MNRF Upgrade: VLBI (Very Long Baseline Interferometry)
Contents of this page;
ATNF VLBI Upgrade
Uni. Tasmania VLBI Upgrade
A general introduction to interferometry and VLBI is given in the Introduction to Radio Astronomy and Interferometry page.
A portion of the MNRF Program funding provides for extension and improvement of the Australian VLBI network and, in particular, facilities managed by the ATNF and the University of Tasmania. Although the two institutions are managing separate (but co-ordinated) upgrades, for reasons of economy and standardization the ATNF has placed some orders of equipment on behalf of the University.
Much of the MNRF VLBI upgrade has already been completed, with 1997-1998 seeing:
Installation of a 22 GHz receiver at Ceduna - the last of the VLBI upgrade projects - is scheduled for March 2000. While this target is expected to be met, studies of the Ceduna antenna optics have revealed complications in providing a receiver at 1.6GHz - the other VLBI frequency extreme. Further investigations are underway, and a final decision on the feasibility of implementing bands below 2 GHz will be made in 1999.
In conjunction with the University of Tasmania and the astronomical community, the ATNF now faces the challenge of operating the VLBI network as a National Facility in a manner which capitalises on the huge scientific promise.
a) 12 mm (22 GHz) Receiver
The ATNF has built and tested a 22 GHz VLBI receiver for the Mopra telescope. The receiver performance is exceptional, with a minimum system temperature of less than 80 K being measured on the 22 m telescope. Astronomical tests indicate a 22 GHz aperture efficiency of around 65%, auguring well for the performance of the very similar ATCA antennas in the 12 mm band. The Mopra receiver forms the basis for two similar receivers to be assembled by the University of Tasmania. It is also regarded by ATNF engineers as a test system for the 22 GHz section of the multi-band ATCA millimetre-wave receiver. With successful installation of the ATNF receiver at Mopra, this section of the MNRF upgrade is complete.
b) Hydrogen-Maser Frequency Standards
For accuracy, each VLBI station uses a Hydrogen-Maser atomic clock to synchronize its clocks. By utilizing a natural resonance of hydrogen atoms as a standard, the stations are able to maintain a timekeeping accuracy within one second in a million years. This accuracy is essential to enable the computers to combine the signals from all the stations during the data analysis stage.
The ATNF purchased three masers from the Russian firm Vremya-CH, together with a fourth unit which is designated a non-operational spare under an agreement with the Russian suppliers. Two of the masers were purchased on behalf of the University of Tasmania and have been installed at Ceduna and Mopra; the third (ATNF) unit has been installed at the ATCA. The Australian experience of the Vremya-CH masers has been of some interest internationally, since the relatively economical Russian units are attractive to research institutions needing alternatives to more expensive Western masers.
In general, the Russian frequency standards are all functioning well, meeting or exceeding vital stability specifications. Two-maser intercomparisons show Allan variances of 2 parts in 1013 and 1 part in 1014 for averaging times of 1 s and 100 s respectively. This order of stability has allowed successful VLBI imaging at frequencies as high as 22 GHz. Minor reliability problems have been experienced with the Ceduna maser, possibly as a result of the poor quality power supply to the site. VLBI data involving Mopra also identified a residual low-frequency modulation problem, prompting the Russian designers to supply upgraded signal distribution hardware. The initial problems are considered minor relative to the complexity of the technology involved and present indications are that all the masers will continue to perform well. This section of the MNRF upgrade is now complete.
c) S-2 Playback Unit
The signals from the VLBI receivers are recorded on ultrafast recorders together with the timing information. Tapes containing the recorded data are then sent from the telescopes to where the data is analysed.
Negotiations are in progress with the Canadian suppliers of an S-2 playback unit and it is expected to be delivered by December 1998, giving the ATNF VLBI correlator the signal processing capacity to match the load generated by the upgraded VLBI network.
d) Timing Units
These units provide accurate timing information (obtained from Global Positioning System satellites) at Australian VLBI observatories. All seven systems (five ATNF, two University of Tasmania) have been installed and associated software developed. The timing units are now in use and the project is complete.
In October 1995 the University of Tasmania acquired the former Telstra 30 m antenna at Ceduna, in South Australia. After an extensive refurbishment effort (refer to 1997 MNRF Anuual Report(pdf file) for details) which included considerable assistance from the ATNF, the Ceduna Radio Astronomy Observatory was opened formally on October 7 1997 by the Hon. Barry Jones, AO.
A number of major improvements were made at Ceduna in 1997-1998. The final S2 VLBI data acquisition system (DAS) has been delivered, replacing the prototype used in initial VLBI experiments. In addition, a new 8K-channel autocorrelation spectrometer, allowing stand-alone spectral-line observations, is operational. Both of these items were supplied by the ATNF and, in fact, the outfitting of Ceduna represents the completion of the ATNF's commitment to establishing an S2-based VLBI network on the Australian continent.
In December 1997 Ceduna Observatory was used in a 1-week national and international VLBI session, including an experiment involving VSOP, the orbiting Japanese VLBI spacecraft. The Observatory is now fully operational in the VLBI national facility mode at 2.3, 4.8 and 6.7 GHz.
a) Ceduna Receivers
The receivers now operational at Ceduna have been developed and constructed by the University of Tasmania. These systems use un-cooled, low-noise amplifiers based on high electron mobility transistors (HEMTs). A program to improve the performance of all receivers at Ceduna and Hobart is progressing well, with a number of new low-noise amplifiers currently being evaluated at the 26 m Mt Pleasant (Hobart) antenna.
b) 12 mm Receivers
Two receivers will be constructed by the University of Tasmania, for use at Ceduna and Mt Pleasant Observatories. While the receivers will use many of the sub-systems developed for the ATNF receivers destined for Mopra and the ATCA, it is expected that the different optical arrangements at the two University telescopes will make customised packaging and installation necessary.
c) Hydrogen Masers
The two Vremya-CH maser units purchased by the University of Tasmania have been installed at Ceduna and Mopra for use as master frequency and time references. Both units are functioning reliably and within specification.
d) Timing Units
Both the Ceduna and Hobart units are installed and working well.
e) Ceduna Antenna Optics
The panels of the 30 m antenna have been surveyed and re-set, giving a surface error of less than 0.4 mm rms, a figure allowing operation at frequencies up to at least 22 GHz with an expected aperture efficiency of better than 50%. At the lower frequency extreme, computer modelling has verified that it is not feasible to construct a 1.6 GHz feed to work at the tertiary (Nasmyth) focus. However, a preliminary design has been completed for a prime focus feed. Part of this design involves a revised, and movable, sub-reflector assembly, allowing both unobstructed prime focus operation and axial focus adjustments. In the intermediate VLBI bands, wideband feeds for 2.3, 4.8 and 6.7 GHz have been fabricated and installed by the University of Tasmania according to ATNF specifications. An 8 GHz feed is currently under construction at the University and is due for installation at Ceduna before the end of 1998.