[alma-config] maximum baseline

[Simon Radford]

Darrel,

The project book does not have a clear specification of the maximum
configuration size or maximum baseline.

Item 2 of table 2.1, which appears in both chapter 2.1 and 2.2,
specifies "0.1'' resolution" and, hence, "baselines longer than 3 km."

[The copy of this table in chapter 2.2 specifies "01 resolution", i. e.,
is missing the double primes for arcsec.]

Section II.4 of chapter 2.1 states, "... array configurations capable
... of 0.1'' angular resolution. Such configurations will have an extent
of 4 km. In addition, a very extended configuration of 12 km diameter is
needed for imaging at ~10 milli-arcsec."

Section 15.2.2 says "The largest configuration is assumed to have a
maximum baseline of 3 km. A separate array of 10 km diameter or larger
is also being cconsidered."

-- Although they are not strictly inconsistent [4 and 12 km are longer
than 3 km], these statements are not the same to the casual reader.

-- Without knowing the observing frequency, the correspondence between
angular resolution and baseline length is undefined.

-- Even when the observing frequency is known, the correspondence
between angular resolution and maximum baseline length is ambiguous. It
depends of the configuration style. For example, the two leading
contenders for ALMA configurations are rings and spirals. For the same
resolution, spirals generally have longer maximum baselines because they
are more centrally concentrated.

-- The engineers need a firm specification on the maximum fiber run. If
the configurations were already known, this could be derived from the
road layout, etc. But the configurations are still being designed while
the engineers already embarked some time ago on development of the fiber
links. Section 7.4.2 states "The longest distance to an antenna will be
25 km." Subsequent sections also mention 25 km. If this is the project
specification, please elevate it to a more prominent place.

Simon

PS. I repeat my plea to abandon letter designations (A, B, C, ...) for
the configurations in favor of their nominal sizes, especially since the
3 km configuration (A) is no longer the largest being planned (Table
15.3).