Major Changes

The interferometer has been stopped on mid-May for major changes:

• New Central Computer System
  All the CAMAC based instrumentation and the VAX computers have been decommissioned. Two Unix-based HP-J200 workstations (one for real-time control and acquisition, one for data reduction which could be used as a backup for the real-time computer) have replaced the VAXes as central computers.
• New VME-based Pointing Control System
  Antenna 5 is equipped with a VME-based interface for azimuth and elevation pointing and tracking, as well as for control of the transporter functions. Antennas 1-4, which were originally equipped with CAMAC, have been retrofitted with a similar equipment for pointing and tracking, but the transporter functions have to be operated under manual supervision.

  VME microcomputers in each antenna interact with the HP workstation through an Ethernet link. They receive time information and astronomical coordinates from the central computer, process them to Az/El and supervise the tracking of the source.

• New IF/LO distribution system
  To handle one additional antenna, the IF/LO distribution system has been completely dismantled and rebuilt. The new system is ready for 6 antennas.
• New 5-antenna correlator
  The 4-antenna correlator has been totally upgraded to handle 5 antennas, with no loss of capabilities. The modification was a major one, since it represents a increase in processing power. Yet, essentially all the original components of the 4-antenna correlator have been re-used.

  The new correlator is equipped with faster microcomputers, and allows atmospheric phase correction at all operating modes.

• Totally New Acquisition and Control software
  All the hardware changes have implied to completely re-write the control and acquisition software. Debugging and testing of the software has been actively going on since early June.

  The software is now available for normal operation, but some improvements expected from the faster computing system still remain to be implemented.

• Commissioning of Antenna 5
  Antenna 5 is now completed, and has been brought outside for testing. First fringes with 5 antennas were obtained on June 15th. After some pointing tests, an initial adjustment of the surface was attempted. However, since weather conditions were not at their best and also because of the necessity to progress on the software tests, the antenna has so far only be adjusted for operation at 3mm.

  The pointing accuracy is currently of order 4-5'' rms, somewhat poorer than for the other antennas, and work is going on to understand the causes of the discrepancy. Antenna efficiency at 3mm is about 24 Jy/K (as compared to 22 Jy/K) for the others). Further surface adjustment will be attempted, but may have to be deferred to next September.

• New Power Generators
  New Diesel generators have been installed to match the increase in power required by antenna 5. Re-cabling of the power distribution of the backend room was also required for the new correlator and computers.

After this very intense activity on the site, the interferometer is now almost ready for normal observations. However, some additional test time will be devoted to the pointing and surface adjustment of antenna 5.