Massaging Steps Performed by UVREAD -- UVSET

  
Table 2.2: Arguments to UVSET, for STATUS=OLD

As mentioned above, the uvread routine can perform, at the programmers request, extra processing steps on the visibility data. These steps consist of averaging and resampling frequency channels, uv coordinate conversion and some corrections for planet observations. The steps are requested by calls to uvset. In the call to uvset, the argument object (a string) gives the general processing step that is being requested. The type argument (another string) gives more specific details, and the arguments n (integer) and p1, p2 and p3 (reals) give any numerical values needed.

Note that the set-up given by uvset only becomes correctly activated during the next call to uvread. Before this next call, the setup is in a somewhat nebulous state. So you should not expect various other routines associated with uvread to work as expected until after the next call to uvread. Associated routines include uvflgwr and uvinfo.

Table 2.2 summarizes the possible values of the arguments to uvset. Here the column titled ``Object'' and ``Type'' are the possible string values that object and type can take on. The third column gives the meaning for the parameters n,p1,p2,p3. Dashes in the third column indicate that the arguments value is ignored in this particular call. While several processing can be performed simultaneously (several calls to uvset will be needed to specify them all), others are mutually inconsistent. When mutually inconsistent steps are requested, the last requested step is honored. Each processing step requires further explanation.

object='data'

This gives operations on the spectral data. Type 'channel' selects the channels to be returned, and possible averaging together of the channel data. If the original channels are numbers from 1 to N, then, by using type='channel', uvread will return nchan massaged channels, where channel i of the massaged channels is formed by averaging width channels of the original data, starting at channel . If uvset is called with nchan being zero, all channels are selected (note that this only makes sense if start, step and width are all 1).

type='wide' is similar, but uses the continuum data rather than the spectral data.

type='velocity' is also similar, returning a weighted sum of the spectral data. However in this case start, width and step are given in units of km/s (rather than channels). This is particularly useful if the spectrometer setup is not constant throughout the data or there is no Doppler tracking, and so the velocity of a given channel changes. Note that 'channel', 'wide' and 'velocity' are mutually exclusive. The default is 'channel' (or wide if there is no spectral data in the file), with start,increment and width of 1.

If there are fewer than nchan channels, then dummy channels, which are flagged as bad, are added. If nchan is specified as 0, then uvread will return as many channels as possible.

object='reference'

The ``reference line'' is a spectral channel, or an average of spectral channels, which the main data is divided by. Typically the reference line would be a strong point source (e.g. a maser). The resultant data is essentially normalized and shifted, but it also has atmospheric-based and instrument-base calibration problems removed. The extra parameters needed to describe the reference line is the same as for object='data', except that the number of channels, and the increment is ignored (there is only ever one reference line). The default is not to have a reference line.

object='coord'

This sets the units of the u and v coordinates returned in the preamble. Using 'wavelength' or 'nanosec' sets the units of the returned u and v. For 'wavelength', the sky frequency used is that of the first channel returned. The default value is 'nanosec'.

object='planet'

This causes the u and v coordinates to be scaled and rotated, and the correlation values to be scaled, to adjust for changes when observing planets. The parameters plmaj, plmin and plangle give the reference size (arcseconds) and position angle (degrees) of the planet. If the reference size is 0, then the size of the first selected data record is used.

object='selection'

This gives extra control over the uv selection process (see 2.6). Currently there is only one possible type, 'amplitude', which enables or disables the amplitude selection process. If the argument n is positive, then amplitude selection is applied (i.e. the normal action), otherwise amplitude selection is not applied.