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 and thus irreplaceable helium is not wasted on unnecessary overheads.
Herschel takes a certain amount of time to slew between targets. The median slew time has been found in the early phases of routine observing, as expected, to be of the order of three minutes (although this depends critically on the density of targets in the sky, which differs for different instruments), thus all unconstrained observations are 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”).
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 5s) to the measurement in each position. This value has been optimised in the light of in-flight experience and is now not likely to change further.
Scan maps have generally been more efficient and added less overhead to an observation than a raster map, although for a scan map the calculation of the overhead is uses a complex formula because several variables are involved. In this case the overhead is the acceleration at the start of a scan and the deceleration at the end of the scan, which will vary according to the length of the scan itself (for short scan legs the telescope will spend a much larger fraction of the time accelerating and decelerating). The telescope then makes a small slew to the start position for the return scan.
Although mini-scan maps look inefficient due to these overheads when compared to point source photometry, in fact they go significantly deeper in the same total time and produce much more accurate photometry (a mini-scan map typically takes less than half the time of an equivalent point-source photometry integration). However, mini-scan maps should only be used for point sources and small targets, as the area that they cover to the maximum depth is only about 1 arcminute in diameter.
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. Obviously, if two observations are concatenated and no slew is involved, the whole of the calibration block has to be charged against the observation; for this reason there may still be a small overhead on concatenated observations.
Constrained observations (see Section 6.4, “Constraints on observations”) limit the telescope scheduling and limit observing efficiency, producing what are effectively hidden overheads (e.g. the telescope is forced to slew to a point on the sky that would not be picked otherwise, making the scheduling less efficient), 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.