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ALMA MEMO #372
An Amplitude Calibration Strategy for ALMA
R.Moreno (IRAM), S.Guilloteau (IRAM/ESO)
2002/05/10
Keywords: Calibration, Receiver
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.
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Last modified: 2002-05-14
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