. • Standard candle redshift-magnitude diagram gives both: • expansion from redshift • lookback time from apparent magnitude • Type Ia supernovae are the best candles known • WD receives mass from binary companion • SN occurs when WD mass exceeds Chandrasekhar limit • This limit is set by electron degeneracy pressure • Empirically, Ia’s can be standardized to < 0.2 magnitudes • therefore, tens to hundreds/bin give few percent precision • Systematics are just as important as statistics • Light curves are important to distinguish variants, trends... • Spectroscopy is important to distinguish variants, trends...
How to achieve these goals? • Huge amounts of uniformly-calibrated observing time => space • Need guaranteed reobservation each SN for light curve => space • Need to probe into the NIR, out to ~1.7 microns => space • Need to go faint => space • detect at 29th AB magnitude at 1 micron • spectrum & precision photometry at 25th magnitude • Spaceborne telescope and instrument complement • approx 2 meter aperture, wide field optics • large format imager/photometer, ~1 deg FOV , 9 bands • repeatedly scans a survey region, ~10 sq deg • multiplex advantage ~half billion pixels • processes entire field regularly, every few days • low dispersion spectrometer • observe each SN at peak for classification
From Science Goalsto Project Design Science • Measure M and • Measure w and w (z) Systematics Requirements Statistical Requirements • Identified and proposed systematics: • Measurements to eliminate / bound each one to +/–0.02 mag • Sufficient (~2000) numbers of SNe Ia • …distributed in redshift • …out to z < 1.7 Data Set Requirements • Discoveries 3.8 mag before max • Spectroscopy with S/N=10 at 15 Å bins • Near-IR spectroscopy to 1.7 m • • • Satellite / Instrumentation Requirements • ~2-meter mirror Derived requirements: • 1-square degree imager • High Earth orbit • Spectrograph • High bandwidth (0.4 m to 1.7 m) • • •
Other Benefits • SNAP main survey will be 6300 x larger (and somewhat deeper) than the HST ACS survey • SNAP will provide 9-band colors of every object within its survey region • SNAP has time resolution • revisit everything every few days • span >2 years • Complementary to NGST: target selection for rare objects • Could survey 3000 sq deg in a year to I=29 or J=28 AB mag
SNAP Reviews/Studies/Milestones Mar 2000 SAGENAP: urged DoE to begin supporting SNAP R&D Sep 2000 NASA Structure and Evolution of the Universe (SEU) Dec 2000 NAS/NRC Committee on Astronomy and Astrophysics Jan 2001 DOE-HEP Reviewed SNAP R&D Program Mar 2001 DOE HEPAP Reviewed SNAP science goals Jun 2001 NASA/GSFC Integrated Mission Design Center July 2001 NAS/NRC Committee on Physics of the Universe July 2001 Snowmass 2001 Workshop: “Resource Book on Dark Energy” Nov 2001 CNES (France Space Agency) IN2P3, U.Marseille Dec 2001 NASA/SEU Strategic Planning Panel Dec 2001 NASA/GSFC Instrument Synthesis & Analysis Lab Jan 2002 AAS Washington: 23 papers Dark Energy & SNAP Mar 2002 SAGENAP: urged continuing support Apr 2002 NAS/NRC CPU (Turner) Report Published NOW ------------------------------------------------------------ July 2002 DOE/SC-CMSD R&D (Lehman) Sept 2002 NASA/SEU Releases Roadmap Oct 2002 CNES Review
R&D Reviews • Jan 2001 DoE Science and R&D Review: • “SNAP will have a unique ability to measure the variation in the equation of state of the universe.” • Look at greatly increasing the near-infrared capabilities • Is the proposed IR spectrograph throughput adequate? • Look at a descoped instrument complement: Can the spectroscopy be done by ground-based facilities? • Develop a calibration strategy and plan. • Address NASA relationship • June 2001 NASA/GSFC Integrated Mission Design Center • Thorough analysis of launcher, shrouds, propellant, link margins, ACS, thermal.... • Generally high marks on mission concept, hardware, maturity • Helped us plan a more cost effective orbit • Nov 2001 NASA/GSFC Instrument Synthesis Analysis Laboratory • Detailed review of telescope, shutter, focus mechanisms, ... • Helped us identify shutter mechanisms and test plans • Generally good marks; urged us to develop and maintain a stray light model
Current Mission Concept • 2.5 x 25 Re orbit, Delta III/IV or Atlas, KSC launch • 29 deg inclination, 3 day period, perigee near Berkeley • Science operations beyond 9 Re for lowest background • Data downlink below 9 Re for best link margins • Single ground station can handle all comm • Survey region near north ecliptic pole • least zodiacal light for best NIR sensitivity • One side of vehicle is always within 45deg of sunward • Opposite side always in shadow, passive cooling radiator • Maneuvering around sun line for other targets (cal, SEP, ...) • Onboard data storage for each orbit’s data • no onboard processing, but 2:1 Rice compression of raw images • 10 Mbit/sec average data generation rate • 2.5 Tbit/orbit data recorder needed • 6 hours AOS per orbit, Berkeley ground station • 150 Mbit/sec actual downlink, Ka band • Nominal 3 year mission, option to extend
Current Observation Concept Imager • Step the entire focal plane through our dedicated observation field. • Fixed length exposures determined by a shutter, typically 200 sec • Multiple exposures per filter. • To implement dithering pattern. • To eliminate cosmic ray hits. • NIR filters have twice the area of visible filters; this combined with time dilation achieves desired S/N in CCDs and HgCdTe. • All stars see all filters (modulo field edge effects). • Field revisited every orbit. SNe will be followed throughout entire mission. • Square-symmetric detector array layout: 90 deg roll each 90 days. Spectrograph • SNe candidates are scheduled for spectrographic measurement near peak luminosity. • Light curve and color analysis done on ground to identify Type Ia and roughly determine z. • Note peak luminosity is 14 days to 40 days after discovery for z = 0 and 1.7 respectively.
