Large Area Surveys with Array Receivers
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Transcript of Large Area Surveys with Array Receivers
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Large Area Surveys with Array Receivers
Robert Minchin
Single Dish Summer School
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A bit of history…
• Array receivers are not new
• NRAO 7-beam receiver was installed on the 91-m telescope at GB in 1986
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A bit of history…
• Array receivers are not new• NRAO 7-beam 4.85 GHz receiver was
installed on the 91-m telescope in 1986• The same receiver was subsequently
used on the 43-m telescope at GB and the 64-m Parkes telescope
• 8-beam 230 GHz receiver installed in 1988 on the 12-m telescope
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Why array receivers?
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Could this be done with a single-pixel receiver?
• Depends on the science objective– If the experiment is to detect the galaxy, a
single-pixel would be as efficient– If the experiment is looking for an extended
halo, then an array is a lot faster
• Array receivers can be used for single pixel observations – although often not as well as dedicated single-pixels
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Why array receivers?
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Could this be done with a single-pixel receiver?
• Here, the source is known to be extended a priori
• Clearly, it will be quicker to survey the region using an array receiver than with a single-pixel receiver
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Why array receivers?
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Could this be done with a single-pixel receiver?
• Here, the presence (or otherwise) of radio sources is not known a priori
• Whether the sources are extended or not, the whole region must be covered before the population is known
• This can be accomplished most efficiently with an array receiver
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Why array receivers?
• The principle reason for building array receivers is to survey large areas more efficiently than single pixel receivers
• Surveys can be used:– to map a known source in detail– to survey for new sources– to do both
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Types of array receiver
• Bolometer cameras– Continuum only– No gaps between pixels
• Examples– MUSTANG (GBT)– BOLOCAM (CSO)– LABOCA (APEX)
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Types of array receiver
• Phased-array feeds– Spectral line, continuum, pulsars– No gaps between pixels– Technology under development
• Examples:– Planned receivers for GBT and Arecibo– A number of prototype receivers
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Types of array receiver
• Heterodyne feed-horn arrays– Spectral line, continuum, pulsars– Gaps between pixels
• Examples:– ALFA– Parkes Multibeam– Green Bank K-band FPA (under
construction)
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Nyquist Sampling
• For a feed array, the separation between the beams on the sky is greater than the half-power beamwidth
• To map the sky with Nyquist sampling, need to observe points separated by a half-power beamwidth or less
• This means either multiple scans or multiple pointings
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Survey strategy
• Best strategy depends on the science:– For pulsar discovery, the P-ALFA strategy
is to track a point on the sky
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Survey strategy
• Best strategy depends on the science:– For pulsar discovery, the P-ALFA strategy
is to track a point on the sky– For galactic hydrogen and continuum, the
I-GALFA and GALFACTS surveys drive the telescope to cover a large area quickly
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Drive vector
Sky drift vector
Resultant
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Without basketweaving
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With basketweaving
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Survey strategy
• Best strategy depends on the science:– For pulsar discovery, the P-ALFA strategy
is to track a point on the sky– For galactic hydrogen and continuum, the
I-GALFA and GALFACTS surveys drive the telescope to cover a large area quickly
– For extragalactic hydrogen, the E-ALFA surveys use drift scans to build up integration time
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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Not all pixels are created equal…
• For even sensitivity, want to make a Nyquist-sampled map with each beam
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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AGES observing stategy
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Raster Mapping
• Can be used to get approximately uniform coverage, even without an array rotator
• Some simulation of raster mapping with the K-band FPA being built for the GBT (Pisano 2008):
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Scanning as position switching
• If the telescope is scanned, the bandpass can be estimated from all or some of the scan points
• This introduces spatial filtering
• The best estimator depends on the science – whether sources are extended or not
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Bandpass estimation
• The HI Parkes All Sky Survey (HIPASS) used the median of points directly surrounding the ‘on’ to form the ‘off’
• Most sources were smaller than the beam and so were correctly measured
• For extended sources, this can lead to errors in the baseline and loss of flux
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Bandpass estimation
• For the HIPASS High Velocity Cloud survey, the MinMed estimator was used to re-analyse the data
• This divides the scan into a number of regions (5 for HIPASS) and takes the median of each region
