A decision tree for the ALMA configuration.


Here are some elements of thinking to prepare the Configuration selection.

Proposition 1)

	Except for the compact configuration, we assume that exact layout of the 
	configuration will not affect the performances: only the style of array matters. 
	Accordingly, details of the topography can be accomodated at a later time,
	once an agreement is reached on which style of array we are building.

Proposition 2)

	The location of the compact configuration will depend on the array style.

Proposition 3)

	ALMA will not perform full tracks on sources, but will rather optimize atmospheric 
	transparency by observing near transit. 

	A consequence of this proposition is that the synthesized beam aspect ratio will 
	depend on the zenith angle for each source. A global optimization of array aspect 
	ratio can be performed on source statistics (fraction of sky as function of zenith
	angle, with ponderation if required for special regions).

Proposition 4)

	Metric to assess configuration quality should include
		a) dynamic range
		b) image fidelity
		c) sidelobe level
		d) noise degradation with tapering
		e) noise degradation in intermediate configurations

Proposition 5)
	
	Single and multi-configuration should be explored in the context of dynamic
	scheduling. Optimization as so far given much importance to the single
	configuration "snapshot", but little is available on multi-configuration yet.

Proposition 6)

	Enhancement due to the Japanese participation should be taken into account.
	This includes possibly an "ultra-compact" array of 7 x 6-8 meter dishes in compact
	hexagon. Location of such an array vis-a-vis the compact array should be discussed.