Topical Issues and Science Questions for the Phase Correction Project
Introduction
The atmospheric phase correction project aims to produce a reliable method
of compensating for interferometer phase fluctuations arising from water-vapour
inhomogeneities in the atmosphere above the Australia Telescope Compact
Array. Several other radio astronomy institutions have similar programs,
the research having become particularly topical - design studies for new-generation
large area, long-baseline, (sub)mm arrays show clearly the need for tropospheric
phase correction if angular resolution and imaging quality targets are to
be met.
The ATNF has had a low-key involvement in phase correction since 1991, with
initial efforts centred on development of a high-stability 225 GHz water
vapour radiometer (WVR) for clear-sky phase correction. Performance targets
for the WVR have been met but, before replicating the prototype for the
ATCA phase correction application, we're assessing a new variant on radiometric
phase correction: water-vapour emission line-shape measurements at 22 GHz.
This "water line monitor" (WLM) approach promises effective phase
correction under a wider range of atmospheric conditions (including at least
light overcast). The ATNF project is maintaining close links with
the developer of the prototype WLM, Dr. David Woody of the Owens Valley
Radio Observatory and, after assessing the effectiveness of 225 GHz and
22 GHz techniques, a final decision on the form of the ATCA system will
be made in early 1998.
While the integration of a 225 GHz atmospheric sounding system is simple
(separate receivers mounted atop the subreflectors on the ATCA antennas),
the 22 GHz WLM has wider system implications. Economy, very demanding
technical performance specifications, and space limitations at the antenna
Cassegrain focus all lead us to favour use of the 22 GHz astronomy receiver
as the front-end of the sounding system. The new mm-wave dewar will
contain at least two receivers and associated feed horns (12 mm and 3 mm
band). The receiver will be much simpler and cheaper if no rapid switching
between 12 and 3 mm is built in. Such a simple design raises at least
two important points:
- With the 3 mm feed on-axis, the 22 GHz feed will be displaced.
How well will this offset feed work as
- a near-field sensor for the atmospheric sounding and,
- a conventional feed for observing 12 mm calibrators for WLM and
array calibration?
- From a system perspective, at what band should we be planning to calibrate
anyway? Our project is looking at international work on the spectral-index
distribution of calibrators. There is, at present, a sustainable case
for regarding 3 mm calibration alone as a viable scheme, despite our initial
attraction to bootstrapping 12 mm calibration to the shorter wavelength.
To answer some questions about the offset 22 GHz feed, we have asked CSIRO
Telecommunications and Industrial Physics (CTIP - formerly Radiophysics)
to analyse the offset geometry; their report is due in early December.
In addition, the ATNF is making measurements using the Mopra antenna and
a prototype 12 mm receiver. Early indications are that both near-field
and far-field characteristics will be acceptable.
The calibration issue has many twists and the ATNF invites comments from
our users and the general astronomical community. Users unfamiliar
with radiometric calibration will find the Background Material Page to
be useful. As well as a thorough review of existing spectral index
statistics, the ATNF plans a group observing project using northern hemisphere
cm-wave and mm-wave arrays to compile better source statistics. Obviously,
our own prototype mm-wave, and existing cm-wave, receivers will also be
used for early southern work.
Go
to MNRF Atmospheric Phase Correction Project Page
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to MNRF Projects Page
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This page is maintained by Michelle
Storey
Last update by Michelle Storey. 18/11/98