Facilities > VLA > Documentation > Manuals > OPT > Using the Observation Preparation Tool (OPT) > Building Your Scheduling Block Scan List

Building Your Scheduling Block Scan List

by Gustaaf Van Moorsel last modified Mar 16, 2017 by Lorant Sjouwerman

The idea is to define a sequence of scans in the left hand side column, each with a source, a resource, an observing mode, a time interval and some reason (intent). Each time a scan is added you need to specify these items. However, it is not always straightforward to assemble this scan list in the sequence you want the first time around, and you will need to move scans around. This is easily done! That is, there is no need to panic if you make scans (a bit) out of order; it is almost straightforward to add, e.g., an extra bandpass calibration scan, to move some scans to the middle of the observation, or to redefine source loops after the main framework of your schedule is set up.


The First Scan

Create a first scan if necessary: FILE - CREATE NEW - SCAN. Select your first scan (click on "[New Scan]" next to the telescope icon and "STD" in the left hand side column); it contains default parameters such as a scan mode "Standard Observing" for 5 minutes "Duration (LST)". The number of tabs at the top depends on the scan mode. Current scan modes are "Standard Observing" (tracking a sidereal position in the sky), "Interferometric Pointing" (for improving telescope pointing), "Tipping" (for measuring opacity curves; not currently in use), "Holography" (for measuring antenna response; internal NRAO use only), and "On The Fly Mosaicking" (for taking data in a constant slew that is different from compensating for Earth rotation). Each mode has a different code that shows next to the telescope icon: STD, IP, TIP, H and OTFM respectively. Next we will describe "Standard Observing" (STD). IP and TIP modes will be described further below; OTFM mode is described in Chapter 5. Be sure to also check the requirements for setup scans and intents as described in the attenuator section of the Guide to Observing with the VLA.

Selecting Standard Observing (STD) for "scan mode" displays two tabs: "Overview" and "Comments". In "Overview" you set up the actual scan (Figure 4.4), whereas in "Comments" you may enter anything specific for this scan for your own reference.

Within the "Overview" tab two tables are displayed. In the first, you name your scan. Note that the scan name is just for the scheduling display in this tree (and in the summary); it is the source name specified in the SCT catalog that ends up in your data. It is followed by scan mode ("Standard Observing"), the antenna wrap, whether or not you want to apply the solution from a previous pointing scan, and whether observing "over the top" is acceptable (most likely not!). Also the "Phase & Delay Cal" and "Record On Mark V" are for specialized scans (VLBI and Pulsar observations) and should not be used (i.e., should be left unchecked) for standard observing! The antenna wrap and reference pointing are described further below.

The second table contains the actual target source, the hardware setup (with Doppler tracking settings), scan timing and intents with this scan. Each of these fields must be completed, and an error would result if any of these fields is unspecified.


Target Source

The "target source" column either shows you the name of the target source (i.e. telescope pointing direction) or tells you that no source is assigned. A source needs to be specified and if it is not the one you want, press the "Import" button. This brings up a dialog box to interact with the source catalogs that are in your SCT database. Select the source catalog and the group within that catalog from which you want to extract a predefined source. Simply tick the source name - you may have to scroll down your list to find the desired one. Note that you cannot define sources "on the fly"; only sources specified previously in a source catalog in the SCT can be selected. You may need to switch to the SCT if you desire to observe a source that was not previously defined and do so at this time.

Figure 4.4: Changing the resource in a scan overview/details page.


As you will be doing this changing of sources potentially for every scan, you probably see that it might be useful to collect all sources that you want to use in this SB in a single catalog (group), i.e., with your target sources but also with your calibrator and tipping sources from, e.g., the "VLA" list. Otherwise you will be switching back and forth and scrolling up and down a lot.


Hardware Setup and Doppler Tracking

The "hardware setup" column is very similar; it shows the hardware setup selected if a resource was previously assigned. Click "Keep Previous Conf." to select the exact resource setting of the preceding scan (it must be defined for that preceding scan of course). Click "Import" to get a familiar dialog box to select your resource catalog, resource group and resource from (only) the predefined resources in the RCT. Resources cannot be defined "on the fly". Also here it is useful to specify all hardware resources (and pointing scan setups) in this SB in a single resource catalog (group), but because resource catalogs typically are not as extensive as source catalogs it is less of a hassle if you don't.

Spectral line resources that were set up with a rest frequency instead of a fixed sky frequency have to be specified with an option for the Doppler Setting in the RCT! The recalculated sky frequencies for the LST starting time and LST starting date of the SB as specified in the Reports page will show in the scan listing mentioned further down.


