Antenna/Baseline Selection

This expression is used to perform baseline based selections. Baseline can be specified as a pair of antenna specifications. Since antenna specification can itself be a list of antennas, the expression allows a rich selection syntax which is simple for simple selections.

ANT in the description below is a comma-separated list of antenna specifications. A baseline specification is an single ANT, ANT followed by an operator or a pair of ANT separated by an operator. A baseline expression is single baseline specification or a semi-colon separated list of baseline specifications.

Syntax

Antenna specification

An ANT can be given as a single string (literal/pattern/regular expression), single integer ID, a range of integer IDs or a comma separated list of integers. For VLA-specific reasons (see Section 3.2), only for antenna specifications, integers are first converted to strings and matched against the antenna names. E.g.

• VLA:N1 corresponds to antenna named "VLA:N1".
• 1,2,3 corresponds to antennas named "1", "2" and "3".
• 1$\sim$ 3 corresponds to antennas named "1", "2" and "3".
• VLA:N* corresponds to all antennas who's name starts with "VLA:N".

The precise logic applied to the antenna specification (ANT) is as follows:

1. If ANT is an integer index (or a list or range), it is first used as a name (e.g. the index 1 is converted to a string "1" etc.) and matched against the NAME column of ANTENNA sub-table. If no match is found, it is treated as an integer index and matched against the antenna indices.

2. If ANT is a string which cannot be converted to an integer, it is first matched against the NAME column of the ANTENNA sub-table. If no match is found, match against the STATION column is attempted.

Antenna@Station specification

A fully qualified antenna specification can also include the station name via the ANT@STATION syntax. STATION specification follows the same rules as antenna specification. When antenna or station in a ANT@STATION syntax is a comma-separated list, the list must be enclosed in a left- and right-parenthesis pair to distinguish it from a comma-separated list of ANT@STATION (i.e., "A1@S1, A2@S2, A3@S3" versus "(A1,A2,A3)@(S1,S2,S3)"). All antennas in the ANT part of the specification on any of the stations in the STATION part of the specification will be selected. The ANT part of the specification is optional and when not specified is replaced with the wild-card "*" (i.e. "@STATION" is equivalent to "*@STATION").

The two forms of specifications that are interpreted differently are:

1. List of ANT@STATION: A1@S1, A2@S2, A3@S3,....
2. (List of ANT)@(List of STATION): (A1, A2, A3)@(S1,S2,S3)

First form is similar to the comma-separated list of antenna specifications used to construct the baseline to be selected (see Section 3.1.3). The latter will select all members of the antenna list that are on any of the stations listed in the station list. E.g. if antenna A1 appears on stations S1 and S3, both will be selected in the second form, but only the antenna A1 on station S1 will be selected in the first form. Note that the second form of specification can itself be a member of a comma-separated list.

Baseline specification

A baseline specification consists of an ANT, an ANT followed by an operator or a pair of antenna specifications separated by an operator. Operator can be one, two or three ampersands ('&') - '&' selects only cross-correlation baselines, '&&' selection cross- and auto-correlation baselines and '&&&' selection only auto-correlation baselines. Formally, baseline specification is of the form [!]ANT[$<$ OP$>$ [ANT]] (where the parts in square brackets are optional and $<$ OP$>$ can be '&', '&&' or '&&&').

Possible forms of baseline specifications are tabulated in Table 1.

Table 1: Baseline specification syntax: ANT, ANT1 and ANT2 represent a comma-separated list of antennas.

