Observation overheads

Overheads are the additional time, on top of the integration time on target, required for calibrator observations and antenna slewing. The latter cannot be easily estimated, so it is advised that the proposed observation be set up and simulated in the Observation Planning Tool (OPT) if the proposed observation has multiple science targets. Setup time at the start of an observation is minimal and can be disregarded in observation requests.

The overhead percentage is defined as follows:

We have found that the typical overhead is 25% for single-target tracks following the standard calibration scheme, if the tracks are longer than 1.5 hours.

Efficiency increases for full tracks of ~9 hours (down to about 18%), but longer tracks lead to less efficient scheduling; hence we generally request users to split their tracks in half (with a rising and setting block in order to increase uv coverage) to facilitate scheduling.

Extremely short tracks, typical for monitoring of bright transients, can incur over 100% overhead due to the time required on the bandpass/flux calibrator. We also recommend that observations with total duration less than ~3 hours be bracketted with two scans of 5 minutes each on the bandpass/flux calibrator (which can be combined if no significant change is noticed in individual scans). Below we show examples of observations of increasing durations, with a single imaging target.

For multi-target observations we recommend bracketing each target scan with a gaincal scan (in order to always have two points between which the gain response can be interpolated), unless sources can be grouped together to use the same gain calibrator. The overheads for such observations should be simulated in the OPT. Below we show examples of a multi-target monitoring block, and a snapshot-like survey of a large number of targets.