Front End Instrument is a the term given to the instruments to distinguish between those directly attached to the telescope from other instruments not so directly attached. It is commonly used for Radio Telescopes. For example, the term "front end" might be applied to those instruments that detect and encode the signal, if further processing is to be carried out. In such a case, CCDs could be termed the front end. The Back End Instrument might be the "Correlator" portion of an Interferometer.
The instrumentation of ALMA demonstrates such an organization. The front end instruments can be thought of as those on the dish and the back end, those in the technical building where signals from multiple dishes are combined.
Back End Instrument