LSST: from Science Drivers to Reference Design and Anticipated Data Products

39 Pages·2014·8.4 MB·English

A living LSST document (arXiv:0805.2366); version 3.1 of Aug 29, 2014 SLAC-PUB-16076
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
LSST: FROM SCIENCE DRIVERS TO REFERENCE DESIGN AND ANTICIPATED DATA PRODUCTS
Zˇ. Ivezic´1, J.A. Tyson2, B. Abel3, E. Acosta4, R. Allsman4, Y. AlSayyad1, S.F. Anderson1, J. Andrew5, R.
Angel6, G. Angeli4, R. Ansari7, P. Antilogus8, K.T. Arndt9, P. Astier8, E´. Aubourg10, T. Axelrod4, D.J.
Bard11, J.D. Barr5, A. Barrau12, J.G. Bartlett13, B.J. Bauman14, S. Baumont15,16, A.C. Becker1, J. Becla11, C.
Beldica17, S. Bellavia18, G. Blanc19,20, R.D. Blandford21, J.S. Bloom22, J. Bogart21, K. Borne23, J.F. Bosch24,
D. Boutigny25,26, W.N. Brandt27, M.E. Brown28, J.S. Bullock29, P. Burchat21, D.L. Burke21, G. Cagnoli30, D.
Calabrese4, S. Chandrasekharan5, S. Chesley31, E.C. Cheu32, J. Chiang21, C.F. Claver4, A.J. Connolly1, K.H.
Cook4, A. Cooray29, K.R. Covey33, C. Cribbs17, W. Cui9, R. Cutri34, G. Daubard8, G. Daues17, F. Delgado35, S.
Digel11, P. Doherty36, R. Dubois11, G.P. Dubois-Felsmann11, J. Durech37, M. Eracleous27, H. Ferguson38, J.
Frank18, M. Freemon17, E. Gangler39, E. Gawiser40, J.C. Geary41, P. Gee2, M. Geha42, R.R. Gibson1, D.K.
Gilmore21, T. Glanzman11, I. Goodenow4, W.J. Gressler5, P. Gris43, A. Guyonnet8, P.A. Hascall11, J. Haupt18,
4
F. Hernandez44, C. Hogan45, D. Huang18, M.E. Huffer21, W.R. Innes21, S.H. Jacoby4, B. Jain46, J. Jee2, J.G.
1
Jernigan47, D. Jevremovic´48, K. Johns32, R.L. Jones1, C. Juramy-Gilles8, M. Juric´1, S.M. Kahn4,21, J.S.
0
Kalirai38, N. Kallivayalil49, B. Kalmbach1, J.P. Kantor4, M.M. Kasliwal50, R. Kessler45, D. Kirkby51, L.
2
Knox2, I. Kotov18, V.L. Krabbendam4, S. Krughoff1, P. Kuba´nek52, J. Kuczewski18, S. Kulkarni53, R.

g Lambert35, L. Le Guillou15,16, D. Levine34, M. Liang5, K-T. Lim11, C. Lintott54, R.H. Lupton24, A. Mahabal53,
u P. Marshall21, S. Marshall21, M. May18, R. McKercher4, M. Migliore12, M. Miller5, D.J. Mills5, D.G.
A Monet55, M. Moniez7, D.R. Neill5, J-Y. Nief26, A. Nomerotski18, M. Nordby11, P. O’Connor18, J. Oliver36,56,
S.S. Olivier14, K. Olsen5, S. Ortiz4, R.E. Owen1, R. Pain8, J.R. Peterson9, C.E. Petry4, F. Pierfederici38, S.

9 Pietrowicz17, R. Pike57, P.A. Pinto6, R. Plante17, S. Plate18, P.A. Price24, M. Prouza52, V. Radeka18, J.
2 Rajagopal5, A. Rasmussen9, N. Regnault8, S.T. Ridgway5, S. Ritz58, W. Rosing59, C. Roucelle13, M.R.
Rumore18, S. Russo8, A. Saha5, B. Sassolas30, T.L. Schalk58, R.H. Schindler21, D.P. Schneider27, G.

] Schumacher35, J. Sebag5, G.H. Sembroski9, L.G. Seppala14, I. Shipsey9,54, N. Silvestri1, J.A. Smith60, R.C.
h Smith35, M.A. Strauss22, C.W. Stubbs36,56, D. Sweeney4, A. Szalay61, P. Takacs18, J.J. Thaler62, R. Van
p Berg11,46, D. Vanden Berk63, K. Vetter18, F. Virieux10, B. Xin4, L. Walkowicz24, C.W. Walter64, D.L. Wang11,
- M. Warner35, B. Willman65, D. Wittman2, S.C. Wolff4, W.M. Wood-Vasey66, P. Yoachim1, and H. Zhan67,
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a A living LSST document (arXiv:0805.2366); version 3.1 of Aug 29, 2014
[
ABSTRACT

4
Major advances in our understanding of the Universe frequently arise from dramatic improvements
v
in our ability to accurately measure astronomical quantities. Aided by rapid progress in information
6
technology,currentskysurveysarechangingthewayweviewandstudytheUniverse. Next-generation
6
surveys will maintain this revolutionary progress. We describe here the most ambitious survey cur-
3
rently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science
2
. will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in
5
the faint time domain. The LSST design is driven by four main science themes: probing dark energy
0
and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and
8
mapping the Milky Way. LSST will be a large, wide-field ground-based system designed to obtain
0
multipleimagescoveringtheskyvisiblefromCerroPach´oninnorthernChile. Thetelescopewillhave
:
v an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg2 field of view, and a 3.2 Gigapixel camera. This
i system can image about 10,000 square degrees of sky in three clear nights using pairs of 15-second
X
exposures twice per night, with typical 5σ depth for point sources of r ∼ 24.5 (AB). The system is
r designed to yield high image quality as well as superb astrometric and photometric accuracy. The
a
projectisintheconstructionphaseandwillbeginregularsurveyoperationsby2022. Thesurveyarea
willbecontainedwithin30,000deg2 withδ <+34.5◦,andwillbeimagedmultipletimesinsixbands,
ugrizy,coveringthewavelengthrange320–1050nm. About90%oftheobservingtimewillbedevoted
to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg2 region about 800 times
(summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map
to r ∼ 27.5. These data will result in databases including 20 billion galaxies and a similar number
of stars, and will serve the majority of the primary science programs. The remaining 10% of the
observingtimewillbeallocatedtospecialprojectssuchasaVeryDeepandFasttimedomainsurvey.
WeillustratehowtheLSSTsciencedriversledtothesechoicesofsystemparameters,anddescribethe
expected data products and their characteristics. The goal is to make LSST data products including
a relational database of about 32 trillion observations of 40 billion objects available to the public and
scientists around the world – everyone will be able to view and study a high-definition color movie of
the deep Universe.
Subject headings: astronomical data bases: atlases, catalogs, surveys — Solar System — stars — the
Galaxy — galaxies — cosmology
1University of Washington, Dept. of Astronomy, Box 351580,
Seattle,WA98195
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025
Work supported in part by US Department of Energy under contract DE-AC02-76SF00515.
