The topic of controlling scan timing is discussed in detail in the Scan Times section.
DURation specifies the length of the current scan. Usually this is used instead of STOP. It is assumed to be a UT time interval unless LST was specified, in which case it was assumed to be an LST time interval. DWELL is a very similar parameter except that, with DURation, the scan starts GAP seconds (perhaps adjusted by the old parameter PRESCAN after the previous stop time while with DWELL, the scan will start at least GAP seconds after the previous stop time, but, if necessary, will wait longer to allow all antennas to reach the source.
Note that DUR and DWELL may be both be used in the same schedule, but cannot both be used for the same scan (that would not make sense). If either is used, the time specified overrides any previous specification of the scan length made with either parameter.
Once the nominal scan start time has been specified as above it can be adjusted further with the old, and mostly obsolete, parameter PRESCAN. Then the actual time that recording starts is adjusted further with MINPAUSE and PRESTART. PRESTART, MINPAUSE, and PRESCAN allow the user to start recording early to give the correlator a chance to synchronize before the start of good data. PRESTART can also be used to delay the scan start to be more sure it starts on good data -- mostly useful with DWELL. These parameters also can help prevent short stoppages which used to cause problems with playback with tape systems. Adjusting the media start time should no longer be needed for correlators and recording systems currently in use (eg MARK5 and DiFX). See the descriptions of the parameters mentioned for more information. The SCHED defaults are generally reasonable so, if you are not an experienced user who wants to exercise fine control of recording management, don't worry about these parameters.
The start and stop times reported in the summary and operator schedule files are the same for all stations. They take into account any adjustments made as a result of specifying PRESCAN, but do not take into account adjustments requested using MINPAUSE and PRESTART, since those can be station dependent. The time the media is started is reported in the sch.xx files under the headnig ''TPStart''.
With the VLBA RDBE recording system and DiFX correlators, both recording and correlation start times ignore most of the efforts by SCHED to adjust recorder start times. They treat the data-good time as given as an offset from the start time in the $SCHED section of the VEX file as the time to start handling data. In the $SCHED section, there can be two start times, one commented out (line starts with ``*''). The commented one is the nominal start time as seen in the SCHED summary file. The active one is what SCHED thinks is the media start time, usually the nominal start time minus PRESTART. Then on each station line, there are two offset times, one for the start and one for the stop. The start is greater of zero and the time when SCHED thinks the data will be good (on-source etc). That is what is used by the VLBA to start recording and DiFX to start correlation. So effectively, even when using duration scheduling, you are fairly likely to actually be using dwell scheduling.
See the Scan Times section for more information on the specification of scans.