The term back end instrument (or just back end) is used to distinguish those indirectly attached to the telescope through others more directly attached, generally when another portion (the front end) is directly attached, between the telescope and the back end. The terms are is commonly used for radio telescopes. For example, the term back end might be applied to those instruments that correlate signal from multiple telescopes, such as a "correlator" in the case 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 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.