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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
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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 • 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
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
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
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
Sky drift vector Drive vector Resultant
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
Not all pixels are created equal… • For even sensitivity, want to make a Nyquist-sampled map with each beam
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):
