After the input files are read and confirmed to be of the same size, they are combined in a single array; each array pixel being thus associated with a multiplet of flux values derived from the inputs. The first processing step is to scale these multiplets with respect to a reference multiplet. The derivation of the reference multiplet is carried out in one of two basic styles: an `internal' style in which a kind of average multiplet is calculated from the values in the image themselves, without reference to any other information, or a `black body' (bb) style in which the reference multiplet is calculated using the widths of the energy bands (in the case of EPIC images) or the filter bandpasses (in the case of the OM). (Future developments may encompass a third style in which the reference multiplet values are entered directly by the user.) In the black body case the reference multiplet is calculated from the integrated effective area (EPIC) or integrated filter bandpass QE (OM), as the multiplet expected from a black body (BB) at at user-definable temperature.
These style choices are controlled firstly by the parameter refclrstyle, which can be either `internal', `bb' or `none'. If this is `internal', the user can further specify via the parameter internalrefstyle whether the reference multiplet is to be calculated by taking the mean or the rms of the image multiplet. Only pixels having a fractional total flux greater than cutoff are used in this calculation. However if refclrstyle=`bb', the user can specify (through the parameter bbrefstyle) the BB curve either directly as a temperature or via the energy or wavelength of the photons occupying the peak of the BB energy spectrum. (Good physics students will not need to be reminded that the peak of the BB curve plotted against energy is not in the same location as the peak of the curve when plotted against wavelength!)
For any of the `bb' reference multiplet styles, colimplot needs access to the boundaries of the energy or wavelength bands within which each of the input images was accumulated. The task looks first in the data subspace (DSS); if nothing is found there it looks for BAND attributes; if these aren't found either, an error is raised. The `bb' style also requires CCF access in order to calculate the integrated effective area etc, hence the environment variable SAS_CCF should be properly set if this style is desired.
Note that colimplot, although designed to operate on XMM images, will accept any fits image. However only the `internal' style of reference multiplet calculation is available for non-XMM images, because these are not likely to store the information about energy bands in the XMM manner.