Each observation that is made with Herschel implies certain overheads. These are detailed in the time estimation breakdown and are charged against the observation. The onus is thus on the observer to make observations as efficient as possible so that precious observing time is not wasted on unnecessary overheads.
Herschel takes a certain amount of time to slew between targets. The median slew time is expected to be of the order of three minutes (although this will depend critically on the density of targets in the sky), thus all unconstrained observations will be charged 180s as observatory overhead for slewing the telescope (for constrained observations a 600s slew overhead is applied - see Section 6.6.4, “Constrained observations”). It is possible that at a later date the 180s median slew overhead will change as the observing database is filled and knowledge of source distribution on the sky becomes better. For concatenated observations on the same target a zero telescope slew overhead is applied.
When making maps there are certain overheads implicit in the process.
In a raster map the telescope must make a slew, stop and wait for the pointing to be stabilised. Due to the satellite's large moment of inertia the process of acceleration, deceleration and stabilisation adds a significant dead time (of the order of 15s) to the measurement in each position.
Scan maps are generally more efficient and add less overhead to an observation than a raster map. In this case the overhead is the acceleration at the start of a scan and the deceleration at the end of the scan. The telescope then makes a small slew to the start position for the return scan.
Each observation requires an internal calibration against black body sources maintained at rigidly controlled temperature. These measurements are essential to the health and success of all observations and are thus charged against the observation. The calibration time is typically in the range 30-300s according to the AOT used.
If the calibration time is less than the slew overhead, it is not charged to the user as an overhead as the calibration is carried out in its entireity during the slew; when this calibration time excedes the slew overhead that has been applied, the excess is charged as an overhead to the astronomer.
Constrained observations (see Section 6.4, “Constraints on observations”) limit the telescope scheduling and limit observing efficiency producing hidden overheads, thus a flat rate of 600s will be charged on all constrained observations in addition to other observational overheads.
If a constrained observation is concatenated, the 600s overhead is applied only to the first observation.
For a fuller definition of what constitutes a constrained observation that will be charged a 600s overhead, please see the (Policies and procedures) document.