Some Intrumental Effects

Looking at the AMPLITUDE and PHASE versus TIME gives lots of informations. Here is a list of some problems you can easily understand by comparing the evolution of the amplitude and phase.

• Decreasing amplitude on some baseline
  The pointing is becoming bad on one antenna (or more). Look at the scanlist and identify the position of the pointing in between the cross-correlations. Usually after the next pointing scan the problem must disappear. If you need to check it, select the next pointing scan and use the command SOLVE POINT/PLOT to determine the pointing corrections, see CLIC Manual for details.

• Amplitude drops to zero and the phase is completely random
  The phase-lock is open on one receiver (or more). If this is confirmed, these data must be flagged on the source.

• Phase jump while the amplitude stays almost constant
  A focus has been done and corrections have been applied on one antenna (or more). Look at the scanlist and identify the focus. Use the command SOLVE FOCUS/PLOT to determine the correction values. Do not forget that the phase jump is twice the focus correction (optical way). For example, if there is a correction in the subreflector position of 0.1 mm on Antenna 1, the phase jump on the corresponding baselines is about $\frac{4 \pi}{\lambda}\times \frac{180}{\pi} \times 0.1 \sim 24^o$. Focus phase jumps must be properly taken into account in the phase calibration curve. In this case, you have to calibrate the phase using the command SOLVE PHASE/BREAK var1 var2 (see details in CLIC manual).

• Smooth phase slope while the amplitude stays constant
  Instrumental thermal drifts due to sun or day-night effects induce such behaviour. During an observing run of several hours, it is normal to see them and one major goal of the relative phase calibration is to correct this effect. Drifts of 10^o per hour are not uncommon, depending on weather conditions (wind, sun, etc...).

  However, should the drift appear quasi-sinusoidal, or exceed 90^opeak to peak, it may originate in a baseline error. This may happens when baselines were measured after your project was performed. For details about this problem, see section 9 and 10. Note that at first order, residual baselines errors are cancelled by the phase calibration. This is why it is of prime importance to use a phase calibrator as near as possible from your source.