MMA Memo #186

Calculation of Anomalous Refraction on Chajnantor

M.A. Holdaway [1]

September 30, 1997

Keywords: pointing, antenna diameter, atmosphere, phase fluctuations

We have investigated the effects of inhomogeneously distributed water vapor, as characterized by our 11.2 GHz site testing interferometer database, upon antenna pointing. This effect, known as ``anomalous refraction'', has been seen at poorer sites with millimeter wavelength telescopes for years (Altenhoff et al. 1987). Because of the structure of atmospheric turbulence, the pointing error in arcseconds will be smaller for larger antennas, but the pointing error will be larger in terms of the fraction of the beam size. The time scale of the pointing error will be nearly the time required for the atmosphere to cross the dish. To first order, water vapor is non-dispersive, so the anomalous refraction pointing errors will be independent of frequency. However, there is mild dispersion in the submillimeter, resulting in slightly larger pointing errors in the submillimeter windows. For an 8 m dish on the Chajnantor site, the atmospheric contribution to the pointing errors will usually be well under an arcsecond, except during poor weather and while observing at the lowest elevation angles.

[1] NRAO

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Last modified: 09 December, 1999