[asac] [almanews] ALMA Memo 371 Released

[Carolyn White]

ALMA Memo #371

Receiver Calibration Schemes for ALMA

S.Guilloteau (IRAM/ESO), R.Moreno (IRAM)<br>

04 June 2001</h2>

Keywords : Calibration, Receiver

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.


View a pdf version of ALMA Memo 371.
http://www.alma.nrao.edu/memos/html-memos/alma371/memo371.pdf

Download a ps version of ALMA Memo 371.
http://www.alma.nrao.edu/memos/html-memos/alma371/memo371.ps


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