Appendix: Scheduling Blocks for Dynamic Time
Considerations for Dynamic Time
Introduction
The process of preparing SBs for dynamic time is described in the general OPT chapters in this document. Much of the previously required information specific to dynamic SBs (in the VLA era) is now more streamlined and can be entered in the SB Information tab. Other information, such as the total length of the SB, is automatically generated. Below are some extra hints you might want to consider while entering this information and creating your SBs for dynamic time.
Logistical Considerations
This section describes some possible pit-falls that you may encounter preparing SBs for dynamic scheduling which may not have been apparent when preparing SBs for fixed-date scheduling.
Slew Time:
More than before, the direction in which the antennas are pointing at the start of an observation is unknown. It may take some time for the antennas to reach the source in the first scan. The antennas slew with a rate of about 20 degrees per minute in elevation and 40 degrees per minute in azimuth. That is, in a bad case your regular observing (taking data) may not start for the first five or so minutes, and it can be longer if your sources are in the north (see antenna wraps below). Do not rely on being able to use this time and therefore do not put critical scans (flux density or bandpass calibration, pointing scans) in this part of your schedule.
Furthermore, during your schedule you will be moving from one source to another. This will take different amounts of time depending on the orientation of your sources in the sky; it changes as function of LST time. Remember that you must use LST durations in your schedule, which means that you will have different on-source (dwell) times for different LST. Include slew times in your durations, especially when you move to a distant source. If you had used on-source time in your scans you would have seen large changes in total time due to this effect, and therefore scan times in LST durations are required.
To determine the change in slew and on-source times as function of LST, you have the option to play around with LST start time for the SB. Use the tab for the SB summary to enter different start LST times from the range you chose in the SB details page. Carefully examine the slew and on-source times in the third (scan) table to make sure you understand your options.
Antenna Wraps:
Similarly, you may encounter the unfortunate occasion where the previous observation left the antennas pointing toward a position where moving to your first source using the shortest slew is not possible. The antennas will have to take the long way in order to unwrap their cables, thereby preventing them from being snapped. This may take up to about 10 minutes if any of your sources are north of ~34 degreesA.1 Declination, which you do not want to have happen during your critical scans. Note that this is not limited to the start of your observation; it may also happen during your own schedule, typically when you are observing sources on either side of a Declination of ~34 degrees. Careful ordering of your sources, or changing your phase calibrator so it is on the same side of 34 degrees Declination will prevent this.
Also here, play around with the LST start time on the SB summary page to get a feel for how to deal with this. You can anticipate and force the slewing to go the other way by using the "Wrap" options on the scan page to limit the impact on your observing, where sometimes it is necessary to shorten the possible LST range in which your schedule may be started. Carefully examine the slew times and wrap modifiers ("CW", "CCW") in the third (scan) table. Use the wrap diagram on the SB page to help you out.
Flux Density Calibrators:
When planning the flux density calibration, be aware of the placing of your flux density calibration scan. Dynamic scheduling may shift your schedule such that your calibrator is below the elevation limit. Also, it may force you to decide on a skewed LST start range other than would be typical for your sources of interest. An example illustrates this: if your sources are observable say from LST 15h to LST 23h and you plan an SB of three hours length, typically your options are:
- start LST between 15 and 19h, where you plan a scan on 3C286 in the first half hour (3C286 sets at 19h30), or:
- start LST between 17h and 23h, where you plan a scan on 3C48 in the last half hour (3C48 rises at 19h30).
Clearly the first option gives you a shorter LST start range and therefore a stricter initial constraint on your SB for being picked, but perhaps may be preferred if the pressure of other projects in the range between 15h and 19h LST is much less than the pressure between LST 17h and 23h (see below).
Tactical Considerations
This section describes possible tweaks that you may apply to increase the chances that your SB will be selected for observation.
Submit early:
If your priority is one of the less favorable ones (i.e, one of the higher numbers), consider creating, checking and submitting your SBs as soon as possible. You may be able to jump the queue by filling a slot before the higher priority SBs are available in the queue. This is more important if the pressure on the required LST range is high, and your priority "color" is near or above the black line in the pressure histogram for your LST range (Figure A.1).
Short SBs:
Highly ranked proposals will typically observe near their optimum LST ranges, and subsequently highly ranked proposals will leave small gaps of observing time for other proposals. The shorter your SB, the more likely it will fit in any of these gaps. Of course, slew time and calibration necessary for many shorter SBs to accumulate observing time on your targets make short SBs less efficient than long observing blocks. Scheduling your total allocation in short SBs will yield less time on source.
LST range:
As mentioned above, perhaps your priority may be such that it would be below the black line for a nearby LST range. If your scientific objectives can be met by observing in a shifted range, toward these less heavily subscribed LST ranges, it may be beneficial to your SB to specify this (extended) range in your starting conditions. Be aware of elevation and slewing effects when considering this option.
Wind:
When the absolute flux of sources is not your primary objective, you possibly wouldn't be upset if occasionally an antenna is not exactly pointed toward your source. In this case you could consider relaxing the wind limit. Your SB can then be picked while others would have to wait for more favorable wind conditions.
Atmospheric phase:
Similarly, when you expect to be able to self-calibrate your sources, you could consider observing in less phase-stable weather conditions by relaxing the atmospheric phase limits. Self-calibration would recover most of the problematic phases. However, realize that the current defaults have been chosen in order to be able to perform well enough for standard observations, and decorrelation may complicate calibration.
