Figure 1 shows what you would input for the sample data in the traditional Perl defineobs GUI, which you invoke simply by typing "defineobs" on the command line. Notice that no suffixes are included in the names for the Mask, Dewar Offset File, or Observation Definition File. The programs will automatically add these suffixes when looking for these files. Since the mask is named CSIfld5, I decide to give the Observation Definition File the same name. Clicking the "Save" button will create CSIfld5.obsdef. After saving, click on the "Quit" button to exit the GUI.
If you find you need to edit CSIfld5.obsdef, just run defineobs as follows to load your existing obsdef file:
defineobs CSIfld5