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Guide to Observing with the VLBA (and HSA/Global VLBI)

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This document provides an overview of observing with the VLBA, HSA, and Global VLBI. For questions regarding this Guide, please submit helpdesk tickets under the VLBA General Queries section. For comprehensive reference material, please consult the VLBA Observational Status Summary (OSS).

1. Disposition Letter and Scheduling Constraints

Interpreting the Disposition Letter

Once the proposal review process is complete, a disposition letter is sent to the principal investigator and co-investigators of each VLBA proposal. This letter contains comments from the cognizant Science Review Panel (SRP), a linear-rank score from the SRP, comments from the NRAO Technical Reviewer and, optionally, comments from the Time Allocation Committee (TAC). Guided by the linear-rank score and taking into account the time available, the TAC assigns a scheduling priority to each session in the proposal. If time is allocated at a scheduling priority of A, B, or C, the proposal is converted to a project which is eligible to compete for time in the dynamic queue.

The VLBA disposition letter is arranged in six portions:

Synopsis This portion lists the proposal id, title, type, authors, cognizant Science Review Panel (SRP), and time allocation summary. The latter tells the proposers whether or not time was allocated to the proposal. If time was allocated, its scheduling priority (A, B, or C) is given and the proposers are responsible for preparing schedules for that time.

Science Review This portion gives the linear-rank score from the SRP, along with comments from the SRP that summarize the proposal, give its strengths and weaknesses and, optionally, note any technical issues. The community-based SRP consists of a chair and five anonymous members. The linear-rank score is on a scale from 0 (a high-ranked proposal) to 10 (a low-ranked proposal).

Comments from the TAC The TAC consists of the chairs of the eight SRPs. Taking into account the time available as a function of Greenwich Sidereal Time (GST), the TAC assigns a scheduling priority to each session in each proposal. The TAC might comment on these assignments. The assigned scheduling priority depends on the linear-rank score of the proposal, the GST ranges involved in the session, the total time requested in the session, and the competition from better-ranked proposals requesting time at similar GST ranges.

Comments from the NRAO These describe the possible scheduling priorities, which are:

  • A = the observations will almost certainly be scheduled

  • B = the observations will be scheduled on a best effort basis

  • C = the observations will be scheduled as filler

  • N*= the observations will not be scheduled because they were explicitly rejected by the TAC

  • N = the observations will not be scheduled because they could not fit in the time available

  • H = not assigned because the proposal is being held for consideration at a future TAC meeting

Time Allocation For each session in the proposal, a table lists the session's name, time in hours, GST range, and scheduling priority. For a given observing semester, time approved at a scheduling priority of A, B, or C may be divided into multiple schedules as appropriate. Factors to consider in this division include the scheduling priorities, the observing frequencies, the tabulated GST ranges for the sessions, and the GST pressure plot at the start of the observing semester.

Further information for proposers and observers, including statistics and important GST pressure plots, is available from the most recent TAC report that is referenced in the disposition letter.

Comments from the NRAO Technical Reviewer Comments concerning the Technical Justification portion of the proposal. These comments are available to the SRP and the TAC.

Scheduling Constraints

Roughly 50% of the operational time on the VLBA is available for “open skies” observing. The other 50% is allocated for use by the US Naval Observatory (USNO). However, the amount of time dedicated to USNO observations varies from month to month.

The VLBA makes daily observations to measure the Earth orientation parameter UT1-UTC for the USNO. These observations are centered between 1730 and 1930 UT, are at most 2 hours long, and currently use only two antennas (usually MK and PT). The UT1-UTc observation may temporarily remove the necessary antennas from an ongoing science observation. The VLBA Schedulers always attempt to minimize the impact of these daily observations on science projects.

Interpreting the VLBA Pressure Plot

The figure below is an example of a VLBA pressure plot, which shows the number of VLBA observing sessions requested for each 1-hour bin in GST. The black line with squares is 52% of the wall-clock time available (after removing time for maintenance and sponsored time). The approximate Pie Town (PT) Local Sidereal Time (LST) is shown on the top horizontal axis.

The colors represent the scheduling priority assigned by the TAC, with the following meaning:

Green = pre-committed time (time allocated via partner observatories such as HST, Chandra, and Global VLBI; time already allocated from previous proposals requesting observations across multiple semesters)
Dark Blue = priority A projects (will almost certainly be observed)
Light Blue = priority B projects (will be scheduled on a best effort basis)
Yellow = priority C projects (will be scheduled as filler)
Red = priority N/N* projects (will not be scheduled)

It is obvious that some GST ranges are heavily over-subscribed, while others have relatively low pressure.

Priority and Schedule Duration

The duration of an individual schedule cannot exceed the time allocated to the project. Beyond this basic fact, the optimal duration of a schedule depends on the scheduling priority (A, B, or C) assigned to the project. Observers are encouraged to inspect the current month’s VLBA schedule, which is posted on the Schedsoc Home Page, for information about how much VLBA time is available in the near future.

Priority A: Dynamic scheduling enables priority A schedules to observe at the first available time. Schedules of any duration are fine. However, there may be other considerations in determining the best schedule duration. The nature of the observation may dictate specific times that are better for a project. For example, observations at high frequencies may require good weather conditions at certain stations.

Priority B: The amount of priority B time approved is designed to almost fit into the available hours per semester. However, if the science goals can be achieved with multiple short observations, it is often advantageous to submit schedules with durations of 6 hours or shorter. This allows these observations to be made in between longer, and possibly higher ranked, observations. Also, as noted above, keeping high frequency observations relatively short will make them easier to fit into times with good weather.

Priority C: These are filler projects. Short duration schedules (4 hours or shorter) are generally easier to slot in, but long schedules can be observed if there is sufficient open time on the telescope.

Tips for More Flexible Schedules

This section is most relevant for projects which have been assigned priority C, but they are also useful for priority B projects which have requested time during high-pressure GST ranges. Some of these tips may also be useful when preparing schedules for priority A projects, particularly those at high frequencies and tight weather constraints.

More flexibility will increase the chance that a particular schedule gets observed. However, if the conditions are too flexible or if the overhead becomes so large that the data cannot be calibrated or yields too little time or u-v coverage on the science target, the data set may not achieve the anticipated science goal. Always ensure that the data set can be calibrated. If there are options to consider, it may be better to choose the option that leans toward a more conservative calibration rather than the option that accrues more observing time on the science target. More conservative calibration will also ensure smoother data reduction.

Tips on how to increase the chance that a schedule will be observed:

  • Submit the schedule early. If the schedule is not available when a scheduling gap occurs, it cannot be selected for observation. Gaps can occur at any time, including at the planned start of the semester.

  • Submit a short schedule. Short gaps occur more often than long gaps. Make schedules short and repeat them as many times as desired to accumulate observing time.

  • Submit schedules with various durations. It is permissible to submit schedules that, in aggregate, exceed the total time allocated to a project. Use this to your advantage by submitting schedules with various durations. For example, if a scheduling gap of 4 hours is available then compete for it with an schedule of duration 4 hours instead of an schedule with a duration of 2 hours. That shorter schedule can be used to compete for a later scheduling gap of 2 hours. Also, as the pressure from priority A and B projects drops a few months into a semester, scheduling gaps can sometimes accommodate priority C schedules of relatively long duration.

  • Break up high and low frequency observations. There is more low frequency time than high frequency time, so it is usually harder to get observed if the weather limits are very low. If it does not matter if low and high frequencies are observed at the same time it is often advantageous to split observations into low and high frequency schedules.

  • Relax the weather limits for the schedule. For example, if the science target is strong enough for self-calibration then the project can be observed in less-than-optimal weather conditions.

  • Widen the possible start GST range(s) of the schedule. Especially at low frequencies, consider observing down to the elevation limit of the antennas.

  • Request fewer stations. If the science goal can be achieved with 6 or 7 stations rather than 8 or 9, that can help get a schedule observed when stations are down due to weather, maintenance, or other issues.

Schedule Life cycle

Project Creation

After the approval of a proposal, the observers are responsible for creating valid schedules to achieve their science goals. About one month prior to the start of an observing semester, NRAO will create the project associated with a successful proposal. The observers may then begin submitting schedules to be observed once the semester begins.

Schedule Creation and Submission

The creation of a schedule is the responsibility of the observer(s). All VLBA schedules must be submitted as SCHED keyin files. Observers should download and install the newest version of SCHED to avoid any issues that may arise from using a different version than the VLBA Data Analysts and Operators. Observers are encouraged to review the SCHED user manual before creating their schedules. NRAO staff are available to answer questions and help solve issues via the NRAO helpdesk.

