Scheduling EVN spectral line observations
This page describes special considerations for planning and scheduling a spectral line observation to be correlated at JIVE and processed in AIPS.
Considerations for Spectral Line Observations
Frequency Setup
The EVN stations with DBBC back-ends have standard band pass filters of 16, 8, 4, 2, 1 MHz. Newer versions of the DBBC firmware may alter this configuration set. The velocity range of individual line sources should be taken into account when choosing the frequency setup. Obervations are performed with fixed frequencies, and Doppler corrections are to be determined in the scheduling process. SCHED automatically calculates appropriate observing frequencies if, in the keyin file, the source velocities and the rest-frame frequencies of the lines are specified (see Spectral line observation setup in SCHED). Note that target lines should be located close to the center of the frequency band because the band edges have lower sensitivity and can have large phase offsets. In particular, an absorption-line observation needs emission/absorption-free frequency ranges bracketting the absorption lines in the same frequency band. In addition, calibration using continuum calibrators will benefit from increased bandwidth, and so the bandwidth should be as large as possible. If a line source has a wide velocity coverage, overlapping a part of the frequency (velocity) coverage between two frequency bands may be a good idea to see a part of the line components in the two bands. In that case, however, the data reduction of the individual bands should be done independently in AIPS.
Radio Frequency Interferance (RFI)
The useful available band width is limited by the RFI distribution, especially at L-band. By looking at total-power spectra of previous L-band observations, which are available on the EVN Pipeline Feedback page, an indication of the RFI environment may be found.
Spectroscopy
The number of spectral channels should be large enough to avoid artificial effects (e.g. RFI, unexpected spurs, etc.) and to divide the true lines into more than a few spectral channels. Currently the The EVN software correlator at JIVE (SFXC) provides options for Hanning, Hamming, top-hat, and cosine spectral-weighting functions. Signal sampling with 2 bits per sample is recommended for obtaining higher sensitivity in each of the spectral channels. You should specify in the schedule file that phase cal signals be turned OFF.
Fringe Finders and Bandpass Calibrators
As always, fringe finders are required for every observation in order to determine the station clock delay offsets and drift rates. As some spectral line observations will use relatively narrow band widths, special care should be taken so that the fringe finders can be detected on all baselines. In addition, the same considerations hold for bandpass calibrators. When several line sources are observed with different frequency setups in one observation, AIPS requires bandpass calibrators to be observed with the same setups, so independently for each target. Thus the fringe finders and bandpass calibrators should be carefully selected, especially in observing bands with low sensitivity. For example, at 22GHz, the following sources are recommended as bandpass calibrators: NRAO150, 0420-014, 4C39.25, 3C273B, 3C279, NRAO530, 2145+067, 3C454.3 .
Phase, Delay and Delay-Rate Calibrators
Relative offsets of delays, rates and phases among frequency bands and polarisations can be removed by fringe fitting of the continuum calibrators independently for individual IF bands and polarisations. Because only phase and rate offsets, not delay offsets, can be determined from maser sources, delay calibrators should be separately inserted. To do this, the continuum calibrators should be in the same part of the sky and strong enough to be detected in each of frequency bands within a coherence time and should be observed every hour or less. Again, to facilitate transparent processing in AIPS, each frequency setup should have its own delay calibrators.
Amplitude Calibration
The EVN telescopes regularly measure system temperatures using noise diodes. At present, some telescopes can measure the temperatures only in time gaps between observing scans. Therefore, individual oberving scans are recommended to be reasonably short, with gaps placed every 10 minutes or so (sched will provide warnings for intervals between gaps of more than 15 minutes). The data are now corrected with an improved 2-bit van Vleck correction to account for the statistics of high/low bits for each IF's data stream at each station. Thus the AIPS task ACCOR shouldn't be run. It should be okay to use auto-correlations for bandplass corrections or to use the task ACFIT.
Phase-referencing Observations
This observation mode probably requires, especially for a lower frequency band, the detection of the continuum sources in a narrow band within the coherence time. At higher frequency, the cycling time should be no longer to 2 minutes (see VLBA Scientific Memo 24 in more detail). For the EVN, the practical minimum cycling time may be about 60-90 seconds, arising from the need to nod the larger (slower) telescopes (e.g., Ef slews typically 20-25 s away from zenith for source separations of 1-2 degrees), while maintaining a reasonable fraction of total observing time on source.
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