2
ing, spectroscopic, and polarimetric – we have been able
to parse radiation detected from distant sources over
1. INTRODUCTION the full electromagnetic spectrum in increasingly subtle
Major advances in our understanding of the Universe ways. Thesedatahaveprovidedthedetailedinformation
have historically arisen from dramatic improvements in needed to construct physical models of planets, stars,
our ability to “see”. We have developed progressively galaxies, quasars, and larger structures.
larger telescopes over the past century, allowing us to Until recently, most astronomical investigations have
peer further into space, and further back in time. With focused on small samples of cosmic sources or individual
the development of advanced instrumentation – imag- objects. This is because our largest telescope facilities
2PhysicsDepartment,UniversityofCalifornia,OneShieldsAv- 32DepartmentofPhysics,UniversityofArizona,1118E.Fourth
enue,Davis,CA95616 Street,Tucson,AZ85721
3Olympic College, 1600 Chester Ave., Bremerton, WA 98337- 331400 W. Mars Hill Rd., Lowell Observatory, Flagstaff, AZ
1699 86001
4LSSTProjectOffice,933N.CherryAvenue,Tucson,AZ85721 34IPAC, California Institute of Technology, MS 100-22,
Pasadena,CA91125
5NationalOpticalAstronomyObservatory,950N.CherryAve, 35CerroTololoInterAmericanObservatory,LaSerena,Chile
Tucson,AZ85719 36Departments of Physics, Center for Astrophysics, Harvard
6StewardObservatory,TheUniversityofArizona,933NCherry University,60GardenSt.,Cambridge,MA02138
Ave.,Tucson,AZ85721 37AstronomicalInstitute,CharlesUniversity,Praha,CzechRe-
7Laboratoire de l’Acc´el´erateur Lin´eaire, IN2P3-CNRS, Univer- public
sit´edeParis-Sud,B.P.34,91898OrsayCedex,France 38Space Telescope Science Institute, 3700 San Martin Drive,
8Laboratoire de Physique Nucl´eaire et des Hautes Ener- Baltimore,MD21218
gies, Universit´e Pierre et Marie Curie, Universit´e Paris Diderot, 39ClermontUniversit´e,Universit´eBlaisePascal,CNRS/IN2P3,
CNRS/IN2P3,4placeJussieu,75005Paris,France Laboratoire de Physique Corpusculaire, BP 10448, F-63000
9DepartmentofPhysicsandAstronomy,PurdueUniversity,525 Clermont-Ferrand,France
40Department of Physics and Astronomy, Rutgers University,
NorthwesternAve.,WestLafayette,IN47907
10AstroParticule et Cosmologie, Universit´e Paris Diderot, 136FrelinghuysenRd,Piscataway,NJ08854
41SmithsonianAstrophysicalObservatory,60GardenSt.,Cam-
CNRS/IN2P3, CEA/lrfu, Observatoire de Paris, Sorbonne Paris
bridgeMA02138
Cit´e, 10, rue Alice Domon et L´eonie Duquet, Paris Cedex 13,
42Astronomy Department, Yale University, New Haven, CT
France
11SLAC National Accelerator Laboratory, 2575 Sand Hill Rd, 06520
43LaboratoiredePhysiqueCorpusculairedeClermont-Ferrand,
MenloParkCA94025
12LaboratoiredePhysiqueSubatomiqueetdeCosmologie,Uni- Universite Blaise Pascal, IN2P3/CNRS, 24 avenue des Landais,
63171Aubi´ereCedex,France
versit´e Grenoble-Alpes, CNRS/IN2P3, 53 av. des Martyrs, 38026
44IN2P3ComputingCenter,CNRS/IN2P3,Lyon-Villeurbanne,
Grenoblecedex,France
13APC,AstroParticuleetCosmologie,Universit´eParisDiderot, France
45Department of Astronomy and Astrophysics, University of
CNRS/IN2P3, CEA/lrfu, Observatoire de Paris, Sorbonne Paris
Chicago,5640SouthEllisAvenue,Chicago,IL60637
Cit´e, 10, rue Alice Domon et L´eonie Duquet, Paris Cedex 13,
46Department of Physics & Astronomy, University of Pennsyl-
France
14LawrenceLivermoreNationalLaboratory,7000EastAvenue, vania,209South33rdStreet,Philadelphia,PA19104-6396
47Space Sciences Lab, University of California, 7 Gauss Way,
Livermore,CA94550
15Sorbonne Universit´es, UPMC Univ Paris 06, UMR 7585, Berkeley,CA94720-7450
48AstronomicalObservatory,Volgina7,P.O.Box74,11060Bel-
LPNHE,F-75005,Paris,France
16CNRS, UMR 7585, Laboratoire de Physique Nuclaire et des grade,Serbia
49Department of Astronomy, University of Virginia, Char-
HautesEnergies,4placeJussieu,75005Paris,France
17NCSA,UniversityofIllinoisatUrbana-Champaign,1205W. lottesville,VA22904
50TheObservatories,CarnegieInstitutionforScience,Pasadena
ClarkSt.,Urbana,IL61801
18BrookhavenNationalLaboratory,Upton,NY11973 CA91101,HubbleFellowandCarnegie-PrincetonFellow,Visiting
19Laboratoire de lAcc´el´erateur Lin´eaire, Univ. Paris Associate,CaliforniaInstituteofTechnology,PasadenaCA91125
51DepartmentofPhysicsandAstronomy,UniversityofCalifor-
Sud/CNRS/IN2P3,Bat. 200,91898OrsayCedex,France
nia,4129FrederickReinesHall,Irvine,CA92697
20Universit´eParisDiderot,SorbonneParisCit´e,F-75013Paris, 52Institute of Physics, Academy of Sciences of the Czech Re-
France
public,NaSlovance2,18221Praha8,CzechRepublic
21Kavli Institute for Particle Astrophysics and Cosmology, 53Astronomy Department, California Institute of Technology,
SLACNationalAcceleratorLaboratory,StanfordUniversity,Stan-
1200EastCaliforniaBlvd.,PasadenaCA91125
ford,CA94025 54Department of Physics, University of Oxford, Denys Wilkin-
22AstronomyDepartment,UniversityofCalifornia,601Camp-
sonBuilding,KebleRoad,Oxford,OX13RH,UK
bellHall,Berkeley,CA94720 55U.S.NavalObservatoryFlagstaffStation,10391NavalObser-
23School of Physics, Astronomy and Computational Sciences,
vatoryRoad,Flagstaff,AZ86001
GeorgeMasonUniversity,4400UniversityDrive,Fairfax,VA22030 56Department of Astronomy, Center for Astrophysics, Harvard
University,60GardenSt.,Cambridge,MA02138
24Department of Astrophysical Sciences, Princeton University, 57GoogleInc.,1600AmphitheatreParkwayMountainView,CA
Princeton,NJ08544
94043
25SLAC National Accelerator Laboratory, Stanford University, 58SantaCruzInstituteforParticlePhysicsandPhysicsDepart-
Stanford,CA94025
ment, University of California–Santa Cruz, 1156 High St., Santa
26CentredeCalculdel’IN2P3,USR6402duCNRS-IN2P3,43
Cruz,CA95064
Bd. du11Novembre1918,69622VilleurbanneCedex,France 59LasCumbresObservatory,6740CortonaDr. Suite102,Santa
27DepartmentofAstronomyandAstrophysics,ThePennsylva-
Barbara,CA93117
niaStateUniversity,525DaveyLab,UniversityPark,PA16802 60AustinPeayStateUniversity,Clarksville,TN37044
28Division of Geological and Planetary Sciences, California In- 61Department of Physics and Astronomy, The John Hopkins
stituteofTechnology,Pasadena,CA91125
University,3701SanMartinDrive,Baltimore,MD21218
29Center for Cosmology, University of California, Irvine, CA 62University of Illinois, Physics and Astronomy Departments,
92697
1110W.GreenSt.,Urbana,IL61801
30Laboratoire des Materiaux Avances (LMA), IN2P3/CNRS, 63SaintVincentCollege,DepartmentofPhysics,300FraserPur-
Universit´edeLyon,F-69622Villeurbanne,Lyon,France
chaseRoad,Latrobe,PA15650
31JetPropulsionLaboratory,CaliforniaInstituteofTechnology, 64DepartmentofPhysics,DukeUniversity,Durham,NC27708
Pasadena,CA91109
3
typically had rather small fields of view, and those with cluding remarks are presented in § 7. This publication
large fields of view could not detect very faint sources. will be maintained at the arXiv.org site71, and will also
With all of our existing telescope facilities, we have still be available from the LSST website (www.lsst.org). The
surveyedonlyasmallfractionoftheobservableUniverse latestarXivversionofthispapershouldbeconsultedand
(except when considering the most luminous quasars). referencedforthemostup-to-dateinformationaboutthe
Over the past two decades, however, advances in tech- LSST system.
