SCHEDULING HINTS

When preparing VLA schedule files, the following facts and guidelines should be noted:

• The NRAO program SCHED assumes that a station name ``VLA27'' refers to phased-array VLBI at the VLA, while a station name ``VLA1'' refers to single-antenna VLBI. A station name of ``VLA'' is ambiguous and is not permitted for VLBI programs.

• No pulse calibration system is presently available at the VLA. If you plan to use more than one BBC, then you should observe a strong and compact source to serve as a ``manual'' pulse calibrator; see the ``VLBA Observational Status Summary''.

• For phased-array programs conducted in spectral-line mode, calibration information are acquired only for the first VLA IF; this software problem will be remedied in the near future.

• Phased-array time on the VLA is very valuable and the VLA should be scheduled in contiguous blocks. Gaps in the VLA schedule should be released for other productive uses by contacting the data analysts as soon as possible.

• Allow 15 m for the VLA to initiate a phased-array program and to acquire the first source. It is unwise to schedule VLBI tape motions until after this interval has elapsed.

• The minimum VLA elevation is $8^{\circ}$. The maximum VLA elevation is $125^{\circ}$ if over-the-top antenna motion is allowed by the observer. However, such antenna motion is not normally recommended and no-over-the-top is the default. At zenith angles less than about $1.5^{\circ}$, source tracking can be difficult.

• The total cable wrap for the VLA antennas is $530^{\circ}$, extending from $-85^{\circ}$ to $445^{\circ}$ azimuth.

• The VLA antennas slew at a rate of $40^{\circ}$ per minute in azimuth and of $20^{\circ}$ per minute in elevation.

• The VLA's Modcomp computers require about 30 s of setup time for the first scan of a program. A maximum of 20 s of setup time is needed to initiate subsequent scans, regardless of whether or not a source change is requested. These setup times should be added to the VLA antenna slew times to estimate when the VLA will deliver its first complete record of good data to the VLA archive tape. However, VLBI observers can assume a somewhat shorter setup time because single-antenna or phased-array data can actually be valid for part of the VLA record prior to the VLA's first completely good record. SCHED assumes a setup time of 15 s for the VLA.

• Very accurate positions ($<0.1''$) are required for phased-array observations, particularly when transferring the phase corrections from a calibrator observed in mode VA to a VLBI target source observed in mode VX.

• A calibration scan with the phased array in mode VA should be in the same coordinate system (J2000 or B1950) as that adopted for the associated VLBI target source scan in mode VX.

• Those using the VLA at 43 GHz should be aware that antenna pointing can be poor at this wavelength. No pointing remedies are available if only one VLA antenna is being used. However, if the phased VLA is being used for VLBI, then reference pointing in VLA mode IR can improve pointing performance at 43 GHz. For more details, see the discussion of the SCHED parameter ``peak'' in ``The SCHED User Manual''. It is now also possible to insert pointing scans automatically; this capability is discussed under reference pointing in ``The SCHED User Manual''.

• Since the VLA LO system is fixed for each scan in the VLA observe file, frequency switching at a faster rate can only be done with an appropriately prepared ``bbbbbcrd.y'' file running on the VME to control the BBC frequency settings with time.

• The final observing file submitted to VLA Operations should be in VLA LST stoptimes. Do not submit a single VLA observing file that runs longer than 24 LST hours. Note that 24 UT hours exceed 24 LST hours.

• The ancillary locations file, distributed with the SCHED release of 2001 January 12 or later, contains 73 station locations and velocities for VLBI at the VLA. One of these sets of locations and velocities refers to phased-array VLBI, with the remainder appropriate to single-antenna VLBI at any of the 72 pads for a VLA antenna. These 73 station locations and velocities have been used by default at the VLBA correlator since 2001 January 8.

• VLA antennas have measureable offsets between their azimuth and elevation axes. These offsets are currently available only in relative terms which allow comparison of a VLA reference antenna with other antennas. These relative offsets are typically in the few millimeter range, and are not used by the VLBA correlator.

• The visibility data acquired by the VLA during a VLBI program are, by default, corrected for system temperature effects. Those who want to derive source flux densities and/or produce images from the VLA's phased-array data should be aware of this fact. These data can be loaded into the NRAO AIPS package using task FILLM.

• If you want to derive source flux densities and/or produce images from the VLA's phased-array data, then your VLA observe file should include at least one scan of a primary flux density calibrator for the VLA; see the ``VLA Calibrator Manual'' for details. It is common to observe the primary calibrator in normal VLA mode ``  ''. Source flux densities, whether from the VLA or elsewhere, are needed to calibrate the phased array as a VLBI station.

• If you want to do polarimetry with the VLA's phased-array data, then consult the ``VLA Calibrator Manual'' for advice.