A discussion of sub-array handling in Difmap. Sub-arrays in Difmap. -------------------- CONTEXT ------- To increase the UV coverage and sensitivity of an experiment it is common to combine one or more observations from different times, telescope arrays, or different configurations of a single interferometer. In each case, the antennas in the different observations usually have to be calibrated separately, so the telescope arrays are treated as being different sub-arrays of a single hypothetical interferometer. Difmap details. -------------- Difmap does handle multiple sub-arrays if they all appear within a single input file, but it does not currently provide facilities for mergeing two or more UV FITS files to produce a multi-sub-array data-set. This should instead be done before entering Difmap, say with DBCON in AIPS (DOPOS(1,1) > 0; DOARRAY < 0). Sub-arrays are actually effectively unrelated, so the visibilities for each sub-array must actually be separately processed. Most Difmap commands do this automatically, processing each sub-array one after the other. Exceptions to this rule are certain plot commands, along with a few sub-array manipulation and informational commands. In these cases, sub-arrays must be refered to by number. The association between sub-array numbers and arrays can be ascertained with the 'header' command. Telescope specification strings. ------------------------------- Since a single telescope may appear in more than one sub-array, Difmap usually requires that when you name a telescope, baseline, or closure-triangle, that you also specify the number of the sub-array in which the telescope(s) belong. The only exception to this rule, is when the telescopes belong to the default sub-array, which is 1 if the specification is given on the command line to a command, or equal to the number of the sub-array that is currently being displayed when given interactively in a plot command. In these cases the sub-array index can be omitted. In particular, if the observation only has a single sub-array, then the default sub-array always refers to that sub-array. The general form of all supported telescope specifications is: [sub-array-number:]telescope_name_a[-telescope_name_b[-telescope_name_c]] Elements of the specification that are enclosed in [] are optional. The actual number of telescope names expected depends upon whether a telescope, baseline or closure-triangle specification is required by the Difmap command. In each case, trailing telescope names can be omitted. Usually when this is done the first recorded telescopes in the sub-array that can be associated with the given telescope name(s), are substituted. Alternatively, commands that accept more than one type of telescope specification note whether a telescope, baseline or complete closure-triangle specification is given and act accordingly. Example telescope specifications are: 1. To specify an antenna called OVRO, in sub-array 1 the specification would be: 1:ovro 2. To specify the baseline formed by telescopes named OVRO and WSRT in sub-array 1, the specification would be: 1:ovro-wsrt 3. To specify the triangle formed by telescopes named OVRO, WSRT and BONN in sub-array 1, the specification would be: 1:ovro-wsrt-bonn Telescope names are matched using a case-insensitive min-match search of all telescopes in the given sub-array. Thus if sub-array 2 contained telescopes named BONN and BOLOGNA, then: 2:bon-bol would be sufficient to select baseline BONN-BOLOGNA, whereas: 2:bo-bol would evoke an error message. Note that the : separator after a sub-array specification is only taken to denote such a separator if it is preceded by an integer at the start of the specification. This makes it possible to select VLA antenna names, which themselves contain a colon. Thus, if the default sub-array is sub-array 1, then: 1:vla:e8 and vla:e8 are both equally acceptable specifications for telescope VLA:E8 in sub-array 1. LIMITATIONS ----------- At the moment all sub-arrays must all have been observed at the same frequency and bandwidth. This translates to the requirement that each sub-array be refered to by the same frequency ID in the input UV FITS file. If this restriction proves to be a problem then I will endeavour to impliment the more general case of allowing a separate FQID per sub-array. Since random-groups UV FITS files also impose the restriction that all visibilities have the same number of frequency channels, polarizations and IFs, removing the above restriction would not be as useful as it might be, so I have not implemented it. Sub-array specific commands and functions. ----------------------------------------- The command that provides the most general information on sub-arrays is the 'header' command. This also shows other information about the observation. Its limitation is that it is given in textual form, so it can not be used as an aid to writing scripts. The following functions can be used to obtain a few bits of information about sub-arrays: nsub - Returns the number of sub-arrays in the data-set. nbase - Returns the number of baselines in a given sub-array. ntel - Returns the number of telescopes in a given sub-array. basename - Returns the name of the n'th baseline in a given sub-array. telname - Returns the name of the n'th telescope in a giveb sub-array. Plot commands that display data for one sub-array at a time include: tplot - Plots the telescope versus time sampling of sub-arrays. vplot - Displays visibilities of sub-array baselines. cpplot - Display closure-phases of sub-array closure-triangles. corplot - Display self-calibration telescope corrections. In these commands, capital N and P can be used to move to the Next or previous Sub-array in order of sub-array index. The 'T' key can also be used to select a given plot via a telescope specification string. Plot commands that plot all sub-arrays at once, but allow one to highlight a given telescope, include: radplot - Display visibilities versus radial UV distance. projplot - Display visibilities versus projected UV distance. uvplot - Display visibility sampling of the UV plane. Each of these commands responds to the 'T' key to allow the user to request that a given telescope be highlighted, via a telescope selection string.