11.2 The ``First Look'' procedure

The procedure ``First Look'' provides information about the weather conditions and a few instrumental parameters at the time of the observations. This information is very important as it helps you to make a first data quality assessment even before you may start with the interactive data calibration procedure. The panel of the procedure is shown on Fig.11.1.

  

Figure 11.1: ``First Look'' panel

Monitoring information is provided on:

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Meteorology: the average and maximum wind velocity, the ambient pressure and temperature. Gusty conditions and observations with wind velocities above 10m/s may show up with high pointing corrections. Take care to tag visibilities which may be affected by such difficult observing conditions.

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Pointing and Focus: the applied corrections are shown for all the antennas in the array. Only differential variations in the corrections play a role, not the absolute amount. Sudden pointing corrections by more than 10'' can considerably spoil the visibilities, especially at the highest observing frequencies. A similar consideration applies for antenna focus corrections, although visibilities are automatically corrected for phase offsets which are generated by focus corrections.

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Total Power: a trace of the total incident atmospheric power recorded by the continuum detectors (one for each receiver, each one second), helps in further evaluating the data quality. As a rule, strong and rapid variations in the total power trace the presence of clouds in the line of sight while a sudden up- or down-stepping on one antenna and on one receiver is a sign of a renewed receiver tuning. The total power increases in general with the air mass.

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Cable Phase: variations in the electrical length of the cables show up in phase variations (referenced to the LO2 phases). These are measured by a phasemeter. Appropriate corrections for the phases of the LO1 rotators are computed taking ntimes the corrections measured by the phasemeters of the 3mm receivers and $3\,n$ times the corrections measured by the phasemeters of the 1mm receivers, where n is the harmonic number of the tuned frequency. Strong and rapid variations while a source is tracked may indicate a fault in a cable (these data should perhaps be flagged), whereas a sudden but steady change is mostly related to a shift in the antenna pointing.

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System Temperature: dependent on the observing conditions and on the frequency. As for the total power detectors, strong and rapid variations in the system temperature trace atmospheric instability, whereas a sudden but steady change on one receiver is a sign of a renewed receiver tuning.

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Water vapor: the content of precipitable water vapor in the atmosphere is a critical parameter on which the quality of most of the high frequency observations depend. Should the measured water vapor content not be consistent (or roughly) on all the high frequency receivers, please check the receiver gain.

Finally, the ``First look'' procedure produces a short list of all the scans recorded at the time of the execution of a project. Such a listing allows you to trace back the sequence of operations during an observing run. Note that the range of scan validity for the calibration procedure sets up with the last GAIN scan in the short list.

Looking at the results of the procedure (called in the example above show-28-feb-2001-x007.ps.gz) should be done simultaneously with the reading of the project.note file (here x007.note).