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

This document provides insights into single-dish spectral-line observing, highlighting its advantages over interferometry, such as total flux density measurement and simplicity. It discusses the importance of bandpass calibration techniques in both compact and extended objects and presents methods like position switching, frequency switching, and load switching. The paper also explores the use of multiple beams for more efficient sky imaging, emphasizing the necessity of accurate calibration for reliable data. This analysis is crucial for astronomers utilizing single-dish telescopes in their research.

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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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