large synoptic survey telescope: from ......serb. astron. j. }176 (2008), 1 - 13 udc 520.2 doi:...

Serb. Astron. J. } 176 (2008), 1 - 13 UDC 520.2DOI: 10.2298/SAJ0876001I Invited review

LARGE SYNOPTIC SURVEY TELESCOPE:FROM SCIENCE DRIVERS TO REFERENCE DESIGN

Z. Ivezic1, T. Axelrod2, W. N. Brandt3, D. L. Burke4, C. F. Claver5, A. Connolly1,

K. H. Cook6, P. Gee7, D. K. Gilmore4, S. H. Jacoby2, R. L. Jones1, S. M. Kahn4,

J. P. Kantor2, V. Krabbendam5, R. H. Lupton8, D. G. Monet9, P. A. Pinto10, A. Saha5,

T. L. Schalk11, D. P. Schneider3, M. A. Strauss7, C. W. Stubbs12, D. Sweeney2,

A. Szalay13, J. J. Thaler14, and J. A. Tyson7 for the LSST Collaboration

1Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195E–mail: [email protected]

2LSST Corporation, 4703 E. Camp Lowell Drive, Suite 253, Tucson, AZ 857123Department of Astronomy and Astrophysics, The Pennsylvania State University,

525 Davey Lab, University Park, PA 168024Kavli Institute for Particle Astrophyics and Cosmology, Stanford Linear Accelerator Center,

Stanford University, Stanford, CA, 943095National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85719

6Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 945507Physics Department, University of California, One Shields Avenue, Davis, CA 95616

8Department of Astrophysical Sciences, Princeton University, Princeton, NJ 085449U.S. Naval Observatory Flagstaff Station, 10391 Naval Observatory Road, Flagstaff, AZ 86001

10Steward Observatory, The University of Arizona, 933 N Cherry Ave., Tucson, AZ 8572111University of California–Santa Cruz, 1156 High St., Santa Cruz, CA 95060

12Departments of Physics and Astronomy, Center for Astrophysics, Harvard University,60 Garden St., Cambridge, MA 02138

13Department of Physics and Astronomy, The John Hopkins University,3701 San Martin Drive, Baltimore, MD 21218

14University of Illinois, Physics and Astronomy Departments,1110 W. Green St., Urbana, IL 61801

(Received: March 5, 2008; Accepted: March 5, 2008)

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Z. IVEZIC et al.

SUMMARY: In the history of astronomy, major advances in our understandingof the Universe have come from dramatic improvements in our ability to accuratelymeasure astronomical quantities. Aided by rapid progress in information technology,current sky surveys are changing the way we view and study the Universe. Next-generation surveys will maintain this revolutionary progress. We focus here on themost ambitious survey currently planned in the visible band, the Large SynopticSurvey Telescope (LSST). LSST will have unique survey capability in the faint timedomain. The LSST design is driven by four main science themes: constraining darkenergy and dark matter, taking an inventory of the Solar System, exploring thetransient optical sky, and mapping the Milky Way. It will be a large, wide-fieldground-based system designed to obtain multiple images covering the sky that isvisible from Cerro Pachon in Northern Chile. The current baseline design, with

an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg2 field of view, and a 3,200Megapixel camera, will allow about 10,000 square degrees of sky to be covered usingpairs of 15-second exposures in two photometric bands every three nights on average.The system is designed to yield high image quality, as well as superb astrometric and

photometric accuracy. The survey area will include 30,000 deg2 with δ < +34.5◦,and will be imaged multiple times in six bands, ugrizy, covering the wavelengthrange 320–1050 nm. About 90% of the observing time will be devoted to a deep-

wide-fast survey mode which will observe a 20,000 deg2 region about 1000 times inthe six bands during the anticipated 10 years of operation. These data will resultin databases including 10 billion galaxies and a similar number of stars, and willserve the majority of science programs. The remaining 10% of the observing timewill be allocated to special programs such as Very Deep and Very Fast time domainsurveys. We describe how the LSST science drivers led to these choices of systemparameters.

Key words. Astronomical data bases: miscellaneous – Atlases – Catalogs – Surveys– Solar system: general – Stars: general – Galaxy: general – Galaxies: general –Cosmology: miscellaneous

1. INTRODUCTION

1.1. Large scale surveys: a new way of seeing

Major advances in our understanding of theUniverse have historically arisen from dramatic im-provements in our ability to ”see”. We have de-veloped progressively larger telescopes over the pastcentury, allowing us to peer farther into space, andfurther back in time. With the development ofadvanced instrumentation – imaging, spectroscopic,and polarimetric – we have been able to parse radia-tion detected from distant sources over the full elec-tromagnetic spectrum in increasingly subtle ways.These data have provided the detailed informationneeded to construct physical models of planets, stars,galaxies, quasars, and larger structures.

Until recently, most astronomical investiga-tions have focused on small samples of cosmic sourcesor individual objects. This is because our largesttelescope facilities have rather small fields of view,typically only a few square arcminutes – a tiny frac-tion (few parts per hundred million) of the sky, andthose with large fields of view could not detect veryfaint sources. With all of our existing telescope fa-cilities, we have still surveyed only a minute volumeof the observable Universe.

Over the past two decades, however, advancesin technology have made it possible to move beyondthe traditional observational paradigm and to un-

dertake large-scale sky surveys. As vividly demon-strated by surveys such as the Sloan Digital SkySurvey (SDSS, York et al. 2000), the Two MicronAll Sky Survey (2MASS, Skrutskie et al. 2006), andthe Galaxy Evolution Explorer (GALEX, Martin etal. 2006), to name but a few, sensitive and accu-rate multi-color surveys over a large fraction of thesky enable an extremely broad range of new scien-tific investigations. These results, based on synergyof advances in telescope construction, detectors, andabove all, information technology, have dramaticallyimpacted nearly all fields of astronomy – and manyareas of fundamental physics. In addition, the re-cent world-wide attention received by Google Sky1

(Scranton et al. 2007) demonstrates that the impactof sky surveys extends far beyond fundamental sci-ence progress and reaches all of society.

