As we enter the 2006 millimetre observing season at the ATNF, the installation of the ATNF's newest and most versatile spectrometer is now complete. The MOPra Spectrometer (MOPS) recently completed the final stages of testing on the Mopra millimetre telescope and observations have commenced for 2006, with fascinating results already being produced. ATNF engineers have produced a frequency-agile spectrometer system that will become the envy of millimetre astronomers worldwide. So much so, that the 2006 winter season at Mopra was already over-subscribed well before the fully functioning and operational system was even announced!
MOPS is largely a by-product of the development being undertaken for
the Compact Array Broadband Backend (CABB) upgrade project. The
CABB project, part of the Major National Research Facility (MNRF)
program, aims to develop signal processing technologies relevant to the
to demonstrate them in a new backend for the Compact Array. It is these
same signal processing technologies which have been applied to MOPS.
Funding for MOPS came from an ARC LIEF (Australian Research Council
Linkage Infrastructure Equipment and Facilities) grant to a consortium of universities,
led by the University of New South Wales (UNSW), and the ATNF. It was
the UNSW astronomers, led by Michael Burton, who originally realised
the potential of applying the CABB developments to single dish
applications at Mopra, particularly as a wide bandwidth backend for the
new Monolithic Millimetre-wave Integrated Circuit (MMIC) based
millimetre receiver. It is not only in the signal processing area where MOPS
has "borrowed" from other ATNF development projects. The system
used to transmit the 8 GHz bandwidth signals over fibre optic cables from the
vertex room to the control building was originally developed by Mark Leach
for the wideband analogue correlator at the Compact Array. The 4
Gigasample/sec samplers used in MOPS were
originally developed by Paul Roberts for the wideband pulsar correlator at
Parkes. They use an indium phosphide MMIC
2-bit digitiser designed by Paul as part of a CSIRO Executive Special Project, the same project which saw the development of the indium phosphide MMIC low-noise amplifiers (LNAs) used in the Compact Array and Mopra 12-mm and 3-mm receivers.
The MOPS signal processing hardware comprises four large circuit boards, one for each 2 GHz quadrant of the total 8 GHz MOPS bandwidth. The boards, which were designed and produced by Dick Ferris and Evan Davis, were developed as prototypes for the CABB system. They are by any measure the most complex Printed Circuit Boards (PCBs) yet produced at the ATNF, each containing seventeen large field programmable gate arrays (FPGAs), which provide the processing power required. Other vital statistics of the boards are: 19 layers, 30,000 pads and 140,000 track segments. The signal processing firmware, i.e. the code which defines how the FPGAs are programmed, was developed by Dick Ferris and Scott Saunders. It is the fact that the FPGAs can be loaded with different firmware for different applications which gives the MOPS system its versatility. Raji Chekkala, Troy Elton, Jennifer Lie and Jamie Daw also made significant contributions to the construction of MOPS.
The spectrometer has an overall bandwidth of 8.3 GHz. This is divided into four overlapping 2.2 GHz sub-bands in a conventional analogue filter and down-conversion system. Each sub-band is digitised in 2-bit samplers operating at the Nyquist sampling rate. The digital data is then fed to the signal processing section, where polyphase digital filterbank processing divides the band into a large number of independent output frequency channels. Two modes are currently available. A wideband mode provides 1024 frequency channels over each 2.2 GHz sub-band, on two polarisations. A narrowband mode offers a choice of up to four 138 MHz wide "zoom" bands within each 2.2 GHz sub-band. It provides 4096 frequency channels on each zoom-band, with up to16 zoom-bands which can be placed at chosen locations throughout the full 8.3 GHz frequency range.
The installation was rather straightforward, being completed in two days in mid-May. First light observations of the complete system were made towards Orion KL, on Monday 22 May 2006.
MOPS may be operated in a `broad band mode' to provide simultaneous access to a completely contiguous 8 GHz range, with 6 km/s channel spacing (at 90 GHz). This kind of configuration is optimal for extragalactic studies, although it will also prove invaluable in obtaining simultaneous, multi-frequency emission-line measurements.
As a test, short (200 s) observations of the circumstellar envelope of the carbon star IRC+10216 have been made across the full 74 - 118 GHz scope of the front-end system at seven offset frequencies. A short part of completely surveyed band is shown in Figure 1, with the stronger transition lines marked (note that at the scales shown here, the plotted results from the entire surveyed range would stretch across 1.85 m!).
Figure 1: An example spectrum from 87 - 95 GHz, taken using the new broadband MOPS system towards the carbon star and its circumstellar envelope, IRC+10216. A few of the important lines are clear even from only 200 sec on-source. The full spectrum, taken in seven, 8 GHz-wide bands and extending from 74 to 118 GHz, would span 1.85 metres!
A second operational mode allows each of the four 2.2 GHz sub-bands to be configured with 4 x 138 MHz "zoom" windows, each with 4000 channels. The beauty of this mode, and the real draw-card for astro-chemists, is that the centre frequencies of the zoom windows will now be completely configurable within each of the 2 GHz windows, providing simultaneous observations of up to 16 different molecular lines across the 8 GHz range of MOPS, at a velocity resolution of ~0.115 km/s (at 90 GHz). This configuration is optimal for Galactic studies, as is evidenced by the rapid progress of the CHaMP (Census of High and Medium mass Protostars) survey which is in the beginning stages of a simultaneous, multi-frequency series of observations of proto-stellar molecular cores.
Some preliminary results from the CHaMP project (and MOPS zoom mode) are shown in Figure 2. These measurements show the integrated N2H+ brightness of an example molecular core as the colour scale, overlaid with contours of integrated HCO+ brightness. Both molecules are excited under similar conditions (critical densities ~105 cm-3), but HCO+ is most likely to be optically thick, while N2H+ is probably optically thin.
Figure 2: A map of a CHaMP target a protostellar core, observed at N2H+ and HCO+ simultaneously. It will soon be possible to observe up to 16 lines simultaneously, accelerating multi-frequency observations of chemically interesting objects such as this one (click on image for larger version).
This means that HCO+ tends to be brightest where there is local heating, while the N2H+ peaks up near the column density peak. As it turns out, there is a luminous IRAS source right at the HCO+ peak in the image, supporting the above interpretation. While the hot HCO+ peak denotes the site of newly-formed stars, the N2H+ peak is possibly a site of star formation in an earlier evolutionary phase. CHaMP intends to measure more than 100 cores with MOPS, at up to 10 different lines simultaneously - a process which would otherwise require years of observing time on any other telescope.
With the newly updated receiver system, and now the new, versatile and broad-band Mopra spectrometer, we look forward to a very prosperous future for the Mopra telescope.
Warwick Wilson, Erik Muller and Dick Ferris