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Re: ALMA Science Software Requirements (SSR) -- Step 1
On Wed, 20 Oct 1999, Robert Lucas wrote:
>
> Ideally with properly tuned scheduling we should always be observing in
> conditions when the array is 100% efficient, i.e. when calibration is
> easy and can be done automatically.
>
I don't believe this will be the case. Many people have the impression
that the ALMA site is so much better than current mm sites that it will
be essentially perfect. Remember that the phase stability at the ALMA
site, while better than the California or PdeB sites, does not show the
same level of improvement that the opacity shows. Rather than assuming
everything will be great, this is actually going to be a hard optimization
problem.
Losses due to efficiency include slewing among sources, observing a flux
calibrator, losses in the phase calibration process (both due to
decorrelation and time spent on the phase calibrator -- for either fast
switching or radiometric cal), losses due to opacity, especially when the
source is far from transit, losses due to pointing errors, and losses due
to surface errors which will change with elevation and solar orientation.
There are many tradeoffs to be made: for example: you win by breaking the
observing chunks into smaller and smaller bits (ie, so you can get closer
to zenith for all data, or so you can observe only when the conditions are
just right), until the slewing to source and flux calibration process
becomes an appreciably large fraction of the observing time for each
chunk.
As we push the array to higher and higher frequencies, these most
demanding experiments eat up the best conditions, leaving the worst
conditions for 230 or 115 (or, God forbid, 70 or 30) GHz. This is
actually a strong argument for including 30 GHz in the ALMA design;
otherwise the ALMA will spend something like 10% of its time just waiting
for better weather or integrating while the phase wraps around
(even after phase calibration).
-Mark
>From - Wed Jan 26 13:47:11 2000
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Date: Wed, 20 Oct 1999 18:38:19 +0200
From: Robert Lucas <lucas@iram.fr>
Organization: IRAM
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To: "ALMA, SSR list" <ALMASSR@eso.org>
Subject: Draft operational concept document
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Hello everyone:
I have prepared a draft document, which is actually no more than a list
of issues (and questions), in order to help stimulate discussions. I'm
waiting for reactions, some of them in Garching next week, but hopefully
even before.
This is at http://iraux2.iram.fr/~lucas/operations/
Remember than most of these issues concern also the science group, and
that some coordination is needed.
Regards
Robert
--
Robert LUCAS, Institut de Radioastronomie Millimetrique
300 rue de la Piscine, F-38406 St Martin d'Heres Cedex (FRANCE)
Tel +33 (0)4 76 82 49 42 - Fax +33 (0)4 76 51 59 38 - Tlx 980753 F
E-mail: mailto:lucas@iram.fr http://iram.fr/~lucas/
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