back end instrument

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 there is another portion (the front end) that is directly attached, sitting between the telescope and the back end. The terms are commonly used for radio telescopes: a generally-used division is to use the term front end for electronics that make the relevant signal easier to transmit over a cable, which may span some distance (i.e., some amplification and modulation), and back end for equipment that carries out the rest of the processing in a more-protected environment. In the case of a radio interferometer, the two terms can be used to distinguish the electronics resident at an individual dish (front end) from that at a central site (back end) that processes the signal-data received via cables from the individual dishes, such as the correlator. The instrumentation of ALMA demonstrates such an organization.

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.