The effect of a radome on submillimeter site testing measurements

We evaluate the effect that radome transparency has on atmospheric opacity measurements performed by the skydip technique. We show that, except at rather high opacities, it is not sufficient to ignore losses in the radome (or “window”) during the data analysis and then subtract them off from the derived atmospheric opacity. Perhaps surprisingly, unless radome transparency is correctly modelled, the atmosphere will appear to have a minimum opacity that is many times greater that the radome losses. Our conclusion is that some previous site studies may have significantly underestimated the quality of the best submillimeter sites, and that the difference between these sites and poorer sites may be much greater that currently believed. We also show that part of the residual 857 GHz opacity at the best sites, currently ascribed to “dry-air opacity”, can in fact be just an artefact caused by not properly modelling the radome during the data analysis. INTRODUCTION The skydip technique (Dicke et al. 1946) is used to measure the atmospheric opacity from radio to infrared wavelengths, both for site testing purposes and to calibrate observations. The technique requires a measurement of the sky flux at different elevations, and derives the zenith sky opacity on the assumption that the atmosphere can be modelled as a single slab at uniform temperature. Skydips have been widely used in the sub-millimeter and millimeter wavelength region (Chamberlin & Bally 1994; Chamberlin, Lane, & Stark 1997; Dragovan et al. 1990; Lane 1998; Matsuo, Sakamoto, & Matsushita 1998; Pardo, Serabyn, & Cernicharo 2001; Radford & Holdaway 1998), because, when observing astronomical sources at these wavelengths, the attenuation introduced by the atmosphere, and its fluctuations, are amongst the main sources of noise and calibration error. We have reviewed the different algorithms that have been used for data reduction, which allow for different contributions. In the general case one should include terms related to the sky, such as the cosmic background radiation, or to the telescope and receiver, like spillover, blockage and ohmic losses (Ulich & Haas 1976). With a finite beam size the non-linearity of the relation between the elevation and the airmass number when approaching the horizon should also be considered. This limits the minimum elevation to