According to a first aspect of the present invention, there is provided an optical filter. The optical filter comprises first and second optical elements, and one or more expansion elements. The first and second optical elements are arranged along a common axis, each terminates in a flat surface perpendicular to the axis. The flat surfaces are separated by a gap having a width d, such that the flat surfaces form a Fabry-Perot etalon and light is transmittable along an optical path through the elements and the etalon. The one or more expansion elements are connected to the optical elements, located outside the optical path and extend parallel to the axis with a length greater than the width of the gap. The expansion elements comprise a material having a linear thermal coefficient of expansion different to that of the optical elements such that a difference in expansion of the expansion elements and the optical elements causes relative movement of the flat surfaces along the axis resulting in a change in the width of the gap.