Once a schedule is created, observers should submit the keyin via email to . The VLBA Data Analysts will verify that the keyin file can be used by SCHED to produce the necessary VLBA station control files. Verified keyin files will then be inserted into the queue for scheduling.

Schedule Observation

Schedules are observed according to priority, station availability, weather conditions, and other considerations. See the Dynamic Scheduling Guide or more information about how observations are scheduled.

Once a schedule has been observed, the observers will receive email notification and an operator’s log for that observation. Any issues with the observation should be noted in the log. If the observers are concerned about any log entries, they should contact the as soon as possible. If the observation is determined to be a failure, it will be re-observed.

Correlation and Data Archiving

The VLBA records observation data on disk packs at each station. The packs are kept at the station until they are full, and then they are shipped to Socorro, NM for correlation.

Once a project has been correlated, the observers will receive an email notifying them that the data are available in the NRAO Data Archive. Observers can then download their data (or request it be shipped on disks, if necessary) and begin the calibration process.

Re-Observation of a FAILED Projects

Occasionally, an observation will fail. This can be due to poor weather at several stations, mechanical failure at a station deemed necessary to the observation, extreme radio frequency interference, or several other possible factors. If a project is in the General Observing category, it will be re-observed when conditions allow. Those projects in the Shared Risk and Resident Shared Risk categories will not be re-observed if they fail.


2. 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:

Freq.Ang. Separation
327 MHz 117. deg
610 MHz 81. deg
1.6 GHz 45. deg
2.3 GHz 36. deg
5.0 GHz 23. deg
8.4 GHz 17. deg
15.0 GHz 12. deg
22.0 GHz 9. deg
43.0 GHz 6. deg

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.

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.

3. Calibration Strategies

The science goals of a given project will guide how the schedules are designed. Different types of observing modes (polarimetry, spectral line, pulsar, etc.) require different approaches to calibration. This chapter will cover the calibration strategies for several observing modes, which will dictate the setup of those observations. These strategies are for VLBA-only observations. High Sensitivity Array (HSA) and Global VLBI observations will be discussed in the next chapter.

Basic Continuum of Bright Sources

The most basic VLBA project would be continuum observations of very bright (> a few Jy) sources at observing frequencies below about 12 GHz. For this case, all of the targets are assumed to have high enough flux density to self calibrate and atmospheric opacity is generally not an issue. Therefore, there is no need to do any phase referencing. The only calibration scans necessary are for fringe-finding and bandpass calibration.

Fringe Finder

Fringe finder sources need to be bright (>1 Jy) and preferably compact in order to calibrate the phases and delays for every baseline in an observation. In general, a 2-minute scan should be made on the fringe finder at every observing band for every 3 to 4 hours of observing. Because the fringe finder scans are so important to proper correlation, it is recommended that they be scheduled near the middle and/or end of the observation. This reduces the chances of any early hiccups in the observation impacting these scans and also helps to ensure that fringe finder will be visible at all stations. Many observers schedule two or three scans on a fringe finder over the course of an observation to ensure good correlation and provide a good source for initial instrumental phase and delay calibration. The NRAO maintains a list of recommended fringe finders, but users are free to choose whichever source they believe will work.

Note that a judicious selection of a fringe finder source could also be used as a bandpass calibration source (discussed more below).

For observations lasting longer than about 4 hours, at least two fringe finder scans should be scheduled. If the first fringe finder is also the bandpass calibrator, the second fringe finder scan should be on the same source as the first fringe finder scan. If the observers have opted to use a separate source for bandpass calibration, the second fringe finder scan can be on a different source than the first fringe finder scan.

Bandpass Calibration

In order to properly characterize the bandpass response for each subband, at least one 2-minute scan should be scheduled on a bright source at every observing band for every 3 to 4 hours of observing. As mentioned above, the bandpass calibration source can be the same as the fringe finder. In fact, it is often a good strategy to use the same source for both because this will reduce overhead in the schedule and leave more time for scans on the science target(s).

As with fringe finder scans, long schedules should include multiple bandpass calibration scans. However, the same bandpass calibrator should be used for every bandpass calibration scan in order to track any changes in the telescope response over the course of the observation.

Basic Continuum at Frequencies Above 12 GHz

Basic continuum observations at frequencies of about 12 GHz and higher require some extra planning in order to track the system temperature as a function of elevation. Observers should ensure that at least one relatively bright source is observed frequently over the entire range of elevation. This is then used to determine the tropospheric opacity corrections during calibration.

Phase Referencing

A slightly more complicated observation involves a science target that is not expected to be bright enough for self calibration. In this case, the phase solutions from a nearby bright source must be transferred to the science target. Therefore, observations of the science target are interspersed with observations of the phase calibration target, which is known as the “phase reference calibrator”. Observers who are creating schedules for phase referencing projects are encouraged to consult Wrobel et al. (2000) and Reid & Honma (2014) for more information on the strategies involved.

The phase reference calibrator should be as close to the science target as possible. This ensures that the phase solutions derived for the phase reference source can be applied to the science target, reduces the uncertainty in the position of the science target, and reduces slew time (which reduces total observation time). The exact observing strategy depends on the observing frequency:

  • For observations below 1 GHz, observers should find a phase reference calibrator that is within the primary beam when observing the science target. Observers will not need to schedule scans on calibrator, but they likely will need to request two correlation passes: 1 on the science target and one on the calibrator.
  • For observations between 1 and 2 GHz, it is highly recommended to use a phase reference calibrator that is within the primary beam when observing the science target. If an in-beam calibrator is not available, observers should choose the nearest appropriate calibrator.  The maximum recommended angular distance between the science target and the calibrator is 4 degrees. 
  • For frequencies between 2 and 5 GHz, observers should choose a phase reference calibrator within about 4 degrees of the science target to reduce the effects of the ionosphere on the calibration.
  • For frequencies above 5 GHz, observers should choose a phase reference calibrator within about 5.7 degrees (0.1 radians) of the science target to ensure that any atmospheric effects can be successfully calibrated out of the data.

The phase reference calibrator should be bright enough to successfully fringe fit, which requires a signal-to-noise ratio (SNR) of at least 7 at the observing frequency. Ideally, the calibrator should have a SNR greater than 7 for all baselines. However, as long as the SNR is at least 7 for all baselines from a reference antenna (usually a station near the center of the array such as FD, PT, or LA), solutions will be found for the longer baselines. The VLBA OSS has a table of the VLBA baseline sensitivities to help determine the SNR at any observing frequency. However, the table assumes that observations are made using the maximum possible data rate and that all data channels will be combined for the calibration. Observers who want to calibrate each data channel independently should use the EVN Sensitivity Calculator to determine the baseline sensitivity in each data channel.

The phase reference calibrator should have a well-known position. It is therefore recommended to choose a calibrator from one of the astrometric catalogs such as the IAU standard International Coordinate Reference Frame 3 (ICRF3) maintained by the International VLBI Service (IVS), or the Radio Fundamental Catalog (RFC) maintained by NASA’s L. Petrov. The NRAO VLBA Calibrators Search and RFC VLBI Calibrator Search tools can help observers find good phase reference calibrators for their science target(s). The positional uncertainty of the phase reference calibrator should be <10 mas in both RA and DEC, but <1 mas is preferred.

Phase referencing is recommended for all astrometric projects because the self calibration process does not preserve the absolute location data of the target source.

For more information on phase referencing strategies, see Wrobel et al. (2000).

Observers are encouraged to contact the VLBA staff via the NRAO helpdesk if they have any questions about choosing a phase reference calibrator.

Phase Referencing Schedules

Building a schedule for a phase referenced observation of a single science target is fairly simple. Every scan on the science target should be bracketed by scans on the phase reference calibrator. Each set of scans (phase cal, science target, phase cal) should be timed such that the mid points of each phase reference calibrator scan are separated by no more than the coherence time at the observing frequency. Observers can estimate the tropospheric coherence time at a given observing frequency using t ~ 2600/(observing frequency in GHz) seconds. Note that this estimate only works for frequencies above 5 GHz; lower frequencies are impacted more by the ionosphere and observers should assume coherence times of ~120 seconds for 300 to 700 MHz, and ~300 seconds for 1 to 3 GHz. For more details on the tropospheric coherence times, see Moran & Dhawan (1995).

As an example, the coherence time at 8.4 GHz (4 cm) is about 310 seconds and the recommended fringe-fit interval is 120 seconds. The “typical” VLBA 8.4 GHz phase referenced observation would have phase reference calibrator scans of 120 seconds and science target scans of 180 seconds (120/2 + 180 + 120/2 = 300). This assumes a fairly dim phase reference calibrator (requiring 120 second scans to get good SNR) which is very close to the science target (such that the slew time is very low). If the phase reference calibrator is bright enough, the calibration scans can be shortened to as little as 30 seconds on-source. If the phase reference calibrator is far away from the science target, the scan times may need to be adjusted to allow for longer slews.

