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Astronomical Surveys Matthew Colless Observational Techniques Workshop April 2001
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Properties of Surveys Type and purpose General-use or highly specific? Targeted or blind? Imaging or spectroscopic? Area and spatial resolution All-sky, wide-field, pencil-beam? Wavelength and spectral resolution Broad-band, narrow-band, spectroscopic? Depth and quality Faintest detections? Reliability? Precision? Analysis, storage and use Reductions, data volume, dissemination?
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Survey Design Factors - product of telescope aperture ( ) and area on sky ( ). Important figure-of-merit for imaging surveys; time taken to cover given area to survey depth. Efficiency - relative system throughput. telescope, instrument, detector throughputs. Multiplex - number of objects that can be observed simultaneously. Important figure-of-merit for spectroscopic surveys (in combination with field-of-view). Overheads - non-survey observing time Field acquisition, calibrations, readout, etc.
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Example I Imaging survey of sky - what combination of telescope + detector is quickest? Telescope/instrument: AAT/WFI has =15m 2, =0.25deg 2, =0.2 so speed = 0.75 UKST/film has =1.5m 2, =25.0deg 2, =0.02 so speed = 0.75 In this case decision would be based on other components of overall survey speed (overheads, telescope availability) or survey goals (spatial resolution, precision of photometry).
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Example II Redshift survey of sky - what combination of telescope + detector is quickest? Survey speed /N where N max(1, /M) is the number of times each field must be observed and is surface density of targets. Telescope/instrument: AAT/2dF - =15m 2, =3deg 2, =0.1, M=400; so speed min(4.5,600/ ) UKST/6dF - =1.5m 2, =30deg 2, =0.2, M=150 so speed min(9,45/ ) Hence the crucial factor here is source density: if > 10deg -2 then use 2dF if < 10deg -2 then use 6dF
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The 2dF Galaxy Redshift Survey 250,000 galaxies over 2000sq.deg magnitude-limited at b J =19.45
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Figures of Merit for 2dFGRS = 15 sq. metres x 3 sq. deg. Multiplex: 400 fibresEfficiency: robotic operation
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2dGRS
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Selection Effects Selection effects - the eternal bane of observational astronomy: spurious trends and correlations biases (e.g. Malmquist bias and its relatives) Good survey design - demands minimizing and controlling selection effects: well-defined target selection criteria well-determined errors on measurements simulated data to determine selection effects on the quantities or relations of interest
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Redshift completeness is a function of magnitude - harder to identify redshift for fainter objects. In fact, completeness depends on both magnitude and S/N of each set of data - lower S/N gives stronger magnitude effect on completeness Example I - redshift incompleteness in the 2dFGRS
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Completeness map for 2dFGRS The map allows correction of the incompleteness by an appropriate weighting of the observed galaxies at each position in the survey region.
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Example II - biased correlation Sample selection effects can also bias physical correlations. Ingredients: two correlated parameters… intrinsic scatter in relation or errors in measurements… selection/observation limits…
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True (x,y) correlation Biased correlation Selection limit on x x y Biased (x,y) correlation Solution requires: accurate knowledge of measurement errors accurate knowledge of selection limits determine correlation allowing for both errors and limits via a full maximum likelihood fit.
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Types of Surveys and Some Useful Examples Imaging surveys (various wavelengths) Spectroscopic surveys (redshift etc.) Surveys of specific object classes General-purpose surveys (‘sky surveys’) Single-goal surveys (‘experiments’) WARNING - acronym soup!
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All-Sky Imaging Surveys Optical sky surveys: UKST Southern Sky Surveys (B and R), esp. APM and SuperCosmos digitized versions DSS - Digitized Sky Survey DPOSS - Digitized Palomar Obs. Sky Survey SDSS - Sloan Digital Sky Survey, ugriz over str to r=23 in north + r=25 in southern deep strips
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All-Sky Imaging Surveys IR sky surveys: 2MASS - Two-Micron All-Sky Survey (JHK) DENIS - Deep Near-Infrared Survey of the Southern Sky (IJK) IRAS - Infra-Red Astronomy Satellite, 60,120 m
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All-Sky Imaging Surveys X-ray surveys: RASS - ROSAT All-Sky Survey, whole-sky map and catalogue of 10 5 sources in 0.1-2.4keV band.
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All-Sky Imaging Surveys Radio surveys: FIRST - Faint Images of the Radio Sky at Twenty-cm, northern sky down to 0.75mJy NVSS - NRAO VLA Sky Survey, continuum survey at 1.4GHz, northern sky above >-40° SUMSS - Sydney U. Molonglo Sky Survey, 843MHz, ~matching NVSS but in south HIPASS - HI Parkes All-Sky Survey; blind HI survey of southern sky
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Deep Imaging Surveys Hubble Deep Fields (N+S) - deepest optical images of the universe (also ISO, Chandra, etc deep fields) EIS - ESO Imaging Survey, O/IR multi-colour imaging of 6 3°x2° patches ELIAS - ISO deep imaging of 13 ° at 7, 15, 90, 175 m
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PSCz - IRAS Point Source Catalog z-survey, 15,000 galaxies over 90% of sky LCRS - Las Campanas Redshift survey, 25,000 galaxies, 800 ° 2dFGRS - 2dF Galaxy Redshift Survey, 250,000 galaxies, b J <19.5, 2000 ° 2QZ - 2dF QSO Redshift Survey, 25,000 QSOs, 750 ° SDSS - Sloan Digital Sky Survey, 10 6 galaxies + QSOs, steradians at NGP Redshift Surveys
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Other Surveys Gravitational Lens Surveys: MACHO - microlensing in LMC, SMC and bulge AGAPE - microlensing in Andromeda EROS - microlensing in the LMC OGLE - microlensing in LMC, SMC and bulge Galaxy Surveys: APM Galaxy Survey - 2x10 6 galaxies with B J <20.5 in South Galactic Cap.
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Other Surveys Astrometric surveys: Hipparcos - positions, photometry, proper motions etc for 10 6 stars down to V=11 Cluster surveys: Abell Cluster Catalogue - 4000 galaxy clusters over the whole sky ENACS - ESO Nearby Abell Cluster Survey, redshifts etc for Abell clusters Secular surveys (time-variability): ASAS - All-Sky Automated Survey, photometric monitoring of 10 7 stars with V<14
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