may stars be the actors and dark energy direct

shoot a movie in the sky

Chihway Chang Oct.8 ‘2008

outline

• Why LSST ?• Science goal and science driven design• The project system

– Telescope– Camera– Data management

• Focal plane problem & weak lensing • Conclusion

The reason

• What is missing in the astronomy society?– Traditional operation of telescopes– Public data available for all science use– “It will not be possible to answer the great

questions in astronomy and cosmology without a technological breakthrough. we need something that goes wider, deeper, and faster than any instrument we have today.” --Anthony Tyson (UCD)

– Large Synoptic Survey Telescope — let’s shoot a movie in the sky

Cast

• ~10 billion galaxies +10 billion stars with redshift

• ~1 million gravitational lenses• ~10,000 asteroids• ~1 million supernovae • Gamma ray bursts• New phenomena

Large and complete 3D sky map

• Probing dark energy & dark matter– Weak lensing on galaxies (WL)– Baryon acoustic oscillation (BAO)– Type Ia Supernovae

• Taking an inventory of the solar system– Near-Earth objects (NEO) survey

• Exploring the transient optical sky– Active galactic nuclei (AGN)

• Mapping the Milky Way– Galaxy formation and evolution

The science goal

Science-driven instrumentation

• Single visit depth : NEO, variable objects• Total visit depth: extragalatic / galatic • PSF, image quality: WL• Single visit exposure time: moving objec

ts, atmosphere, readout noise• Filter components: photometric z

• Telescope– Telescope optical and mechanical

design, calibration, building and site• Camera

– Electronics, filter, shutter, cryostat, controller, guider, detectors, simulation and calibration

• Data management– Image processing pipeline, data storing

and public access

Crew

Telescope basics

• 9.6 degree2 field of view(Keck ~ 0.2)

• Etendue = collecting area * sky coverage ~ 320 m2 degree2 (Keck ~ 4)

• Three mirror Paul-Baker:– M1 8.4 m primary– M2 3.4 m convex secondary– M3 5.0 m tertiary (monolithic design) – L1 L2 L3 (refractive corrector)

• University of Arizona's Steward Observatory Mirror Lab

Too much data!

• (4 byte per pixel) * (32 billion pixels per exposure) * (continuous 15 or 1 sec exposures) ~ 1.6 GB/sec

• One pass ~ 20,000 square degrees ~ three nights of observation ~ 150 TB

• Overtime ~ 31,000 square degrees ~ 5 years of observation ~ 30 PB (whole sky ~ 41,253 degree2 )

No big deal…

Challenge

• Technology: high data rate, real-time analysis, later data exploration

• Computational cost: PB disk storage system $1 millionin five years, this price should drop to well below $100,000

• National Virtual Observatory

A man-size camera

• 1.6 * 1.6 * 3 m3, 2800 kg• (64 cm)2 flat focal plane with 3.2G pixels• Focal plane operate at -100 degree C• Six 75cm filters UVBRIY

Filters

• 5 band from SDSS ugriz + y• Photometric redshift: linear regression fi

tting of spectral energy distribution (SED) templates

• Y band: designed to probe high z objects

Photometric redshift

By Anthony Tyson

Cryostat and contamination test

Focal plane

Focal plane flatness and weak lensing

Dark energy and dark matter

• The visible mass and known matter cannot explain the why the Universe behaves

• How to “see” DM:– rotational speeds of galaxies– orbital velocities of galaxies in clusters– gravitational lensing

• How to “see” DE:– baryon acoustic oscillation– SN

Weak lensing basics• Gravity bends light• Map of dark matter• Method:

– Use stars to construct PSF map– Deconvolve galaxy with this PSF map– Measure residual ellipticity to infer shear

• Lensing signals are typically WEAK• Accurate “shape” measurement is crucial• The misalignment of the optics can easily distort imag

e shape and mimic shear• LSST may have more difficulties because the focal pla

ne is enormous

10 um defocus

The simulator

• John Peterson @ Purdue• Include science:

– Kolmogorov density screen generator multi-layer frozen screen atmosphere

– ray-tracing refraction and reflection of mirrors and lensing

– Zernike distortions on mirror surfaces – 6 degree of freedom motions for the optical elemen

ts refraction– photo-electron conversion and diffusion in silicon – charge saturation and blooming

Basic checks

• Optics• Background• PSF changes

Build in

• Potato chip shapes• Characterize and analyze PSF

90

100

110

94

96

98

100

102

-2

0

90

100

110

Where is this going?

• Removing instrument signature from weak lensing data

• Understand the limit of weak lensing using LSST

• Set specs on CCD manufacture

• LSST is based on the concept of “fast, wide, deep” as opposed to traditional astrophysics projects.

• The instrumentation of LSST require high technology and complete understanding of the physics involved.

• Good instrumentation makes doing science easier.

• The data of LSST will be available on line to anyone who is interested in it.

• 2014 – Let the movie begin…

Conclusion

Reference

• http://discovermagazine.com/2008/may/13-movie-camera-to-the-stars

• http://www.lsst.org/lsst_home.shtml (LSST official website)

• LSST: FROM SCIENCE DRIVERS TO REFERENCE DESIGN AND ANTICIPATED DATA PRODUCTS (LSST overview paper)

THANKS