Note that observations at low elevations will have shorter coherence times, so observers should adjust their schedules accordingly. See Ulvestad (1999) for more details on this effect.

Check Sources

It is often a good idea to include a handful of scans on one or two additional bright sources, which are referred to as “phase-check sources” or just “check sources”. These sources should be about as far from the phase reference calibrator as the science target. The check sources should be calibrated in the same way as the science target (i.e., do not fringe fit on the check sources, but apply the fringe fit solutions from the phase reference calibrator). The check sources should be bright enough to allow for self calibration. This allows the observer to check that the solutions derived from the phase reference calibrator properly calibrate the phases for the check source. A check source with a well-known position (uncertainties <1 mas) will also allow the observer to check the astrometric precision of the calibration.

Polarimetry

Polarimetric calibration requires three additional steps beyond continuum calibration:

1. Determining the leakage terms (i.e., the polarization impurity between the R and L polarizations), a.k.a. the “D-term” calibration.

2. Calibrating the absolute electric vector polarization angle (EVPA) by determining the phase difference between the right circular polarization (RCP) and left circular polarization (LCP).

3. Calibrating the RCP-LCP delays.

For the VLBA, polarimetric observations must include scans on calibrator sources for each of these steps.

Note that ionospheric Faraday rotation may be an issue for observations at 20cm and longer wavelengths.

Leakage/D-term Calibration

The strategy for determining the leakage terms depends on the total length of the observation. For short (<4 hours) observations, at least one scan on a strong unpolarized (>1 Jy, less than 1% polarized) calibrator source will be required. For longer observations, at least 5 scans on one strong calibrator (>1 Jy) over a wide range (at least 100 degrees) of parallactic angle should be scheduled throughout the observation. It is even better if the strong calibrator has a known and constant EVPA. Some D-term calibrators may also work as fringe finders and/or bandpass calibrators. Each scan on the D-term calibrator should last at least 2 minutes, but 5 to 10 minutes is better.

Some recommended VLBA D-term calibrators are:
Bright, unpolarized: OQ208 (J1407+2827) Note – steep spectrum, only recommended for observations below ~9 GHz.
Bright, low polarization fraction: 3C84 (J0319+4130)
Bright, polarized: NRAO150 (J0359+5057), DA193 (J0555+3948), 3C286 (J1331+3030), BL Lac (J2202+4216), 3C454.3 (J2253+1508)

EVPA Calibration

To accurately determine the EVPA of a target, the observation must include several scans on 1, 2, or 3 sources with known EVPA. Each EVPA calibrator should be observed at least twice, and preferably 4 times, over as wide a range of parallactic angle as possible. Each scan should be at least 3 minutes long. The VLBA OSS Polarimetry section lists some recommended EVPA calibrators for observing frequencies between 1 and 5 GHz. A collection of EVPA calibrators useful for observing frequencies of 5 GHz and above can be found at the VLA/VLBA Polarization Calibration page.

Observers planing to use polarimetric calibration should request VLA observations of their EVPA calibrators within +/- 2 weeks of the VLBA observation. The closer in time the VLA observations are made, the better. The measured EVPAs in the VLA observations must then be transferred to the VLBA data using the AIPS task CLCOR or RLCOR, as described in the AIPS Cookbook.

RCP-LCP Delay Calibration

To determine the RCP-LCP delays, observers should schedule a single 2-minute scan on a very strong (~10 Jy) source. This source can often also serve as the fringe finder and bandpass calibrator.

Spectral Line Observations

Spectral line projects should include scans on fringe finders and bandpass calibrators. The same source may be used for both purposes. The bandpass calibrator should be bright enough, or observed long enough, so that the bandpass calibration does not significantly contribute to the noise in the image. For a given channel width,

[display] ( S_{cal} \cdot \sqrt{t_{cal}} ) / ( S_{obj} \cdot \sqrt{t_{obj} } ) \gt 2  [/display]

where [inline]S_{cal}[/inline] is the flux density of the bandpass calibrator, [inline]t_{cal}[/inline] is the total time on the bandpass calibrator, [inline]S_{obj}[/inline] is the flux density of the science target, and [inline]t_{obj}[/inline] is the total time on the science target. 

The exact ratio between the two values will be determined by the science goals, but it should be no smaller than 2. Note that for extremely narrow channels or very weak bandpass calibrators, those typical flux requirements can lead to extremely long integration times.

If the science target has strong continuum emission (e.g., absorption line studies) or a strong emission line (e.g., a maser line), phase referencing may not be necessary. For such cases, self-calibrating the continuum source or the strong emission line would be sufficient to calibrate the delay and fringe-rate. In all other cases, including when preserving the absolute position of the target (i.e., astrometry) is necessary, phase referencing as described previously should be utilized.

Observations on the spectral line target should have the pulse cal generators disabled: in SCHED .key file, set pcal=’off’. If the pulse cal is not set to ‘off’, the pulse tones will corrupt the spectral line data. It is highly recommended that the pulse cal generators be turned off for all scans in the observation, including scans on calibration sources (bandpass calibrator, phase reference calibrator, etc.), otherwise the pulse cal tones on those sources may contaminate the spectral calibration of the science target(s).

Spectral line projects often require the bandpass to be very well-calibrated. It is therefore highly recommended to include scans on the bandpass calibrator once every two hours. This ensures the availability of bandpass calibration information even if one or more antennas fail throughout the observation.

Pulsar Observations

The strategy for pulsar observations is essentially identical to a phase referenced observation. The schedules will require scans on a fringe finder, a bandpass calibrator, and a phase reference calibrator. However, absolute amplitude calibration is lost during correlation for observations using the pulsar gating and pulsar binning modes.

Observers conducting pulsar observations will need to submit two additional files for correlator configuration. First, a pulsar ephemeris file (.polyco) to predict the phase of the pulsar. Second, a bin configuration (.binconfig) file containing details for the correlation of the observation. These two files should be submitted no later than 1 week after the pulsar observation took place, but can be submitted along with the normal scheduling files. Note that a separate pair of ephemeris and bin configuration files must be submit for each pulsar in a multi-pulsar observation.

Details of the DiFX pulsar modes can be found in Brisken & Deller (2010).

4. High Sensitivity Array (HSA), Global Millimeter VLBI Array (GMVA), and Global cm VLBI

VLBA+ Arrays

If your science project involves adding additional telescopes to the 10 standard VLBA stations, it is considered a VLBA+ observation. These observations involve extra steps. First, the observations must be coordinated with the other observatories.

Y1: Adding a single VLA antenna to the VLBA

Y1 observations are the easiest VLBA+ observations to schedule. Because the VLA has 27 antennas, pulling a single antenna out and adding it to the VLA can often be done fairly easily. However, the VLBA schedulers will still need to coordinate with the VLA and determine the best date/time to perform such an observation. The VLA control staff must also ensure that the VLBA recording media is properly mounted at the VLA.

Y27: Adding the phased VLA

Because the VLBA and VLA scheduling staff are all located in the same office building, it is relatively painless to coordinate VLBA observations with the phased VLA. However, the observations must still be scheduled as “fixed date” in order to ensure everything is set up properly ahead of time for the observation.

Adding the VLA to the VLBA involves some special scheduling considerations.

First, all of the rules for scheduling a VLA observation must be followed. See the Guide to VLA Observing for more details. The two items that will have an impact on most observers are:

  1. Dummy scans: The VLA must have a 90 second scan for each frequency setup used before that frequency is used. This 90 second does not have to be on-source. The best practice is to put the dummy scans at the beginning of schedule to get them out of the way and so they can be in the possible slew time at the beginning of the schedule.

  2. Pointing: The VLA must do reference pointing at all frequencies higher than 15 GHz. See the high frequency observing strategy guide for details but below are the most important notes. The VLA:

    • should perform the reference pointing at X or C band and 1s integrations to determine pointing solutions for the high frequencies; the default X band setup is recommended for this.

    • must repoint if the antennas move ~15 degrees, so after about an hour and/or the telescopes move to a different source. This changes if you are near zenith (see VLA documentation).

    • needs at least 2.5 minutes on source to point, this can be tricky at the start of a schedule since you do not know where the telescopes are moving from. It is generally recommended that you assume there will be a very long slew (~10 minutes).

