4 Telescope Modes

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The WSRT can be used in a number of different observing modes. The most characteristic is local synthesis observing (continuum or spectral line). In this mode of operation the WSRT is used as an aperture synthesis array with a total length of 2.7 km and a maximum of 91 interferometers.

It is also possible to use it as an array of 14 standalone dishes for high-resolution spectroscopy. In addition, there are facilities for coherently adding the signals from a subarray or all telescopes, Tied Array mode, for sensitive pulsar observations (detection and timing) using the Pulsar Machine (Puma, see §7) and for participation in VLBI.

  • Local synthesis or snap-shot for continuum and spectral line observations: both type of observations uses the new IVC+DZB backend that can provide a total bandwith from 0.15 MHz up to 160 MHz (8x20 MHz). See below for more details.
  • Single Dish. The DZB offers the possibility to provide 14 autocorrelations in addition to the crosscorrelations used for local synthesis. This implies that one can use the WSRT as the equivalent of 14 separate single dishes, or indeed that 1 telescope could be used for this purpose while the others pursue another programme. Special «on-off»calibration is required for subtracting the receiver contributions from the observed auto-correlation spectra, similar to procedures followed when using a single dish.
  • Tied Array Adding Box. This is a special interface which coherently adds the signals from all telescopes or any (up to 8) subsets into one coherent output. Regular VLBI observations will generally add all telescopes, but separate subarrays at different frequencies can be used simultaneusly for pulsar observations or are used for specialised phase referencing VLBI. A total of 8 "arrays" can be formed, in each of which a 10 MHz bandwidth is available; the maximum total bandwidth is 80 MHz, by selecting the same subarray in each of the bands. The outputs can be thought of as the signals one would measure in the center of the field of a local synthesis observation, but more accurately, it should be realised that at any instant the added signal reflects the response of the fan beam of the East-West array; extended target sources, or those in a complex field, or with strong nearby sources, are therefore not suitable for tied array observations. Regular calibration scans are needed to maintain phase coherence; experience suggests that this is only needed every 12 hours or even less often at 21 cm, but cautious observers might wish to calibrate rather more often under poor tropospheric (short wavelength) or ionospheric (long wavelength) conditions or if high spectral dynamic range (>1000:1 ) is needed. 

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