Scan Loops

Setting up a "scan loop" is done using the menu strip: FILE - CREATE NEW - NEW SCAN LOOP. It will show you a "Scan loop details" page in the main editing window; assign a descriptive name to it and specify the number of iterations of this loop. The tick-box for bracketed means to copy the first scan in the loop to the end of the loop, i.e., add another calibrator scan so that the last target scan is also bracketed between two calibrator scans when the first scan in the loop is on a calibrator source. The four tree setups of scans in the table of examples below are equivalent; they all enclose scans on a target with a scan on a calibrator source before and after each target scan, i.e., they all result in the sequence Cal - Target - Cal - Target - Cal.

No Loop	Normal Loop		Bracketed Loop
Individual Scans	two different orderings		should start with Cal

In the example on the right hand side (the most compact, bracketed loop) the double-star after the loop icon, in front of the number of iterations of this loop, indicates that this loop is a bracketed loop. To achieve bracketing of the target source(s) with scans on the calibrator source, the bracketed loop must begin with the calibrator source scan. Of course one is free in choosing any of the possible scheduling solutions; the resulting observing script is the same either way, but the scan listing summary will differ in compactness and clarity. Also, if there is a long slew to the sources in the loop, the best way to absorb the slew in the scan duration of the first calibrator scan is using "No Loop" or "Loop1", as otherwise this extra slew time is also added to the duration of the loop calibrator scan.

Note that you can NOT use the "No Change" resource option in the first scan of a loop! This is because when unfolding the loop, the "previous scan" in the first source of the loop can refer to the resource of the last scan outside the loop (first occurrence) as well as the last scan inside the loop (any following occurrences).

A loop can contain any number of sources, not necessarily only a calibrator scan and a single target scan. If your target sources are near in the sky and you can get away with a single calibrator for all of these targets you can group them in a loop with more than one, say four, target scans before returning to your calibrator. Keep in mind that the total loop time should be shorter than the anticipated coherence time at our observing frequency. Loops may also contain loops. If your loop is selected, adding a new scan will place this new scan in the scan loop. The only difference with a normal scan is that this scan will be scheduled as many times as the "Loop iterations" specified, consecutively in a loop with the other sources in the loop. When finished with defining a loop, you may want to highlight it and then collapse it (using from the icon menu) for a more compact display in the tree.