If an input NDF has more pixel axes than the reference NDF, then the extra pixel axes are retained unchanged in the output NDF. Thus, for instance, if an input RA/Dec/velocity cube is aligned with a reference two-dimensional galactic-longitude/latitude image, the output NDF will be a galactic-longitude/latitude/velocity cube.
The transformations needed to produce alignment are derived from the co-ordinate system information stored in the WCS components of the supplied NDFs. For each input NDF, alignment is first attempted in the current co-ordinate Frame of the reference NDF. If this fails, alignment is attempted in the current co-ordinate Frame of the input NDF. If this fails, alignment occurs in the pixel co-ordinate Frame. A message indicating which Frame alignment was achieved in is displayed.
Two algorithms are available for determining the output pixel values: resampling and rebinning (the method used is determined by the REBIN parameter).
Two methods exist for determining the bounds of the output NDFs. First you can give values for Parameters LBND and UBND which are then used as the pixel index bounds for all output NDFs. Second, if a null value is given for LBND or UBND, default values are generated separately for each output NDF so that the output NDF just encloses the entire area covered by the corresponding input NDF. Using the first method will ensure that all output NDFs have the same pixel origin, and so the resulting NDFs can be directly compared. However, this may result in the output NDFs being larger than necessary. In general, the second method results in smaller NDFs being produced, in less time. However, the output NDFs will have differing pixel origins which need to be taken into account when comparing the aligned NDFs.
This parameter is only accessed if the INSITU parameter is FALSE.
PARAMS(1) is required by all the above schemes. It is used to specify how many pixels are to contribute to the interpolated result on either side of the interpolation or binning point in each dimension. Typically, a value of 2 is appropriate and the minimum allowed value is 1 (i.e. one pixel on each side). A value of zero or fewer indicates that a suitable number of pixels should be calculated automatically. 
PARAMS(2) is required only by the Gauss, SombCos, SincSinc, SincCos, and SincGauss schemes. For the SombCos, SincSinc and SincCos schemes, it specifies the number of pixels at which the envelope of the function goes to zero. The minimum value is 1.0, and the run-time default value is 2.0. For the Gauss and SincGauss scheme, it specifies the full-width at half-maximum (FWHM) of the Gaussian envelope measured in output pixels. The minimum value is 0.1, and the run-time default is 1.0. On astronomical NDFs and spectra, good results are often obtained by approximately matching the FWHM of the envelope function, given by PARAMS(2), to the point-spread function of the input data. 
The resampling algorithm steps through every pixel in the output image, sampling the input image at the corresponding position and storing the sampled input value in the output pixel. The method used for sampling the input image is determined by the METHOD parameter. The rebinning algorithm steps through every pixel in the input image, dividing the input pixel value between a group of neighbouring output pixels, incrementing these output pixel values by their allocated share of the input pixel value, and finally normalising each output value by the total number of contributing input values. The way in which the input sample is divided between the output pixels is determined by the METHOD parameter.
Both algorithms produce an output in which the each pixel value is the weighted mean of the near-by input values, and so do not alter the mean pixel values associated with a source, even if the pixel size changes. Thus the total data sum in a source will change if the input and output pixel sizes differ. However, if the CONSERVE parameter is set TRUE, the output values are scaled by the ratio of the output to input pixel size, so that the total data sum in a source is preserved.
A difference between resampling and rebinning is that resampling guarantees to fill the output image with good pixel values (assuming the input image is filled with good input pixel values), whereas holes can be left by the rebinning algorithm if the output image has smaller pixels than the input image. Such holes occur at output pixels which receive no contributions from any input pixels, and will be filled with the value zero in the output image. If this problem occurs the solution is probably to change the width of the pixel spreading function by assigning a larger value to PARAMS(1) and/or PARAMS(2) (depending on the specific METHOD value being used).
Both algorithms have the capability to introduce artefacts into the output image. These have various causes described below.
KAPPA --- Kernel Application Package