Liz Buckley-Geer have found a bug in GSCUT. The problem arises if the last slit of an MDF file contains an acquisition object. This means that the EXTVER for that slit to be zero because these objects are ignored. The value contained in EXTVER is read into the n_j variable in GSCUT using scan and so it gets set to zero. So when the code comes to insert n_j into the NSCIEXT keyword it finds a value of zero and exits with an error. She worked around it by setting the priority of the acquisition object to 3 so that it wouldn’t ignore it.