The Australia Telescope Compact Array (ATCA) is likely to be the first operational mm-wave interferometer in the Southern Hemisphere. This paper examines some possible atmospheric phase correction strategies for the array, notes the applicability of these to future large mm-wave arrays, and points to the usefulness of the ATCA in assessing the various approaches.
Water Vapour Sounding
The ATNF Water Vapour Radiometer
Other ATNF Phase Correction Schemes
The ATCA is an array of six
22 m dishes arranged in a linear east-west configuration and
located near Narrabri, in northern NSW. Five antennas are movable
on a 3 km rail track; the sixth is located a further 3 km west.
Available baselines range from 6 km to 30 m. At frequencies
below 50 GHz the full 22 m aperture is available on each antenna,
while a precision 15 m inner section forms the effective reflector
at frequencies up to 115,GHz. The array currently operates in
a variety of modes (including mosaicing and real-time, one-dimensional
imaging) in four broad observing bands below 9.2,GHz. Antenna
and system tests show that the ATCA will function effectively
at its highest design frequency of 115 GHz, making it viable
as the first-generation southern millimetre array. This paper outlines
possible solutions to the greatest challenge
facing ATCA mm-wave observers: phase fluctuation induced by atmospheric
water vapour instabilities. Some of the approaches are
likely to be applicable in future large mm and sub-mm wave arrays.
The ATCA site is only 200,m above sea level but, notwithstanding the low altitude, the Narrabri climate is an inland continental one characterized by a clear and still winter season, with night-time temperatures often reaching freezing point. First site evaluation results, based on 30 GHz water vapour radiometry opacity measurements and 9 GHz interferometric stability tests, indicate that approximately 60 winter nights per year will support 3,mm band synthesis, assuming that only existing observing and calibration techniques are used. Precipitable water vapour under such conditions is often < 4 mm. As with other mm-wave interferometers, the challenge is to increase the array coherent integration time to the point where the resulting signal-to-noise ratio allows an armoury of imaging tools (e.g. self-calibration) to be used. At Narrabri the search is particularly tantalizing in that most winter (and many late autumn and early spring) days are cloud-free. A phase fluctuation correction scheme which accounts for atmospheric water vapour instability under such conditions promises substantial return, possibly more than doubling the available mm-wave observing time.
In common with other groups, the ATNF has been examining ways of correcting phase fluctuations induced by the motion of tropospheric water vapour inhomogeneities passing the array. At present, four techniques are being considered:
Water Vapour Sounding
The basis of this technique is the use of the emission brightness
of tropospheric water vapour as a measure of the integrated vapour
column encountered by cosmic radio waves propagating towards
an astronomy antenna. The amount of WV encountered determines
the so-called "wet delay'' and, in large measure, the electrical
path (or phase) defining the transmission path. By observing
the water vapour seen by each antenna in a synthesis array, it
should be possible to make differential phase corrections to
account for inhomogeneities in the vapour distribution above
the array. Frequencies near atmospheric WV resonance lines are
obvious choices in the design of sounding equipment. Practical
considerations relating to brightness saturation and pressure
broadening effects make it desirable to operate a little away
from the lines, at least with robust phase correction algorithms
based on straightforward atmospheric models.
After reviewing earlier (disappointing) attempts at phase correction based on 22 GHz sounding and considering the relative sensitivity of emission brightness to water vapour content, the wide range of precipitable water vapour encountered at Narrabri, the required degree of insensitivity to fluctuations in other tropospheric constituents, and available stable receiver technology, the ATNF chose 225 GHz as an operating frequency for its water vapour radiometers (WVRs). A subsequent collaboration with IRAM (Bremer 1994) confirmed, on the basis of more sophisticated atmospheric modelling, that 220-230 GHz operation was indeed a good choice.
While the results of Welch (1994) taken with the BIMA array provided good initial impetus for the renewed interest in water vapour sounding, recent results from IRAM (Bremer 1995) demonstrating the efficacy of the technique on the Plateau de Bure Interferometer (PdBI) have shown beyond doubt the merit of the approach. The IRAM-ATNF collaboration continues to explore the subject, including parallel investigations of astronomy antenna and dedicated WVR sounding. Specifically, the ATNF programs are designed to implement a pilot scheme which involves placing two precision 225 GHz WVRs on elements of the ATCA as well as investigating the merit of WV sounding using a stable continuum channel in future 3 mm band receivers. At present, one prototype 225,GHz WVR has been built and tested, and initial PdBI 110 GHz data, together with that of Welch (1994), encourages further the pursuit of sounding investigations at that frequency.
Table 1 lists the WVR major specifications and measured performance. The WVR is fairly conventional in design, with the front-end components being supplied by Millitech Inc. The Dicke reference load is cooled, rather than heated, to bring the physical temperature closer to typical sky brightness temperatures, thereby permitting more effective gain fluctuation cancellation in the quasi-optic Dicke switching process. An ambient load can be positioned in the sky path, allowing load-load switching and explicit gain and T_sys calibration. This load also serves as a fail-safe shutter, protecting the internal optical path from residual solar radiation in the event of a power failure. Within the optics box, mountings with high thermal mass ensure that critical components experience a minimum of short-term temperature variation.
Operating frequency 225 GHz
Architecture DSB superheterodyne, un-cooled Schottky mixer
Optics Classical cassegrain
Primary reflector aperture 0.5 m
Beamwidth (-3 dB) 11deg.
Aperture efficiency 50%
System temperature 1600 K
Sensitivity 0.08 K (1 sec. integration)
Stability 0.08,K over 10,sec. (total power mode) 0.15,K thereafter (switched mode)
Control/Data acquistion system TMS320C25 DSP muP based
I/O system Serial data, control and monitor via fibre-optic link
The digital signal processing backend, together with hardware controllers designed around a field programmable gate array, give great flexibility in data acquisition, on-the-fly calibration, and WVR control and monitoring. The fibre-optic serial data link allows a galvanically isolated connection to the remote package.
The performance of the prototype WVR is encouraging and the next stage of the project is to produce a second unit so that interferometric phase correction trials can commence.
As well as receiver alternation, the next-generation ATCA packages will include a 12 mm band (<18 - 25 GHz) and a shorter-wavelength receiver (either lambda = 7 or 3,mm) within the same dewar. Thus, simultaneous 12 mm and shorter-wavelength observations will be possible, conferring dual-band calibration advantages without the time penalty involved in turret rotation. A particularly interesting possibility arises in the observation of (e.g.) CO sources that have H_2O masers located in the 22 GHz field. In these cases, ATCA observations with the longest baselines will be possible as the phase correction process can be essentially perfect. True sub-arcsecond angular resolution should therefore be obtainable.
With regard to the use of a stable 3 mm continuum channel to obtain phase correction data, it is worth noting that ATCA 3 mm receivers will use HEMT amplifiers cooled to 20 K (in contrast with the 4 K SIS systems in use elsewhere), either in discrete form or integrated into MMIC packages. A project to demonstrate this technology is already under way and it remains to be seen whether expected gains in receiver stability can be realised in practice.
Bremer, M. (1994): The Phase Project - First Results, IRAM Technical Note, April 1994.
Bremer, M. (1995): IRAM Newsletter, November 1995.
Hall, P., Abbott, D. (1993): Interferometer Phase Correction Using Millimetre Wave Water Vapour Radiometry, ATNF Technical Note, AT/31.6.7/018.
Welch, W. (1994): in Astronomy with Millimetre & Submillimetre Interferometry, eds. Ishiguro M. & Welch W., Astron. Soc. Pac. Conf. Series, 59.