A description of the format of telescope specifications.

Many Difmap commands expect you to select one or more sub-arrays,
antennas, baselines, or closure triangles. To make this easier, a
consistent syntax is used to specify all such telescope aggregates.
This specification of an aggregate of n>=0 telescopes is of the form:

 [sub-array:][antenna_1 antenna_n...]

Where "sub-array" is a number between 1 and nsub, and antenna_n is an
exactly matching or an otherwise unambiguous prefix of an antenna
name. 

[Sub-array]
-----------
The sub-array index is optional and defaults to a context dependent
value. From the command line it generally defaults to sub-array 1.
Within interactive commands, such as vplot, it defaults to whatever
sub-array is currently being viewed.

If both a sub-array index and one or more antenna names are specified,
the first antenna name must be separated from the sub-array index with
a colon (:). Otherwise the colon is optional. A colon without a
preceding sub-array index implies selection of the default sub-array.

[antenna_1 ...antenna_n]
-------------------------
If a telescope aggregate is made up of n telescopes then there may be
up to n antenna names specified. Omitted trailing antenna names are
treated as wild-cards.

Omitting trailing components of a specification
------------------------------------------------
For non-interactive commands, omitting one or more trailing components
from a specification is interpreted to mean that all telescope
aggregates of the expected type (eg. a sub-array or baseline
aggregate...) that match in the initial specified components, are
acceptable. The omission is thus effectively a wild-card. For
interactive commands, such as vplot, which won't plot all the
wild-carded aggregates without further user direction, the first
aggregate that matches in the initial given indexes is sought and the
selection is then forgotten. Thus to the vplot command an empty
specification string "" means plot all baselines of all sub-arrays
when plotting to a non-interactive device, whereas on an interactive
device the same specification would mean start plotting from the first
baseline in the default sub-array.

EXAMPLES
--------
The following examples illustrate how specifications are interpreted
in different contexts. Note that while each specification is shown
enclosed within quotes, this is to make it possible to display an
empty string. In practice quotes are only legal for specifications
that are given as command-line arguments to commands, and even there
they are rarely necessary. Also note that all difmap commands
interpret the omission of a trailing telescope specification argument
as equivalent to specifying an empty string.

Sub-array specifications
------------------------

  ""                All sub-arrays.
  ":"               The default sub-array.
  "2"               Sub-array 2.

Telescope specifications
------------------------

  ""                 All telescopes of all sub-arrays.
  ":"                All telescopes of the default sub-array.
  "2"                All telescopes of sub-array 2.
  "an1"              Telescope AN1 of the default sub-array.
  "2:an1"            Telescope AN1 of sub-array 2.

Baseline specifications
-----------------------

  ""                 All baselines of all sub-arrays.
  ":"                All baselines of the default sub-array.
  "2"                All baselines of sub-array 2.
  "an1"              All baselines of telescope AN1 in the default sub-array.
  "2:an1"            All baselines of telescope AN1 in sub-array 2.
  "an1 an2"          Baseline an1-an2 in the default sub-array.
  "2:an1 an2"        Baseline an1-an2 in subarrary 2.

Closure triangle specifications
-------------------------------

  ""                 All triangles of all sub-arrays.
  ":"                All triangles of the default sub-array.
  "2"                All triangles of sub-array 2.
  "an1"              All triangles of telescope AN1 in the default sub-array.
  "2:an1"            All triangles of telescope AN1 in sub-array 2.
  "an1 an2"          All triangles of baseline an1-an2 in the 
                     default sub-array.
  "2:an1 an2"        All triangles of baseline an1-an2 in subarrary 2.
  "an1 an2 an3"      Triangle an1-an2-an3 in the default sub-array.
  "2:an1 an2 an3"    Triangle an1-an2-an3 in sub-array 2.

SPECIFIC EXAMPLES
-----------------

1. Inspection of the currently selected self-cal constraints
   for one or more telescopes.

   0>selfant "" 
   1:AN1      self-cal  status=correctable  weight=1
   1:AN2      self-cal  status=correctable  weight=1
   1:AN3      self-cal  status=correctable  weight=1
   2:AN1      self-cal  status=correctable  weight=1
   2:AN2      self-cal  status=correctable  weight=1
   2:AN3      self-cal  status=correctable  weight=1

   0>selfant 2
   2:AN1      self-cal  status=correctable  weight=1
   2:AN2      self-cal  status=correctable  weight=1
   2:AN3      self-cal  status=correctable  weight=1

   0>selfant an3  
   1:AN3      self-cal  status=correctable  weight=1

2. Non-interactive visibility plotting to a hard-copy device.

   0>dev /ps

   0>vplot 0, ""
   Using default options string "efbm3"
   Page 01: baselines of 1:AN1
   Page 02: baselines of 1:AN2
   Page 03: baselines of 1:AN3
   Page 04: baselines of 2:AN1
   Page 05: baselines of 2:AN2
   Page 06: baselines of 2:AN3

   0>vplot 0, 1
   Using default options string "efbm3"
   Page 01: baselines of 1:AN1
   Page 02: baselines of 1:AN2
   Page 03: baselines of 1:AN3

   0>vpl 0, 2:an2
   Using default options string "efbm3"
   Page 01: baselines of 2:AN2

   0>vpl 1, 2:an2 an3
   Using default options string "efbm3"
   Page 01: baselines of 2:AN2