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Single-dish spectral-line observing Lister Staveley-Smith, ATNF

Single-dish spectral-line observing Lister Staveley-Smith, ATNF. Why use a single-dish? Analogy with interferometer Demonstration - bandpass calibration Multiple beams Imaging & calibration. Why use a single-dish?.

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Single-dish spectral-line observing Lister Staveley-Smith, ATNF

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  1. Single-dish spectral-line observingLister Staveley-Smith, ATNF • Why use a single-dish? • Analogy with interferometer • Demonstration - bandpass calibration • Multiple beams • Imaging & calibration ATNF Synthesis Workshop - September 2001

  2. Why use a single-dish? • Provide zero-spacing  total flux density (IDd=0 for interferometer) • Provides structure with  >  /Bmin, where Bmin=30 m for ATCA  complementary to interferometer • Sensitivity (often larger diameter, more beams, better receivers) • Simplicity (beams formed in hardware)

  3. Single-dish/interferometer analogy -I Interferometer: Asymmetric cross-correlations Cij()  Cij(-)  Complex visibilities V() Single dish: Symmetric auto-correlations  Real visibilities

  4. Single-dish/interferometer analogy -II ground source • Interferometer: Cij = A1(t)A2(t) = A1,src(t)A2,src(t) Let Voltage A(t)=Asrc (t)+Aatmos (t)+Agrnd (t)+Arx (t) atmosphere receiver • Single dish (A1=A2): C= A(t)A(t) = A2src(t)+A2atmos(t)+A2grnd(t)+A2rx(t)

  5. Parkes 21-cm X and Y spectra as telescope scans at 1o min-1

  6. Continuum and gain variations • Autocorrelations sensitive to: • Atmospheric emission • Background continuum sources

  7. Parkes 21-cm X and Y spectra as telescope scans at 1o min-1

  8. Spectra normalised by Tsys (to remove sky continuum)

  9. Bandpass calibration techniques for compact obects ON SOURCE • Position switching • Calibrate using a nearby off-source position OFF SOURCE • Beam switching • Calibrate using an off-source beam • (needs symmetric beams)

  10. Normalised with a reference spectrum (position-switching)

  11. Bandpass calibration techniques for extended objects • Frequency switching • Change 1st local oscillator so spectral line moves • Load switching • Switch to a cold load (e.g. Penzias & Wilson) • Noise adding

  12. Frequency-switching whilst scanning the LMC

  13. Bandpass calibration using frequency-switching

  14. Frequency-switch reconstruction Frequency Frequency Shift & subtract Deconvolve

  15. Liszt (1997)

  16. Reconstructing a multi-frequency shift Bell (1997)

  17. Multiple beams • Many beams in focal plane  faster imaging of sky • A typical multibeam array is NOT an interferometer (beams don’t overlap) • An n-beam array is equivalent to n single dishes pointing in n different locations

  18. Imaging • Sky will not be regularly sampled, so Fourier interpolation not useful • Interpolation in general not useful if PSF allowed to vary • Generally prefer to convolve data onto a regular grid

  19. Calibration - I • If you want to accurately calibrate your images, image the calibrator so that it goes through same processes LMC in HI (Parkes multibeam)

  20. Calibration - II • To accurately calibrate an extended source, use an extended calibrator (e.g. S8, S9 for 21-cm images) as beams, in general,are not Gaussian Luks & Rohlfs (1992) Parkes multibeam

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