Motivated by the evident scientific progressmade possible by large sky surveys, three recentnationally endorsed reports by the U.S. NationalAcademy of Sciences2 concluded that a dedicatedground-based wide-field imaging telescope with aneffective aperture of 6–8 meters is a high priority forplanetary science, astronomy, and physics over thenext decade. The Large Synoptic Survey Telescope(LSST) described here is such a system. The LSSTwill be a large, wide-field ground based telescope de-signed to obtain multi-band images over a substan-tial fraction of the sky every few nights. The surveywill yield contiguous overlapping imaging of over half

1http://earth.google.com/sky/

2Astronomy and Astrophysics in the New Millennium, NAS 2001; Connecting Quarks with the Cosmos: Eleven Science Ques-tions for the New Century, NAS 2003; New Frontiers in the Solar System: An Integrated Exploration Strategy, NAS 2003.

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LSST: FROM SCIENCE DRIVERS TO REFERENCE DESIGN

the sky in six optical bands, with each sky locationvisited about 1000 times over 10 years.

The purpose of this paper is to provide a sum-mary of the main LSST science drivers and how theyled to the current system design parameters, as de-scribed in §2. A project status report and concludingremarks are presented in §3. For detailed and up-to-date information, please consult the LSST website(www.lsst.org).

2. THE LSST REFERENCE DESIGN

The most important characteristic that deter-mines the speed at which a system can survey a givensky area to a given depth (faint flux limit) is itsetendue (or grasp), the product of its primary mir-ror area and the field-of-view area (assuming thatobserving conditions such as seeing, sky brightness,etc., are fixed). The effective etendue for LSST willbe greater than 300 m2 deg2, which is more thanan order of magnitude larger than that of any exist-ing facility. For example, the SDSS, with its 2.5-mtelescope (Gunn et al. 2006) and a camera with 30imaging CCDs (Gunn et al. 1998), has an effectiveetendue of only 7.5 m2 deg2.

The range of scientific investigations whichwill be enabled by such a dramatic improvement insurvey capability is extremely broad. Guided by thecommunity-wide input assembled in the report of theScience Working Group of the LSST3, the LSST de-sign is focused to achieve goals set by four main sci-ence themes:

(1) Constraining Dark Energy and Dark Matter(2) Taking an Inventory of the Solar System(3) Exploring the Transient Optical Sky(4) Mapping the Milky Way

Each of these four themes itself encompassesa variety of analyses, with varying sensitivity to in-strumental and system parameters. These themesfully exercise the technical capabilities of the system,such as photometric and astrometric accuracy andimage quality. The working paradigm is that all sci-entific investigations will utilize a common databaseconstructed from an optimized observing program,such as that discussed in Section 3. Here we brieflydescribe the science goals and the most challengingrequirements for the telescope and instruments thatare derived from those goals, which led to the overallsystem design decisions discussed below. For a moredetailed discussion, we refer the reader to the LSSTScience Requirements Document4, as well as to thenumerous LSST poster presentations at the recent211th Meeting of the AAS5.

2.1. The Main Science Drivers

The main science drivers are used to optimizenumerous system parameters. Ultimately, in thishigh-dimensional parameter space, there is a one-dimensional manifold defined by the total projectcost. The science drivers must both justify this cost,as well as provide guidance on how to optimize var-ious parameters while staying on the cost manifold.Here we summarize a dozen most important inter-locking constraints on data properties imposed bythe four main science themes:

o The depth of a single visit (an observation con-sisting of two back-to-back exposures of thesame region of sky)

o Image qualityo Photometric Accuracyo Astrometric Accuracyo Optimal exposure timeo The filter complemento The distribution of revisit times (i.e. the ca-

dence of observations)o The total number of visits to a given area of

the skyo The coadded survey deptho The distribution of visits on the sky, and the

total sky coverageo The distribution of visits per filtero Data processing and data access (e.g. time

delay for reporting transient sources and thesoftware contribution to measurement errors)

We present a detailed discussion of how thesescience-driven data properties are transformed tosystem parameters below.

2.2. Constraining Dark Energy andDark Matter

Current models of cosmology require the exis-tence of both dark matter and dark energy to matchobservational constraints (Spergel et al. 2007). Darkenergy affects the cosmic history of both the Hub-ble expansion and mass clustering. If combined, dif-ferent types of probes of the expansion history andstructure history can lead to percent level precisionin dark energy and other cosmological parameters.These tight constraints arise because each techniquedepends on the cosmological parameters or errors indifferent ways. These probes include weak gravita-tional lens (WL) cosmic shear, baryon acoustic os-cillations (BAO), supernovae, and cluster counting –all as a function of redshift. Using the cosmic mi-crowave background as normalization, the combina-tion of these probes can yield the needed precisionto distinguish between models of dark energy (Zhan2006, and references therein). In addition, time-resolved strong galaxy and cluster lensing probes thephysics of dark matter. This is because the positions

3Available as http://www.lsst.org/Science/docs/DRM2.pdf

4Available at http://www.lsst.org/Science/docs.shtml

5Available at http://www.lsst.org/Meetings/AAS/2008/AAS211.shtml

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Z. IVEZIC et al.

and profiles of multiple images of a source galaxydepend sensitively on the total mass distribution, in-cluding the dark matter, in the lensing object.

While LSST WL and BAO probes will yieldthe strongest dark energy and dark matter con-straints, two major programs from this science themethat provide unique and independent constraints onthe system design are

o Weak lensing of galaxies, ando Type Ia Supernovae.