nology have made it possible to move beyond the tra-
ditional observational paradigm and to undertake large- 2. FROMSCIENCEDRIVERSTOREFERENCEDESIGN
scale sky surveys. As vividly demonstrated by surveys Themostimportantcharacteristicthatdeterminesthe
such as the Sloan Digital Sky Survey (SDSS; York et speedatwhichasystemcansurveyagivenskyareatoa
al. 2000), the Two Micron All Sky Survey (2MASS; given flux limit (i.e., its depth) is its´etendue (or grasp),
Skrutskie et al. 2006), and the Galaxy Evolution Ex- the product of its primary mirror area and the angular
plorer (GALEX; Martin et al. 2006), to name but a few, area of its field of view (for a given set of observing con-
sensitive and accurate multi-color surveys over a large ditions,suchasseeingandskybrightness). Theeffective
fraction of the sky enable an extremely broad range of ´etendueforLSSTwillbegreaterthan300m2deg2,which
new scientific investigations. These projects, based on a is more than an order of magnitude larger than that of
synergy of advances in telescope construction, detectors, any existing facility. For example, the SDSS, with its
andaboveall,informationtechnology,havedramatically 2.5-m telescope (Gunn et al. 2006) and a camera with
impacted nearly all fields of astronomy – and many ar- 30 imaging CCDs (Gunn et al. 1998), has an effective
eas of fundamental physics. In addition, the world-wide ´etendue of only 5.9 m2 deg2.
attention received by Sky in Google Earth68 (Scranton The range of scientific investigations which will be en-
et al. 2007) and the hundreds of thousands of volunteers abledbysuchadramaticimprovementinsurveycapabil-
classifyinggalaxiesintheGalaxyZooproject(Lintottet ity is extremely broad. Guided by the community-wide
al.2011)anditsextensionsdemonstratethattheimpact input assembled in the report of the Science Working
of sky surveys extends far beyond fundamental science Group of the LSST in 200472, the LSST is designed to
progress and reaches all of society. achieve goals set by four main science themes:
Motivatedbytheevidentscientificprogressenabledby
large sky surveys, three nationally-endorsed reports by 1. Probing Dark Energy and Dark Matter;
the U.S. National Academy of Sciences69 concluded that
a dedicated ground-based wide-field imaging telescope 2. Taking an Inventory of the Solar System;
with an effective aperture of 6–8 meters is a high pri-
ority for planetary science, astronomy, and physics over 3. Exploring the Transient Optical Sky;
the next decade. The Large Synoptic Survey Telescope
4. Mapping the Milky Way.
(LSST) described here is such a system. The LSST will
bealarge,wide-fieldground-basedtelescopedesignedto
Each of these four themes itself encompasses a vari-
obtain multi-band images over a substantial fraction of
ety of analyses, with varying sensitivity to instrumental
the sky every few nights. The survey will yield contigu-
and system parameters. These themes fully exercise the
ousoverlappingimagingofoverhalftheskyinsixoptical
technical capabilities of the system, such as photomet-
bands, with each sky location visited close to 1000 times
ric and astrometric accuracy and image quality. About
over 10 years. The recent 2010 report “New Worlds,
90% of the observing time will be devoted to a deep-
New Horizons in Astronomy and Astrophysics” by the
wide-fast (main) survey mode. The working paradigm
Committee for a Decadal Survey of Astronomy and As-
is that all scientific investigations will utilize a common
trophysics70 ranked LSST as its top priority for large
database constructed from an optimized observing pro-
ground-based projects, and in May 2014 the National
gram (the main survey mode), such as that discussed in
Science Board approved the project for construction.
§ 3.1. Here we briefly describe these science goals and
Thepurposeofthispaperistoprovideanoverallsum-
the most challenging requirements for the telescope and
maryofthemainLSSTsciencedriversandhowtheyled
instrument that are derived from those goals, which will
tothecurrentsystemdesignparameters(§2),todescribe
inform the overall system design decisions discussed be-
anticipateddataproducts(§3),andtoprovideafewex-
low. Foramoredetaileddiscussion,wereferthereaderto
amples of the science programs that LSST will enable
the LSST Science Requirements Document73, the LSST
(§ 4). The community involvement is discussed in § 5,
ScienceBook(2009;hereafterSciBook),aswellastonu-
and broad educational and societal impacts in § 6. Con-
merous LSST poster presentations at recent meetings of
the AAS74.
65Department of Astronomy, Haverford College, 370 Lancaster
Avenue,Haverford,PA19041
66Department of Physics and Astronomy, University of Pitts- 2.1. The Main Science Drivers
burgh,3941O’HaraStreet,PittsburghPA15260 The main science drivers are used to optimize various
67National Astronomical Observatories, Chinese Academy of
system parameters. Ultimately, in this high-dimensional
Sciences,A20DatunRd,ChaoyangDistrict,Beijing100012,China
parameterspace,thereisamanifolddefinedbythetotal
68 http://earth.google.com/sky/
69 Astronomy and Astrophysics in the New Millennium, NAS 71 http://arxiv.org/abs/0805.2366
2001; ConnectingQuarkswiththeCosmos: ElevenScienceQues- 72 Availableashttp://www.lsst.org/files/docs/DRM2.pdf
tionsfor theNew Century, NAS2003; NewFrontiers inthe Solar 73 Availableathttp://www.lsst.org/files/docs/SRD.pdf
System: AnIntegratedExplorationStrategy,NAS2003. 74 See http://www.lsst.org/lsst/content/aas2013 and further
70 http://www.nap.edu/catalog.php?record id=12951 linksathttp://www.lsst.org/lsst/news.