Second, in order to improve the amplitude calibration of both the VLBI data and the VLA only data, it is good practice to include a few minute scan on a standard VLA flux density scale calibrator in a phased-array observation. This scan does not need to be phased nor have to be observed by the VLBA, however, if the observations are at a frequency requiring pointing at the VLA then the flux density scale calibrator scan should have pointing before it. The standard VLA flux density scale calibrators are 3C286, 3C48, 3C147 and 3C138. Pick one with the shortest slew. If you do not include such a calibrator scan, you are depending solely on the VLA switched power which is only good to ~10%. The amplitude calibration will either be attached to the data, or the observer will receive an ANTAB style text file that includes the amplitude calibration for the VLA as a VLBI station. This can be loaded with the AIPS task ANTAB into the TY and GC calibration tables attached to the correlated VLBI data. [NOTE: If no flux density scale calibrator scan is scheduled, or if the scan is not observed correctly, observers can use gain constraints in the AIPS task CALIB during calibration (e.g., to exclude the VLA in the overall normalization).]

Third, in order to get the highest sensitivity from the VLA, the VLA antennas must be autophased before they are summed. The phasing takes out the delay and phase differences between antennas. In order to autophase the array the corrections are determined by observing a point-like calibrator, then these can be applied to a weaker or resolved target source. If your target is bright and unresolved (with the VLA) then it can be used as its own autophase calibrator. The autophase source should be strong, unresolved at the VLA and near the target. How strong and near the autophase source should be depends on the frequency of observing, rough guidelines can be found on the Guide to VLBI at the VLA page. The old VLA calibrator manual is a good place to look for autophase sources. You can also use the calibrator search in the VLA Observation Preparation Tool (OPT).

Details of including the phased VLA in a SCHED keyin file, including examples of setups and scans, can be found in the Building a Schedule File in SCHED chapter of this guide.

Adding other external telescopes

Coordination with observatories outside of NRAO (including Green Bank Observatory, Arecibo, and Effelsberg) requires some lead time. Each observatory has different demands on their available open-skies time. While NRAO will make every attempt to schedule an approved project as proposed, users should be aware that circumstances outside the control of NRAO staff may prevent a project from having access to all requested telescopes (e.g., when hurricane Maria caused damage to Arecibo).

Due to the coordination required for these types of observations, they must be scheduled as “fixed date”. In the case of high frequency observations, NRAO will attempt to schedule a small collection of possible observation times. If the weather forecast is not conducive to observing at the designated frequency, the observation will be delayed to a later date.

Note that the NRAO has no control over the hardware at other observatories. If an observation fails due to a hardware malfunction at one of the non-NRAO telescopes, it may not be possible to re-observe.

GMVA projects

Projects making use of the GMVA are only scheduled during the pre-determined GMVA sessions, as set by JIVE. Observations are coordinated by the Max Planck Insitut fur Radioastronomie (MPIfR). Note that weather conditions may not be optimal during these specified times. Observations will be attempted regardless of weather conditions at any particular telescope. Failed observations may not be re-observed.

GMVA observations are correlated at the MPIfR in Bonn, Germany. The correlation process is much more complicated than a usual due to the very heterogeneous array of antennas used and the impacts of weather on each individual station. Several correlation passes are often necessary for each observation. Therefore, the time to receive correlated data may be longer than a normal VLBA observation, and GMVA data is typically delivered to observers about two months after observing.

Global cm VLBI

Observations requiring the VLBA to participate in Global cm VLBI are coordinated by JIVE. Similar to GMVA observations, the dates for Global cm VLBI observations are determined well in advance of the observations. As such, the observing conditions may not be optimal during the observations. Failed observations may not be re-observed. The time to correlate Global cm VLBI observations is likely to be longer than a typical VLBA observation due to the more complicated correlation process, which is similar to the GMVA.

5. Radio Frequency Interference

RFI at the VLBA

As the VLBA pushes the recording media capabilities to higher and higher bit rates, astronomers will have access to more and more bandwidth for their observations. While that increase in bandwidth improves continuum sensitivity and allows access to more spectral lines in one observation, it comes at the cost of making the telescope more susceptible to radio frequency interference (RFI).

In addition, an ever increasing number of potential interfering sources are being created every day. New satellite-based internet providers such as SpaceX’s Starlink plan to place tens of thousands of radio-emitting satellites in orbit in the next decade. The radar necessary for emergency braking and self-driving cars, the proliferation of cellular communications and wireless data access, and the general expansion of human habitats have the potential to dramatically increase sources of RFI on the ground, as well.

Considerable effort has gone into making the VLBA's electronics as linear as possible so that the effects of any RFI will remain limited to the actual frequencies at which the RFI exists. Non-linear effects, such as receiver saturation, should occur only for those very unlikely, and usually very brief, times when the emitter is within the antenna primary beam.

RFI is primarily a problem within the low frequency (C, S, L, and the low-band system) bands, and is most serious at less remote stations. With increasing frequency and increasing resolution comes an increasing fringe rate, which is often very effective in reducing interference to tolerable levels.

The bands within the tuning range of the VLBA which are specifically protected for radio astronomy are: 322.0-328.6 MHz, 1400–1427 MHz, 1660.6–1670.0 MHz, 4990–5000 MHz, 15.35–15.4 GHz, 22.21–22.5 GHz, 23.6–24.0 GHz, and 42.5–43.5 GHz. No external interference should occur within these bands.

RFI seen in VLBA data can be internal or external. Great effort has been expended to eliminate all internally-generated RFI. Nevertheless, some internal RFI remains, which we are working hard to eliminate. Nearly all such internally-generated signals are at multiples of 128 MHz. So far as we know, all such internal signals are unresolved in frequency, and therefore will affect only a single channel.

The VLBA has installed filters to reduce RFI from known sources at some stations (e.g., the US Border Patrol radio communications channels at Kitt Peak and Fort Davis). If observers identify any persistent RFI in their data, they are encouraged to contact the VLBA staff either directly or via the helpdesk.

Examples of known strong RFI and given in the table below.

Frequency
[MHz]
SourceComments
1337 Aeronautical radar
1376 - 1386 GPS L3 Intermittent
1525 - 1564 INMARSAT satellites
1598 - 1609 GLONASS L1
1618 - 1627 IRIDIUM satellites
1683 - 1687 GOES weather satellite
1689 - 1693 GOES weather satellite
1700 - 1702 NOAA weather satellite
1705 - 1709 NOAA weather satallite
2178 - 2195 Satellite Downlink Very Strong
2320 - 2350 Sirius/XM Satellite Radio Very Strong

The most current RFI measurements at the VLBA stations can be found on the VLBA RFI webpage.

Satellite Transmissions and the Clarke Belt

The last two entries in the table deserve extra discussion. These are satellite transmissions, whose severity is a strong function of the angular offset between the particular satellite and the antenna. It appears that significant degradation can occur if the antennas are within ~10° of the satellite. The great majority of the satellites are along the Clarke Belt—the zone of geosynchronous satellites. As seen from the Pie Town station, this belt is at a declination of about −5.5°. There are dozens—probably hundreds—of satellites parked along this belt, transmitting in many bands: at a minimum, S, C, Ku, K, and Ka-bands. Observations of sources in the declination rate of +5° to −15° can expect to be significantly degraded due to satellite transmission.

C-band (4–8 GHz), X-band (8–12 GHz), and Ku band (12–18 GHz) are subject to strong RFI from satellites in the Clarke Belt.

The Sirius digital radio system, and probably the satellites in the 2178–2195 MHz band, comprises three satellites in a 24-hour, high eccentricity orbit with the apogee above the central U.S. For the Sirius system, the orbit is arranged such that each of the three satellites spends about eight hours near an azimuth of 25° and an elevation of 65°. The corresponding region in astronomical coordinates is between declinations 50° to 65°, and hour angles between −1 and −2 hours. Observations at S-band within that area may—or may not—be seriously affected. Due to the very long baselines of the VLBA, only one station is typically affected by RFI from these satellites at a time (although the PT, LA, and Y27/Y1 baselines may be impacted simultaneously).

Although most of the stronger sources of RFI are always present, it is very difficult to reliably predict their effect on observations. Besides the already noted dependence on frequency and baseline, there is another significant dependency on sky location for those satellites in geostationary orbit. For these transmitters (e.g., the frequency range from 3.8–4.2 GHz), the effect on observing varies dramatically on the declination of the target source. Sources near zero declination will be very strongly affected, while observations north of the zenith may well be nearly unaffected, especially at the highest resolutions.

Observing and Post Processing Considerations

The VLBA electronics, including the DiFX correlator, have been designed to minimize gain compression due to very strong RFI signals, so that in general it is possible to observe in spectral windows containing RFI, provided the spectra are well sampled to constrain Gibbs ringing and spectral smoothing (such as Hanning) is applied. Both AIPS and CASA provide useful tasks which automatically detect and flag spectral channels/times which contain strong RFI.

