3.5 or 2.7 mm?

One of the most exciting aspects of this upgrade is that we will be able to extend the AT Compact Array to work at millimetre wavelengths. At millimetre wavelengths, as well as getting higher resolution for the objects we study at present, we will also be able to study other processes (e.g. thermal dust emission) and exciting molecular spectral lines that we haven't been able to study before. The AT Compact Array will be the first Array in the Southern Hemisphere able to do this.

However, these millimetre wavelengths also present more of a technical challenge than the longer wavelengths. The ATNF sites at Narrabri and Mopra are at low altitude, and so the AT will suffer from atmospheric absorption and phase degradation at 3 mm. We do not yet know exactly how much of the time this will be a problem, but we do know that it will be worse at 2.7 mm than at 3.5 mm. Instrumental problems such as dish efficiency and pointing also become more severe at the shorter wavelengths. At present, we believe that 115 GHz (2.7 mm) is likely to be so corrupted by oxygen absorption that we are not planning to build a 2.7 mm receiver. The atmospheric problems will be far less at 90 GHz (3.5 mm) however, and so at present we plan to build a 3.5 mm receiver. Depending on technological developments, we may end up extending the frequency range to 115 GHz anyway, but our primary goal is 90-100 GHz.

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Last updated: 9-August-1996

Ray Norris (rnorris@atnf.csiro.au)