4
project cost. The science drivers must both justify this onthetotalmassdistribution,includingthedarkmatter,
cost,aswellasprovideguidanceonhowtooptimizevar- in the lensing object.
ious parameters while staying within the cost envelope. The three major programs from this science theme,
Here we summarize the dozen or so most important WL,BAOandSN,provideuniqueandindependentcon-
interlocking constraints on data and system properties straints on the system design (SciBook Ch. 11–15).
placed by the four main science themes: Weaklensing(WL)techniquescanbeusedtomapthe
distributionofmassasafunctionofredshiftandthereby
1. The depth of a single visit; trace the history of both the expansion of the Universe
and the growth of structure (e.g., Hu & Tegmark 1999;
2. Image quality;
for reviews see Bartelmann & Schneider 2001 and Wein-
berg et al. 2013). Measurements of cosmic shear as a
3. Photometric accuracy;
function of redshift allow determination of angular dis-
4. Astrometric accuracy; tances versus cosmic time, providing multiple indepen-
dent constraints on the nature of dark energy. These
5. Optimal exposure time; investigations require deep wide-area multi-color imag-
ing with stringent requirements on shear systematics in
6. The filter complement;
at least two bands, and excellent photometry in at least
five bands to measure photometric redshifts (a require-
7. The distribution of revisit times (i.e., the cadence
ment shared with BAO, and indeed all extragalactic sci-
of observations), including the survey lifetime;
encedrivers). ThestrongestconstraintsontheLSSTim-
8. The total number of visits to a given area of sky; age quality arise from this science program. In order to
control systematic errors in shear measurement, the de-
9. The coadded survey depth; sireddepthmustbeachievedwithmanyshortexposures
(allowing for systematics in the measurement of galaxy
10. The distribution of visits on the sky, and the total
shapes related to the PSF and telescope pointing to be
sky coverage;
diagnosed and removed). Detailed simulations of weak
11. The distribution of visits per filter; and lensingtechniquesshowthatimagingover∼20,000deg2
to a 5σ point-source depth of r ∼27.5 gives adequate
AB
12. Data processing and data access (e.g., time de- signal to measure shapes for of order 4 billion galaxies
layforreportingtransientsourcesandthesoftware for weak lensing. These numbers are adequare to reach
contribution to measurement errors). Stage IV goals for dark energy, as defined by the Dark
Energy Task Force (Albrecht et al. 2006). This depth,
We present a detailed discussion of how these science- and the corresponding deep surface brightness limit, op-
drivendatapropertiesaretransformedtosystemparam- timize the number of galaxies with measured shapes in
eters below. ground-based seeing, and allow their detection in sig-
nificant numbers to beyond a redshift of two. Analyz-
2.1.1. Probing Dark Energy and Dark Matter ing these data will require sophisticated data processing
Current models of cosmology require the existence of techniques. For example, rather than simply coadding
both dark matter and dark energy to match observa- all images in a given region of sky, the individual expo-
tionalconstraints(Riessetal.2007;Komatsuetal.2009; sures,eachwiththeirownPSFandnoisecharacteristics,
Percival et al. 2010; LSST Dark Energy Science Collab- shouldbeanalyzedsimultaneously tooptimallymeasure
oration 2012, and references therein). Dark energy af- theshapesofgalaxies(Tysonetal.2008a;JeeandTyson,
fects the cosmic history of both the Hubble expansion 2011).
and mass clustering. Distinguishing competing mod- Type Ia supernovae provided the first robust evidence
els for the physical nature of dark energy, or alterna- that the expansion of the Universe is accelerating (Riess
tive explanations involving modifications of the General et al. 1998; Perlmutter et al. 1999). To fully exploit
Theory of Relativity, will require percent level measure- the supernova science potential, light curves sampled in
ments of both the cosmic expansion and the growth of multiple bands every few days over the course of a few
dark matter structure as a function of redshift. Any months are required. This is essential to search for sys-
given cosmological probe is sensitive to, and thus con- tematic differences in supernova populations (e.g., due
strains degenerate combinations of, several cosmological to differing progenitor channels) which may masquerade
parameters. Thereforethemostrobustcosmologicalcon- as cosmological effects, as well as to determine photo-
straints are the result of using interlocking combinations metric redshifts from the supernovae themselves. Unlike
ofprobes. Themostpowerfulprobesincludeweakgravi- other cosmological probes, even a single object gives in-
tationallenscosmicshear(WL),baryonacousticoscilla- formation on the relationship between redshift and dis-
tions(BAO),andphotometryoftypeIasupernovae(SN) tance. Thus a large number of SN across the sky allows
–allasfunctionsofredshift. Usingthecosmicmicrowave one to search for any dependence of dark energy prop-
background fluctuations as the normalization, the com- erties on direction, which would be an indicator of new
bination of these probes can yield the needed precision physics. The results from this method can be compared
to distinguish among models of dark energy (Zhan 2006, with similar measures of anisotropy from the combina-
and references therein). In addition, strong galaxy and tion of WL and BAO (Zhan et al. 2009). Given the ex-
cluster lensing as a function of cosmic time probes the pected SN flux distribution at the redshifts where dark
physicsofdarkmatter,becausethepositionsandshapes energy is important, the single visit depth should be at
of multiple images of a source galaxy depend sensitively least r ∼24. Good image quality is required to separate
5
SN photometrically from their host galaxies. Observa- per day for the most distant TNOs) places strong con-
tionsinatleastfivephotometricbandswillallowproper straints on the cadence of observations, requiring closely
K-corrected light curves to be measured over a range spaced pairs of observations (two or preferably three
of redshift. Carrying out these K-corrections requires times per lunation) in order to link detections unam-
that the calibration of the relative offsets in photomet- biguously and derive orbits (SciBook Ch. 5). Individual
ric zero points between filters and the system response exposures should be shorter than about 30 seconds each
functions, especially near the edges of bandpasses, must to minimize the effects of trailing for the majority of
be accurate to about 1% (Wood-Vasey et al. 2007), sim- moving objects. The images must be well sampled to
ilar to the requirements from photometric redshifts of enableaccurateastrometry,withabsoluteaccuracyofat
galaxies. Deeper data (r > 26) for small areas of the least 0.1 arcsec in order to measure orbital parameters
skycanextendthediscoveryofSNtoameanredshiftof of TNOs with enough precision to constrain theoretical
0.7 (from ∼0.5 for the main survey), with some objects models and enable prediction of occulations. The pho-
beyond z ∼1 (Garnavich et al. 2005; Pinto et al. 2005; tometryshouldbebetterthan1-2%tomeasureasteroids’
SciBook Ch. 11). The added statistical leverage on the colors and thus determine their types. The different fil-
“pre-acceleration”era(z (cid:38)1)wouldimproveconstraints ters should be observed over a short time span to reduce
onthepropertiesofdarkenergyasafunctionofredshift. apparent variations in color due to changes in observing
Finally, there will be powerful cross checks and com- geometry,butshouldberepeatedovermanylunationsin
plementarities with other planned or proposed surveys, order to determine phase curves and allow shape model-
suchasEuclid(Laureijsetal.2011)andWFIRST-AFTA ing.
