Front end instrument (or just front end) is a the term given to the instruments to distinguish between those directly attached to the telescope from other instruments not so directly attached (the back end). 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, the sensors such as bolometers could be termed the front end. The back end might be the correlator portion of an interferometer.
There is a generally-used division of processing between the front end and back end of a radio telescope: the front end performs operations that make the relevant signal easier to transmit over a cable which may cover some distance, i.e., some amplification and modulation, and the back end has equipment carry out the rest, in a more protected environment. 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.