VLBA schedules are prepared using the SCHED software, which has an extensive on-line help file and template input files. Most users start with a template input (or "key-in") file, then modify it as needed. SCHED also can compute the times at which sources are up at different stations, and can plot the (u,v) coverage for a particular draft schedule; these capabilities may be useful in writing the proposal, as well as in scheduling.

Preparation of schedule files is nominally the responsibility of the PI, but NRAO also offers assistance in schedule preparation if necessary.

Source Positions
Setup Files
Clock and Fringe Calibration
Amplitude Calibration
Phase Referencing
Cross-Polarization Observations
Readback Gaps
Dynamic Scheduling
Schedule Submission

1. Source Positions
   The most accurate possible source positions are needed for generating the proper correlator models for data processing. NRAO maintains a list of milliarcsecond positions for strong sources that appear in astrometric VLBI or connected-element interferometer catalogs. (See phase-referencing.) For other sources, positions generally are taken from the schedule file, so it is essential that the schedule file have the most accurate possible source positions. A limiting rule of thumb is that the source position error, σ_θ, must be
   
   
   
   If possible, it is desired that positions be better than this by a factor of at least 3-5, to provide the best results. The correlator model is very detailed, and used to best advantage when source positions are as accurate as possible.
2. Setup Files
   Setup files control most parameters of data acquisition, such as observing band and channelization. Most observations use one of the standard files produced by NRAO; see the SCHED user manual. File names typically describe the nature of the setup; for example, v2cm-128-4-2.set is the standard file for a 2-cm observation recording 128 Mbit/s in 4 channels (2 RCP and 2 LCP), with digitization of 2 bits per sample. The file v2cm-128-4-2-L.set is a similar setup file, except that all 4 channels are LCP. The standard files set up the default VLBA frequencies; observers who wish to use non-standard setups (frequencies or otherwise) should consult the SCHED documentation or NRAO staff. Default frequencies are below.

   Table 2. Default VLBA Observing Frequencies in SCHED
   Band	Center Frequency
   90 cm	330.49 MHz
   50 cm	610.98 MHz
   21 cm	1465.49 MHz	for 128 MHz total bandwidth
   21 cm	1435.49 MHz	for 64 MHz total bandwidth
   21 cm	1416.49 MHz	for narrow-band HI observations
   18 cm	1658.49 MHz	most 18cm observing, including with phased VLA
   18 cm	1653.99 MHz	32 MHz bandwidth with Jodrell, but not phased VLA
   13 cm	2295.49 MHz
   6 cm	4990.49 MHz
   4 cm	8415.49 MHz
   2 cm	15285.49 MHz
   1 cm	22235.49 MHz
   7 mm	43135.49 MHz
   3 mm	...	default not yet established; ~86 GHz
   sx	2295.49 & 8415.49 MHz	dual-frequency, usually used for astrometry

3. Clock and Fringe Calibration
   The user must determine the relative delays and rates of change of the delay between the stations during the data reduction. The largest delay errors are due to clock offsets between the stations (typically kept under 100 nsec for the VLBA, but with final calibration of better than a nanosecond required). Other offsets may be attributable to causes such as atmospheric propagation or incorrect source positions. The bulk of the delay errors can be removed either by the use of pulse-calibration tones injected at the front ends, or by observations of appropriate strong sources in the schedule. Pulse calibration is generally used for continuum observations, but strong artificial line sources are unacceptable for spectral-line experiments. In the latter case, occasional (every few hours) observations of strong continuum sources are required for the calibration. Such sources also are used when non-VLBA telescopes are employed, since they may not have useful pulse calibration systems.

   After the initial delay calibration, more specific calibration of the delays and possibly delay rates in a particular direction may be called for. In particular, weaker sources require phase referencing for this fringe calibration. Phase referencing is very similar to the phase calibration done at the VLA, and is described in detail in Chapter 17 of "Very Long Baseline Interferometry and the VLBA", referenced previously, and in VLBA Scientific Memo No. 24.

