Urquhart et al. 2010. - Australia Telescope Compact Array - Australia ...

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322 J. S. Urquhart et al. replacement of the outer perforated panels of the dish in 1999 with solid panels, and the receiver optics were modified to illuminate the full 22 m. Several rounds of holography using a 30-GHz satellite beacon were used to reduce the surface RMS error from 270 μm in 1999 to 180 μm in 2004. In 2004, observing capabilities were enhanced with the addition of an on-the-fly mapping mode, enabling efficient mapping of larger regions of interest. The 3-mm SIS receiver was the main receiver used for single-dish observing during this period, and the telescope was characterised in the 3-mm band by Ladd et al. (2005). Following the replacement of the 3-mm SIS receiver in 2005, and the addition of a 12-mm receiver in 2006 and 7-mm receiver (Moorey et al. 2008) in 2008, Mopra now has a suite of three receivers for singledish observations, covering the frequency ranges 16–28, 30–50 and 76–117 GHz. The receivers are cryogenically cooled Indium Phosphide (InP) High Electron Mobility Transistor (HEMT) Monolithic Microwave Integrated Circuits (MMICs) low-noise amplifiers. The receiver systems do not require any manual tuning and include noise diodes for system temperature determination. The 3 mm receiver also has a paddle for atmospheric attenuation (τ) corrections. Physically, the three receivers are mounted in a single dewar (Moorey et al. 2008), with a short turret rotation required to change between 12-mm and 3-mm observing frequencies. Changing to and from 7mm observing requires a physical translation of the dewar by about 10 cm, which takes several minutes. In 2006, the Mopra Spectrometer (MOPS) 2 , was installed as the main back-end for single-dish observing. MOPS is a broadband digital filter bank with four overlapping sub-bands, each with a bandwidth of 2.2 GHz providing a total of 8.3 GHz of continuous bandwidth. The spectrometer can be used in two observing modes: wideband mode and zoom mode. The wideband mode provides 8.3 GHz of continuous coverage with a total of 32 384 spectral channels in each polarisation, providing a frequency resolution of 256 kHz per channel and a velocity resolution of 3.4, 1.7 and 0.9 km s −1 at 22, 44 and 90 GHz, respectively. The zoom mode provides a total of 16 zoom windows — four of which can be placed in each of the four sub-bands — each with 137 GHz of bandwidth and 4096 spectral channels providing approximately eight times higher frequency and velocity resolution than the wideband mode. Observations at 3 mm are restricted to the winter months (May–October) when the atmosphere is generally more stable and the amount of water vapour in the atmosphere is generally lower. However, observations at 12 and 7 mm are less affected by atmospheric fluctuations and water vapour content than at 3 mm, and thus, can be scheduled throughout most of the year. Therefore, the installation of the 12- and 7-mm receivers has significantly 2 MOPS was funded, in part, through a grant instigated by UNSW under the Australian Research Council Grants scheme for Linkage, Infrastructure, Equipment and Facilities (LIEF), and in part by CSIRO ATNF. increased the potential science output of the telescope (e.g., Walsh et al. 2007, 2008; Urquhart et al. 2009). In this paper we will present a combination of continuum and spectral-line observations of Jupiter and H2O and SiO masers chosen to characterise the telescope’s beam size, shape and efficiency between 16 and 50 GHz. To maintain consistency across all three millimetre bands we have followed the procedures and analyses used by Ladd et al. (2005) to characterise the 3-mm system. 2 Observations and Data Reduction The observations were made between October and December 2009 and consisted of a combination of two orthogonal cross-scans, beam maps, and positionswitched observations. During all of these observations the pointing was checked, using the standard five-point procedure, approximately once an hour with corrections found to be