Scan Timing

The scan timing determines the length of the scan, either in LST (sidereal) or in UT. The difference is about ten seconds in an hour. You must keep your schedule in LST duration when observing with the VLA only. Options are to set the exact time the scan has to end ("Stop Time", only useful for fixed date schedules), the total (maximum) time the scan may take from the end of the previous scan including telescope slewing time ("Duration (LST)"), or the time the telescopes should track the source regardless of telescope slewing time ("On Source (LST)"). Dynamically allocated observing time schedules must be in "Duration (LST)". It is possible to create schedules in On Source time to investigate slew times between sources, etc., but the SB must be converted to LST duration prior to submitting. This is done best by picking a sensible LST start time in the Reports page, clicking "Update" to recalculate the slew times and to make use of Edit-To Durations selection from the menu at the top.


Scan Intents

In the intents you should indicate the intent of the scan. By default it is set to "Observe Target" (for "Standard Observing"), but you can add more than one intent to it. For example for your phase calibrator you would tick "Complex Gain Calibration", for 3C286 you would choose "Flux Calibration" and for any suitable source you intend to use for bandpass calibration you would select "Bandpass Calibration". The most common options are shown, and the more specialized options that you probably would not want to use are hidden behind the "More" button. More than one intent may be ticked, and will be useful in particular for the automated calibration pipeline. Note that if you leave the intent to the default (Observe Target), you will not have calibrator codes with your data which may complicate your data reduction; in particular it will prevent the automated pipeline from processing your data. On the other hand, the current practice of having to schedule "dummy" scans that take care of setting up the hardware (frequency tuning, local oscillators, attenuators, etc), usually do not contain useful scientific data. We now have an intent labeled "Setup Intent" that should be chosen for such scans so that the pipeline knows to skip this data during the data reduction.


Comments Tab

Write anything you would like here; it is a comments and notes field for your own reference.

Subsequent Scans

There are a few ways to add extra scans. A blank-slate scan can be obtained using the menu strip: FILE - CREATE NEW - SCAN. The options are to place it before or after a selected scan, or in a selected loop.

Another way to obtain a new scan is by using the icon menu. It has several icons dealing with creating scans. Using the icons for copy and paste, a new scan can be created from a previously created scan, and be pasted at any position in the scan tree after selecting (highlighting) the scan it has to follow or precede in the tree. The same can be achieved using the icon with the little green dot (\includegraphics[height=3mm]{psimg/page_add.png.eps}), which will paste a new scan directly after the scan that is copied. Useful when building your scans sequentially.

You probably want to change your source of the scan if you place the new scan directly after the previous one (otherwise it is the same scan). Please take an effort to fill out the correct scan intent for each scan.


Most observers would want to include calibration scans next to their target source scans. Almost always you would schedule one or two scans on a flux density scale ("amplitude" or "primary") calibrator (e.g., 3C286, J1331+3030) somewhere in the schedule where it is convenient. Spectral line observers would also include one or more scans on a bandpass calibrator if the flux density calibrator is not suited for this (if it is, please select both flux density scale and bandpass calibration as "Intents" for this calibrator source).

The target source position scan is typically sandwiched between complex gain ("phase" or "secondary") calibrator scans in order to interpolate the phase changes between the beginning and end of the target scan. If observing more than one frequency setting (resource) and switching between them within an SB, however, there is no guarantee that returning to the same resource the phase from a previous scan using that resource is preserved. That is, using more than a single resource regardless of the resources are within a single receiver band or not, might cause phase jumps when interpolating between the scans. Therefore, it is extremely important to bracket the target with complex gain calibrator scans that use the same frequency setting as the target to avoid experiencing any phase jumps.

Because of the short coherence times at high frequencies and/or longer baselines and therefore the many calibrator-target source scan switches necessary, it is customary to do this using a scan loop between your calibrator and target sources to increase integration time beyond the coherence time.

Scan Loops

Setting up a "scan loop" is done using the menu strip: FILE - CREATE 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 LoopNormal LoopBracketed Loop
Individual Scans two different orderings should start with Cal
No Loop Loop 1 Loop 2 Bracketed Loop


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. Note that the first scan of a loop must have its own Resource; your SB will not validate if you attempt to set "Keep Previous Conf." in the Resource setting for the first scan of a scan loop.

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 \includegraphics[height=3mm]{psimg/collapseall.png.ps} from the icon menu) for a more compact display in the tree.