Weak lensing (WL) techniques can be used tomap the distribution of mass as a function of redshiftand thereby trace the history of both the expansionof the universe and the growth of structure (e.g. Huand Tegmark 1999, Wittman et al. 2000, for a reviewsee Bartelmann and Schneider 2001). These investi-gations require deep wide-area multi-color imagingwith stringent requirements on shear systematics inat least two bands, and excellent photometry in allbands. The strongest constraints on the LSST imagequality come from this science program. In order tocontrol systematic errors in shear measurement, it ismandatory to obtain the desired depth with manyshort exposures (which effectively enables ”random-ization” of systematic errors). Detailed simulationsof weak lensing techniques show that, in order toobtain a sample of ∼3 billion lensing galaxies, thecoadded map must cover ∼20,000 deg2, and reach adepth of r ∼ 27.5 (5σ for point sources), with severalhundred exposures per field and sufficient signal-to-noise in at least five other bands to obtain accuratephotometric redshifts (Zhan 2006). Because of theirlow surface brightness, this depth optimizes the num-ber of detected galaxies in ground-based seeing, andallows their detection in significant numbers to be-yond a redshift of two. It is anticipated that opti-mal science analysis of weak lensing will place strongconstraints on data processing software, such as si-multaneous analysis of all the available data (Tysonet al. 2008).

Type Ia supernovae (SN) provided the first ev-idence that the expansion of the universe is acceler-ating (Riess et al. 1998, Perlmutter et al. 1999). Tofully exploit the supernovae science potential, light-curves sampled in multiple bands every few days overthe course of a few months are required. This isessential to search for systematic differences in su-pernovae populations which may masquerade as cos-mological effects, as well as to determine photomet-ric redshifts from the supernovae themselves. Un-like other cosmological probes, even a single objectcan provide useful constraints and, therefore, a largenumber of SN across the sky can enable a high an-gular resolution search for any dependence of darkenergy properties on direction, which would be anindicator of new physics.

Given the expected SN flux distribution, thesingle visit depth should be at least r ∼ 24. Goodimage quality is required to separate SN photomet-rically from their host galaxies. Observations in atleast five photometric bands are necessary to ensurethat, for any given supernova, light-curves in several

bands will be obtained (due to the spread in red-shift). The importance of K-corrections to supernovacosmology implies that the calibration of the relativeoffsets in photometric zero points between filters andthe knowledge of the system response functions, es-pecially near the edges of bandpasses, must be accu-rate to about 1% (Wood-Vasey et al. 2007). Deeperdata (r > 26) for a small area of the sky can ex-tend the discovery of SN to a mean redshift of 0.7,with some objects beyond z ∼1. The added sta-tistical leverage on the ”pre-acceleration” era wouldimprove constraints on the properties of dark energyas a function of redshift.

2.3. Taking an Inventory of the Solar System

The small-body populations in the Solar Sys-tem, such as asteroids, trans-Neptunian objects(TNOs) and comets, are remnants of its early as-sembly. The history of accretion, collisional grind-ing, and perturbation by existing and vanished giantplanets is preserved in the orbital elements and sizedistributions of those objects. In the main asteroidbelt between Mars and Jupiter collisions still occur,and occasionally objects are ejected on orbits thatmay take them on a collision course with the Earth.

As a result, the Earth orbits within a swarmof asteroids; some number of these objects will ul-timately strike Earth’s surface. In December 2005,the U.S. Congress directed6 NASA to implement anear-Earth object (NEO) survey that would catalog90% of NEOs larger than 140 meters by 2020. About20% of NEOs, the potentially hazardous asteroids orPHAs, are in orbits that pass sufficiently close toEarth’s orbit, to within 0.05 AU, that perturbationswith time scales of a century can lead to intersec-tions and the possibility of collision. In order to ful-fill the Congressional mandate using a ground-basedfacility, a 10-meter class telescope equipped with amulti-gigapixel camera, and a sophisticated and ro-bust data processing system are required (Ivezic etal. 2007). The search for NEOs also places strongconstraints on the cadence of observations, requiringclosely spaced pairs of observations (two or prefer-ably three times per lunation) in order to link obser-vations unambiguously and derive orbits. Individ-ual exposures should be shorter than about 1 minuteeach to minimize the effects of trailing for the major-ity of moving objects. The images must be well sam-pled to enable accurate astrometry, with absolute ac-curacy of at least 0.1 arcsec. The images should reacha depth of at least ∼24.5 (5σ for point sources) in ther band in order to probe the ∼ 0.1 km size range atmain-belt distances, and to fulfill the CongressionalNEO mandate. The photometry should be betterthan 1-2% to enable color-based taxonomic classifi-cation.

2.4. Exploring the Transient Optical Sky

Recent surveys have shown the power of vari-ability for studying gravitational lensing, searchingfor supernovae, determining the physical propertiesof gamma-ray burst sources, and many other projects

6For details see http://neo.jpl.nasa.gov/neo/report2007.html

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at the forefront of astrophysics (Tyson 2006, and ref-erences therein). A wide-area dense temporal cov-erage to deep limiting magnitudes would enable thediscovery and analysis of rare and exotic objects suchas neutron star and black hole binaries, gamma-raybursts and X-ray flashes, at least some of which ap-parently mark the deaths of massive stars; AGNsand blazars; and very possibly new classes of tran-sients, such as binary mergers and stellar disruptionsby black holes. It is likely that such a survey woulddetect numerous microlensing events in the LocalGroup and perhaps beyond, and open the possibilityof discovering planets and obtaining spectra of lensedstars in distant galaxies as well as our own.

Time domain science requires large area cov-erage to enhance the probability of detecting rareevents; good time sampling, since light curves arenecessary to distinguish certain types of variablesand in some cases to infer their properties (e.g. de-termining of the intrinsic luminosity of supernovaeType Ia depends on measurements of their rate of de-cline); accurate color information to assist with theclassification of variable objects; good image qualityto enable discerning of images, especially in crowdedfields; and rapid data reduction, classification andreporting to the community in order to flag interest-ing objects for spectroscopic and other investigationswith separate facilities. Time scales ranging from 1min (to constrain the properties of fast faint tran-sients such as optical flashes associated with gamma-ray bursts (Kaspi et al. 2007) and transients recentlydiscovered by the Deep Lens Survey, Becker et al.2004) to 10 years (to study long-period variables andquasars) should be probed over a significant fractionof the sky. It should be possible to measure colors offast transients, and to reach faint magnitude limitsin individual visits (at least the Deep Lens Surveylimit of r ∼ 24.5).

