Astron. Astrophys. 339, 183-186 (1998)

1. Introduction

The atmospheric extinction of radio waves, which is negligible in the meter-to-centimeter range, becomes significant at wavelengths short of 7 mm. The attenuation of the signal is highly weather dependent, especially close to atmospheric absorption lines of water and oxygen. A proper calibration of mm-wave measurements thus requires a proper knowledge about the atmospheric opacity, which is usually done by measuring the atmospheric emission relative to the emission from a cold and a warm load. Including information about atmospheric parameters, the application of an atmospheric model allows the determination of the water wapour content and hence the optical depth can be derived.

Even if the calibration procedure appears to work properly, independent cross-checks of the temperature scale are necessary in order to check the system set-up, e.g. the determination of the sideband ratios and the receiver temperatures. For spectral line observations, catalogues with calibrated reference spectra exist (e.g. Mauersberger et al. 1989) whereas continuum calibration checks are done by cross-scans on planets.

Only few planets can be used as reliable primary calibrators: Venus and Mecury show strong phase-dependent flux variations and the flux of Saturn is affected by the viewing angle of its rings. Uranus and Neptune are relatively weak and currently at low declinations which (at sites in the northern hemisphere) introduces an additional uncertainty. The disk temperature of Jupiter is stable but its disk is resolved by large single-dish mm/submm-telescopes, like the IRAM-30m telescope and the JCMT, and thus requires a complex deconvolution of the observed temperature profile which introduces additional errors into the calibration procedure. Mars appears to be the only planet which is mostly small enough not to be resolved by mm-wave telescopes and whose (frequency-dependent) disk temperature is well determined and stable. Proper continuum calibration checks are therefore restricted to a few hours per day when Mars is at sufficiently high elevations. In order to provide a set of reliable calibrators which are distributed more uniformly over the sky, we have undertaken sensitive narrowband (500 MHz) continuum measurements in the frequency range 86-245 GHz of the galactic HII-regions W 3(OH) and K 3-50A. Moreover, we present broadband (60 GHz) maps of the 1.2 mm dust continuum emission of these sources which provide information of the source sizes and their morphology.

© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998
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