[alma-config] Topography mask

[Adrian Webster]

Incorporating topography constraints.

Simon Radford and John Conway have both suggested approaches to
incorporating present knowledge of the topography of Chajnantor
into the design process for the configurations of ALMA. The subject
is not at all straightforward when looked at in detail, when more
and more considerations and complications present themselves. 
This becomes clear, as Simon points out, when considering the form 
of an appropriate cost function to be associated with each possible 
site for an antenna station.

I would here suggest a very simple interim measure for consideration as the 
best way to proceed initially. It is to adopt an unsophisticated binary 
mask, labelling different terrain as either suitable for stations or 
unsuitable, with the threshold between the two choices set fairly high,
so that ground that is at all dubious or difficult is deemed unsuitable
and only ground that is clearly good is deemed acceptable. The idea
is that it is just possible that a configuration design with good
electromagnetic properties can be found which falls exclusively in the
suitable terrain, 
or nearly so, in which case the extra effort involved in devising and 
implementing a more sophisticated characterization of the terrain would 
have been wasted. Furthermore, if no such configuration can be found, that 
will at least become obvious at an early stage, so that there would be 
time available to implement a more sophisticated characterization.

I think that this will help reach an early understanding of which general
designs
are feasible and which are not, so that a more detailed design study
optimizing
the fine details may be undertaken on a much more restricted range of
designs than
are being considered at present.

Adrian

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Adrian Webster,
Institute for Astronomy,
Department of Physics and Astronomy,
University of Edinburgh,
Royal Observatory,
Blackford Hill,
Edinburgh EH9 3HJ,
Scotland.
asw at roe.ac.uk
tel: 0131-668-8391
fax: 0131-668-8416

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