ATNF Synthesis Workshop - September 20011 Single-dish spectral-line observing Lister Staveley-Smith, ATNF  Why use a single-dish?  Analogy with interferometer.

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Presentation transcript:

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

2 Why use a single-dish?  Provide zero-spacing  total flux density (  I D d  =0 for interferometer)  Provides structure with  > /B min, where B min =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 C ij (  )  C ij (-  )  Complex visibilities V( ) Single dish: Symmetric autocorrelations  Real visibilities

4 Single-dish/interferometer analogy -II  Interferometer: C ij =  A 1 (t)A 2 (t)  =  A 1,src (t)A 2,src (t)  Let Voltage A(t)=A src (t)+A atmos (t) +A grnd (t) +A rx (t)  Single dish (A 1 =A 2 ): C =  A(t)A(t)  =  A 2 src (t)  +  A 2 atmos (t)  +  A 2 grnd (t)  +  A 2 rx (t)  source atmosphere ground receiver

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

6 Continuum and gain variations  Autocorrelations sensitive to: –Atmospheric emission –Background continuum sources

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

Spectra normalised by T sys (to remove sky continuum)

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

Normalised with a reference spectrum (position-switching)

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

Frequency-switching whilst scanning the LMC

Bandpass calibration using frequency-switching

14 Frequency-switch reconstruction Shift & subtractDeconvolve Frequency

15 Liszt (1997)

16 Reconstructing a multi-frequency shift Bell (1997)

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

19 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

20 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)

21 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 Parkes multibeam Luks & Rohlfs (1992)