Compact Array and Mopra report

Narrabri operations and development

During the Australian summer (from November to February), the 3- and 12-mm systems have not been scheduled on the Compact Array. Although there will be no major changes to the 3-mm systems in the 2005 winter season which begins in May, the summer months have provided an opportunity for ATNF staff to improve some issues that were apparent with the millimetre systems during 2004.

One issue that has been rectified is the optics of antenna 1. When the 3-mm system was installed on antenna 1 in October 2004, it became apparent that the gain and beam pattern of antenna 1 was substantially poorer than for the other antennas. Following photogrammetry on antennas 1, 2 and 6, the reasons for this poor performance were clarified. All of the subreflectors on the Compact Array antennas are somewhat under-engineered. They are not naturally rigid and conform to the design shape only to about one millimetre; this is a large error for 3-mm observations. Following holographic measurements, previous adjustments have shaped the main dish surface to counter the subreflector errors. (Holography measures the combined effect of the main dish and subreflector.) However, more recent work on antenna 1 modified the subreflector, so that the subreflector and main surface were no longer a matched pair. Further holographic measurements and main-surface adjustments for this antenna were done in early February 2005. This reduced the rms surface error of the subreflector/main-surface combination from 290 μm to 150 μm. This implies an improvement in the Ruze efficiency factor at 100 GHz from 23% to 67%. Before the start of the millimetre season, alignment of the subreflectors on antennas 1 and 5 will also be performed.

Other development work in progress includes modifying the water vapour radiometers to allow them to be used as opacity metres and developing 16-MHz filters for the second frequency chain.

On the software front, the LINUX observing system is nearing completion. Many of the observing programs (including CAOBS, VIS, CACAL, CAGET and CATAG) now operate under LINUX. Watch this space for the demise of VMS at Narrabri.

A source of radio interference near 1403 MHz, which affected some HI observers, has been located outside CSIRO property. Interim shielding for this has substantially improved the situation. A replacement of the offending gear is pending.

A special trailer that will be used to monitor the radio frequency interference (RFI) environment at Mileura, recently spent a few days at the Observatory undertaking some shakedown tests, and independently assessing the local RFI environment.

Narrabri storms

The Narrabri district has had its share of extreme weather over the summer. During the night of 9 – 10 December, 160 mm of rain fell in the immediate catchment area, resulting in significant flooding. A section of road near the Narrabri township was partially washed away, and a 100-m section of road on the main access route to the Observatory completely disappeared. Neighbours of the Observatory were rescued by helicopter. The worst flood waters disappeared within a few days. By 12 December it was possible to access the town and antenna 6 using a four-wheel drive. The staff and observers present at the observatory at the time took the isolation in their stride, and the telescope operated normally throughout the episode. From the time of the floods until late January, the road damage meant a different route needed to be taken between the Observatory and the town. This route was again closed on 28 – 29 December after a more minor flood. In late January, the main road to the Observatory was re-opened. However this still has flood-damaged sections. We ask all visitors to drive carefully!

On 20 January the district experienced a severe storm, with 155 km/hour winds. This caused appreciable damage in the town, with many houses unroofed, sheds demolished and most trees losing branches.

Fortunately the Observatory sustained little damage in the storms. Unfortunately some staff members were less lucky. One had 40 cm of water flooding their house, while another had a section of their home blown in and furniture blown away.

Mopra

Developments at Mopra will be a major focus during 2005. In 2004, the antenna control system and parts of the observing system were replaced or enhanced, and a preliminary version of the new digital filterbank backend was installed. During 2005 we aim to replace the SIS 3-mm receiver with an "ATCA-style" MMIC receiver. We also plan to install the 8-GHz multi-line digital filterbank backend, upgrade other parts of the observing system, and enable remote observations from Narrabri.

An essential component of these developments is the MMIC receiver. The unique capability of the filterbank cannot be exploited without this receiver, nor can remote observing be performed. As with the ATCA 3-mm system, a critical component in the receiver development has been the local oscillator system. Some thought was given to whether the SIS local oscillator system could be used with the new receiver. A more detailed analysis showed this was not practical.

For 2005 APRS Mopra will initially be operated with the SIS receiver. The MMIC receiver will be installed later in the semester when it becomes available. Observations that do not require 115-GHz operation or that would benefit most from the multi-line aspect of the digital filterbank will be scheduled later in the winter.

Staff

In November we welcomed Belinda Adamson to the office at Narrabri. Belinda is with us on a one-year contract while Allison Fairfull is on maternity leave. Also in November, we bid farewell to Debbie Rowe-McDonald. Debbie had been with us for about 30 months, working hard to decrease the entropy of the observatory, by tidying up after the rest of us. In her place we welcome Pam Kelly who has quickly learnt the ropes.

After about 20 months at Narrabri, in early February, Stuart Robertson and Eva Boralv left us to return to Europe. Stuart worked hard to raise Mopra to a higher performance level, through long and tireless stints helping observers. Eva, who worked at the Observatory for one day a week, is largely responsible for the fresh appearance of the Narrabri and Mopra web pages. We wish them the best in their new positions in Europe. In Stuart's place, we welcome Jürgen Ott. Jürgen, currently a Bolton Fellow at Marsfield, will be providing Mopra support over the coming year. Although Jürgen's home base will stay at Marsfield, he will be spending extended periods at Narrabri and Mopra over the winter.

Finally we welcome Philip Peitsch. Philip, a student from Swinburne, will be working in the Electronics Group over the coming six months, with his focus being on Mopra remote operation.

Observer feedback

Figures 2 and 3 show Narrabri and Mopra user feedback for the year 2004, obtained from the online feedback forms (see www.narrabri.atnf.csiro.au/feedback). The feedback is shown for all Narrabri observations (108 responses), millimetre observations at Narrabri (16 responses), and Mopra observations (7 responses). Feedback is given on a scale of 1 (extremely poor) to 10 (excellent).

The Compact Array feedback results are very similar to those of 2003. The only significant changes were an improved response to millimetre documentation and a poorer response to weather during millimetre observations.

This was the first year that Mopra feedback responses were collected. Comparing the Mopra responses to the equivalent Narrabri ones, three areas show markedly poorer responses: the observing system, documentation/web information and offline processing software. Efforts are being made to improve all three areas of concern.

Figure 3: Mopra observer feedback for 2004

The fault report system is another way for observers to give useful feedback. Observers are encouraged to use this system: it ensures that faults are responded to appropriately. On average, 1.2 faults reports are received per day. The faults in this system are being addressed at a somewhat higher rate: the number of active fault reports has decreased markedly over the last 30 months.

The user and fault reporting feedback from observers is highly valued and helps the ATNF to identify and address issues promptly.

Outreach

Figure 4: Indicative sketch of the representation of the Milky Way being constructed as part of the Visitors Centre redevelopment.

Development of the garden area around the Narrabri Visitors Centre is proceeding well. A design company has been contracted to develop a variety of displays and exhibits for this area. Work on an observing deck for viewing the antennas is now finished. The centre piece of the redevelopment will be a representation of the Milky Way, located just to the east of the observing deck.

With the recent floods and disruption to the roads, visitor numbers at the Visitor Centre over the Christmas-New Year period were appreciably lower than in previous years. However, these returned to "normal" in January.

Bob Sault
Officer-in-Charge, Narrabri Observatory
(Bob.Sault@csiro.au)