2.5. Mapping the Milky Way

A major objective of modern astrophysics isto understand when and how galaxies formed andevolved. Theories of galaxy formation and evolutioncan be tested and influenced by a significantly im-proved understanding of the distribution and kine-matics of stars in our own Galaxy, the Milky Way,which is a complex and dynamical structure that isstill being shaped by the infall (merging) of neigh-boring smaller galaxies. We still lack robust answersto two basic questions about the Milky Way Galaxy:

o What is the detailed structure and accretionhistory of the Milky Way?

o What are the fundamental properties of all thestars within 300 pc of the Sun?

Key requirements for mapping the Galaxy arelarge area coverage, excellent image quality to max-imize the photometric and astrometric accuracy, es-pecially in crowded fields; photometric precision ofat least 1% to separate main sequence and giantstars; astrometric precision of about 10 mas per ob-

servation to enable parallax and proper motion mea-surements; and dynamic range that allows measure-ments of astrometric standard stars at least as brightas r = 16. In order to probe the halo out to itspresumed edge at ∼ 100 kpc using numerous main-sequence stars, the total co-added depth must reachr > 27, with a similar depth in the g band. To studythe metallicity distribution of stars in the Sgr tidalstream (see e.g. Majewski et al. 2003) and other halosubstructures at distances beyond the presumed in-ner vs. outer halo boundary (at least ∼ 40 kpc), theco-added depth in the u band must reach ∼ 24.5.To detect RR Lyrae stars beyond the Galaxy’s tidalradius at ∼ 300 kpc, the single-visit depth must ber ∼ 24.5. In order to constrain the tangential ve-locity of stars at a distance of 10 kpc, where halodominates over disk, to within 10 km/s needed tobe competitive with large-scale radial velocity sur-veys, the required proper motion accuracy is at least0.2 mas/yr. The same accuracy follows from therequirement to obtain the same proper motion ac-curacy as Gaia (Perryman et al. 2001) at its faintlimit (r ∼ 20). In order to produce a complete sam-ple of solar neighborhood stars out to a distance of300 pc (the thin disk scale height), with 3σ or bettergeometric distances, trigonometric parallax measure-ments accurate to 1 mas are required. To achieve therequired proper motion and parallax accuracy withan assumed astrometric accuracy of 10 milliarcsecper observation per coordinate, approximately 1,000observations are required. This requirement on thenumber of observations is in good agreement withthe independent constraint implied by the differencebetween the total depth and the single-visit depth.

2.6. A Summary and Synthesis ofScience-drivenConstraints on Data Properties

The goals of all the science programs dis-cussed above (and many others, of course) can be ac-complished by satisfying the following minimal con-straints7

o The single visit depth should reach r ∼ 24.5.This limit is primarily driven by the NEOsurvey and variable sources (e.g. RR Lyraestars), and by proper motion and trigono-metric parallax measurements for stars. Indi-rectly, it is also driven by the requirements onthe coadded survey depth and the minimumnumber of exposures placed by weak lensingscience.

o Image quality should maintain the limit setby the atmosphere (the median seeing is 0.7arcsec in the r band at the chosen site), andnot be degraded appreciably by the hardware.In addition to stringent constraints from weaklensing, good image quality is driven by re-quired survey depth for point sources and byimage differencing techniques.

7For a more elaborate listing of various constraints, including detailed specification of various probability distributions, pleasesee the LSST Science Requirements Document (http://www.lsst.org/Science/docs.shtml).

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Z. IVEZIC et al.

o Photometric repeatability should achieve 5 mil-limag precision at the bright end, with zero-point stability across the sky of 10 millimagand band-to-band calibration errors not largerthan 5 millimag. These requirements aredriven by the photometric redshift accuracy,the separation of stellar populations, detec-tion of low-amplitude variable objects (such aseclipsing planetary systems), and the searchfor systematic effects in type Ia supernovalight-curves.

o Astrometric precision should maintain thelimit set by the atmosphere, of about 10 mil-liarcsec per visit at the bright end (on scalesbelow 20 arcmin). This precision is driven bythe desire to achieve a proper motion accuracyof 0.2 mas/yr and parallax accuracy of 1.0 masover the course of a 10 year long survey.

o The single visit exposure time should be lessthan about a minute to prevent trailing of fastmoving objects and to facilitate control of var-ious systematic effects induced by the atmo-sphere. It should be longer than ∼ 20 secondsto avoid efficiency losses due to finite readoutand slew time.

o The filter complement should include at leastsix filters in the wavelength range limited byatmospheric absorption and silicon detectionefficiency (320–1050 nm), with roughly rectan-gular filters and no large gaps in the coverage,in order to enable robust and accurate photo-metric redshifts, and stellar typing. An SDSS-like u band is extremely important for sepa-rating low-redshift quasars from stars, and forestimating metallicity of F/G main sequencestars. A bandpass with an effective wave-length of about 1 micron would enable studiesof sub-stellar objects, high-redshift quasars,and regions of the Galaxy that are obscuredby interstellar dust.

o The revisit time distribution should allow SNlight curves to be sampled every few days; thisconstraint is needed to obtain orbits of SolarSystem objects as well, while accomodatingconstraints set by proper motion and trigono-metric parallax measurements.

o The total number of visits of any given areaof sky, when accounting for all filters, shouldbe of the order of 1,000, as mandated by weaklensing science, the NEO survey, and propermotion and trigonometric parallax measure-ments. Studies of transient sources also bene-fit from a larger number of visits.

o The coadded survey depth should reach r ∼27.5, with sufficient signal-to-noise ratio inother bands to address both extragalactic andGalactic science drivers.

o The distribution of visits per filter shouldenable accurate photometric redshifts, sepa-ration of stellar populations, and sufficientdepth to make detection of faint extremelyred sources posible (e.g. brown dwarfs andhigh-redshift quasars). Detailed simulationsof photometric redshift estimates suggest anapproximately flat distribution of visits among

bandpasses (because the system throughputand atmospheric properties are wavelength de-pendent, the achieved depths are different indifferent bands). The adopted time allocation(see Table 1) gives a slight preference to ther and i bands because of their dominant rolefor star/galaxy separation and weak lensingmeasurements.

o The distribution of visits on the sky should ex-tend over at least ∼20,000 deg2 to obtain therequired number of galaxies for weak lensingstudies, with attention paid to ”special” re-gions such as the Ecliptic, Galactic plane, andthe Large and Small Magellanic Clouds.

o Data processing, data products and data accessshould enable efficient science analysis with-out a significant impact on the final uncertain-ties. To enable a fast and efficient responseto transient sources, the processing latencyshould be less than a minute, with a robustand accurate preliminary classification of re-ported transients.

