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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Large Area Surveys with Array Receivers. Robert Minchin Single Dish Summer School. A bit of history…. Array receivers are not new NRAO 7-beam receiver was installed on the 91-m telescope at GB in 1986. A bit of history…. Array receivers are not new - PowerPoint PPT Presentation

Transcript of Large Area Surveys with Array Receivers

Page 1: Large Area Surveys with Array Receivers

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