rgssources can also deduce the exposure number from the standard file name formats described above. However this is not the preferred option, which is to give the exposure number directly via the parameter instexpid. However, the information is optional. If the file name is not in a standard format, and the exposure number is not otherwise known, its use can be turned off by setting writeexpkwds to `no' or `false'. In this case the only effect is that some exposure-specific keywords are not written to the file header. (Note that the exposure number is required if it is desired to recalculate the attitude.)