Move periods:
There is less pressure on dynamic time during the move periods between configurations. If your scientific objectives can be met with a less regular configuration, some missing antennas and a bit more careful data reduction, then you could ask schedsoc@nrao.edu to shift a portion of your dynamic allocation from a principal or hybrid configuration to a move period between these configurations.
Dynamic Observing Time
All proposers will receive an email with the disposition of their proposal about three months after the proposal deadline. General information from the EVLA/VLBA Proposal Selection Committee (PSC) can be found at http://www.aoc.nrao.edu/~schedsoc/covermemo.shtml . If your application for observing time has been approved by the PSC, the PSC communicates this to you at the top of the disposition, followed by the individual referee reports. You might read something like:
Proposal status (January 1, 2010):
Approved for dynamic scheduling at priority 5.
This line includes at least two important (obvious) pieces of information:
- Your application is approved for dynamic scheduling.
- Your application is ranked in the approved-proposal pool at priority "5".
Following you will see the breakdown in observing sessions, something like:
Current time allocation:
Up to 5.0 hours dynamically scheduled in D configuration.
Time requested:
1 times 2.5 hrs in D config centered at 6.0
1 times 2.5 hrs in D config centered at 21.5
That is, you are expected to submit SBs that are formatted to be selected for observing without any modifications only moments before the actual observation takes place. In the particular example above, you would submit SBs with an LST range roughly centered on your target sources (which are around 6h and 21.5h), adding up to 5 hours of observations in the D configuration. The breakdown may be more complex, more specific for LST ranges, than the above example. The default treatment of SBs in the OPT is to assume dynamic scheduling (the other possibility is fixed-date scheduling). For dynamic scheduling, the SBs must be in LST durations with the total time of each SB being an integer number of 30 minutes (or slightly less) in LST. Longer blocks will be cut short during observations, meaning that you may loose the last part of your schedule without any control (which would be unwise). Some hints specific to dynamic scheduling are given below, and more about the actual scheduling can be found in the OPT manual part of this document.
You are competing with other proposals for the available dynamic time in a trimester. You now have a rough indication of your proposal's priority compared with those other proposals. To gauge the pressure on dynamic time as a function of LST, consult the pressure histograms for the trimester at http://science.nrao.edu/evla/sched/schedsoc.shtml . Figure A.1 shows an example for the D configuration that starts on 2010 March 1 and ends on May 24. The time available per LST hour is shown by the thick black line. Maintenance, software and approved fixed-date proposals cause the black line to be less than the total number of LST days in the configuration and also to vary with LST. The figure shows that the D configuration is oversubscribed for LSTs ranging between 3h and 19h. For example, a SB for a priority 9 proposal is very unlikely to be observed in this LST range. But if that SB could also be observed at, say, an LST of 20h, you can increase the likelihood of it getting selected by allowing this option in the SB details.
Figure A.1: Pressure on the dynamic time for the D configuration (2010 March 1 to May 24). Color codes show the priorities of proposals approved for dynamic scheduling, but excluding triggered transient proposals that have targets at unknown sky locations. Priority 9 proposals have the lowest probability of being observed.
SBs for Dynamic Time and Doppler Tracking
Currently Doppler tracking and Doppler setting is not supported in Array Operations. We thus have disabled Doppler tuning in the OPT. Use sky frequencies and cancel/resubmit new SBs with updated sky frequencies if the change of frequency due to the movement of the earth requires this. Note that for some experiments, this may be on time scales as short as a week (or even worse) to ensure that the frequency/velocity span of our line is located fully in your observed bandwidth. Ask the staff (through the helpdesk) if you need help. You can calculate your sky frequencies using the Online Dopset Tool at http://www.vla.nrao.edu/astro/guides/dopset/ .
We are currently working on a solution to avoid having to redefine the frequency setup every now and then. Please check again every few weeks as this section is the place where we will keep you informed about this.
Introduction
This appendix does not directly relate to the OPT but does expect you are familiar with it and with its jargon.
Starting with the WIDAR observations in the spring of 2010, the majority of observing with the EVLA will be done by dynamically allocating observing time to the most appropriate SB in the observing queue. Here "most appropriate" typically depends on scientific priority, weather conditions and state of the array. This appendix describes some considerations and choices that might increase your chances to get more of your SBs observed successfully. This assumes that your scientific objectives can be met with a flexible approach while competing with other projects for the LST range to be observed.
Important note: for line observations currently (as of March 2010) there are restrictions on Doppler tracking. See section A.3.
Submitting SBs for Dynamic Time
There is no deadline for submitting your SBs, perhaps other than the end of the array configuration in the current observing trimester for which your proposal was approved. How to submit your SB can be found in the OPT part of this document. Generally speaking, it is to your benefit to submit your schedule sooner rather than later, especially if your priority is not one of the highest ones. However, do not submit SBs which you know are wrong. You can seek help from your collaborators, use the NRAO helpdesk to obtain help from the NRAO staff, or use the help buttons in the OPT (SB submission, third tab). Also therefore, please plan your scheduling to allow for ample time before expecting your SB to be run on the telescope (please also plan similarly for fixed-date observations).
Note that, unlike previous practice, when your SB has been successfully submitted, there is no confirmation email sent to you. You can check the status of your SBs through the PB page of your project. For the first month(s), your SB will initially be put on "Hold" temporarily in order for NRAO staff to check the WIDAR setup before releasing it to the queue.
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