Considerations for Dynamic Time

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.

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 degrees^A.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:

1. start LST between 15 and 19h, where you plan a scan on 3C286 in the first half hour (3C286 sets at 19h30), or:
2. 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).

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.