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Astronomical Surveys

Astronomical Surveys. Matthew Colless. Observational Techniques Workshop April 2001. 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?

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Astronomical Surveys

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  1. Astronomical Surveys Matthew Colless Observational Techniques Workshop April 2001

  2. 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?

  3. Survey Design Factors • A-product of telescope aperture (A) and area on sky (). • Important figure-of-merit for imaging surveys; time taken to cover given area to survey depth. • Efficiencye - 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.

  4. Example I • Imaging survey of sky - what combination of telescope + detector is quickest? • Telescope/instrument: • AAT/WFI has A=15m2, =0.25deg2, e=0.2 so speed  Ae = 0.75 • UKST/film has A=1.5m2, =25.0deg2, e=0.02 so speed  Ae = 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).

  5. Example II • Redshift survey of sky - what combination of telescope + detector is quickest? • Survey speed Ae/N where N  max(1,s/M) is the number of times each field must be observed and s is surface density of targets. • Telescope/instrument: • AAT/2dF - A=15m2, =3deg2, e=0.1, M=400;so speed  min(4.5,600/s) • UKST/6dF - A=1.5m2, =30deg2, e=0.2, M=150 so speed  min(9,45/s) • Hence the crucial factor here is source density: • if s > 10deg-2 then use 2dF • if s < 10deg-2 then use 6dF

  6. The 2dF Galaxy Redshift Survey 250,000 galaxies over 2000sq.degmagnitude-limited at bJ=19.45

  7. A = 15 sq. metres x 3 sq. deg. Multiplex: 400 fibres Efficiency: robotic operation Figures of Merit for 2dFGRS

  8. 2dGRS

  9. 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

  10. Example I - redshift incompleteness in the 2dFGRS • 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

  11. 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.

  12. 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…

  13. 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. True (x,y) correlation Biased (x,y) correlation Selection limit on x Biased correlation y x

  14. 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!

  15. 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 p str to r=23 in north + r=25 in southern deep strips

  16. 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, 60,120mm

  17. All-Sky Imaging Surveys • X-ray surveys: • RASS - ROSAT All-Sky Survey, whole-sky map and catalogue of 105 sources in 0.1-2.4keV band.

  18. 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 d>-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

  19. 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, 175mm

  20. Redshift Surveys • 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, bJ<19.5, 2000° • 2QZ - 2dF QSO Redshift Survey, 25,000 QSOs, 750° • SDSS - Sloan Digital Sky Survey, 106 galaxies + QSOs, p steradians at NGP

  21. 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 - 2x106 galaxies with BJ<20.5 in South Galactic Cap.

  22. Other Surveys • Astrometric surveys: • Hipparcos - positions, photometry, proper motions etc for 106 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 107 stars with V<14

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