VLBI in 1999

During the final session of 1998 the first observations were done with the MFFEs in the Westerbork tied-array. In 1999 all four network sessions had the tied-array available for observing. Westerbork also took part in a few VLBI observations outside the network sessions. Thick and thin tape recording continued to be made with the MkIV tape drive. During 1999 most of the array control and monitoring software has been changed. This is part of the project to run observations with an object-orientated Telescope Management System (TMS) and replace outdated HP-1000 based hardware and software. This project continues into 2000.

System upgradess

Frequency Flexibility

During 1999 the software for controlling the MFFEs was upgraded to allow parallel setting of wavebands and frequencies. This allows new wavebands to be chosen in about 1 minute, and re-locking to another frequency in the same waveband in about 15 seconds. In it possible to do true dual frequency observations with the tied-arrayin the following bands:

3.6 cm and 13 cm ('S/X')
3.6 cm and 92 cm
3.6 cm and UHFlow
6 cm and 49 cm
6 cm and UHFlow
18/21 cm and UHFlow

However these setups have not been tried in real VLBI sessions, and only one linear polarization would be given by the tied-array at present.

New maser

During 1999 we installed a new Datum (formerly Sigma Tau) hydrogen maser. This was to replace the old maser (a reconditioned prototype EFOS maser) which had a known sensitivity to atmospheric pressure. The clock rate has reduced dramatically and clock offsets become much more predictable.

New Field System

The field system computer and its software were replaced in 1999 to be Y2K compliant.

More filters

A 'standard' set of MkIV filters were placed in the first 8 baseband converters (BBCs). These give 0.5MHz and 0.125MHz bandwidths (USB only) for some spectral line observations. All 15 BBCs have 16,8,4 and 2MHz in both sidebands. Some other filters are available for attaching via the 'external' connectors.

Hardware repairs

Diodes for bandwidth switching: In the baseband converters the bandwidths are switched with diodes. However the diodes initially used were unsuitable for wider bandwidth switching (8MHz and 16MHz). These were replaced during 1999. For tied-array observations the local bandwidth limitations dominated before these we apparent.
Tape unit: Several problems occurred with the VLBI tape unit during 1999. Most of these could be attributes to mechanical wear, and in particular the write-only headstack was badly worn during session 4. It was replaced at the beginning of 2000.

Future prospects

New tied-array

A new tied-array is planned to go with the new IF system for Westerbork. This will be implemented in stages, to allow the old tied-array to be retired as soon as possible. When implemented:

Total Bandwidth need no longer to be limited to 80MHz, but will allow up to the full 160MHz from MFFEs above 1GHz.
16MHz per sideband will be available as the new IF will have 20MHz filters.
Circular polarization will be made at earlier stage, and hopefully have less crosstalk.

Circular polarizers for the 3.6cm band

Before mid 2000 there will be circular polarizers for the 3.6cm band, so that these receivers can be used in geodetic experiments.

Recording with 2 headstacks

With 2 headstacks (write-only and read-write) recording simultaneously we can increase the recording bandwidth from the current VLBA standard of 256Mbit/s to 512MBit/s without increasing tape speed. This mode has been locally demonstrated, but scheduling software will not be available until 2000.

Tony Foley <foley@nfra.nl>

Last modified: Mon Jan 17 10:58:15 2000