Displaying Spectropolarimetry Data

The techniques used in the examples in the previous section can also be used to display a single spectral channel when using spectropolarimetry data, but only if the data has some spatial coverage (i.e. has more than 1 pixel in the x-y plane). To do this, specify the spectral channel to be displayed by POLPLOT, using parameter ZAXVAL or ZCOLVAL. These two parameters specify the spectral channel in different ways:

• ZCOLVAL requires the spectral channel to be given as a value for the Z column in the catalogue. This column usually holds floating point pixel coordinates which describe a position on the third pixel axis of the original intensity NDFs. These will usually have a fractional part of 0.5, indicating that the position refers to the centre of a pixel.

• ZAXVAL requires the spectral channel to be given as a value for the third axis in the current Frame of the WCS component of the original intensity NDFs. If the spectral axis has been calibrated, this axis may specify the spectral channel in some physical units such as Hertz, Angstroms, nanometres, microns, km/s, etc. If the spectral channel has not been calibrated, then ZAXVAL will usually require a value in pixel coordinates to be supplied (just like ZCOLVAL). The units required by ZAXVAL can be determined by entering a colon (``:'') when prompted for ZAXVAL. This will produce a description of the current coordinate Frame (you should supply a value for axis 3). The details of the description depend on the nature of the WCS calibration supplied in the original NDFs (which is outside the control of POLPACK).

If a ZAXVAL value is supplied, it is converted into the corresponding ZCOLVAL value. Either way, only vectors for which the Z column value in the catalogue equals the specified ZCOLVAL value are plotted. If the catalogue contains no vectors with the specified ZCOLVAL, then the nearest ZCOLVAL for which some vectors are available is used instead of the supplied ZCOLVAL. The value actually used is reported to the user and included in the default title for the plot.

An alternative method for displaying spectropolarimetry data is to use KAPPA LINPLOT to display line plots of the quantities of interest, as a function of spectral channel. This method has the advantage that, unlike POLPLOT, it can be used even if your data has no spatial coverage (i.e. covers only a single pixel in the x-y plane). Before using LINPLOT you need to create an NDF containing the data to be plotted. To do this, use POLIMAGE. As an example, to plot polarization against wavelength at ( x=1.5, y=2.5 ) for the data in catalogue mycat.FIT, do:

   % polimage in=mycat out=pdata coldat=p colvar="dp*dp" box="[5,5,3\]"
   % linplot ndf="pdata(2,3,)" errbar

POLIMAGE creates the file pdata.sdf which is a 3D NDF containing the values from the ``P'' column in mycat.FIT. The VARIANCE component of the NDF is filled with the square of the values in the ``DP'' column (do not supply a value for parameter COLVAR if the catalogue does not contain a suitable error column). The values assigned to the BOX parameter should usually be the same as the bin sizes used to create the catalogue (use 1,1,1 if the catalogue has not been binned). The LINPLOT command then plots the NDF data values at X and Y pixel indices of 2 and 3 (i.e. pixel coordinates ( 1.5, 2.5 ) ), and covering the entire third (Z) axis. If the catalogue contains data only for a single x-y position, then the LINPLOT parameter NDF could be assigned the value ``pdata'' (without any section specified) since there is no ambiguity about which x-y pixel to use. The ERRBAR parameter indicates that error bars are to be plotted (this would not be possible if we had not assigned a value to the COLVAR parameter when running POLIMAGE).

There are many other options for using these applications. See there reference documentation for details.