When you concatenate two or more observations you oblige them to be observed consecutively without an intermediate slew making observatory use more efficient. This is particularly important when requesting scan maps so that scan and cross-scan are taken together in time.
No slew overhead is applied to the second and subsequent AORs in the concatenation, saving normally 180s per AOR. However, occasionally, the time saving may be less than 180s.
This happens because HSpot allows the time taken to carry out instrument set-up and internal calibration to be included as part of the slew overhead. In other words, when the calibration time is less than the slew overhead that HSpot applies, the calibration time will not be added to the observation. If an internal calibration or additional set-up is required between the AORs of a concatenation, this time will be added to the observation.
Similarly, even on the first observation of a concatenation, it is possible that an extra overhead will be added beyond the standard 180s.
This may happen because, in some cases, the required calibration time may be longer than 180 seconds. If this is the case, HSpot will apply a slew overhead of 180s plus the excess calibration time over and above this slew time (e.g. if the calibration time is 210s and the slew overhead is 180s, HSpot will apply 180 + (210-180) = 210s as the total overhead).