VLBI status March 2004

Tied Array

The default observing mode of Westerbork is as a tied array (also known as a phased array). For this mode we add signals from the 14 telescopes up after first doing phase and delay compensation. This makes the effective area of the telescope N times larger for the N telescopes added - provided the phase and delay are correctly compensated

This of course decreases the field of view of the telescope considerably - from that of a single dish to that of the instantaneous fanbeam of the telescope. PIs who need a large field of view can either time switch around the fields of interest or (for < 8 fields) split the array into subarrays or use the single dish observing mode. It is of course possible to make an array of only 1 telescope.
It is also possible to have different subarrays observe at different wavelengths. All subarray observing modes should be considered non-standard and special contact should be made with the Friend of VLBI

Phasing up the array before observations is vital. This is handled by the observatory staff. PIs are however advised to include occasional fringe finder sources in their schedules to allow us to check on the phasing during the observing run. While observing in the tied-array mode the Westerbork correlator is also running (as it is used for checking the phasing). It is possible to request this data, although this should be arranged with the Friend of VLBI before observing

At present this mode supports 8 bands at 16MHz each in 2 polarizations. That corresponds to a peak rate of 

2(pol) × 8(bands) × 16(MHz) × 2(samples/Hz) × 2 (bits/sample) = 1024Mbit/s

Single Dish

For geodetic observations or observations of large fields it may be useful to observe in single dish mode. This uses different hardware to the tied-array and so takes slightly longer to set up. The maximum bandwidth of the single-dish system is 100MHz per polarization.

Westerbork only has one 5cm (methanol) receiver so we will use it by default in the single-dish mode.

Receivers

The following receivers are available:
Wavelength Frequency range/MHz FEs Tsys/K
 Gain / K/Jy
 Notes
3.6cm
 8150-8650
 >10
 120
 N×0.050 Usually phase up only the inner array.
5cm
 6000-6900
 1
 95
 0.060 Default is single dish
6cm
 4760-5020
 14
 65
 N×0.085
13cm
 2150-2375
 14
 60
 N×0.090 Circularly polarized feeds
Lband (18-21cm)
 1160-1800
 14
 30
 N×0.095 RFI at Glonass and Iridium frequencies
UHFhigh (30cm)
 750-1200
 14
 120
 N×0.100 RFI at frequencies<800MHz & 930-970MHz
49cm
 605-615
 14
 75
 N×0.100 very strong RFI just outside the band
92cm
 310-390
 14
 150
 N×0.105 Occasional very strong RFI
UHFlow
 250-460
 >10
 >200
 N×0.105 Only small section free of strong RFI

Gain values are given as multiples of the number (N) of frontends (FEs) - assuming perfect phasing-up of the array
At 3.6cm the atmospheric phase stability is reasonably good for baselines < 1.5km but rapidly degrades beyond that limit. Three of the 3.6cm receivers also have circular polarization and so cannot be combined with the other linearly polarized receivers.
The presence of RFI (radio frequency interference) makes phasing the array very difficult. PIs should avoid large bandwidths where this is a problem and use by preference larger numbers of smaller bandwidths

Recording

Westerbork has both a Mark4 (tape) and Mark5A (disk-based) recording system. The peak recording bit rate is 512Mbit/s for the Mark4 and 1024Mbit/s for the Mark5A. In some test experiments we have used the Mark5A and the fibre link to JIVE for eVLBI.
In eVLBI the various datastreams are correlated in (very near) realtime at the correlator, so allowing for rapid fringe-tests and potentially rapid production of VLBI maps.
The acquisition rack has the following bandwiths available:
Bandwidth
 Number
 Notes
16MHz
 14
 both sidebands
8MHz
 14
 both sidebands
4MHz
 14
 both sidebands
2MHz
 14
 both sidebands
1MHz
 3
 external
0.5MHz
 8
 upper sideband + 2 external
0.25MHz
 6
 external
0.125MHz
 2
 upper sideband
0.0625MHz
 1
 external

"External" filters may be attached to the external terminals at the back of the BBCs, but these cannot easily be changed
Tony Foley <foley@astron.nl>
Last modified: Fri Mar 26 11:06:37 2004