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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 scheduingscheduling.
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.
Time on target | Bpcals | Overhead | Total time |
|---|---|---|---|
900 s (15 min) | 2 x 300 s | 110 %110% | 1889 s (32 mmin) |
1800 s (30 mmin) | 2 x 300 s | 55 %55% | 2790 s (47 mmin) |
3600 s (1 hr) | 2 x 300 s | 32 %32% | 4740s 4740 s (79 mmin) |
5400 s (1.5 hr) | 2 x 300 s | 24 %24% | 6691 s (1.6 hr) |
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.
Type of observation | Total time on targets | Calibration strategy | Overhead | Total time |
|---|---|---|---|---|
3 targets, 900 s on each. Each with own gaincal. | 2708 s (45 mmin) | bp cal (300s) at start and end of block. Gaincal bracketing each obsobservation | 50 %49% | 4029 s (67 mmin) |
Multiple (22) targets in pointings - 180 s on each, repeated to increase uv coverage | 19800 s (5.5 hr) 900s 900 s per target | bp cal 600 s each every 3 hoursgain calibrator . Gaincal cadences varied | 48 %48% | 29255 s (8.1 hrshr) |