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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). The terms are 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.
The front end versus back end distinction can be used in any case where a telescope instrument feeds into another (or something generally referred to as a single instrument has such subsections). A back end might be a sensor (e.g., a CCD), and among the possible equipment in front of it which might be termed its front end include an adaptive optics unit, a disperser, a coronagraph or a set of selectable filters (e.g., a filter wheel). In some cases, there can be more than two such devices in succession and "front end versus back end" is not fully descriptive.