(Spergel et al. 2013) (wide-field optical-IR imaging from The Congressional mandate can be fulfilled with a
space), DESI (Levi et al. 2013) and PFS (Takada et 10-meter-class telescope equipped with a multi-gigapixel
al. 2014) (spectroscopic BAO with several millions of camera, and a sophisticated and robust data processing
galaxies), and SKA75 (radio). Large survey volumes are system (Ivezi´c et al. 2007a). The images should reach a
keytoprobingdynamicaldarkenergymodels(withsub- depthofatleast24.5(5σ forpointsources)intherband
horizon dark energy clustering or anisotropic stresses). to reach high completeness down to the 140 m mandate
The cross-correlation of the three-dimensional mass dis- for NEOs. Such an instrument would probe the ∼100 m
tribution (as probed by HI in SKA, or galaxies in DESI size range at main-belt distances, and discover rare dis-
and PFS) with the gravitational growth probed by to- tantTNOssuchasSedna(Brown,Trujillo&Rabinowitz
mographic shear in LSST will be complementary way to 2004) and 2012 VP113 (Trujillo & Sheppard 2014).
constrain dark energy properties beyond simply charac-
terizing its equation of state. 2.1.3. Exploring the Transient Optical Sky
Recent surveys have shown the power of measuring
2.1.2. Taking an Inventory of the Solar System variability for studying gravitational lensing, searching
for supernovae, determining the physical properties of
The small-body populations in the Solar System,
gamma-ray burst sources, probing the structure of ac-
such as asteroids, trans-Neptunian objects (TNOs) and
tive galactic nuclei, studying variable star populations,
comets, are remnants of its early assembly. The his-
discovering exoplanets and many other subjects at the
tory of accretion, collisional grinding, and perturbation
forefrontofastrophysics(SciBookCh.8;Lawetal.2009;
byexistingandvanishedgiantplanetsispreservedinthe
Djorgovski et al. 2012; Rowe et al. 2014). Wide area,
orbital elements and size distributions of those objects.
dense temporal coverage to deep limiting magnitudes
Catalogingtheorbitalparameters,sizedistributions,col-
would enable the discovery and analysis of rare and ex-
ors and light curves of these small-body populations re-
otic objects such as neutron stars and black hole bina-
quires a large number of observations in multiple filters,
ries, novae and stellar flares, gamma-ray bursts and X-
and will lead to insights into planetary formation and
ray flashes, active galactic nuclei, stellar disruptions by
evolution by providing the basis and constraints for new
black holes (Bloom et al. 2011; Gezari et al. 2012), and
theoretical models. In addition, collisions in the main
possibly new classes of transients, such as binary merg-
asteroid belt between Mars and Jupiter still occur, and
ers of black holes (Shields & Bonning 2008; Thornton
occasionally eject objects on orbits that may place them
et al. 2013). Such a survey would likely detect numer-
onacollisioncoursewithEarth. Studyingtheproperties
ous microlensing events in the Local Group and perhaps
ofmainbeltasteroidsatsub-kilometersizesisimportant
beyond, and open the possibility of discovering popula-
forlinkingthenear-EarthObject(NEO)populationwith
tions of binaries and planets via transits (e.g., Beaulieu
itssourceinthemainbelt. About20%ofNEOs,thepo-
et al. 2006; Drake et al. 2010; Choi et al. 2013; Batista
tentially hazardous asteroids (PHAs), are in orbits that
et al. 2014), as well as obtaining spectra of lensed stars
passsufficientlyclosetoEarth’sorbit,towithin0.05AU,
in distant galaxies.
thatperturbationswithtimescalesofacenturycanlead
Time-domain science requires large area coverage to
to intersections and the possibility of collision. In De-
cember 2005, the U.S. Congress directed76 NASA to im- enhance the probability of detecting rare events; good
time sampling, because light curves are necessary to dis-
plement a NEO survey that would catalog 90% of NEOs
tinguish certain types of variables and in some cases to
with diameters larger than 140 meters by 2020.
infertheirproperties(e.g. determiningtheintrinsiclumi-
Discovering and linking objects in the Solar System
nosity of Type Ia supernovae depends on measurements
moving with a wide range of apparent velocities (from
of their rate of decline); accurate color information to
several degrees per day for NEOs to a few arc seconds
assistwiththeclassificationofvariableobjects;goodim-
75 https://www.skatelescope.org agequalitytoenabledifferencingofimages,especiallyin
76 Fordetailsseehttp://neo.jpl.nasa.gov/neo/report2007.html crowded fields; and rapid data reduction, classification
6
andreportingtothecommunityinordertoflaginterest-
ing objects for spectroscopic observations and follow-up
in other wavebands.
Time series ranging between one minute and ten years
cadence should be probed over a significant fraction of
the sky. Rapid cadence makes it possible to serendipi-
tously catch very short-lived events, such as eclipses in
ultra-compact double degenerate binary systems (An-
derson et al. 2005), to constrain the properties of fast
faint transients (such as optical flashes associated with
gamma-ray bursts; Bloom et al. 2008), to detect elec-
tromagnetic counterparts to gravitational waves (Nis-
sanke, Kasliwal & Georgieva 2013) and to further con-
strain the properties of new classes of transients discov-
ered by programs such as the Deep Lens Survey (Becker
et al. 2004), the Catalina Real-time Transient Survey
(Drake et al. 2009) and the Palomar Transient Factory
(Law et al. 2009). Observations over a decade would
enable the study of long period variables and quasars
(Kaspi et al. 2007; MacLeod et al. 2010; Graham et
al.2014). Thenextfrontierinthisfieldwillrequiremea-
suring the colors of fast transients, and to probe vari- Fig. 1.— TheimagequalitydistributionmeasuredattheCerro
ability at faint magnitudes. Classification of transients Pach´onsiteusingadifferentialimagemotionmonitor(DIMM)at
in close to real time will require access to the full photo- λ=500nm,andcorrectedusinganouterscaleparameterof30m
metric history of the objects, both before and after the overan8.4maperture. Fordetailsabouttheouterscalecorrection
see Tokovinin (2002). The observed distribution is well described
transient event (e.g., Mahabal et al. 2011).
by a log-normal distribution, with the parameters shown in the
figure.
2.1.4. Mapping the Milky Way
detect RR Lyrae stars beyond the Galaxy’s tidal radius
A major challenge in extragalactic cosmology today at ∼300 kpc, the single-visit depth must be r ∼24.5.
concerns the formation of structure on sub-galactic In order to measure the tangential velocity of stars at
scales,wherebaryonphysicsbecomesimportant,andthe a distance of 10 kpc, where the halo dominates over the
nature of dark matter may manifest itself in observable disk, to within 10 km s−1 (comparable with the accu-
ways(e.g.,Weinbergetal.2013). TheMilkyWayandits racy of large-scale radial velocity surveys), the proper
environment provide a unique dataset for understanding motion accuracy should be 0.2 mas yr−1 or better. This
the detailed processes that shape galaxy formation and is the same accuracy as will be delivered by the recently
for testing the small-scale predictions of our standard launchedGaiamission77 (Perrymanetal.2001; deBrui-
cosmological model. However, we still lack answers to jne 2012) at its faint limit (r ∼ 20). In order to mea-
two basic questions about our Galaxy: sure distances to solar neighborhood stars out to a dis-
tance of 300 pc (the thin disk scale height), with geo-
• What is the detailed structure and accretion his- metric distance accuracy of at least 30%, trigonometric
tory of the Milky Way? parallax measurements accurate to 1 mas (1σ) are re-
quiredover10years. Toachievetherequiredpropermo-
• What are the fundamental properties of all the tion and parallax accuracy with an assumed astromet-
stars within 300 pc of the Sun? ric accuracy of 10 mas per observation per coordinate,
approximately 1,000 separate observations are required.
