5 Technical Information
wsrtGuide
3 Telescope Parameters
and Contents
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.
5 Technical Information
wsrtGuide
3 Telescope Parameters
and Contents