AAS 199th meeting, Washington, DC, January 2002
Session 103. Instrumentation for Radio Wavelengths
Display, Wednesday, January 9, 2002, 9:20am-6:30pm, Exhibit Hall

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[103.08] Phase Correction for the Sub-Millimeter Array (SMA) or Getting a clearer view through the Earth atmosphere

M.C. Wiedner (CfA), C. Cunningham, G. Rodrigues (HIA), C.D. Wilson (McMaster), SMA Team

The angular resolution that can be obtained by ground-based aperture synthesis telescopes at millimeter and sub-millimeter wavelengths is limited by wave front distortions caused by water vapor in the Earth's atmosphere, analogous to ``seeing'' in optical astronomy. For the Sub-Millimeter Array (SMA) on Mauna Kea, Hawaii, we expect average path fluctuations of 300 microns (phase fluctuations of 125 deg at 350GHz) for baselines of 500m. This would reduce the signal of a point source by about 90% at 350GHz and more at higher frequency. Clearly, a phase correction method needs to be found to make high resolution observations feasible.

In one possible correction method the amount of water vapor above each element of the interferometer is measured in real-time by monitoring the strength of the 183 GHz water vapor line. Any difference in water vapor above the individual antennas, which will result in wave front distortions, can be corrected for.

Previously, we obtained encouraging results with this method at the joint Caltech Submillimeter Observatory (CSO) and James Clark Maxwell Telescope (JCMT) interferometer (Wiedner et al., 2001). Now, 183 GHz phase correction is investigated in more detail at the SMA, where two WVMs have been installed. Extrapolating from the earlier tests we hope to correct the path lengths fluctuations to 60 microns (25 deg rms at 350 GHz), which would translate into a signal loss of less than 10% at 350GHz even at the longest baselines. We will report about the phase correction initiatives at the SMA and present our latest results.

We thank the SMA for supporting M.C. Wiedner, NSERC and the NRC of Canada for financing the building of one WVM, and the CSO for lending us their WVM.

References: Wiedner, M.C., Hills, R.E., Carlstrom, J.E., & Lay, O.P., 2001, ApJ 553, 1036.

The author(s) of this abstract have provided an email address for comments about the abstract: mwiedner@cfa.harvard.edu

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