• The minimum of the medians at each spectral point is then used as the ‘off’
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Bandpass estimation
• For very extended sources, frequency switching may be best– Used by GALFA-HI surveys at Arecibo– Introduces a degree of frequency filtering,
so not suitable for very broad sources
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Bandpass stability
• As in ordinary position switching, the bandpass must be similar in the ‘on’ and the the ‘off’ point
• Variability can be due to a number of reasons, including:– the telescope moving– receiver gain/temperature changing– changes in the atmosphere
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Bandpass stability
• Deep spectral-line surveys at Arecibo use drift scans to ensure stable baselines– At most telescopes, variation with alt-az
position is less important that at Arecibo
• At higher frequencies, the atmosphere is important– Atmosphere is common across pixels, which
can be used to remove this
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Survey science is the driver
• Survey strategy is driven by science
• Data analysis is driven by science
• The receivers are primarily used for surveys
• The receiver design should be driven by the survey science
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Design considerations
• Array receivers are large, expensive instruments
• Servicing is harder than for single-pixels• KISS – Keep It Simple, Stupid• The simplest design that will accomplish
the survey science goals is the best• Array receivers are not traditional
‘general science’ instruments
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Design considerations
• Is an array rotator needed?– Rotators make life easier for survey
design, but cause reliability problems
• Is polarisation data needed?– A single-polarisation receiver is simpler,
and the system temperature improvement may offset the √2 sensitivity loss
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Design considerations
• How wide a band is needed?– Array receivers often have a narrower band
than equivalent single-pixel receivers– IF and spectrometer bandwidth needed is BW
× Npixels
• What spectral resolution is needed?– Can current back-ends deliver this?– Can new back-ends be available by the time
the instrument is ready?
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Design considerations
• Feed design– If close-packing is important, this will
impact the efficiency of the feed– Should the feed be more optimised for
extended sources than a normal feed?
• There are no universal right answers!
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ALFA
• Array rotator needed for pulsar survey
• Full Stokes data needed for continuum survey
• Bandwidth needed for continuum, pulsar and E-ALFA surveys
• GALSPECT and Mock Spectrometer back-ends built to match instrument
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ALFA for ALFALFA
• Fixed rotation angle (19°)
• 100 MHz bandwidth
• Use pre-existing WAPP correlators
• Could still have carried out an all (AO) sky survey for HI (13,000 sq. deg.)
• Would have found ~40,000 galaxies
• Would not have done pulsars or continuum
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ALFA vs LBW
• ALFA does have its limitations:– 300 MHz vs 600 MHz– Higher system temperature
• ALFA cannot do OH– LBW is used for monitoring OH/IR stars,
observations of OH in comets, etc.
• ALFA cannot do HI at z > 0.16– LBW has seen HI at z ~ 0.25
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Examples of array-receiver science
• HI surveys– HIPASS, HIZOA, HIDEEP
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Examples of array-receiver science
• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA
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ALFALFA pie-slices
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Examples of array-receiver science
• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA– GALFA-HI
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Examples of array-receiver science
• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA– GALFA-HI
• Pulsar surveys– Parkes Multibeam– P-ALFA
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J1903+0327
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Examples of array-receiver science
• Continuum surveys– NRAO & PMN 4.85 GHz surveys– GALFACTS
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Examples of array-receiver science
• Continuum surveys– NRAO & PMN 4.85 GHz surveys– GALFACTS– Orion M42 region
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Dicker et al. (astro-ph/0907.1300)
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Summary
• Large area surveys are the science driver behind survey receivers
• The science drives the receiver design, the back-end design, and the software
• This is very different from traditional ‘build it and they will come’ single pixels
• The science returns can justify the cost
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Bibliography
• Multi-Feed Systems for Radio Telescopes, ASP Conf. Proc 75, eds. D. T. Emerson & J. M. Payne
• Single-Dish Radio Astronomy: Techniques and Applications, ASP Conf. Proc. 278, eds. S. Stanimirović, D. R. Altschuler, P. F. Goldsmith & C. J. Salter
• Parkes MB (HI): http://www.atnf.csiro.au/research/multibeam/• Parkes MB (pulsars): http://www.atnf.csiro.au/people/pulsar/pmsurv/• ALFA: http://www.naic.edu/alfa/• K-FPA: https://safe.nrao.edu/wiki/bin/view/Kbandfpa/WebHome• MUSTANG: http://www.gb.nrao.edu/mustang/• Extragalactic HI Surveys at Arecibo: the Future, R. Giovanelli,
http://www.arxiv.org/abs/0806.1714• Minihalos in and Beyond the Local Group, Astro 2010 white paper, R. Giovanelli• Comets to Clusters: Wide-field Multi-pixel Camera Development for the GBT,
Astro 2010 white paper, K O’Neil, J. Lockman, J. Ford, M. Morgan, J. Fisher & B. Mason