"On-the-fly" mapping with the Mopra telescope
We have recently implemented an On-the-fly (OTF) observing mode at Mopra to permit more efficient mapping of large regions in molecular lines. In this mode, the telescope takes data continuously while scanning across the sky and accurate coordinate information is written into the datafile with each spectrum. Not only does OTF mapping help to smooth out systematic errors as in pointing, but it also provides a net gain in efficiency because a single reference spectrum can be applied to multiple source spectra, as the effective on-source integration time is very short.
The OTF mode has been made possible by several recent developments: (1) adjustment of the surface panels and subreflector to produce a more symmetric beam pattern at 3 mm, (2) installation of a new antenna control computer (ACC) that can handle shorter cycle-times, and (3) development of software to facilitate rapid scanning, schedule file preparation and processing of OTF data into spectral cubes.
The first development was needed to correct a significant coma lobe which had been a long-standing problem with the Mopra beam. After re-alignment of the optics, a significant improvement in the main-beam efficiency was noted, and the overall beam-shape could be much better approximated as Gaussian. Further discussion of the Mopra beam shape and size, as well as measured beam efficiencies and standard spectra, can be found in a recent paper submitted to PASA by Ladd et al. A copy can be downloaded from the Mopra website (www.narrabri.atnf.csiro.au/mopra/).
The second development, installation of a new Compact Array-style antenna control computer, has allowed us to shorten the default cycle-time from ten to two seconds, which in turn allows us to scan much faster across the sky. This has also cut down significantly on overheads, including time spent making system temperature measurements and time lost due to the "missing cycle" problem (having to request one more cycle than is actually recorded).
Last but not least, a significant amount of software development has occurred, based in large part on the Livedata and Gridzilla packages used to process Parkes Multibeam data in aips++. Livedata has been upgraded to recognise Mopra RPFITS files automatically, identify reference scans, and extract the proper coordinate information for each spectrum. It then takes care of bandpass calibration and baseline fitting and writes the spectra out to an SDFITS file. Gridzilla can take one or more SDFITS files and generate a FITS data cube based on the user's choice of coordinate frames, projections, gridding techniques and pixel size. A number of widely available visualisation tasks can be used subsequently to examine the data.
Figure 1: 6 arcmin square map of the Horsehead Nebula in 13CO, overlaid on an optical image. The image has been rotated so that east is up.
Some results from this season are shown in the accompanying figures. Figure 1 shows a 13CO integrated intensity map of the Horsehead Nebula overlaid on the well-known optical image. The dust extinction that is apparent in the optical image is beautifully traced by the molecular-line emission. Figure 2 shows the current state of the most ambitious OTF project undertaken thus far, 13CO mapping of a 1.2° × 0.6° field in the southern Galactic plane near Galactic coordinates (333.3, -0.3). This UNSW project, called the Delta Quadrant Survey, is focused on understanding the interplay between massive star formation and the molecular interstellar gas and will be followed up with mapping in higher density molecular tracers next year.
Figure 2: Mosaic of 94 fields (each 5 arcmin square) observed as part of the Delta Quadrant Survey in 13CO. The black dot in the lower left is the size of Mopra's beam. Credit: UNSW Star Formation group.
OTF mapping is suitable for many applications, and is the preferred observing mode for almost any type of mapping at Mopra. If one oversamples the 35-arcsec primary beam for high-fidelity imaging, a sensitivity of about 0.75 K (main-beam temperature) can be obtained over a 5-arcmin square region in about 80 minutes (this yields about 1500 spectra). For larger fields the telescope can be made to scan faster (at the expense of sensitivity and oversampling) or a number of smaller fields stitched together using Gridzilla. For better sensitivity a field can be repeated several times. Note that you should not observe for more than two hours without stopping to refine the pointing. In order to make best use of OTF mapping, we suggest that users spend a few days in Marsfield after observing to work on their data with local experts. Documentation on how to conduct OTF mapping will appear on the Mopra website (www.narrabri.atnf.csiro.au/mopra ) about a month before the next proposal deadline of 1 December.
We thank Ned Ladd for helping to develop the OTF strategy, Ravi Subrahmanyan for directing the effort to improve the beam shape, and Stuart Robertson and Cormac Purcell for helping to test and debug the technique.
Tony Wong (University of New South Wales/ATNF), Michael Kesteven (ATNF and, Mark Calabretta (ATNF)
(Tony.Wong@csiro.au), (Micheal.Kesteven@csiro.au), (Mark.Calabretta@csiro.au)