4. Amplitude Calibration
   Standard antenna gain files are maintained, and system temperatures are measured every 1-2 minutes during an observation. Therefore, there is no need to schedule a standard flux calibrator as is done for VLA observations by including 3C 48 or 3C 286. (There are no constant-flux, unresolved sources on VLBI baselines!) However, an amplitude-check source should be observed several times to check for consistency among the antennas. In addition, gains are measured regularly only for standard frequencies in each band. If a frequency far from the standards is employed, an amplitude-check source should be observed at both standard and non-standard frequencies in order to correct for any variation as a function of frequency. See the OSS for more details.
5. Phase Referencing
   Phase referencing permits imaging of sources that are too weak to give detectable fringes in a coherent integration time. Many details of this process are given in VLBA Scientific Memo No. 24. Typically, sources weaker than ~50 mJy at centimeter wavelengthswith 128 Mbps bandwidths, and 100-200 mJy at 22 and 43 GHz, require phase referencing; to date, there has been no successful (out-of-beam) phase-referencing at 0.3, 0.6, or 86 GHz, due to the extreme problems with the ionosphere and troposphere. When using phase-referencing, a VLA-like sequence of calibrator-source-calibrator is recommended. The calibrator should be within ~2°-3° of the program source, dominated by a point source, and have a compact flux of at least ~150 mJy. Smaller source/calibrator separations are desirable, and may be essential at 1.4, 22, and 43 GHz. Users should consult the VLBA Calibrator Survey. Cycle times can be estimated from equations (17-9) and (17-10) of the article by Beasley and Conway, on page 337 of ASP Conf. Series 82. Tabulated and plotted estimates based on those equations, as a function of source elevation and weather, also are in VLBA Scientific Memo No. 20. The time estimates in Scientific Memo No. 20 only take the troposphere into account; modifications to account for the ionosphere may be found in VLBA Scientific Memos No. 18 and 22.
6. Cross-Polarization Observations
   Data reduction of dual-polarization observations depends critically on the observational strategy. The basic strategy is to observe a strong source (typically the program source or a phase-reference source) over a wide range of parallactic angles to solve for the polarization leakage ("D" terms). In addition, observations of a point source of known polarization position angle (PA) are needed to calibrate the absolute polarization PA. NRAO is carrying out a VLA flux/polarization monitoring program that can be used to calibrate the polarization position angles. For more information, see the Polarimetry section of the OSS and VLBA Scientific Memo No. 26.
7. Readback Gaps
   Recording gaps (i.e., tape stoppage) of at least two minutes should be scheduled at least once per two hours of observing, so that the recording quality can be checked at the telescopes.
8. Dynamic Scheduling
   At present, more than half of all VLBA observations are done dynamically, with time allocation made approximately 1-2 days in advance, depending on weather forecasts and telescope availability. For dynamic schedules, observers must generate a schedule input file starting at a particular sidereal time. The proposer submits only this SCHED input file (often called a "key" file). NRAO staff will create the final schedule files once the exact time allocation is established shortly before the observation.
9. Schedule Submission
   For observations at fixed times, the control files (*.crd output from SCHED) are placed in a subdirectory to the vlbiobs account on aspen.aoc.nrao.edu. The password for this account is available from the data analysts by fax or phone; they can be contacted by phone at 505-835-7238, by fax at 505-835-7027, or by e-mail. For example, files for the observation bm157c carried out in October 2001 are located in the directory /home/archive/e2e/archive/operations/VLBA/observe/oct01/bm157c/ or on the web at http://www.vlba.nrao.edu/astro/VOBS/astronomy/. After file deposition, e-mail notification must be sent to vlbiobs.

   For dynamically scheduled observations, the input file for SCHED, incorporating any non-standard setup files, is e-mailed to vlbiobs, then placed in a subdirectory of /home/archive/e2e/archive/operations/VLBA/observe/dynamic or on the web at http://www.nrao.edu/astro/VOBS/astronomy/dynamic/ by the NRAO data analysts. Comments at the start of this file must be used to specify required conditions for the observation (e.g., weather and antenna availability). There is a required template for the comments. The user-specified requirements depend on the scientific goals, but the more flexible the specified conditions are, the greater the likelihood that an observation with a medium or low priority will be carried out expeditiously (or at all!).

   After the schedule is submitted, the correlation parameters in the schedule (e.g., source positions and number of spectral channels) are assumed to be correct, and are used in the data correlation. Therefore, it is important that the correlation parameters are checked carefully when the schedule is submitted. Correlation with incorrect parameters may result in the failure of the observation.

Introduction Why use the VLBA? How to propose Proposal selection: refereeing and observing allocation Schedule preparation Observations, correlation, data validation, and distribution Calibration Final imaging and analysis Staff collaboration, financial support for data reduction, scientific results and publications, graduate student programs

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