ALMA Memo #371
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Receiver Calibration Schemes for ALMA 
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S.Guilloteau (IRAM/ESO), R.Moreno (IRAM)
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04/06/2001
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Keywords : Calibration, Receiver 
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We re-investigate the two receiver calibration schemes which have been proposed for ALMA: dual loads located in the sub-reflector, or semi transparent vane in front of the receiver. We show that the dual-load system needs to switch with a cycle time of less than 0.1 second to be efficient in the submillimeter regime. Moreover, integrations in excess of 20 seconds are required to get sufficient calibration accuracy at these frequencies. The semi-transparent vane must have an absorption coefficient of only 0.06--0.10 below 100 GHz, but is no longer required at frequencies above 300 GHz. The vane transmission can be calibrated in 2-3 minutes by measurement of an astronomical source (nearby quasar). Two vanes, with absorption coefficient 0.06 and 0.12 respectively, are recommended to minimize the calibration time. Given the simplicity of the vane system (passive, slow device, in the receiver cabin), compared with the complexity, speed and location of the dual-load system, we strongly recommend that ALMA develops and adopts such a scheme for the receiver calibration. 
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