It is remarkable that, even with these joint re-quirements, none of the individual science programsis severely overdesigned. That is, despite their sig-nificant scientific diversity, these programs are highlycompatible in terms of desired data characteristics.Indeed, any one of the four main science drivers couldbe removed, and the remaining three would still yieldvery similar requirements for most system parame-ters. As a result, the LSST system can adopt a highlyefficient survey strategy where a single dataset servesall science programs (instead of science-specific sur-veys executed in series). One can think of this asmassively parallel astrophysics. The vast majority(about 90%) of the observing time will be devotedto a deep-wide-fast survey mode, with the remaining10% of observing time allocated to special programswhich will also address multiple science goals. Beforedescribing these surveys in more detail, we discussthe main system parameters.

2.7. The Main System Design ParametersGiven the minimum science-driven constraints

on the data properties listed in the previous section,we now discuss how they are translated into con-straints on the main system design parameters: theaperture size, the optimal exposure time, and the fil-ter complement. We also briefly describe the LSSTreference design.

2.8. The Aperture Size

The product of the system’s etendue and thesurvey lifetime, for given observing conditions, deter-mines the sky area that can be surveyed to a givendepth, where the etendue is the product of the pri-mary mirror area and the field-of-view area. TheLSST field-of-view area is maximized to its practicallimit, 10 deg2, determined by the requirement thatthe delivered image quality be dominated by atmo-spheric seeing at the chosen site (Cerro Pachon inNorthern Chile). A larger field-of-view would leadto unacceptable deterioration of the image quality.This leaves the primary mirror diameter and survey

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lifetime as free parameters. The adopted survey life-time of 10 years is a compromise between a shortertime that leads to an excessively large and expensivemirror (15 m for a 3 year-long survey and 12 m fora 5-year long survey), and a smaller telescope thatwould require more time to complete the survey, withthe associated increase in operations cost.

The primary mirror size is a function of therequired survey depth and the desired sky coverage.By and large, the anticipated science outcome scaleswith the number of detected sources. For practicallyall astronomical source populations, in order to max-imize the number of detected sources, it is more ad-vantageous to maximize the area first, and then thedetection depth. For this reason, the sky area forthe main survey is also maximized to its practicallimit, 20,000 deg2, determined by the requirementto avoid large airmasses (which would substaintiallydeteriorate the image quality and the survey depth).

With the adopted field-of-view area, the skycoverage and the survey lifetime fixed, the primarymirror diameter is fully driven by the required surveydepth. There are two depth requirements: the final(coadded) survey depth, r ∼ 27.5, and the depth of asingle visit, r ∼ 24.5. The two requirements are com-patible if the number of visits is several hundred (perband), which is in good agreement with independentscience-driven requirements on the latter.

The required coadded survey depth provides adirect constraint, independent of the details of sur-vey execution such as the exposure time per visit, onthe minimum primary mirror diameter, as illustratedin Fig. 1.

Fig. 1. The co-added depth in the r band vs. aper-ture and the survey lifetime (r ∼ V , where V is theJohnson visual magnitude). It is assumed that 22%of the total observing time (corrected for weather andother losses) is allocated for the r band, and that theratio of the surveyed sky area to the field-of-view areais 2,000.

2.9. The Optimal Exposure Time

The single visit depth depends on both the pri-mary mirror diameter and the chosen exposure time.In turn, the exposure time determines the time in-terval to revisit a given sky position and the totalnumber of visits, and each of these quantities hasits own science drivers. We summarize these simul-taneous constraints in terms of single-visit exposuretime:

o The single-visit exposure time should not belonger than about a minute to prevent trail-ing of fast Solar System moving objects, andto enable efficient control of atmospheric sys-tematics.

o The mean revisit time (assuming uniform ca-dence) for a given position on the sky, n(days), scales as

n =(

texp

10 sec

)(Asky

20, 000 deg2

)(10 deg2

AFOV

),

(1)where the losses for realistic observing condi-tions have been taken into account. Sciencedrivers such as SN and moving objects in theSolar System require that n < 4, or equiva-lently texp < 40 seconds for the nominal valuesof Asky and AFOV. Note that normalizationby 20,000 deg2 is equivalent to two visits pernight over 10,000 deg2.

o The number of visits to a given position onthe sky, Nvisit, with losses for realistic observ-ing conditions taken into account, is given by

Nvisit =(

3000n

)(T

10 yr

). (2)

The requirement Nvisit > 800, again impliesthat n < 4 and texp < 40 seconds if the surveylifetime, T ∼ 10 years.

o These three requirements place a firm up-per limit on the optimal exposure time oftexp < 40 seconds. Surveying efficiency (theratio of open-shutter time to the total timespent per visit) considerations place a lowerlimit on texp due to finite read-out and slewtime (the longest acceptable read-out time isset to 2 seconds, and the slew and settle timeis set to 5 seconds, including the read-out timefor the second exposure in a visit):

ε =(

texp

texp + 9 sec

). (3)

To maintain efficiency losses below 30% (i.e. at leastbelow the limit set by the weather patterns), andto minimize the read noise impact, texp > 20 sec-onds. Taking these constraints simultaneously intoaccount, as summarized in Fig. 2, yielded the follow-ing reference design:

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Z. IVEZIC et al.

