Described herein is an apparatus for guiding an elongated flexible instrument, the apparatus comprising a variable-length support assembly adapted to maintain a length of the elongated flexible instrument in a fixed configuration relative to the variable-length support assembly as the variable-length support assembly is moved along a longitudinal axis. The variable-length support assembly includes a plurality of linkages connected in series along the longitudinal axis, and the variable-length support assembly has a compact configuration and an expanded configuration.