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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This page is maintained by Michelle Storey
Last update by Michelle Storey. 18/11/98

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