The term back end instrument is used to distinguish those indirectly attached to the telescope through others more directly attached. It 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.
The instrumentation of ALMA demonstrates such an organization. The front end instrument 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.