The task etruecolor generates from a set of input event list (for EPIC MOS, EPIC pn or a combination of both) a spatial image in which the value of the scalar event attribute energy is three-color coded. The actual generation of the red, green, and blue component images is done through the task evselect (see § 4.7.1), where etruecolor filters the input table using the min and max parameter assigning each energy value a color index. More information on the task etruecolor can be found on the online SAS documentation on individual SAS tasks.
An example is given below as to how this task works. This example combines images of the three EPIC cameras in the energy range between 3 and 7 keV.
etruecolor colortable=RGB_Scheme tablelist='pn_evfile mos1_evtfile \ mos2_evtfile' min=300 max=7000 fileset='MY_COLOR_IMAGE.FIT'
where pn_evfile, mos1_evtfile and mos2_evtfile are filtered event files corresponding to the three EPIC instruments, and min and max the explicit minimum and maximum values respectively for the energy in eV. The energy bands (coded in the pre-defined color table RGB_Scheme) for the red, green and blue channels of the true color image have been chosen so that a power law spectrum with photon spectral index 1.7, absorbed by galactic gas with neutral hydrogen column density of , results in approximately the same number of counts in each band. The user can load his/her own color table.
The output of this particular example is a data set containing three arrays with R/G/B components respectively, which can be viewed by the image viewer ds9 , version 2.3 or later, by typing:
ds9 -rgbcube MY_COLOR_IMAGE.FIT
Figure 19 shows the resultant three color coded images (red, green and blue) and the combined image.