Scheduling Considerations

VLBA Scheduling

The VLBA schedules observations in 2 ways: dynamic and fixed date. By default, observations are scheduled dynamically. If necessary, observations are scheduled on a specific date and time. Some examples of fixed date observations are coordinated observations with the VLBA and other non-radio telescopes (HST, Chandra, Gemini, etc.), high sensitivity array (HSA) observations, Global VLBI observations that are coordinated with the EVN and other radio telescopes, and resident shared risk observations that must be observed while the observers are present at NRAO.

Dynamic Scheduling

Most schedules are observed dynamically, which means the VLBA staff decide which schedules to observe based on the projects’ priority, the available time on the telescope, and the current/forecast conditions. For additional information, consult the VLBA Dynamic Scheduling Guide.

Dynamic scheduling means that observers may not know exactly when their observations will occur. However, information about upcoming observations is available in the VLBA schedule for the current month.

Fixed Date Scheduling

If a project requires observations to be made at a specific time or in coordination with other telescopes, the observations will be scheduled as “fixed date”. To qualify for fixed date observing, the proposal must have requested and justified the need for observations at specific times and the proposal must have received a high priority from the TAC. HSA, Global VLBI, and GMVA observations are nearly always scheduled as fixed date observations due to the necessity of coordinating with other observatories.

The observers will need to work with the VLBA scheduling officer and operators to determine the date and time for the observation(s). Because the observation(s) will be planned weeks to months in advance, they may be impacted by less-than-optimal weather conditions. Unless there is some flexibility in the schedules, they will be observed at the predetermined time regardless any other factors.

Doppler Correction

For some spectral line setups, Doppler correction (position and velocity) should be applied. To turn on the Doppler correction in SCHED, the user needs to invoke the DOPPLER setting before the first scan. The benefit of using DOPPLER, since the VLBA is dynamically scheduled, is that it calculates the sky frequency at the beginning of an observation and keeps this fixed for the duration of the observation. The Doppler correction can be turned on and off in a SCHED keyin file using the DOPPLER and NODOP parameters. For more details, refer to the Spectral Line Observations section of the SCHED user’s manual.

Avoiding the Sun

Observations of objects close to the Sun can have extra difficulties in calibration due to increased phase fluctuations and elevated system temperatures. Unless a project’s science goals specifically require pointing at an object near the sun (e.g., testing the effects of General Relativity on the astrometric position of a background quasar), it is strongly recommended that observers avoid pointing the telescope near the Sun. The lower the observing frequency, the further from the Sun the telescope should point. The following table contains the recommended minimum angular distance from the Sun to avoid scattering issues for select observing frequencies:

These limits are on the conservative side and apply mainly to the active Sun (i.e., around solar maximum). Observers should consult space weather monitoring sites such as the Space Weather Prediction Center and SpaceWeather.com for information about current solar activity and forecasts. The VLBA schedulers will not take the solar constraints into consideration for dynamically scheduled observations. Observers who require strict solar constraints should clearly specify their desired limits in the Preferred Dynamic Constraints section of the keyin schedule file.

3mm Observations - Reference Pointing

Observations using the 3mm receiver (80 to 90 GHz) need to include reference pointing scans to ensure the antennas maintain accurate pointing. Pointing scans are typically done at 7mm (41 to 45 GHz), and often use SiO masers as targets. Pointing scans need to occur at least once every hour during the observation, or any time the antennas slew 25 degrees or more. The truly cautious observer will do the reference pointing scans once every 30 minutes, or after the antennas slew 20 degrees or more.

Additional information is available in the Reference Pointing section of the SCHED manual.

For an example of a SCHED .key file with referent pointing, see the 3mm section at the end of the hsaddc.key example file.

u-v Coverage and Target Elevation

Prior to building a SCHED keyin file, observers may wish to determine the optimal times to observe their target(s). The most straight-forward way to do this is to use the extremely simplified keyin file uvcov.key. Observers should change the sources in the uvcov.key file to their target sources, then run SCHED on the keyin file.

To obtain a simple plain-text summary of when each target is visible to each VLBA station, type

sched < uvcov.key

and look for the uvcov.sum file which will be created in the current working directory.

To plot the u-v coverage, times above a minimum elevation, and elevation vs. time for the target(s), type

sched
plot sch=uvcov.key

and SCHED will bring up the plotting GUI that will allow the user to create various figures. The plotting section of the SCHED User Manual has more details on producing these and other plots.

If an observation includes multiple science targets, scans on the targets should be interspersed with one another in order to obtain the best possible u-v coverage on each target.