As of April 9 2008, Doppler tracking can be used again but for VLA - VLA or EVLA - EVLA baselines only

Our original recommendation was not to use Doppler tracking only when crossed (VLA - EVLA) baselines were being used. This was because the Fluke synthesizers which control the last frequency conversion for VLA antennas do not preserve phase when changing frequency. This is not a problem for VLA-VLA baselines, since any change in phase from the Flukes affect all VLA antennas identically. However, the Fluke synthesizers are not used for the EVLA antennas, so any phase change does not propagate to the EVLA antennas, and thus there is a (random) phase jump on EVLA-VLA baselines when the Flukes change frequencies. This problem will be with us for the entire time of the transition period, when we are using both VLA and EVLA antennas in the array.

We have identified another problem, in that the Jobserve program (and indeed the older Observe program) do not make the same Doppler calculation as the online system. Depending on the sky frequency and the sky position, it appears that these two calculations can differ by at most a few tenths km/sec. Thus an observer should not use the frequencies computed by Jobserve (or Observe) for the absolute sky frequency unless such an error is not important (for example, if the channel widths are significantly larger than this error).

We have therefore implemented a web-based version of the "gold-standard" Dopset program to calculate the sky frequency, given a sky position, rest frequency, velocity, rest frame and velocity type, and an LST day and time. This program has been carefully compared to both the on-line system of the VLA and that of the EVLA antennas. The velocities thus derived with all three of these systems agree to within 0.01 km/s for all the cases we have tested. This web-based version, implemented by Bryan Butler, can be found here.

We recommend that observers use this web tool to calculate absolute sky frequencies for their spectral line observations, and to copy these frequencies directly into the Frequency Calculations page in Jobserve. There are two possible ways to observe a spectral line in fixed frequency mode:

• replicate online Doppler tracking by calculating a new observing frequency for the target source periodically throughout the observing run. Bear in mind, however, that we have observed phase jumps on VLA-EVLA baselines on frequency changes (caused by the VLA system), so we strongly recommend including calibration scans at every change in observing frequency. These scans should be at the same observing frequency as the source scan they refer to.
• if the spectral line is narrow compared with the total bandwidth the entire observation can be made at a single fixed frequency, and the Doppler corrections applied in post-processing using the task CVEL in AIPS. If the line is well-resolved spectrally, this will not result in any degradation of spectral resolution; if the line is very narrow and strong relative to the channel width, some spectral smoothing may be needed to eliminate the Gibbs phenomenon.

For dynamically scheduled projects, we advise users to submit a valid observe file for the start of the configuration during which the project is scheduled. As soon as the frequency shift due to the Earth's orbital motion affects the position of the line of interest in the band significantly, we advise users to submit an updated version, and to continue to do so until the project has been executed.

For further assistance in planning these kind of spectral line experiments, please contact one of our support scientists.