Requirements Motivate Current R&D • Telescope • SNR => aperture, efficiency, stray light.... • SNR => point spread function, Strehl ratio, ... • discovery rate => field of view • PSF, focal plane size => pixel sizes <=> focal length • materials limitations => thermal control, focussing mechanisms... • Instrumentation • shutter precision • detector performance • spectroscopy performance • Spacecraft systems • attitude control system => 0.02 arcsecond stability • data generation and orbit downlink plan => 2Tbit onboard storage • Downlink Plan • Orbit AOS Berkeley => 6h contact time, 150 Mbit/sec link rate • Ka band transponder, transmit power, antenna size, ground station... • Ground Computing • must turn around SN detection in < 14 days for spectroscopy • sustained throughput requirement of 100Tbyte/year sizes systems
Ongoing SNAP R&D Efforts • Mission Development & Optimization • Telescope Development • Payload structural static & dynamic models • Spacecraft ACS performance • Shutter technology • IFU/Spectrometer technology • Bandpass filter stability • Calibration Plan • Si CCD’s • HgCdTe’s • Detector electronics
Mission Development & Optimization • Start with any Universe • Populate it with matter, dark energy, supernovae, lensing, ... • host galaxy, reddening, evolution, .... • Use a SNAP mission performance model to harvest that crop • aperture, point spread function, attitude jitter, ... • detector noise, linearity, CR hits, dithering,... • produce simulated photometry data record • perform triggering, spectroscopy, categorization • Fit the Hubble diagram with model universes • How constrained are they? • Repeat for various SNAP designs • wider / narrower survey region? • more VIS? versus NIR? • more objects followed? versus fewer objects, more time on each? • These sims have driven (and will drive) our mission design
Telescope Development • Three-mirror anastigmat does the job • Existing manufacturing and test technologies are entirely suitable • Policy: Build, test, fly at 290 K • R&D phase task is to work with industry to create a biddable requirements document including comprehensive end-to-end test plan. • Ongoing trade studies: aperture, Strehl ratio, focal length, mfr/test plans
Structural/Dynamic Model Forward baffle Passive Radiator SolarArray Booster Attachment
NASA GSFC/IMDC Spacecaft Packaging Secondary Mirror and Active Mount Optical Bench Primary Mirror Solar Array Wrap around, body mounted 50% OSR & 50% Cells Thermal Radiator Sub-system electronics Detector/Camera Assembly Propulsion Tanks from GSFC - IMDC study
Attitude Control System Development • Requirement: 0.02 arcsecond RMS, 200sec exposure • IMDC Recommendations • Need complete flexural FEM to understand resonance modes and to guarantee system stability • Use dedicated star tracker for coarse acquisition, gyros for dynamic feedback, and feedback from focal plane star guider for fine guidance • Aerospace industry contractor Recommendations • Compared SNAP to similar-size payload flown by another customer • our planned “rigid” spacecraft will deliver needed stability • Complete attitude model will be developed • propellant slosh • sensor noise spectra • wheel rumble • predict jittter, settling times, maneuver rates
Rotary Shutter Concept 1 sec to open 1 sec to close timing error <0.01sec reliability study 2003 zero angular momentum
Calibration technology • Identified as a mission driver • Overall “absolute relative” errors < 2%, 0.4 to 1.7microns • SNAP working group is preparing an R&D plan • Four thrusts are being explored: • interpixel flat fielding by dedicated illumination system • frequent comparisons with well-studied reference stars: KOIII, DA WDs • absolute irradiance standard comparison NIST reference sources: ground, SOFIA, balloon, GAS • adoption of hot DA WDs model atmospheres as known-slope calibrators
Bandpass filter technology • Technology: multilayer dielectric thin-film stack • ion assisted deposition • Fixed filters/substrates suspended above detector chips • potential light loss from interface reflection • Fixed filters deposited onto detector chips • could offer improved QE • could reduce detector yield • Rotary filter wheel • downside is additional moving part • Goal: extreme stability of bandpass curves • ESA: SUVIM/ISS, 2003, 7 bandpasses <0.1% • NASA: SORCE, Pegasus, 2003, 3 bandpasses <0.03% • Berkeley: have begun test & evaluation of sample filters that have been directly deposited onto silicon
Conclusions • Many aspects of the proposed SNAP mission have been reviewed • Most of these do not require any R&D • However, development and/or definition are needed in these areas... • Need >2 terabit SSR, flight proven onboard data storage • Spaceborne Ethernet? router? TCP/IP protocol? (CHIPS!) • Posix/Linux in space? • Need 5 watt solid state Ka band transmitter for high speed downlink • Need thorough study & test plan of our shutter • Several calibration issues need planning • accurate bandpass filters VIS-NIR • absolute spectrophotometric standards • benefits to many other missions NASA and community • Ground data processing issues: volume of data = 100TBytes/year • Next up: Chris Bebek, Detector R&D