ALMA Memo 372 An Amplitude Calibration Strategy for ALMA

We propose an amplitude calibration strategy for ALMA which attempts to reach the 1 % precision for absolute calibration. Key points and major difficulties are identified. Polarization is ignored in this first step. We first review the properties of possible calibration sources. Planets and specially giant planets satellites are the preferred reference sources for flux calibration. Asteroids are too complex for such a purpose, but are useful as e.g. targets for relative pointing & focus determination. Stars are too weak. Quasars have to be used for various intermediate steps: bandpass, pointing, focus, and relative amplitude calibration. The amplitude calibration precision may be divided into 3 major items: absolute flux scale, relative amplitude calibration (as function of time and antenna), and bandpass (dependence on frequency). Absolute amplitude depends on receiver gain and atmospheric opacity correction, and on pointing & focus control. The semi transparent vane is found to be 3 to 10 times less sensitive to atmospheric parameters than the dual-load calibration system. Accurate pointing & focus control can be done within reasonably short time (1 minute or so). Relative amplitude calibration can be checked to 1% accuracy at mm wavelengths by observation of a nearby quasar, but integration time considerations prevents reaching better than about 3% at submm wavelengths. Bandpass calibration, and specially the sideband ratio determination, is the longest calibration procedure. Within 3 minutes, this can be calibrated to 1% at mm wavelengths, but only to 3 % at submm frequencies. Based on these considerations, we recommend 1) to further develop the semi-transparent vane system, 2) to keep the 1 % accuracy goal at mm wavelengths, 3) to relax the specification to 3 % in the submm domain, and 4) implement the coherent signal from the subreflector for bandpass calibration purpose.