Compact Array and Mopra report
Since the last newsletter, we have had two 10-day shutdowns at the Compact Array. While there were many jobs done during these shutdowns, the most notable is the near-completion of the 12-mm upgrade.
During the first two weeks of April, 12-mm receivers were installed in antennas 1, 5 and 6, and the 12/3-mm receiver package in antenna 4 underwent a major overhaul. Unallocated time in the following week was used for further system integration and commissioning work. At 4.01 p.m. on 16 April, for the first time we measured fringes to all six antennas simultaneously. John Whiteoak snuck in to make the first 12-mm astronomy observation with the full system, observing Sagittarius B2. Dick Manchester followed shortly afterwards observing SN1987A. On 23 April, the receivers were re-tuned to allow observations at the ammonia lines. Figure 1 shows one of the first ammonia spectra with the full 12-mm system.
Figure 1: A spectrum of the NH3 1-1 transition in Orion.
A shutdown during 26 May 5 June essentially completes the 12-mm upgrade. After this shutdown it is intended that full automatic tuning across the 12-mm band will be possible, and the module swaps needed at 12- and 3-mm will no longer be required. A restriction on the final system will be that, when observing at two frequencies, the band centres cannot differ by more than about 1 to 2 GHz. An integral part of the wideband tuning is a major upgrade of the local oscillator system. The local oscillator upgrade is now complete. With the exception of the primary monitor system (which is still on copper), all communications and data paths between control building and antennas is now on single-mode fibre.
Although there are still some rough edges to the 12-mm system, observing at 12 mm is proving to be only little more difficult than at 3 cm, and so it promises to be a very popular use for the Compact Array.
The shutdowns in April and May/June were used for a broad range of activities in addition to the 12-mm upgrade. These include:
New antenna control computers (see separate article).
Antenna holography and panel adjustment: At high frequencies, surface irregularities in the Compact Array dishes significantly affect the overall efficiency. Using the new 30-GHz holography system, antenna 2 was surveyed in December 2002, and the panels adjusted in late April. During the May/June shutdown, all antennas except 2 and 6 were surveyed and adjusted. The rms surface accuracy is 150 to 180 mm. Figure 2 shows an image of the alignment error on antenna 3.
Figure 2: A roughness image of the surface of CA03. The contour levels show deviations from the ideal dish surface of ± 200, 400, 600 and 800 micron.
Each Compact Array antenna has four gearboxes two each in azimuth and elevation. Over the last 10 months (and mainly during the two shutdowns) the elevation gearboxes on each antenna were removed, stripped down and cleaned, and some components replaced.
Structural repairs: A small crack on two gussets on five of the six antennas became apparent during structural inspections in March. Following advice from structural engineers, the gussets have been welded and strengthened on all antennas.
Staff, visitors and special events
After six years of sterling work, Margaret Guest, our weekend cook, has retired. We have recently welcomed three new staff members to Narrabri: Maxim Voronkov, Stuart Robertson and Donna Brennan. Maxim joins us as a postdoc, and will work on SKA simulation and imaging studies (page 3). Stuart's background is in space physics, and he is in the final stages of completing his PhD. His work with us will revolve around the operation and upgrade of Mopra. Donna will be familiar to a number of our visitors as she has acted as a casual cook at Narrabri for nearly two years. Following Margaret Guest's retirement, Donna is now taking on the role of weekend cook.
The highly successful synthesis imaging workshop was held at Narrabri during the week of
12 - 16 May (page 6).
Figure 3: Ray Norris with Science Minister Peter McGauran at the Narrabri Visitors Centre.
On 20 February the federal Minister for Science, Peter McGauran, and his group of staff, visited the observatory (Figure 3). The minister toured an antenna, the Visitors Centre and control building, and talked with the team observing at the array at the time. In addition the minister also met with some students from Narrabri High School working with the SEARFE project (a radio-spectrum measurement project). At the end of May, we hosted a visit by chiefs from three CSIRO divisions (Telecommunications and Industrial Physics, Mathematics and Information Science, and Molecular Science), along with some of their staff. After an antenna and control building tour, the chiefs enjoyed a campfire dinner during a mild Narrabri evening.
Figure 4: Compact Array use from January to April 2003.
The pie-chart shown in Figure 4 gives statistics for the use of the Compact Array during the 2003 January to April term. The amount of time allocated for observations was slightly lower than usual to allow time for the installation and commissioning of the new 12-mm systems. The downtime during observations was also a little higher than usual, largely due to a nearby lightning strike on 22 February, a disk crash on the correlator control computer, and unusually windy weather. The old antenna control computers remain our single-biggest "non-weather" source of lost time.
On 2 May, one of the old Culgoora heliograph dishes was reborn, when St Columba High School in the Blue Mountains officially "opened" a refurbished dish (Figure 5). The restoration of the antenna was funded through the school's Parents and Friends Association, and involved the hard work of the students, teachers and parents to bring it to a conclusion. The project required the persistence of these people since the project started in August 1997. The antenna has been used to detect the sun and thunderstorms as well as a range of man-made emission.
Figure 5: One of the old Culgoora heliograph dishes. This is one of an array of 96, 13.7-m dishes that circled the present location of the Compact Array. The heliograph array imaged the Sun at 80 and 160 MHz with several discoveries to its credit.
Following a failure in the HALCA spacecraft, use of Mopra in conjunction with this space-VLBI mission was suspended in February. Control of the spacecraft was recovered in early June. From February until May 2003, Mopra was used mainly for three VLBI sessions.
Recognizing the UNSW astronomy group's financial commitment to Mopra, both in the panel upgrade some years ago, and in the coming wideband correlator upgrade, UNSW has been allocated six weeks of Mopra time during the winter (May to August) term. In addition to this, a similar amount of "National Facility" 3-mm time and one week of VLBI has been scheduled.
In readiness for the winter season, the 3-mm SIS receiver used at Mopra underwent maintenance at Marsfield during April and was reinstalled at Mopra on 19 May. In addition during the lead-up to the season, the subreflector was adjusted to reduce the telescope coma lobe (see Figure 6), and some software/communication problems in the 3-mm tuning system were resolved. To introduce new observers to the telescope, and to aid the interaction between the engineers and observers, a "Mopra indoctrination" workshop was held at the Mopra site on 1 2 June. Twelve people attended this workshop, seven being University of NSW students and astronomers. The workshop was very successful, despite uncharacteristically heavy rain!
Figure 6: Beam shape of Mopra dish before (upper panel) and after (lower panel) subreflector adjustment. The decrease in the coma lobe on the right shoulder of the beam is very evident (the horizontal scales of the two plots are slightly different).
During the coming months, we plan a round of panel adjustments to improve the surface accuracy of the Mopra telescope. The rms surface accuracy is currently about 260 mm while experience at the Compact Array suggests we should be able to improve this to 160 mm. We also intend to install a new antenna control computer and a prototype of the wideband correlator.