The task merge merges two EPIC event lists or two additional files (attitude files, orbit files etc.) from two exposures (even from different observations and instruments), pointing to the same or an adjacent direction. The output files can be used by evselect for the product generation, higher level detection and time analysis tasks.
In the attitude and orbit cases, any files can be merged together, whereas in the events case, some care should be taken: For instance event files from different modes should not be merged, though in some cases event files from different instruments can. Warnings are given when the event files are deemed not entirely compatible (i.e. their INSTRUME or FILTER keywords are different). Here it is up to the user to interpret the output files sensibly.
Also, in the event files case, the two pointings need to be close to each other. The accuracy of the reprojection degrades with increasing offset between the two pointings, and warnings will be given when this offset becomes significantly large. Only if the input files pass all the checks does the merging proceed and an output file (event list, attitude file, orbit file) is produced. In the event files case, the sky coordinates are deprojected and reprojected again onto a common sky position (given either by the user, via the parameters ra and dec, or as the mean of the two input pointings), and new WCS attributes are calculated. The size of the image is given by the parameter imagesize.
For example, two event files can be merged reprojecting X/Y coordinates to a mean reference point using the following command.
merge set1=inevlist1.fits set2=inevlist2.fits outset=outevlist.fits
The two event files can also be merged reprojecting X/Y coordinates to a reference point defined by the user.
merge set1=inevlist1.fits set2=inevlist2.fits outset=outevlist.fits \ withradec=Y ra=1.1 dec=-0.7
The user has to be careful when using any merged output for a quantitative analysis, as this task is not yet optimal for this purpose. There are many open questions regarding merging files originated in different exposures, and the handling of EXPOSURE, GTI and BADPIX extensions (check the task description of merge for further details).