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• 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

Why array receivers?

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

Why array receivers?

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

Why array receivers?

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

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

Types of array receiver

• Bolometer cameras– Continuum only– No gaps between pixels

• Examples– MUSTANG (GBT)– BOLOCAM (CSO)– LABOCA (APEX)

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

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)

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

Survey strategy

• Best strategy depends on the science:– For pulsar discovery, the P-ALFA strategy

is to track a point on the sky

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

Drive vector

Sky drift vector

Resultant

Without basketweaving

With basketweaving

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

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

Not all pixels are created equal…

• For even sensitivity, want to make a Nyquist-sampled map with each beam

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

AGES observing stategy

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):

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

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

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’

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

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

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

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

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

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

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?

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!

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

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

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

Examples of array-receiver science

• HI surveys– HIPASS, HIZOA, HIDEEP

Examples of array-receiver science

• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA

ALFALFA pie-slices

Examples of array-receiver science

• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA– GALFA-HI

Examples of array-receiver science

• HI surveys– HIPASS, HIZOA, HIDEEP– ALFALFA, AGES, AUDS, A-ZOA– GALFA-HI

• Pulsar surveys– Parkes Multibeam– P-ALFA

J1903+0327

Examples of array-receiver science

• Continuum surveys– NRAO & PMN 4.85 GHz surveys– GALFACTS

Examples of array-receiver science

• Continuum surveys– NRAO & PMN 4.85 GHz surveys– GALFACTS– Orion M42 region

Dicker et al. (astro-ph/0907.1300)

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

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