Mapping the Galaxy requires large area coverage, ex-
Thisrequirementforalargenumberofobservationsisin
cellent image quality to maximize photometric and as-
good agreement with that from minimizing systematics
trometric accuracy, especially in crowded fields, photo-
in weak lensing observations (§ 2.1.1).
metricprecisionofatleast1%toseparatemainsequence
andgiantstars(e.g.,Helmietal.2003)aswellastoiden- 2.1.5. A Summary and Synthesis of Science-driven
tify variable stars such as RR Lyrae (Sesar et al. 2010, Constraints on Data Properties
Sharma et al. 2011), and astrometric precision of about
The goals of all the science programs discussed above
10 mas per observation to enable parallax and proper
(and many more, of course) can be accomplished by
motion measurements (SciBook Ch. 6,7). In order to
satisfying the minimal constraints listed below. For a
probe the halo out to its presumed edge at ∼ 100 kpc
more elaborate listing of various constraints, including
(Ivezi´c et al. 2003) using main-sequence stars, the total
detailed specification of various probability density dis-
coaddeddepthmustreachr >27,withasimilardepthin
tributionfunctions,pleaseseetheLSSTScienceRequire-
the g band. The metallicity distribution of stars can be
ments Document78 and the LSST Science Book.
studiedphotometrically inthe Sgrtidal stream(e.g., see
Majewski et al. 2003, Chou et al. 2007) and other halo
1. The single visit depth should reach r ∼ 24.5. This
substructures (∼ 30 kpc, Carollo et al. 2007), yielding
limit is primarily driven by the search for NEOs,
new insights into how they formed. These metallicities
arelimitedbythecoaddeddepthintheuband. Toreach 77 http://sci.esa.int/gaia/
thelimitofthestellarhalo,thismustreachu∼24.5. To 78 http://www.lsst.org/files/docs/SRD.pdf
7
variable sources (e.g., SN, RR Lyrae stars), and SNlightcurveseveryfewdays,whileaccommodat-
by proper motion and trigonometric parallax mea- ing constraints set by proper motion and trigono-
surements for stars. Indirectly, it is also driven by metric parallax measurements.
therequirementsonthecoaddedsurveydepthand
theminimumnumberofexposuresrequiredbyWL 8. The total number of visits of any given area of sky,
science. We will split a single visit into two expo- when accounting for all filters, should be of the
suresofequallengthtoidentifyandremovecosmic order of 1,000, as mandated by WL science, the
rays. search for NEOs, and proper motion and trigono-
metricparallaxmeasurements. Studiesoftransient
2. Image quality should maintain the limit set by the sources also benefit from a large number of visits.
atmosphere (the median free-air seeing is 0.65 arc-
9. The coadded survey depth should reach r ∼ 27.5,
secintherbandatthechosensite,seeFig.1),and
with sufficient signal-to-noise ratio in other bands
not be degraded appreciably by the hardware. In
to address both extragalactic and Galactic science
additiontostringentconstraintsfromweaklensing,
drivers.
goodimagequalityisdrivenbytherequiredsurvey
depth for point sources and by image differencing
10. The distribution of visits per filter should enable
techniques.
accurate photometric redshifts, separation of stel-
lar populations, and sufficient depth to enable de-
3. Photometric repeatability should achieve 5 mmag
tection of faint extremely red sources (e.g., brown
precision at the bright end, with zeropoint stabil-
dwarfs and high-redshift quasars). Detailed sim-
ity across the sky of 10 mmag and band-to-band
ulations of photometric redshift estimates sug-
calibration errors not larger than 5 mmag. These
gest an approximately flat distribution of visits
requirements are driven by the need for high pho-
amongbandpasses(becausethesystemthroughput
tometric redshift accuracy, the separation of stel-
and atmospheric properties are wavelength depen-
lar populations, detection of low-amplitude vari-
dent, the achieved depths are different in different
able objects (such as eclipsing planetary systems),
bands). The adopted time allocation (see Table 1)
and the search for systematic effects in type Ia su-
includes a slight preference to the r and i bands
pernova light curves.
because of their dominant role in star/galaxy sep-
aration and weak lensing measurements.
4. Astrometricprecisionshouldmaintainthelimitset
by the atmosphere, of about 10 mas per visit at 11. The distribution of visits on the sky should extend
the bright end (on scales below 20 arcmin). This over at least ∼20,000 deg2 to obtain the required
precisionisdrivenbythedesiretoachieveaproper number of galaxies for WL studies, with attention
motionaccuracyof0.2masyr−1andparallaxaccu- paidtoinclude“special”regionssuchastheEclip-
racy of 1.0 mas over the course of a 10-year survey tic and Galactic planes, and the Large and Small
(see §3.2.3). MagellanicClouds(ifintheSouthernHemisphere).
Notethat20,000deg2iscomparabletothefullarea
5. The single visit exposure time (including both ex- observablefromanygivenground-basedsiteatrea-
posures in a visit, which are required for cosmic sonably low airmass (the LSST survey area will be
ray rejection) should be less than about a minute contained within 30,000 deg2 with δ <+34.5◦).
topreventtrailingoffastmovingobjectsandtoaid
controlofvarioussystematiceffectsinducedbythe 12. Data processing, data products and data access
atmosphere. It should be longer than ∼20 seconds should result in data products that approach the
to avoid significant efficiency losses due to finite statistical uncertainties in the raw data; i.e., the
readout, slew time, and read noise. processingmustbeclosetooptimal. Toenablefast
andefficientresponsetotransientsources,thepro-
6. The filter complement should include at least six cessing latency for variable sources should be less
filters in the wavelength range limited by atmo- than a minute, with a robust and accurate prelim-
spheric absorption and silicon detection efficiency inary characterization of all reported variables.
(320–1050 nm), with roughly rectangular filters
and no large gaps in the coverage, in order to en- Remarkably, even with these joint requirements, none
ablerobustandaccuratephotometricredshiftsand of the individual science programs is severely over-
stellar typing. An SDSS-like u band (Fukugita et designed,i.e.,despitetheirsignificantscientificdiversity,
al.1996)isextremelyimportantforseparatinglow- theseprogramsarehighlycompatibleintermsofdesired
redshift quasars from hot stars, and for estimating datacharacteristics. Indeed,anyoneofthefourmainsci-
the metallicities of F/G main sequence stars. A ence drivers could be removed, and the remaining three
bandpass with an effective wavelength of about 1 would still yield very similar requirements for most sys-
micron would enable studies of sub-stellar objects, temparameters. Asaresult,theLSSTsystemcanadopt
high-redshiftquasars(toredshiftsof∼7.5),andre- a highly efficient survey strategy where a single dataset
gions of the Galaxy that are obscured by interstel- serves most science programs (instead of science-specific
lar dust. surveysexecutedinseries). Onecanviewthisprojectas
massivelyparallelastrophysics. Thevastmajority(about
7. Therevisittimedistributionshouldenabledetermi- 90%) of the observing time will be devoted to a deep-
nationoforbitsofSolarSystemobjectsandsample wide-fastsurveymode,withtheremaining10%allocated
8
reason, the sky area for the main survey is maximized
TABLE 1
The LSST Baseline Design and Survey Parameters to its practical limit, 18,000 deg2, determined by the re-
quirement to avoid airmasses less than 1.5, which would
Quantity BaselineDesignSpecification substantially deteriorate the image quality and the sur-
OpticalConfig. 3-mirrormodifiedPaul-Baker
vey depth (see eq. 6).