1. A primary mirror effective diameter of ∼ 6.5m. With the adopted optical design, describedbelow, this effective diameter corresponds toa geometrical diameter of ∼ 8 m. Motivatedby characteristics of the existing equipment atthe Steward Mirror Laboratory, which is cast-ing the primary mirror, the adopted geomet-rical diameter is set to 8.4 m.

2. A visit time of 30 seconds (using two 15 sec-ond exposures to efficiently reject cosmic rays;ε = 77%).

3. A revisit time of 3 days on average per 10,000deg2 of sky, with two visits per night.

To summarize, the chosen primary mirror di-ameter is the minimum diameter that simultaneouslysatisfies the depth (r ∼ 24.5 for single visit andr ∼ 27.5 for coadded depth) and cadence (revisittime of 3-4 days, with 30 seconds per visit) con-straints described above.

Fig. 2. The single-visit depth in the r band (5σdetection for point sources) vs. revisit time, n (orexposure time, texp = 10 n seconds), as a functionof aperture size. In addition to direct constraints onoptimal exposure time, texp is also driven by require-ments on the revisit time, n, the total number of vis-its per sky position over the survey lifetime, Nvisit,and the survey efficiency, ε (see eqs.1-3). Note thatthese constraints result in a fairly narrow range ofallowed texp for the main deep-wide-fast survey.

2.10. The Filter Complement

The LSST filter complement (ugrizy, seeFig. 3) is modeled after the Sloan Digital Sky Sur-vey (SDSS) system (Fukugita et al. 1996) because ofits demonstrated success in a wide variety of appli-cations, including photometric redshifts of galaxies(Budavari et al. 2003), separation of stellar popula-

tions (Lenz et al. 1998, Helmi et al. 2003), and pho-tometric selection of quasars (Richards et al. 2002).The extension of the SDSS system to longer wave-lengths (the y band at ∼ 1 micron) is driven by theincreased effective redshift range achievable with theLSST due to deeper imaging, the desire to study sub-stellar objects, high-redshift quasars, regions of theGalaxy that are obscured by interstellar dust, andthe scientific opportunity offered by modern CCDswith high quantum efficiency in the near infrared.

Fig. 3. The current design of the LSST band-passes. The vertical axis shows the overall systemthroughput. The computation includes the atmo-spheric transmission, optics, and the detector sen-sitivity.

2.11. The LSST Reference Design

We briefly describe the reference design for themain LSST system components. Detailed discussionof the flow-down from science requirements to sys-tem design parameters, and extensive system engi-neering analysis can be found in Claver et al. (2008,in prep.). Additional discussion of science drivers,description of data products and examples of sci-ence programs can be found in Ivezic et al. (2008,in prep.). Both publications will be maintained atthe astro-ph site8, and should be consulted for thedetailed and most up-to-date information about theLSST system.

2.12. Telescope and Site

The large LSST etendue is achieved in a novelthree-mirror design (modified Paul-Baker, Davisonand Angel 2002) with a very fast f/1.25 beam. Theoptical design has been optimized to yield a largefield of view (9.6 deg2), with seeing-limited imagequality, across a wide wavelength band (350–1050nm). Incident light is collected by the primary mir-ror, which is an annulus with an outer diameter of8.4 m (an effective diameter of 6.5 m), then reflectedto a 3.4 m convex secondary, onto a 5 m concavetertiary, and finally into three refractive lenses in acamera (see Fig. 4). All three mirrors will be ac-

8http://arxiv.org/archive/astro-ph

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tively supported to control wavefront distortions in-troduced by gravity and environmental stresses onthe telescope.

Fig. 4. The LSST baseline optical design with itsunique monolithic mirror: the primary and tertiarymirrors are so positioned that they form a continu-ous compound surface, allowing them to be polishedinto a single substrate.

The telescope mount is a compact, stiff struc-ture with a fundamental frequency of nearly 10 Hz,which is crucial for achieving the required fast slew-and-settle times. The telescope sits on a concretepier within a carousel dome that is 30 m in diam-eter. The dome has been designed to reduce domeseeing (local air turbulence that can distort images)and to maintain a uniform thermal environment overthe course of the night. The LSST Observatory willbe sited atop Cerro Pachon in northern Chile, nearthe Gemini South and SOAR telescopes (latitude: S30◦ 10’ 20.1”; longitude: W 70◦ 48’ 0.1”; elevation:2123 m; the median r band zenith seeing: 0.7 arcsec).

Fig. 5. The LSST camera with a person to indicatescale size. The camera is positioned in the middle ofthe telescope and will include a filter mechanism andshuttering capability.

2.13. CameraThe LSST camera provides a 3.2 Gigapixel

flat focal plane array, tiled by 4K x 4K CCD sen-sors with 10 µm pixels (see Figs. 5 and 6). Thispixel count is a direct consequence of sampling the∼ 10 deg2 field-of-view with 0.2×0.2 arcsec2 pixels(Nyquist sampling). The sensors are deep depleted,back-illuminated devices with a highly segmented ar-chitecture that enables the entire array to be read in2 seconds.

Fig. 6. The LSST focal plane. Each cyan squarerepresents one 4096 × 4096 pixel large sensor. Ninesensors are assembled into a raft; the 21 rafts areoutlined in red. There are 189 science sensors, eachwith 16.8 Mpix, for a total pixel count of 3.2 Gpix.

2.14. Data Management

The rapid cadence of the LSST observing pro-gram will produce an enormous volume of data, ∼30 TB per night, leading to a total database over theten years of operations of 60 PB for the raw data,and 30 PB for the catalog database. The total datavolume after processing will be several hundred PB,processed using substantial computing power (∼ 100TFlops). Processing such a large volume of data,converting the raw images into a faithful represen-tation of the universe, and archiving the results inuseful form for a broad community of users is a ma-jor challenge.

The data management system is configured inthree levels: an infrastructure layer consisting of thecomputing, storage, and networking hardware andsystem software; a middleware layer, which handlesdistributed processing, data access, user interface,and system operations services; and an applicationslayer, which includes the data pipelines and productsand the science data archives. There will be bothmountain summit and base computing facilities, as

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Table 1. The LSST Baseline Design and Survey Parameters.

