VLBA correlator

  Reading data from the VLBA correlator is much simpler than the process described in § 3.1.1. The task FITLD is used. Data from the VLBA correlator comes on DAT tape (by default) in several FITS format files. A CL table will be created when these files are read in. The size of this table is controlled by the adverb CLINT, which sets the time interval in this table. As discussed in § 3.1.1, a 1 minute interval is usually sufficient. Because of the size of the files, which are much larger than files out of the MarkIII correlator (usually 16 channels per IF instead of 4 for the MarkIII data and 2 second averages rather than 4 or more seconds for the MarkIII), the data must be written in compressed format (DOUVCOMP=1). The most important adverb in FITLD is DIGICOR, which applies corrections to the data as it is read in to compensate for losses arising from the correlation of digitized signals rather than analog. This is done by setting DIGICOR=1. As many tape files as possible should be concatenated into one disk file, to make book-keeping easier.

When reading data that are all at one frequency, the remaining adverbs can be set to default values. For multi-frequency data, additional steps must be taken. In principle, ${\cal AIPS}$ can process multiple frequencies in the same file, each frequency labeled by a unique FREQID which can be selected in most tasks. However, for book-keeping reasons, each frequency should be saved in a separate file. This can be done in two ways. Adverbs SELBAND and SELFREQ can be specified to choose each frequency separately. This necessitates running FITLD multiple times, one for each frequency. Since digital corrections require computations for every visibility, every channel, and every time stamp, this can result in a lot of extra time reading the data. If one has plenty of disk space, the other alternative is to run FITLD once, saving all frequencies in the same file. Then run the task UVCOP to copy out each FREQID into a separate file. This saves CPU time since FITLD is only run once, but requires a lot of extra disk space since the result is effectively two copies of the same data on disk at the same time.

Before proceeding, the flagging table provided by the online monitoring system at the VLBA stations should be read with the task UVFLG. Little editing of the text file should be required before running this task.