MountConfig. Alt-azimuth
With the adopted field-of-view area, the sky coverage
Finalf-ratio,aperture f/1.234,8.4m
and the survey lifetime fixed, the primary mirror diam-
Fieldofview,´etendue 9.6deg2,319m2deg2
eter is fully driven by the required survey depth. There
PlateScale 50.9µm/arcsec(0.2”pix)
Pixelcount 3.2Gigapix are two depth requirements: the final (coadded) survey
WavelengthCoverage 320–1050nm,ugrizy depth,r ∼27.5,andthedepthofasinglevisit,r ∼24.5.
Singlevisitdepths,designa 23.9,25.0,24.7,24.0,23.3,22.1 The two requirements are compatible if the number of
Singlevisitdepths,min.b 23.4,24.6,24.3,23.6,22.9,21.7 visitsisseveralhundredperband,whichisingoodagree-
Meannumberofvisitsc 56,80,184,184,160,160 ment with independent science-driven requirements on
Final(coadded)depthsd 26.1,27.4,27.5,26.8,26.1,24.9 the latter.
a Design specification from the Science Requirements Document The required coadded survey depth provides a direct
(SRD)for5σdepthsforpointsourcesintheugrizybands,respec- constraint,independentofthedetailsofsurveyexecution
tively. ThelistedvaluesareexpressedonABmagnitudescale,and such as the exposure time per visit, on the minimum
correspondtopointsourcesandfiducialzenithobservations(about
effectiveprimarymirrordiameterof6.5m, asillustrated
0.2maglossofdepthisexpectedforrealisticairmassdistributions,
see Table 2 for more details). b Minimum specification from the in Fig. 2.
Science Requirements Document for 5σ depths. c An illustration
of the distribution of the number of visits as a function of band- 2.2.2. The Optimal Exposure Time
pass,takenfromTable24intheSRD.dIdealizeddepthofcoadded
The single visit depth depends on both the primary
images,basedondesignspecificationfor5σdepthandthenumber
ofvisitsinthepenultimaterow(takenfromTable24intheSRD). mirror diameter and the chosen exposure time, tvis. In
turn, the exposure time determines the time interval to
revisitagivenskypositionandthetotalnumberofvisits,
to special programs which will also address multiple sci- and each of these quantities has its own science drivers.
ence goals. Before describing these surveys in detail, we We summarize these simultaneous constraints in terms
discuss the main system parameters. of the single-visit exposure time:
2.2. The Main System Design Parameters • Thesingle-visitexposuretimeshouldnotbelonger
than about a minute to prevent trailing of fast So-
Given the minimum science-driven constraints on the
lar System moving objects, and to enable efficient
data properties listed in the previous section, we now
control of atmospheric systematics.
discuss how they are translated into constraints on the
main system design parameters: the aperture size, the • Themeanrevisittime(assuminguniformcadence)
surveylifetime,theoptimalexposuretime,andthefilter for a given position on the sky, n, scales as
complement. (cid:18) t (cid:19)(cid:18) A (cid:19)(cid:18)10deg2(cid:19)
n= vis sky days, (1)
2.2.1. The Aperture Size 10sec 10,000deg2 AFOV
The product of the system’s ´etendue and the survey where two visits per night are assumed (required
lifetime, for given observing conditions, determines the for efficient detection of Solar System objects, see
sky area that can be surveyed to a given depth. The below),andthelossesforrealisticobservingcondi-
LSST field-of-view area is maximized to its practical tionshavebeentakenintoaccount(withtheaidof
limit, ∼10deg2, determinedbytherequirementthatthe the Operations Simulator described below). Sci-
delivered image quality be dominated by atmospheric ence drivers such as supernova light curves and
seeing at the chosen site (Cerro Pach´on in Northern moving objects in the Solar System require that
Chile). A larger field-of-view would lead to unaccept- n < 4 days, or equivalently t < 40 seconds for
vis
able deterioration of the image quality. This constraint the nominal values of A and A .
sky FOV
leaves the primary mirror diameter and survey lifetime
• Thenumberofvisitstoagivenpositiononthesky,
as free parameters. The adopted survey lifetime of 10
N ,withlossesforrealisticobservingconditions
years is a compromise between a shorter time that leads visit
taken into account, is given by
to an excessively large and expensive mirror (15 m for
(cid:18) (cid:19)(cid:18) (cid:19)
a 3 year survey and 12 m for a 5 year survey) and not 3000 T
N = . (2)
as effective proper motion measurements, and a smaller visit n 10yr
telescope that would require more time to complete the
The requirement N > 800 again implies that
survey, with the associated increase in operations cost. visit
n < 4 and t < 40 seconds if the survey lifetime,
The primary mirror size is a function of the required vis
T ∼10 years.
surveydepthandthedesiredskycoverage. Byandlarge,
the anticipated science outcome scales with the num-
survey area, the number of sources increases by a factor of two.
ber of detected sources. For practically all astronomical If the survey area is kept fixed, the increased exposure time will
source populations, in order to maximize the number of resultin∼0.4magdeeperdata(seeeq.6). Forcumulativesource
detected sources, it is more advantageous to maximize counts described by log(N) = C+k∗m, the number of sources
willincreasebymorethanafactoroftwoonlyifk>0.75. Apart
the area first, and then the detection depth79. For this
fromz<2quasars,practicallyallpopulationshavek atmost0.6
(the Euclidean value), and faint stars and galaxies have k < 0.5.
79 Ifthetotalexposuretimeisdoubledandusedtodoublethe Formoredetails,pleaseseeNemiroff(2003).
9
• These three requirements place a firm upper limit
on the optimal visit exposure time of t < 40
vis
seconds. Surveying efficiency (the ratio of open-
shutter time to the total time spent per visit) con-
siderations place a lower limit on t due to fi-
vis
nitedetectorread-outandtelescopeslewtime(the
longestacceptableread-outtimeissetto2seconds,
the shutter open-and-close time is 2 seconds, and
the slew and settle time is set to 5 seconds, includ-
ing the read-out time for the second exposure in a
visit):
(cid:18) (cid:19)
t
(cid:15)= vis . (3)
t +9sec
vis
To maintain efficiency losses below ∼30% (i.e., at
least below the limit set by the weather patterns),
and to minimize the read noise impact, t > 20
vis
seconds is required.
Taking these constraints simultaneously into account,
as summarized in Fig. 3, yielded the following reference
design:
1. A primary mirror effective diameter of ∼6.5 m.
Fig. 2.— The coadded depth in the r band (AB magnitudes)
With the adopted optical design, described below,
vs. the effective aperture and the survey lifetime. It is assumed
thiseffectivediametercorrespondstoageometrical that 22% of the total observing time (corrected for weather and
diameter of ∼8 m. Motivated by characteristics of otherlosses)isallocatedforther band, andthattheratioofthe
theexistingequipmentattheStewardMirrorLab- surveyedskyareatothefield-of-viewareais2,000.
oratory, which is fabricating the primary mirror,
the adopted geometrical diameter is set to 8.4 m.
