OTF maps

In principle, data reduction for OTF maps is the same as described in the previous section. However, for switch mode "totalPower", there are different modes to weight the off-source reference measurement. This is to get the best estimate of the atmospheric total power contribution during an OTF subscan. The modes are specified as arguments for option /OFF of command MIRA\CAL (see the MIRA language internal help for the command syntax). This command subtracts the atmospheric total power, and concatenates spectral basebands if necessary.

• Mode NONE: No atmospheric reference signal is subtracted, but spectral basebands are concatenated.

• Mode AVER: Uses the unweighted average of all available off-source reference measurements.

• Mode EQUAL: As mode AVER, but only the reference measurements done immediately before and after the OTF subscan are used.

• Mode TIME: Uses as reference signal a weighted mean of the off-measurements taken before respectively after the on-subscan. The first reference has a weight decreasing with the time elapsed between the on-the-fly record and the first reference measurement (and the weight of the second reference increases correspondingly).

• Mode TOTAL: as mode TIME, but uses the total power rather than the elapsed time as weighting criterion, such that the total power of the on-the-fly record equals the weighted sum of the total power of the reference measurements. A spectral line contributing significantly to the total power may be masked (see the MIRA language internal help for the command syntax).

The default (also used by the automatic online data processing) is mode AVER. Depending on weather conditions, you may get flatter and lower baselines using one of the other options.










 




 



For HERA data, command VIEW displays the pixel offsets (in PaKo's projection system, with respect to the reference pixel) in the plot's boxes, and (schematically) in the plot header. The pixel offsets are also given in the box labels, together with the offsets (pixel offsets plus scan offsets) of the ON position (and/or the OFF position, depending on the plot produced).

Figure 5: Example of a calibrated, large OTF map, as displayed by MIRA. The records masked by a blanking value correspond to the off-source reference measurements. The offset position of the reference measurement is displayed.
Figure 6: The same plot after defining a sub-region with the cursor, using VIEW /ZOOM. Note that now the offsets of the first and last record shown are also displayed.