Error Handling

The program checks for a number of error conditions from the user and from the input files. An appropriate message is written either on the program report file or at the terminal. The program terminates if necessary. The conditions tested for are listed here.

• Obvious errors such as missing input files, invalid sky region and incompatible coordinate systems are caught while the window is displayed and the user is requested to correct the problem.
• The EGRET_DIR and FITS_DIR and EGRET_PROGRAMS environment variables need to be set for the program to find the executables files, the FITS directory and the sequence file. In general, these are set by the .egretrc file which EGRET users should execute with their .cshrc. In addition, the FITS directory has to be writable by the user. The user may not ask that the output file replace the first input file unless the input file is an addmap file (has a + before the viewing period number in the file name).
• An error is written on the terminal if any needed file cannot be opened. This may happen if there is problem with the addmap.list file or if the counts file cannot be found.
• The input maps must have the standard FITS format and the data must either be 2 or 4 bytes integers. The number of axis and the axis range must be reasonable (3 axis, range 3 to 360 or 720). There must be at least 1 energy level but no more than 10. The data type (keyword BUNIT) must be COUNTS or EXPOSURE. The coordinate system must be either galactic, celestial, earth, instrument, moon or sun centered. It may also be transformed galactic or transformed celestial (produced by TRANSMAP). However if the map coordinate systems differ, they must either be galactic or celestial. The map grid type (keyword HISTORY GRID) must be either RECT or AITF.
• An error occurs if the total number of bins added is null.
• A warning condition occurs if the number of bins on an axis is uneven. The value entered is then decremented by 1.
• Another warning message is generated if the energy levels and the zenith angles of the 2 input maps do not match exactly. However, the maps are added using the energy levels indexes. The warning is also written on the terminal.