Extracting astronomy data from frequency channels in which the RFI is present is much more difficult. Testing of algorithms which can distinguish and subtract RFI signals from interferometer data is ongoing.

Calibration of VLBA data when strong RFI is present within a subband can be difficult. Careful editing of the data, using newly available programs within CASA and AIPS, will be necessary before sensible calibration can be done. The use of spectral smoothing, typically Hanning, prior to editing and calibration is strongly recommended when RFI is present within a subband.

Identification and removal of RFI is always more effective when the spectral and temporal resolutions are high. However, the cost of higher spectral and temporal resolution is in database size and, especially, in computing time. A good strategy is to observe with high resolution then average down in time and frequency once the editing is completed.

6. Frequency Dependent Observing Strategies

Observations Above 12 GHz

The troposphere has a larger impact on frequencies above about 12 GHz. The higher the frequency, the more observers need to worry about weather. In the Technical Justification section of the VLBA proposal submission tool, proposers are asked to specify any restrictions on when an observation may occur, including under what weather conditions. If an observer needs to update these conditions, they should contact the VLBA Scheduling Officer () and explain any changes that need to be made. Occasionally, observers decide their original constraints were too demanding and will prevent a project from being observed in the desired time frame. In this case, the weather constraints can be relaxed and allow the observation to take place at the risk of the data being slightly degraded. Weather constraints also depend on the science targets and whether self-calibration can be used.

Because the troposphere affects high frequency waves more, small changes in the structure of the troposphere (turbulence) must be accounted for when observing at these frequencies. The primary way observers correct for this turbulence is to shorten the time between scans on calibrators and scans on science targets. See the Phase Referencing section of the Calibration Strategies chapter for more information on tropospheric coherence time and the associated switching time at high frequencies.

The effects of elevation are also larger at higher frequencies because observing at lower elevations means observing through more of the troposphere. Observers encouraged to use very short cycle times and sufficiently bright phase reference calibrators when performing observing at low elevations at frequencies above about 12 GHz.

While the VLBA is able to slew significantly faster than the VLA, there will still be gaps of a few to several seconds between the target and calibrator scans. For this reason, it is imperative that the observer chooses calibrators which are as close as possible to the science target when observing at higher frequencies.

Observations Between 1 and 3 GHz

Observations at frequencies between 1 and 3 GHz are often impacted by radio frequency interference (RFI). Because the VLBA antennas are widely separated, it is unlikely for multiple stations to experience the same RFI at the same time (unless a target happens to be close to a broadcasting satellite such as SirusXM, which broadcast between 2332 and 2345 MHz). However, more commercial systems are beginning to make use of frequencies in this range, which means each station is at greater risk of suffering from local RFI. Observers should be aware that portions of the observed bandwidth may be unusable at several stations. Plots of RFI for several VLBA observing bands are available on the VLBA RFI webpage. When possible, it is best to design the observing setup such that it avoids the worst of the known RFI (e.g., don’t observe near 2.3 GHz). Having a large number of spectral channels in each data channel can be useful for flagging RFI in the data while keeping as much of the bandwidth as possible.

Another concern at lower frequencies is the impact of the ionosphere. Fluctuations in the electron content of the ionosphere and turbulence within the ionosphere have larger impacts on observation below about 5 GHz. It is therefore recommended that observers use phase reference calibrators as close to the science target as possible, and no more than 4 degrees away. Whenever possible, it is best to use a phase reference target that will be in the primary beam while observing the science target.

Observations Below 1 GHz

At frequencies below 1 GHz, the fluctuations in the ionosphere have major impact on observations. At these frequencies, observers should use in-beam phase reference calibrators as close as possible to the science target(s). This will require at least 2 correlator passes (one at the location of the science target, and one at the location of the phase reference calibrator).

7. Building a Scheduling File in SCHED

The major calibration strategies were covered in a previous chapter. Here, the details for applying those strategies in a SCHED .key file are presented.

Some General Points

The easiest way to create a SCHED .key file to start with an example .key file or a .key file that has successfully been run previously. The SCHED release contains many example .key files in the “examples” directory. Some of them are out-of-date and will not be useful to many observers.

The filename for the .key file should be the project name (e.g., BL267) plus the letter corresponding to which observation in the project it is (A=1, B=2, C=3, etc.). For example, the .key file for the fifth observation under project code BL267 would be named “bl267e.key”.

SCHED will not execute any line in a .key file that is preceded by a “!” (i.e., this is the comment symbol).

Observers are encouraged to consult the SCHED User Manual for many more details and tips. Additionally, observers can always contact the VLBA staff via the NRAO helpdesk if they have any questions or encountered any problems.


Sections of a .key File

At the very top of a .key file, users may enter notes for themselves and the VLBA Data Analysts. This is done using the “!” symbol.

The sections can be in any order, with the exception that the “Scans” section must be the last section. It is often useful to have the Preferred Dynamic Constraints and Cover Information sections near the top so the VLBA Data Analysts and schedulers can see them easily.


Preferred Dynamic Constraints

This section is not included in many of the example .key files, but it is very helpful to the Data Analysts and schedulers for determining the best time to schedule an observation. An example of the Preferred Dynamic Constraints section is available from an online template and a downloadable text file. It is also possible to copy this section from the vips11.key example file in the “examples” directory of SCHED.

At the very least, this section should identify which antennas are essential to the observation (many users require both MK and SC in order to get the maximum angular resolution), how many antennas are required, and what weather constraints should be used for scheduling. If a specific observing cadence is desired (e.g., one observation every 30 +/- 3 days), it should be stated in this section as well. Specifying both MK and SC as required antennas will reduce the chances of an observation being interrupted by a Daily UT1-UTC observation.

All constraints provided in this section should be consistent with those that were specified in the proposal. An observer who submits a .key file with tighter constraints than specified in the proposal may be asked to change the schedule to be more in line with the proposal, or the VLBA Schedulers may edit the .key file themselves and inform the observers of the changes.

Cover Information

This section should have the contact information for the PI and observer for the project. It can also contain notes to the Analysts, schedulers, and operators.

The expcode should be set to match the filename of the .key file. This should be the project code plus the letter corresponding to the observation number (1=A, 2=B, 3=C, etc.). For example, if the project code is BL267 and the .key file is for the fifth observation under that project, the user should set

expcode = bl267e

After running SCHED on the .key file, it will produce control and summary files based on the expcode, NOT the filename. For example, if a schedule is named bl267d.key but has expcode=bl276a, SCHED will produce a summary file named bl267a.sum. If the Program Control keyword “overwrit” is set, SCHED will overwrite any pre-existing files with the same names, so it is important to remember to change the expcode for every new observation.

Correlator Information

This section describes how the observation will be correlated once the data arrives at the NRAO Pete V. Domenici Science Operations Center (DSOC) in Socorro, NM.

Most VLBA observations will have CORREL = ‘Socorro’, which means the observation will be correlated using the DiFX correlator at the DSOC.

Observers can set the correlator average time, number of spectral channels per data channel, the correlator weighting function, and several other parameters in this section of the .key file. See the Correlator Information section of the Schedule File chapter in the SCHED User Manual.

Spectral Zooming

The Correlator Information section of the keyin file is where observers inform VLBA staff about their choices for zooming in on particular regions of the observed frequency range. Observers should specify their zoom options using the corenote entries. In the following example, the first correlation pass is for continuum, and the second pass zooms in on the HI line.

correl = Socorro
coravg = 2
corchan = 256
cornant = 10
corpol = on
corwtfn = uniform
corsrcs = standard
cortape = FTP
corship1 = 'G. Observer'
corship2 = '123 Example St’
corship3 = 'Beverly Hills, CA 90210'
cornote1 = 'First correlation pass: 256 channels for all IFs.'
cornote2 = 'Second correlation pass: 32000 channels for IF-pair 1 (of 2) for the HI line.'
cornote3 = 'Output line data from channel 23001 to 25000 in zoom band mode (4 MHz total; covers the frequency range 1349-1353 MHz).'

The second correlation pass will deliver a 4 MHz chunk of data spanning the frequency range of 1349 - 1353 MHz, which is at 75% of the first 64 MHz IF pair. The 64 MHz IF itself will require 32000 channels to deliver 2 kHz channel spacing. The spectral region of interest (the 4 MHz) will be between channels 23001-25000 of the 32000 channels.

Program Control

This section allows the user to set up certain controls for their observation. Some options here are overwriting preexisting files (overwrit), starting the disks syncing before the first scan (prestart), and setting the minimum gap between entries (minpause).

Setting the sumitem variable will determine what information is included in the summary (.sum) file once SCHED is run on the .key file.