2. A visit exposure time of 30 seconds (using two 15
second exposures to efficiently reject cosmic rays),
yielding (cid:15)=77%.
3. A revisit time of 3 days on average for 10,000 deg2
of sky, with two visits per night.
To summarize, the chosen primary mirror diameter is
the minimum diameter that simultaneously satisfies the
depth (r ∼24.5 for single visit and r ∼27.5 for coadded
depth) and cadence (revisit time of 3-4 days, with 30
seconds per visit) constraints described above.
2.3. System Design Trade-offs
We note that the Pan-STARRS project (Kaiser et
al.2002),withsimilarsciencegoalsasLSST,hasadopted
a distributed aperture design, where the total system
´etendue is a sum of ´etendue values for an array of small
telescopes (the prototype PS1 telescope has an ´etendue
1/24th that of LSST). Similarly, the LSST system could
perhaps be made as two smaller copies with 6m mirrors,
or 4 copies with 4m mirrors, or 16 copies with 2m mir-
rors. Each of these clones would have to have its own 3 Fig. 3.— The single-visit depth in the r band (5σ detection
Gigapixel camera (see below), and given the added risk forpointsources,ABmagnitudes)vs. revisittime,n(days),asa
function of the effective aperture size. With a coverage of 10,000
andcomplexity(e.g.,maintenance,dataprocessing),the
deg2 intwobands,therevisittimedirectlyconstrainsthevisitex-
monolithic design seems advantageous for a system with posuretime,tvis=10nseconds. Inadditiontodirectconstraints
such a large ´etendue as LSST. on optimal exposure time, tvis is also driven by requirements on
It is informative to consider the tradeoffs that would therevisittime,n,thetotalnumberofvisitsperskypositionover
be required for a system with a smaller aperture, if the the survey lifetime, Nvisit, and the survey efficiency, (cid:15) (see eqs.1-
3). Note that these constraints result in a fairly narrow range of
science requirements were to be maintained. For this allowedtvis forthemaindeep-wide-fastsurvey.
comparison, we consider a four-telescope version of the
Pan-STARRS survey (PS4). With an ´etendue about 6 7.2 deg2), and all observing conditions being equal, the
times smaller than that of LSST (effective diameters of PS4systemcouldinprincipleuseanidenticalcadenceas
6.5 m and 3.0 m, and a field-of-view area of 9.6 deg2 vs. that of LSST. The main difference in the datasets would
10
be a faint limit shallower by about 1 mag in a given
survey lifetime. As a result, for Euclidean populations
1.0
the sample sizes would go down by a factor of 4, while
for shallower populations (e.g., galaxies around redshift Airmass 1.2
of 1) the samples would be smaller by a factor 2-3. The
distance limits for nearby sources, such as Milky Way 0.8 u g r i z y
stars, would drop to 60% of their corresponding LSST
values, and the NEO completeness level mandated by 1)
0-0.6
the U.S. Congress would not be reached. ut (
If instead the survey coadded depth were to be main- hp
g
tsaminaellde,rt(h∼en3,t5h0e0sduergv2e)y.skIyf athreeaswurovuelydhsianvgelet-ovbiseit6dtiemptehs Throu0.4
were to be maintained, then the exposure time would
have to be about 6 times longer (ignoring the slight dif-
0.2
ferenceinthefield-of-viewareaandsimplyscalingbythe
´etendue ratio), resulting in non-negligible trailing losses
for Solar System objects, and either i) a factor of six
0.0
smaller sky area observed within n = 3 days, or ii) the 300 400 500 600 700 800 900 1000 1100
Wavelength (nm)
same sky area revisited every n = 18 days. Given these
conflicts, one solution would be to split the observing Fig. 4.—TheLSSTbandpasses. Theverticalaxisshowstheto-
time and allocate it to individual specialized programs talthroughput. Thecomputationincludestheatmospherictrans-
mission (assuming an airmass of 1.2, dotted line), optics, and the
(e.g., large sky area vs. deep coadded data vs. deep
detectorsensitivity.
single-visit data vs. small n data, etc.), as is being done
by the PS1 Consortium80.
In summary, given the science requirements as stated
here, there is a minimum´etendue of ∼300 deg2m2 which 5.5x1e-14
enables our seemingly disparate science goals to be ad-
dressed with a single data set. A system with a smaller 5.0
´etenduewouldrequireseparatespecializedsurveystoad- m) 4.5
dressthesciencegoals,whichresultsinalossofsurveying ro
t
efficiency81. The LSST is designed to reach this mini- gs4.0
n
mum ´etendue for the science goals stated in its Science A
Requirements Document. 2/s/ 3.5
m
2.4. The Filter Complement s/c3.0
g
The LSST filter complement (ugrizy, see Fig. 4) is er 2.5
modeled after the Sloan Digital Sky Survey (SDSS) sys- x (
u 2.0
tem (Fukugita et al. 1996) because of its demonstrated Fl
success in a wide variety of applications, including pho-
1.5
tometricredshiftsofgalaxies(Budav´arietal.2003),sep-
aration of stellar populations (Lenz et al. 1998; Helmi et 1.0
5000 6000 7000 8000 9000 10000 11000
al.2003),andphotometricselectionofquasars(Richards Wavelength (Angstrom)
etal.2002; Rossetal.2012). TheextensionoftheSDSS
Fig. 5.—Anexampleofdeterminationoftheatmosphericopac-
system to longer wavelengths (the y band at ∼1 micron)
itybysimultaneouslyfittingathree-parameterstellarmodelSED
isdrivenbytheincreasedeffectiveredshiftrangeachiev- (Kurucz1979)andsixphysicalparametersofasophisticatedatmo-
able with the LSST due to deeper imaging, the desire sphericmodel(MODTRAN,Andersonetal.1999)toanobserved
to study sub-stellar objects, high-redshift quasars, and F-typestellarspectrum(Fλ). Theblacklineistheobservedspec-
trum and the red line is the best fit. Note that the atmospheric
regions of the Galaxy that are obscured by interstellar
water feature around 0.9-1.0 µm is exquisitely well fit. The com-
dust, and the scientific opportunity enabled by modern ponents of the best-fit atmospheric opacity are shown in Fig. 6.
CCDs with high quantum efficiency in the near infrared. AdaptedfromBurkeetal.(2007).
The chosen filter complement corresponds to a design
separation of stars and quasars, and the telluric water
“sweet spot”. We have investigated the possibility of absorption feature at 9500˚A effectively defines the blue
replacing the ugrizy system with a filter complement
edge of the y band. Of the remaining four filters (griz),
that includes only five filters. For example, each filter
thegbandisalreadyquitewide. Asalastoption,theriz
width could be increased by 20% over the same wave-
bandscouldberedesignedastwowiderbands. However,
length range (neither a shorter wavelength range, nor
this option is also undesirable because the r and i bands
gaps in the wavelength coverage are desirable options),
are the primary bands for weak lensing studies and for
butthisoptionisnotsatisfactory. Placingtherededgeof
star/galaxy separation, and chromatic atmospheric re-
theubandbluewardoftheBalmerbreakallowsoptimal
fraction would worsen the point spread function for a
wider bandpass.
80 More information about Pan-STARRS is available from
http://pan-starrs.ifa.hawaii.edu.
81 The converse is also true: for every´etendue there is a set of
optimal science goals that such a system can address with a high
efficiency.