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Large Area Surveys with Array Receivers

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

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  1. Large Area Surveys with Array Receivers Robert Minchin Single Dish Summer School

  2. A bit of history… • Array receivers are not new • NRAO 7-beam receiver was installed on the 91-m telescope at GB in 1986

  3. 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

  4. Why array receivers?

  5. 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

  6. Why array receivers?

  7. 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

  8. Why array receivers?

  9. 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

  10. 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

  11. Types of array receiver • Bolometer cameras • Continuum only • No gaps between pixels • Examples • MUSTANG (GBT) • BOLOCAM (CSO) • LABOCA (APEX)

  12. 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

  13. 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)

  14. 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

  15. Survey strategy • Best strategy depends on the science: • For pulsar discovery, the P-ALFA strategy is to track a point on the sky

  16. 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

  17. Sky drift vector Drive vector Resultant

  18. Without basketweaving

  19. With basketweaving

  20. 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

  21. AGES observing stategy

  22. AGES observing stategy

  23. AGES observing stategy

  24. AGES observing stategy

  25. AGES observing stategy

  26. AGES observing stategy

  27. Not all pixels are created equal… • For even sensitivity, want to make a Nyquist-sampled map with each beam

  28. AGES observing stategy

  29. AGES observing stategy

  30. AGES observing stategy

  31. AGES observing stategy

  32. AGES observing stategy

  33. AGES observing stategy

  34. 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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