Quantity Baseline Design SpecificationOptical/mount Configuration 3-mirror modified Paul-Baker; alt-azimuthFinal f-Ratio, aperture f/1.25, 8.4 mField of view area, etendue 9.6 deg2, 318 m2deg2

Plate Scale, pixel count 50.9 µm/arcsec (0.2” pix), 3.2 GigapixWavelength Coverage, filters 320 – 1050 nm, ugrizySingle visit depths (5σ) u : 23.9, g : 25.0, r : 24.7, i : 24.0, z : 23.3, y : 22.1Mean number of visits u : 70, g : 100, r : 230, i : 230, z : 200, y : 200Final (coadded) depths (5σ) u : 26.3, g : 27.5, r : 27.7, i : 27.0, z : 26.2, y : 24.9

well as a central archive facility and multiple data ac-cess centers. The data will be transported over exist-ing high-speed optical fiber links from South Americato the U.S.

2.15. The Baseline Main Deep-Wide-FastSurvey

The fundamental basis of the LSST concept isto scan the sky deep, wide, and fast, and to obtain adataset that simultaneously satisfies the majority ofscience goals. This concept, so-called ”universal ca-dence”, will yield the main deep-wide-fast survey anduse about 90% of the observing time. The observingstrategy will be optimized to maximize the scientificthroughput by minimizing slew and other downtimeand by making appropriate choices of the filter bandsgiven the real-time weather conditions. As often aspossible, each field will be observed twice, with vis-its separated by 15-60 minutes. This strategy willprovide motion vectors to link detections of movingobjects in the Solar System, and fine-time samplingfor measuring short-period variability. The resultingsky coverage for LSST baseline cadence, based ondetailed operations simulations, is shown for the rband in Fig. 7. The anticipated total number of

Fig. 7. The distribution of the r band visits on thesky for the baseline main survey. The sky is shownin Aitoff projection in equatorial coordinates and thenumber of visits for a 10-year survey is color-codedaccording to the inset. The two regions with smallernumber of visits than the main survey are the Galac-tic plane (arc on the left) and the so-called ”northernEcliptic region” (upper right). It is likely that the re-gion around the South Celestial Pole will also receivesubstantial coverage.

visits for a ten-year LSST survey is 2,767,595 (∼ 5.5million 15-second long exposures). The per-band al-location of these visits is shown in Table 1. The re-maining 10% of observing time will be used to obtainimproved coverage of parameter space such as verydeep (r ∼ 26) observations, observations with veryshort revisit times (∼ 1 minute), and observations of”special” regions such as the Ecliptic, Galactic plane,and the Large and Small Magellanic Clouds.

3. CONCLUSIONS

Until recently, most astronomical investiga-tions have focused on small samples of cosmic sourcesor individual objects. Over the past decade, however,advances in technology have made it possible to movebeyond the traditional observational paradigm andto undertake large-scale sky surveys, such as SDSS,2MASS, GALEX and many others. This observa-tional progress, based on synergy of advances in tele-scope construction, detectors, and above all, infor-mation technology, has a dramatic impact on nearlyall fields of astronomy, many areas of fundamentalphysics, and the society in general.

The LSST builds on the experience of thesesurveys and addresses the broad goals stated in sev-eral nationally endorsed reports by the U.S. NationalAcademy of Sciences. The realization of the LSSTinvolves extraordinary engineering and technologicalchallenges: the fabrication of large, high-precisionoptics; construction of a huge, highly-integrated ar-ray of sensitive, wide-band imaging sensors; and theoperation of a massive data management facility han-dling tens of terabytes of data each day. The projectis scheduled to have first light in 2014 and the begin-ning of survey operations in 2015.

The LSST survey will open a movie-like win-dow on objects that change brightness, or move, ontimescales ranging from 10 seconds to 10 years. Thesurvey will have a data rate of about 30 TB/night(more than one complete Sloan Digital Sky Sur-vey per night), and will collect over 60 PB of rawdata over its lifetime, resulting in an incredibly richand extensive public archive that will be a treasuretrove for breakthroughs in many areas of astronomy.About 10 billion galaxies and a similar number ofstars will be detected – for the first time in history,the number of cataloged celestial objects will exceedthe number of living people!

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LSST has been conceived as a public facility:the database that it will produce, and the associ-ated object catalogs that are generated from thatdatabase, will be made available to the world’s sci-entific community and to the public at large with noproprietary period. LSST will be a significant mile-stone in the globalization of the information revolu-tion. The LSST data management system will pro-vide user-friendly tools to access this database andto support user-initiated queries, run on LSST com-puters, either at the archive facility or at the dataaccess centers. We expect that many, perhaps eventhe majority of LSST discoveries will come from re-search astronomers with no formal affiliation to theproject, from students, and from interested ama-teurs, intrigued by the accessibility to the universethat this facility uniquely provides.

Acknowledgements – In 2003, the LSST Corporationwas formed as a non-profit 501(c)3 Arizona corpora-tion with headquarters in Tucson, AZ. Membershiphas since expanded to more than twenty membersincluding Brookhaven National Laboratory, Califor-nia Institute of Technology, Carnegie Mellon Uni-versity, Columbia University, Google Inc., Harvard-Smithsonian Center for Astrophysics, Johns HopkinsUniversity, Kavli Institute for Particle Astrophysicsand Cosmology - Stanford University, Las Cum-bres Observatory Global Telescope Network, Inc.,Lawrence Livermore National Laboratory, NationalOptical Astronomy Observatory, Princeton Univer-sity, Purdue University, Research Corporation, Stan-ford Linear Accelerator Center, The PennsylvaniaState University, The University of Arizona, Uni-versity of California at Davis, University of Cali-fornia at Irvine, University of Illinois at Urbana-Champaign, University of Pennsylvania, Universityof Pittsburgh, and the University of Washington.LSST is a public-private partnership. Design anddevelopment activity is in part supported by the Na-tional Science Foundation under Scientific ProgramOrder No. 9 (AST-0551161) and Scientific ProgramOrder No. 1 (AST-0244680) through CooperativeAgreement AST-0132798. Portions of this work aresupported by the Department of Energy under con-tract DE-AC02- 76SF00515 with the Stanford LinearAccelerator Center, contract DE-AC02- 98CH10886with Brookhaven National Laboratory, and contractDE-AC52-07NA27344 with Lawrence Livermore Na-tional Laboratory. Additional funding comes fromprivate gifts, grants to universities, and in-kind sup-port at Department of Energy laboratories and otherLSSTC Institutional Members. NOAO is operatedby the Association of Universities for Research in As-tronomy, Inc. (AURA) under cooperative agreementwith the National Science Foundation. KHC’s workwas performed under the auspices of the U.S. D.O.E.by LLNL under contract DE- AC52-07NA27344.