Standard Source and Station Catalogs

In this section, observers select which file to use for the source names and coordinate, and the antenna locations for their observation. The Catalogs section of the Schedule File chapter in the SCHED user manual lists all of the catalogs and other external input observers can use in this section of the .key file.

In Line Source Catalogs

Observers can define sources to be observed using custom coordinates. This is where the science targets are defined. It is often useful to define all of the target sources in one place rather than throughout the Scans section. At minimum, all sources should include the source name, RA, Dec, and the equinox used for the coordinates (usually “J2000”).

NOTE: If observers will be using AIPS to calibrate their data, it will be very useful to define source names using all capital letters.

Setup Information

In this section, observers define their observing setups. Many example setups are provided with the SCHED release in the “setups” directory. For more details on creating setups, see Linford & Brisken (2020).

Example setups for the new 4096 Mbps observing mode are included at the end of this chapter. Any observers who require assistance designing their setups are encouraged to contact the NRAO helpdesk and a member of the VLBA staff will contact them.

Initial Scan Information

The schedule must specify a start date and time. For dynamically scheduled projects, this will be a placeholder. The VLBA schedulers will update the the .key file with the appropriate date and time when the project is preparing to observe.

Optimization Information

SCHED can determine scan orders to optimize a schedule for many parameters. Usually, this section is only useful for projects observing many targets across a large range of RA. See the vips11.key file for an example optimizing for hour angle selection.

NOTE: The optimization controls are still labeled as “experimental features” and the output files should be inspected carefully to ensure they create schedules appropriate for a given observation.

The Scans

This must be the last section in the .key file. It is in this section that observers define the scans on the various sources. Unless optimization is being used, the scans will be executed in the order listed.

The first entry in The Scans section should be the list of stations to be used. The list can contain up to 30 stations. For most observers, this will be the ten standard VLBA stations and will look like:

stations = VLBA_SC, VLBA_HN, VLBA_NL, VLBA_FD, VLBA_LA,
           VLBA_PT, VLBA_KP, VLBA_OV, VLBA_BR, VLBA_MK

Other stations can be found in the stations.dat, stations_RDBE.dat, and stations_VLA.dat files located in the “catalogs” directory included in the SCHED release.

The list of stations can be changed before any scan. Each scan without a new list of stations will default to the previous list that was provided (i.e., observers using a single set of stations only need to define the list once before the first scan).

Each scan requires a frequency setup to be associated with it. Observers can switch frequency setups as often as necessary. However, changing the frequency will usually involve rotating the sub-reflector on the telescopes. This takes between 5 and 20 seconds, depending on the bands. When switching bands, it is usually prudent to include gaps in the schedule to give the telescopes time to switch receivers.

Scans can be defined similar to loops in computer programming using the “group” and “rep” commands. For example, to observe a phase reference source and a science target 10 times each in an alternating patter (e.g., phase ref, science target, phase ref, science target, …) with each scan lasting 2 minutes, an observer would enter:

group=2 rep=10
source=‘PHASEREF’ dwell=2:00 /
source=’SCIENCETARGET’ dwell=2:00 /

Observers have two options for defining the scan times: dwell and dur. Dwell means the scan time starts once the antennas get on source and lasts for the specified time. Dur (short for duration) means that the scan time starts immediately following the previous scan and will include the time it takes to slew to the next source. If observers want to have exactly a certain amount of time on source, it is recommended to use dwell. If observers want the total observation time to be set regardless of differences in slew times, it is recommended to use dur.

Observers can also use the "preempt" keyword to ensure that a scan or group of scans is not interrupted by a Daily UT1 observation. Setting preempt = 'NO' will ensure that all available antennas are used for a scan. For more information on using "preempt", see the Daily UT1-UTC page and the egdelzn.key example keyin file.

Running Sched

Once the .key file is complete, observers should run SCHED to ensure that everything is entered correctly and that all scans are scheduled properly.

To run SCHED on a .key file, enter:

sched < file.key

SCHED will produce several messages as it prepares the file. Once complete, if there are no errors, it will create several control files and a summary file. Observers should inspect the summary file to determine the total observing time, the total time on source for each source, and to see that the scans are scheduled properly.

If observers encounter any issues or have any questions while creating their schedules, they can contact the NRAO helpdesk and a member of the VLBA staff will provide assistance.

Phased VLA Special Considerations

Autophasing in SCHED

It will probably take about a minute to autophase. Every scan with the VLA in it must have a "vlamode". There are three modes that will be use most often:

  1. vlamode = ' ', used when the VLA is not determining nor applying the autophase. E.g., dummy scans, pointing scans.
  2. vlamode='va', determine autophase.
  3. vlamode='vx', apply the autophase determined in the last vlamode='va' scan.

Reference Pointing for the VLA in SCHED

See the high frequency observing strategy guide for details on how to and when to reference pointing at the VLA. In SCHED the command "vlapeak" controls determining and applying pointing at the VLA, there are two modes used most often:

  1. vlapeak= 'determine', determine reference pointing solution.

  2. vlapeak='apply', apply last reference pointing solution.

Frequency Setups

Frequency setups compatible with VLBA PFB and DDC (4- and 8-channel) observing systems are available for the VLA. For more information see the table of phased array modes. The VLA's LO system is much more flexible than the VLBA's DDC system. See the RDBE section of the VLBA OSS for a description of the DDC/PFB. When putting together a frequency setup for a VLBA+VLA observation, start with the VLBA setup and just copy most of that setup for the VLA. The VLA setup should be correct in the channel (subband in VLA speak) width, edge frequencies, net sideband, IF channels and FE (front end specifications), but the details of the LO do not matter. Possible major differences between the VLA and the VLBA:

  1. The VLA is always dual polarization, even in shared risk modes.

  2. The VLA is always net upper side band (netside in SCHED).

  3. The IF channels (ifchan in SCHED) on the VLA are always A, C, B, D

  4. The Front End (FE) must be specified. This tells the telescope which receiver to use.

Examples of VLBA+VLA setups and scans are below.

Example VLBA+VLA Setups and Schedules

The SCHED User Manual has many example schedules, in particular see jvla.key and any DDC examples which will help you set up the VLBA DDC. Following are a few examples of frequency setups and sequences of phasing up and pointing.

Frequency Setup Example #1: 8 GHz

setinit = rdbe_ddc_8540_dual.set /
! top section is for the VLBA DDC
dbe = rdbe_ddc
sideband = U
netside = U
bits = 2
bbfilt = 128
nchan = 4
pol = dual
firstlo = 7900.0
bbsyn=640., 640., 768., 768.
station = vlba
/
! bottom section is for the VLA
sideband = L
netside = U
pcal = off
ifchan = A, C, B, D
fe(1)='4cm' fe(2)='4cm' fe(3)='4cm' fe(4)='4cm'
firstlo = 9436.0
bbsyn=896., 896., 768., 768.
dbe = widar
station = vla27
/
endset /

 

Frequency Setup Example #2: 22 GHz

setinit = rdbe-128-k.set /
! Again, top section for the VLBA DDC
dbe = rdbe_ddc
sideband = U
netside = U
bits = 2
pcal = off
bbfilt = 128
nchan = 4
pol = dual
firstlo = 21300.00
bbsyn = 512.0,512.0, 640., 640.
station = vlba
/
! bottom part for the VLA
pcal = off
nchan = 4
sideband = U
netside = U
bbfilt = 128.0
ifchan = A, C, B, D
firstlo = 21300.00
bbsyn = 512.0,512.0, 640., 640.
dbe = widar
station = vla27
/
endset /

 

Scans example #1: Simple phasing up including Flux density scale calibrator scan

norecord
setup=rdbe_ddc_8540_dual.set
stations = VLA27
!! dummy scan for VLA
source='3C273' dur = 90 vlamode='' /

!! now observe with everybody
record
stations = VLBA_BR, VLBA_OV, VLBA_KP, VLBA_FD,
VLBA_PT, VLBA_LA, VLBA_NL, VLBA_HN, VLA27
!! Phase up with vlamode='va'
source='3C273' dur = 1:00 gap = 0 vlamode='va' /

!! Apply phased with vlamode='vx'
!! VLA scans cannot be longer than 10 minutes
!! D array so only need to phase up every ~20 minutes
source='M87' dur = 10:00 gap = 0 vlamode='vx' /
source='M87' dur = 10:00 gap = 0 vlamode='vx' /

!! Phase and apply loop
group=3 repeat=5
source='3C273' dur = 1:00 gap = 0 vlamode='va' /
source='M87' dur = 10:00 gap = 0 vlamode='vx' /
source='M87' dur = 10:00 gap = 0 vlamode='vx' /

