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Appendix 3: EVN CAL Observations

There are a number of questions that can be addressed from routine calibrator
observations:

At the correlator:

     Tapes: when do the tapes and log files get there ? 
     At playback, what is the error rate (all tracks) ?
     What are the clock offsets and rates ?

Post-correlation:

     Phase cal signal: do the signals line up across the band?
                       how does this compare to using a strong 
                       calibrator source to align bands ? 
                       what is its level?

     Which stations suffer radio frequency interference, and at what 
     frequencies?

Calibration:

     Do TSYS values look reasonable? -- what is the multiplicative 
     factor for each station (c.f. the FT factors in the calibration file), 
     obtained from amp self--cal ?

     Does each antenna's sensitivity (in Jy) tally with that ``advertised'' 
     in the EVN status file (evn.sts) ?  (!)

     Is the gain curve correct? It should be possible to construct 
     independent gain curves for each antenna from amplitude self-cal 
     solutions derived from data for which the polynomial gain curves 
     have not been applied.

     [Aside - How important are opacity effects at 6cm and 
              above ?
            - How can we correct for these effects (see VLBA)
            - How well does a polynomial function fit a typical 
              gain curve ]

     Which stations are easiest to calibrate ? Does the calibration 
     coversheet/info easy to interpret ? ANCAL/CAL files supplied (?),
     gain curves in polynomial form, continous T$_{sys}$?

Data Editing (see also at correlator):

     What % of data had to be deleted from each station?