Specification	Meaning
ANT	Select only cross-correlation baselines between all the
	antennas in ANT and all other available antennas
ANT&	Select only cross-correlation baselines between antennas
	in ANT only
ANT1 & ANT2	Select only cross-correlation baselines between
	antennas in ANT1 and ANT2
ANT&&	Select cross- and auto-correlation baselines
	between all the antennas in ANT only
ANT&&*	Select cross- and auto-correlation baselines
	between all the antennas in ANT and all other
	available antennas
ANT1 && ANT2	Select cross- and auto-correlation baselines
	between antennas in ANT1 and ANT2
ANT&&&	Select only auto-correlation baselines for
	antennas in ANT
!ANT	Excludes all baselines involving antennas in ANT.
	ANT can be any of the above expressions
ANT1 ; !ANT2	ANT1 and ANT2 can be any of the above expressions.
	This selects only cross-correlation baselines
	between all the antennas in ANT1 and all
	other available antennas except those involving
	antennas in ANT2.

Baseline negation operator

The negation operator '!' can be used to exclude a baseline specification from the final selection. Note that this is a baseline negation operator and not an antenna negation operator. The negation operator applies only to the baseline specification immediately following it. E.g.,

• baseline=!15 excludes all baselines with the antenna named "15"
• baseline=!1$\sim$ 10 excludes all baselines involving the antennas named between "1" and "10"
• baseline=!1,3,5,7,9 excludes all baselines involving antennas named "1", "3", "5", "7" and "9"
• baseline=1$\sim$ 10;!5 selects all baselines involving antennas named between "1" and "10" except those involving antenna named "5".

Note that the negation operator cannot be applied to individual members of the antenna specification/antenna list. E.g. the expression "1,2,!5,8" is syntactically incorrect.

Examples

In the most common usage, an atomic ANT selects all baselines containing all the antennas in ANT. ANT& selects only baselines between the list of antennas in the antenna specification. ANT1&ANT2 selects baselines between antennas in ANT1 and ANT2 only. E.g.

• baseline=1,2,3 selects baseline between antennas 1, 2 and 3 and all other antennas.
• baseline=1,2,3& selects baseline between antennas 1,2 and 3 only.
• baseline=1,2,3 & 4,5,6 selects baselines between antennas 1,2,3 and 4,5,6.

Following are more examples of baseline specification using ranges and names:

• baseline=1$\sim$ 3 same as baseline=1,2,3
• baseline=1$\sim$ 3& same as baseline=1,2,3&
• baseline=1$\sim$ 3 & 4$\sim$ 5 same as baseline= 1,2,3&4,5,6
• baseline=VLA:N* & VLA:E* selects all baselines between antennas with names starting with "VLA:N" and "VLA:E".
• baseline=21;!15 selects baselines between antenna named "21" and all other available antennas, except antenna named "15". I.e, baseline "21-15" (or its conjugate, if present in the database) is excluded.

The full baseline selection expression is any of the examples shown above or a semi-colon-separated list of baseline specifications. E.g.

• baseline=1$\sim$ 3& ; 4$\sim$ 5&10$\sim$ 15 ; VLA:N* & VLA:E*

with each elements of the semi-colon separated list being interpreted as explained above.

Integers-as-names VLA naming convention

Antenna naming convention for VLA is such that the antenna names are actually valid integers converted to strings. While we feel that this is indeed a bad idea and it will be best to translate the VLA antenna names to something like VLA1, VLA2 in the CASA VLA filler (or something that does reflect that its a name and not get confused with integer indexes), for now, to accommodate the VLA tradition, the following logic is used:

Just for antenna selection, a user supplied integer (or integer list) is converted to a string and matched against the antenna name. If that fails, the normal logic of using an integer as an integer and matching it with antenna index is done.

For example, if the antenna with index 17 is named "21", the string "21,VLA22" will expand into an antenna index list of 17,22 (assuming that the antenna named VLA22 has index 22).

If we conclude that this style of antenna selection is indeed the way we wish to go, users should be aware that the antenna selection will behave differently for telescopes other than VLA. For example, assuming that antenna with names "21","17", and "11" have indices 1,2 and 3, for VLA only a selection string "21,17,11" will select antenna with indices 1, 2 and 3. For other instruments where this is not the naming convention, the same selection string ("21,17,11") will select antennas with indices 21, 17, and 11.