!! Now lets look at a VLA flux density scale calibrator
source='3C286' dur = 7:00 gap = 0 vlamode='' /

 

Scans example #2: Pointing and Phasing the VLA at beginning of schedule

start=22:00:00
stations= VLA27
norecord

!! Dummy scans for the VLA, 90 seconds for each setup (yes
!! even for pointing setups).
setup = rdbe-128-x.set !x-band pointing dummy scan for the vla
source='0234+285' dur = 1:30 vlamode='' /
setup = rdbe-128-k.set ! k-band dummy scan for the vla
source='0234+285' dur = 1:30 vlamode='' /

!! Point at the VLA, the pointing scan is so long because
!! this is the start of the experiment and the VLA might
!! have to slew a long time to get to the pointing source.
!! Later in the schedule when you have to point again and
!! you know the slew times, you can usually make this shorter
!! The VLA needs 2.5 minutes on-source to point
setup = rdbe-128-x.set ! vla pointing setup
source = '0234+285' dur = 10:00 vlamode = '' vlapeak = 'determine' /

!! While the VLA points, VLBA will start observing.
setup = rdbe-128-k.set
stations=VLBA_HN,VLBA_NL,VLBA_FD,VLBA_PT,VLBA_KP,
VLBA_OV,VLBA_BR,VLBA_MK,VLBA_LA,VLBA_SC
nopeak
start = 22:00:00
source='J0437+24' dwell=12:00 record /

!! VLA is back from pointing, now start phasing
stations=VLBA_HN,VLBA_NL,VLBA_FD,VLBA_PT,VLBA_KP,
VLBA_OV,VLBA_BR,VLBA_MK,VLBA_LA,VLBA_SC,VLA27

!! Scan to phase up the VLA with vlamode='va', 70 seconds
!! are used to account for slewing. The VLBA is included
!! in this scan because it is better not to force subarrays
!! unless absolutely necessary. VLA pointing is applied with
!! vlapeak='apply'. The VLBA does not need to point at 22GHz.
source = '0234+285' dur = 1:10 gap=0 vlamode = 'va' vlapeak='apply' /

!! Scan to apply phases to the VLA with vlamode='vx'.
source = '0234+285' dur = 3:30 gap=0 vlamode = 'vx' vlapeak='apply' /

 

Example 4096 Mbps Setups

6cm Receiver

!4 Gbps C-band, standard
setini = 4gb_ddc.6cm /
nchan = 8
bbfilt = 128.0
pol = dual
bits = 2
dbe = rdbe_ddc
netside = L
freqref = 4612.0
freqoff = 128.0, 128.0, 256.0, 256.0, 384.0, 384.0, 512.0, 512.0 /
endset /
!4 Gbps C-band, split between 4.5 and 6.7 GHz
setini = 4gb_ddc.6cm_split
nchan = 8
bbfilt = 128.0
pol = dual
bits = 2
dbe = rdbe_ddc
netside = U
freqref = 0
firstlo = 3900, 3900, 3900, 3900, 5900, 5900, 5900, 5900
freqoff = 4412, 4412, 4540, 4540, 6450, 6540, 6668, 6668 /
endset /

4cm Receiver

!4 Gbps X-band, standard
setini = 4gb_ddc.4cm /
nchan=8
bbfilt= 128.0
pol=dual
bits=2
freqref = 8112.25
dbe = rdbe_ddc
sideband = U
netside = U
freqoff = 0.0, 0.0, 128.0, 128.0, 256.0, 256.0, 384.0, 384.0 /
endset /

2cm Receiver

!4 Gbps Ku-band, standard
setini = 4gb_ddc.2cm /
nchan=8
bbfilt= 128.0
pol=dual
bits=2
freqref = 15039.75
dbe = rdbe_ddc
sideband = L
netside = L
freqoff = 0.0, 0.0, 128.0, 128.0, 256.0, 256.0, 384.0, 384.0 /
endset /

1cm Receiver

!4 Gbps K-band, standard
setini = 4gb_ddc.1cm /
nchan=8
bbfilt= 128.0
pol=dual
bits=2
freqref = 23312.0
dbe = rdbe_ddc
sideband = U
netside = U
freqoff = 0.0, 0.0, 128.0, 128.0, 256.0, 256.0, 384.0, 384.0 /
endset /

7mm Receiver

!4 Gbps Q-band, standard
setini = 4gb_ddc.7mm /
nchan=8
bbfilt= 128.0
pol=dual
bits=2
freqref = 42912.75
dbe = rdbe_ddc
sideband = U
netside = U
freqoff = 0.0, 0.0, 128.0, 128.0, 256.0, 256.0, 384.0, 384.0 /
endset/

3mm receiver

!4 Gbps W-band, standard
setinit = 4gb_ddc.3mm /
nchan=8
bbfilt=128.0
pol=dual  
bits = 2     
samprate = 256
freqref = 85500.0
dbe=rdbe_ddc
firstlo =85500,85500,85500,85500,85500,85500,85500,85500 pol     =RCP,LCP,RCP,LCP,RCP,LCP,RCP,LCP bbc     =  1,  2,  3,  4,  5,  6,  7,  8 netside =  U,  U,  U,  U,  U,  U,  U,  U ifchan  =  B,  D,  B,  D, B,  D,  B,  D freqoff  = 512,512,640,640,768,768,896,896 /

Additional Example setups

For more information on tuning the VLBA, see Linford & Brisken (2020).

Two data channels spanning 1476-1484 MHz and 1512-1520 MHz

!20cm, two 8 MHz channels
setini = ddc_2_8_20cm /
dbe = rdbe_ddc
nchan = 4
pol = dual
bbfilt = 8
firstlo = 2100
freqref = 0
netside = U
fredoff = 1476, 1476, 1512, 1512 /
endset /

Two data channels spanning 21879-21901 MHz and 21922-21937 MHz

!1cm, two 32 MHz channels
setini = ddc_2_31_1cm
dbe = rdbe_ddc
nchan = 4
pol = dual
bbfilt = 32
firstlo = 21200
freqref = 0
netside = U
fredoff = 21876, 21876, 21914, 21914 /
endset /

Continuous coverage from 6624-6880 MHz using the PFB

!6cm, 2048 Mbps, PFB
setini = pfb_8_32_6cm
dbe = rdbe_pfb
nchan = 16
pol = dual
bbfilt = 32
firstlo = 7600
freqref = 0
sideband = L
netside = U
fredoff = 6624, 6624, 6656, 6656,
               6688, 6688, 6720, 6720,
               6752, 6752, 6784, 6784,
               6816, 6816, 6848, 6848 /
endset /

Alternating frequency channels at 20cm using the PFB

!20cm, 2048 Mbps, PFB
setini = pfb_8_32_20cm_alt
dbe = rdbe_pfb
nchan = 16
pol = dual
bbfilt = 32
firstlo = 2400
freqref = 0
netside = U
fredoff = 1392, 1392, 1456, 1456,
               1520, 1520, 1584, 1584,
               1648, 1648, 1712, 1712,
               1776, 1776, 1840, 1840 /
endset /

8. Submitting the Schedule

Pre-submission Checklist

Observers are encouraged to go through the following checklist to make sure their schedule is correct and will properly generate the control files.

1. Are the filename and the expcode in the Cover Information section correct?

2. In the Preferred Dynamic Constraints section:
    A. Are the required and optional antennas correctly identified?
    B. Is the minimum number of antennas correct?
    C. Are the observing bands correctly marked with the proper polarization codes?
    D. Are the weather constraints clearly specified?
    E. Are the date constraints clearly specified?
    F. If a certain cadence is desired, is that cadence clearly specified?
    G. Are any other necessary constraints clearly specified?

3. Are the entries in the Correlator Information section correct?

4. Are all coordinates for user-defined sources correct? (It is a horrible feeling to spend 8 hours of telescope time staring at a blank patch of sky.)

5. Are all of the necessary stations included in the station list(s)?

6. Are the pulse cal settings appropriate for the observations? Recall that spectral line observations should have pcal=’off’ to avoid corrupting the line of interest.

7. Are any necessary zoom windows properly defined in the Correlator Information section?

8. [For pulsar observations only] Are the ephemeris and bin configuration files correct?

9. [For phased VLA observations only] Are all necessary dummy scans, calibration scans, and pointing scans properly scheduled?

10. After running SCHED on the .key file:
    A. Does SCHED produce the control files and the summary file?
    B. Are the observing frequencies in the summary file what is expected?
    C. Were all scans successfully scheduled with at least 2 stations on source?
    D. Were all calibrator scans scheduled with the necessary time on source?
    E. Do the science targets have the necessary total on-source times?
    F. Is the total elapsed time for the schedule correct?