Characterisation of the Mopra Radio Telescope at 16–50 GHz 323 function of frequency. To minimise the effects of atmospheric attenuation due to water vapour these mapping observations were made at an elevation of ∼70 ◦ . The map sizes were approximately 9 ′ × 9 ′ and 6 ′ × 6 ′ for the 12 and 7 mm observations, respectively. The mapping observations were produced using the on-the-fly mode and consisted of 35 rows scanned in right ascension, with each scan separated in declination by approximately onequarter beam width (∼30 ′′ and ∼20 ′′ at 12 and 7 mm, respectively), with an off-source reference position being observed before each row. The position of Jupiter was updated at the beginning of each scan to compensate for its orbital motion. The beam maps took ∼20 minutes each to complete, with the telescope pointing being checked before and after each map. The temperature scale for the planet observations is set by the noise calibration system, which must be stable and well-characterised. For the 7-mm band, the noise source is a commercial unit, ELVA-1 model ISSN-22, whereas for the 12-mm band the noise source was built by the ATNF. The electronic drive circuitry necessary to ensure a stable and flat noise output spectrum is internal to the 7-mm band noise source, whereas the 12-mm noise source is driven by an external constant current power supply. For both bands the units were tested and characterised before being installed and found to be stable, essentially flat-spectrum noise sources. The noise diode outputs are introduced into the signal path by waveguide couplers, which also make a contribution to the overall performance. The typical performance of the 7-mm couplers (installed both at Mopra and the ATCA) is described by Moorey et al. (2008). The 12-mm coupler has comparable characteristics, with some amplitude variability above 25 GHz. 2.2 H2O and SiO Maser Beam Maps Although planets are the preferred astronomical sources for determining beam efficiencies, their relatively low signal to noise ratio (SNR), and the difficulty of removing the sky emission due to atmospheric variations, makes continuum observations less suitable for determining the telescope’s beam pattern.A better choice is to make observations of strong astronomical masers which provide high SNR maps of the telescope beam patterns, primarily due to the better baseline subtraction. However, unlike continuum observations of a planet, that can be made at all available frequencies to each receiver, observations of masers can only be made at a few well defined frequencies, and consequently only provide information at these frequencies. We have therefore made beam maps of the bright H2O and SiO masers (at 22.24 and 43.12 GHz, respectively) associated with Orion SiO. These two masers allow us to investigate the telescope beam pattern in both the 12- and 7-mm bands. These maps are centred on the position of Orion SiO (αJ2000 = 5:35:14.5 and δJ2000 =−6:22:29.6) and covered a 20 ′ × 20 ′ anda10 ′ × 10 ′ region at 22.24 and 43.12 GHz, respectively. Figure 1 Example of the Jupiter cross-scan results. The profile is an average of the two orthogonal scans made at 37 GHz. The Gaussian fit is shown as a solid red line. Each map consisted of a total of 25 on-source scans, each individual scan was separated by 51 ′′ and 26 ′′ for the 22.24 and 43.12 GHz maps, respectively. Each map required approximately one hour to complete. Two maps were produced for each maser frequency using orthogonal scan directions. These were subsequently combined to reduce striping (which can result from fluctuations in the sky emission during the observations) and improve the signal to noise in the final map. 2.3 Data Reduction The cross-scans and position-switched observations were reduced using the ATNF spectral analysis package (ASAP). 3 Each cross-scan consisted of 17 separate measurements taken across the planet. For each point, the average antenna temperature measurement was calculated across the zoom bands. The background antenna temperature obtained from the reference position is then subtracted to produce a plot of antenna temperature as a function of angular offset across the planet (see example presented in Figure 1). The beam maps were reduced using LIVEDATA and GRIDZILLA packages available from ATNF. 4 LIVEDATA performs a bandpass calibration for each row using the off-source data followed by fitting a user-specified polynomial to the spectral baseline. GRIDZILLA grids the data to a user-specified weighting and beam parameter input. The data were weighted using the periodic Tsys measurements, and a cell size of 0.25 ′ and 0.15 ′ was used to grid the 12- and 7-mm data, respectively. A Gaussian smoothing kernel was used to produce the beam maps with a full-width at half maximum (FWHM) and a radial cutoff of 2 ′ for the 12-mm data and 1 ′ for the 7-mm data; these values were chosen to minimise smoothing and obtain a resolution in the reduced maps similar to the native resolution of the telescope at the observed 3 http://www.atnf.csiro.au/computing/software/ asap/. 4 Both software packages are available from http://www.atnf. csiro.au/people/mcalabre/livedata.html.
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