An optical instrument with temperature-related focal length variations compensation mechanism is provided herein. The optical instrument includes: a housing within a bore of the optical instrument, wherein the bore and the housing have each a distal end and a proximal end; a set of lenses located on a common optical axis and affixed within the housing, wherein the set of lenses is associated with a specified focal length for each specified temperature; and a temperature compensation member connecting the distal end of the housing to the distal end of the bore of the optical instrument, wherein the temperature compensation member comprises one or more sections whose thermal expansion coefficient and length along the optical axis are selected such that for a specified range of temperatures, the expansion of the temperature compensation member along the optical axis compensates for a change of the focal length of the lenses.