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LSST: OD NAUQNIH CIǈEVA DO DIZAJNA

Z. Ivezic1, T. Axelrod2, W. N. Brandt3, D. L. Burke4, C. F. Claver5, A. Connolly1,

K. H. Cook6, P. Gee7, D. K. Gilmore4, S. H. Jacoby2, R. L. Jones1, S. M. Kahn4,

J. P. Kantor2, V. Krabbendam5, R. H. Lupton8, D. G. Monet9, P. A. Pinto10, A. Saha5,

T. L. Schalk11, D. P. Schneider3, M. A. Strauss7, C. W. Stubbs12, D. Sweeney2,

A. Szalay13, J. J. Thaler14, and J. A. Tyson7 for the LSST Collaboration

1Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195E–mail: [email protected]

2LSST Corporation, 4703 E. Camp Lowell Drive, Suite 253, Tucson, AZ 857123Department of Astronomy and Astrophysics, The Pennsylvania State University,

525 Davey Lab, University Park, PA 168024Kavli Institute for Particle Astrophyics and Cosmology, Stanford Linear Accelerator Center,

Stanford University, Stanford, CA, 943095National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85719

6Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 945507Physics Department, University of California, One Shields Avenue, Davis, CA 95616

8Department of Astrophysical Sciences, Princeton University, Princeton, NJ 085449U.S. Naval Observatory Flagstaff Station, 10391 Naval Observatory Road, Flagstaff, AZ 86001

10Steward Observatory, The University of Arizona, 933 N Cherry Ave., Tucson, AZ 8572111University of California–Santa Cruz, 1156 High St., Santa Cruz, CA 95060

12Departments of Physics and Astronomy, Center for Astrophysics, Harvard University,60 Garden St., Cambridge, MA 02138

13Department of Physics and Astronomy, The John Hopkins University,3701 San Martin Drive, Baltimore, MD 21218

14University of Illinois, Physics and Astronomy Departments,1110 W. Green St., Urbana, IL 61801

UDK 520.2Pregledni rad po pozivu

U istoriji astronomije, veliki po-maci u naxem razumevaǌu Vasione qestosu proizlazili iz dramatiqnog napretka umogu�nostima preciznog mereǌa astronom-skih veliqina. Zahvaǉuju�i brzom razvojuinformacionih tehnologija, savremeni pre-gledi neba meǌaju naqin na koji posmatramo iprouqavamo Vasionu. Pregledi neba slede�e

generacije nastavi�e ovim putem revolu-cionarnog napretka. U ovom radu usredsre-�ujemo se na najambiciozniji planirani pro-jekat pregleda neba u vidǉivom delu spektra,Veliki sinoptiqki teleskop za pregled neba(skr. LSST, od eng. Large Synoptic Survey Tele-scope). LSST �e imati jedinstvene mogu�nosti

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pregleda u kratkim vremenskim intervali-ma. Dizajn LSST odre�uju qetiri primarnanauqna zadatka: ograniqavaǌe na parametrevezane za tamnu energiju i tamnu materiju,pravǉeǌe inventara objekata Sunqevog sis-tema, istra�ivaǌe kratkotrajnih pojava nanebu u vidǉivom delu spektra i mapiraǌeMleqnog puta. Teleskop �e predstavǉati ve-liki, zemaǉski, xirokougaoni sistem dizajni-ran za dobijaǌe vixestrukih snimaka koji biu potpunosti pokrili nebo vidǉivo iz mestaCerro Pachon u severnom Qileu. Aktuelniosnovni dizajn predvi�a primarno ogledalopreqnika 8.4 m (efektivno 6.5 m), vidnopoǉe od 9.6 kvadratnih stepeni i kameru sa3200 megapiksela, xto �e omogu�iti da se udve ekspozicije od po 15 sekundi, u dva fo-tometrijska filtera, za tri no�i u prose-ku, pokrije ukupno 10 000 kvadratnih stepenineba. Sistem je dizajniran tako da obezbedivisok kvalitet snimaka, kao i izuzetnu as-

trometrijsku i fotometrijsku taqnost. Pre-gled �e pokriti ukupnu povrxinu od 30 000kvadratnih stepeni, u oblasti deklinacijaδ < +34.5◦, snimaju�i vixe puta u xest fil-tera, ugrizy, koji pokrivaju oblasti talas-nih du�ina od 320–1050 nm. Oko 90% posma-traqkog vremena bi�e iskorix�eno za rad utzv. dubokom-xirokom-brzom modu, pri qemu�e se, tokom predvi�enih 10 godina radateleskopa, otprilike 1000 puta u xest fil-tera posmatrati oblast od 20 000 kvadrat-nih stepeni. Prikupǉeni podaci �e biti poh-raǌeni u bazu koja �e ukǉuqivati oko 10milijardi galaksija i pribli�no isti brojzvezda, i koja �e slu�iti ve�ini nauqnih pro-grama. Preostalih 10% posmatraqkog vremenapredvi�eno je za posebne programe kao xto suVrlo Duboki i Vrlo Brzi pregledi. Ovdeopisujemo kako se od nauqnih zadataka pro-grama LSST doxlo do ovih izbora parametarasistema.

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large synoptic survey telescope: from ......serb. astron. j. }176 (2008), 1 - 13 udc 520.2 doi:...

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