Reading and Writing Visibilities

The routines uvread and uvwrite are routines used to read and write the correlation data (and the associated flagging information). Here preamble is an array of four double precision elements, data is an array of n complex elements, whereas flags is an array of n logical values. Preamble, data and flags are output from uvread, whereas they are input to uvwrite. The four elements of preamble are the u coordinate, v coordinate (measured in nanoseconds), time (Julian date) and baseline number. The baseline number, $Bl$, is calculated as:

$$Bl = 256 A_1 + A_2$$

where $A_1$ and $A_2$ are the numbers of the first and second antennae respectively (antenna numbers vary from 1 to $N_{antenna}$). The array data is used to store the complex correlation data, whereas the logical values of the array flags indicate whether the corresponding correlation is deemed good or bad (true or false, respectively). For uvread, n limits the number of correlations that can be read; the actual number of correlations read is passed back as nread. Uvread can perform a number of additional processing steps - see the description of uvset (Section 2.5.11).

The flags of a data file can be modified using the uvflgwr subroutine. When called, the flags associated with the previous call to uvread and uvwrite are changed to those values given in the flags array. Using uvflgwr when reading a visibility file, is the method used to develop flagging tasks. Currently uvflgwr has the limitation that the linetype is either `channel' or `wide', and that the `start' and `width' linetype parameters are 1 (see Section 2.5.11). Also uvflgwr aborts if no flagging file exists