Schedule Submission

Once the observer has confirmed that the .key file is correct and complete, it should be submitted to the VLBA Schedulers at vlbiobs@nrao.edu. The Schedulers will reply with a confirmation that the .key file was received.

Observers doing pulsar observations should also submit their ephemeris (.polyco) and bin configuration (.binconfig) files to the VLBA Data Analysts using the same email as above. While it is not necessary to submit these files until after the observation has occurred, it is recommended to submit them as early as possible to avoid delays in correlating the observations.

After Submission

The VLBA Data Analysts may contact the observer with questions regarding the schedule. This is especially common when observers omit information from the Preferred Dynamic Constraints section. However, the Analysts will also contact observers with tips on getting the schedule observed sooner. For example, if one of the antennas that an observer marked as “required” is down for maintenance, the Analysts may ask if they can change that antenna to “optional” in order to schedule the observation that week.

Observers can check the status of their observation by looking at the current VLBA schedule. Dynamically scheduled observations are typically scheduled 24 hours in advance, except for weekend schedules which are usually made the Thursday before observing. Fixed date observations can be scheduled several months before the observation begins.

9. Frequently Asked Questions

Observing with a certain cadence

My science objective requires that the observations be done with a specific cadence. How do I schedule these?

If your project requires that the observations are separated by a very specific amount of time (e.g., exactly 28 days), they should be scheduled as “fixed date” observing. We recommend that you contact the VLBA scheduling officer as soon as possible to arrange your observations. It is also a good idea to prepare your SCHED keyin files well in advance of the observations in order to avoid any delays.

Note that fixed date observations will take place regardless of weather conditions or other constraints. Therefore, there is a possibility that these observations will take place under less-than-optimal conditions.

It is recommended to schedule repeating observations with a flexible cadence, if the science goals allow for it. For example, instead of requiring an observation every 30 days, the request could be for every 30 +/- 3 days. This allows the observations to be scheduled dynamically and allows the schedulers to avoid poor weather conditions, antennas down for maintenance, and other conditions that can impact an observation. This type of flexibility is especially important for projects with B priority.

All cadence requirements should be included in the Dynamic Constraints section of the keyin file.

Polarization Products and Noise

How does the number of polarization products affect the noise?

Choices are single polarization (RR or LL), dual polarization (RR and LL), and full polarization (RR, RL, LR, and LL). If the emission is unpolarized, dual polarization gives twice the amount of independent data as single polarization does, and therefore decreases the rms noise by a factor of √2, as the sensitivity calculator will show. Full polarization does not improve this any further since RL and LR do not contribute to Stokes I.

Phase Referencing

When should I use phase referencing?

If there is any chance that your science target will not be bright enough for self calibration, you should use phase referencing.

Calibrators

How should I pick calibrators for my project?

The VLBA Calibrator List  will provide you with a list of potential calibration sources.  It is generally a good idea to check for recent information on these sources to ensure that variability is not an issue at your desired observing frequency.  Also, check that the positions of the sources are known to an acceptable uncertainty for your project.  If you cannot find recent information calibrators at your desired frequency, you can request extra observing time to identify a good calibrator (see the Technical Justification section regarding whether targets can be self-calibrated).

What calibration sources do I need for polarimetric observations?

Polarimetric observations require scans on at least one leakage (D-term) calibrator, and at least one electric vector polarization angle (EVPA) calibrator (although 2 or 3 EVPA calibrators are recommended).  Several (at least 5) calibration scans should be made covering as large a range in parallactic angle as possible.  Also, at least one 2-minute scan should be scheduled on a very bright (~10 Jy) calibration source to permit the calibration of the RCP-LCP delays.  See the VLBA Polarimetry page in the VLBA OSS for more details.

Setup Restrictions

How do current observing setup restrictions impact my project?

The VLBA can only be tuned to certain allowed frequencies.  Check the SCHED example setup files for examples of how to tune the frequencies in your observation.  For details on creating setups, see Linford & Brisken (2020).

If a target is observed at multiple frequencies during a single observation, there will be increased calibration overhead since it takes a finite amount of time for the telescope to switch receivers. For phase reference observations, each target source scan at a certain frequency has to be bracketed by gain calibrator scans at that frequency.

Assistance with VLBA Observations

Can I get help calibrating and analyzing my VLBA observations?

The NRAO is now offering assistance with certain VLBA programs. Currently, this offer is for standard VLBA observations only (no HSA, GMVA, or Global VLBI). This assistance program is intended for new or novice users. If you wish to request assistance, please check the text box on the bottom of the Proposal Cover Sheet in the PST and provide a brief statement about why the extra help is needed.

Even if a project was not awarded assistance under this new program, any VLBA observer can always contact the VLBA staff via the NRAO helpdesk if they have any questions or encounter any problems.

CASA for VLBA Calibration

Can I use CASA to calibrate my VLBA observations?

CASA releases 5.3 and later contain tools for VLBI calibration. However, these tools were primarily developed for use with the EVN and EHT. Some tools may not work properly with VLBA data. Users who wish to calibrate VLBA data in CASA are strongly encouraged to use CASA 5.7 because it will have the most current bug fixes and patches. CASA 6.1 can be used, but it lacks the ability to plot fringefit solutions due to plotcal being deprecated in the CASA 6.X versions. VLBA staff are currently testing the CASA VLBI tools.

Tutorials for VLBI calibration in CASA were developed by Anita Richards (University of Manchester) and Jack Radcliffe (SARAO / University of Pretoria) for the Development in Africa with Radio Astronomy project. There are two VLBI data reduction tutorials: one for EVN continuum observations, and one for EVN spectral line observations.

The EVN Data Reduction Guide provides a more general overview of the calibration process and compares AIPS tasks with related CASA tools.

In addition to the main CASA program, VLBA users will also need a script to convert the VLBA gain curve table into a format usable in CASA. This script (gc2.py, written by Mark Kettenis) is available from JIVE’s CASA VLBI Github repository.

Acronyms

AIPS = Astronomical Image Processing System
BBC = Baseband Channel
CASA = Common Astronomy Software Applications
DDC = Digital Downconverter
DDT  = Director’s Discretionary Time
EHT = Event Horizon Telescope
EVN = European VLBI Network
EVPA = Electric Vector Polarization Angle
GBT = Green Bank Telescope (officially, the Robert C. Byrd Green Bank Telescope)
GMVA = Global Millimeter VLBI Array
GO = General Observing
GST = Greenwich Sidereal Time
HSA = High Sensitivity Array
ICFR3 = International Coordinate Reference Frame 3
IEEE = Institute of Electrical and Electronics Engineers
IVS = International VLBI Service
JIVE = Joint Institute for VLBI in Europe
LCP = Left-hand Circularly Polarized
LST = Local Sidereal Time
NASA = National Aeronautics and Space Administration
NRAO = National Radio Astronomy Observatory
OPT = Observation Preparation Tool (for creating VLA schedules)
OSS = Observational Status Summary
PFB = Polyphase Filter Bank
PFT = Proposal Finder Tool
PST = Proposal Submission Tool
RCP = Right-hand Circularly Polarized
RDBE = ROACH Digital Backend
RFC = Radio Fundamental Catalog
RFI = Radio Frequency Interference
ROACH = Reconfigurable Open Architecture Computing Hardware
RSRO = Resident Shared Risk Observing
SEFD = System Equivalent Flux Density
SNR = Signal-to-Noise Ratio
SRO = Shared Risk Observing
SRP = Science Review Panel
TAC = Time Allocation Committee
TEC = Total Electron Content
TLA = Three Letter Acronym
TJ = Technical Justification
ToO = Target of Opportunity
UHF = Ultra High Frequency
VHF = Very High Frequency
VLA = Very Large Array (officially, the Karl G. Jansky Very Large Array)
VLBA = Very Long Baseline Array
VLBI = Very Long Baseline Interferometry

NRAO Bands: Letter Codes, Frequencies, and VLBA Receivers

VLBA Bands

Band
Designation
Frequency
Range
Associated
Receiver
P 312 - 342 MHz 90 cm
UHF 596 - 626 MHz 50 cm
L 1.35 - 1.75 GHz 20 cm
S 2.2 - 2.4 GHz 13 cm
C 3.9 - 7.9 GHz 6 cm
X 8.0 - 8.8 GHz 4 cm
Ku 12.0 - 15.4 GHz 2 cm
K 21.7 - 24.1 GHz 1 cm
Q 41.0 - 45.0 GHz 7 mm
W 80.0 - 90.0 GHz 3 mm

See the Frequency Band & Performance section of the VLBA OSS for more details.