[mmaimcal] forwarded message from Stephane Guilloteau

[Al Wootten]

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From: Stephane Guilloteau <guillote at iram.fr>
Sender: guillote at iram.fr
To: Wolfgang Wild <wild at astro.rug.nl>
CC: Al Wootten <awootten at NRAO.EDU>, sguillot at eso.org
Subject: Re: phase calibration
Date: Fri, 27 Aug 1999 14:17:10 +0200

Dear Wolfgang,

	Although this would require a more complete document about
calibration strategies, I guess one of the possible outline is the
following

- - Phase calibration around 90 GHz. Sensitivity is described in an ALMA
  memo by R.Lucas. I'll rewrite that later as a "pathlength" error
  as function of calibration frequency and Tsys (or Trec)
	- Typical frequency 90 GHz
	- Typical time on phase cal < 30 sec
	- Time to switch to phase cal < 2 sec
 	
- - Sensitivity : to be studied in more details. One would like to
	phase cal on 70 mJy or so sources

- - Dual polarisation : needed only for sensitivity reason. In other
words, only the total bandwidth matters

- - Exact frequency unimportant

- - Receiver amplitude stability unimportant 

However, there are other calibration operations. One of these is
to calibrate the relative phases / pathlengths (not quite the same,
in fact) of the observing receiver and calibration receiver. Another
one if amplitude calibration at the observing frequency. Yet another
one is the polarisation calibration (which in fact is mixed with the
amplitude, because most calibrators are linearly polarized). Bandpass
calibration is also needed, and can be decoupled in two stages
(correlator and IF bandpass).

None of these additional observations drive any serious spec on
receiver (besides the normal phase and gain stability). The important
specification is the setup time from one frequency to the other:
1.5 sec.

Another important consequence is the possible very high number of
switching cycles that may result out of this "fast" calibration 
strategy. It may be worth to feed continuously the calibration and
observation receivers with LO power rather than trying to switch
them too often.


	Best regards
		Stephane

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Stephane Guilloteau                       | Tel: (33) (0)4 76 82 49 43
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