A tunable filter stage having a selected temperature range, comprising: a variable retarder element with an adjustable first retardance; a fixed retarder element with a second retardance, wherein the second retardance exhibits a thermally dependent characteristic such that the second retardance varies with temperature; wherein it further comprises: a control element in communication with the variable retarder element, the control element being configured to control adjustment of the first retardance, wherein the control element is configured to select a target retardance for the variable retarder element, and adjust the first retardance to the target retardance, based on a selected operating temperature range and the thermally dependent characteristic of the second retardance, the target retardance for the variable retarder element being selected so as to impose a compensating shift in the variable retarder element to compensate for thermal drift in the second retardance of the fixed retarder, such that the net retardance produced by first and second retardance is independent of temperature over the selected operating temperature range and does not change, for a certain wavelength λ, the order N(λ) over the selected operating temperature range, the order N(λ) being an integer counting the number of full wavelength periods of wavelength λ of the net retardance, so as to maintain a bandwidth B(λ) of the tunable filter stage over the selected operating temperature range. a variable retarder element with an adjustable first retardance; a fixed retarder element with a second retardance, wherein the second retardance exhibits a thermally dependent characteristic such that the second retardance varies with temperature; wherein it further comprises: a control element in communication with the variable retarder element, the control element being configured to control adjustment of the first retardance, wherein the control element is configured to select a target retardance for the variable retarder element, and adjust the first retardance to the target retardance, based on a selected operating temperature range and the thermally dependent characteristic of the second retardance, the target retardance for the variable retarder element being selected so as to impose a compensating shift in the variable retarder element to compensate for thermal drift in the second retardance of the fixed retarder, such that the net retardance produced by first and second retardance is independent of temperature over the selected operating temperature range and does not change, for a certain wavelength λ, the order N(λ) over the selected operating temperature range, the order N(λ) being an integer counting the number of full wavelength periods of wavelength λ of the net retardance, so as to maintain a bandwidth B(λ) of the tunable filter stage over the selected operating temperature range. a control element in communication with the variable retarder element, the control element being configured to control adjustment of the first retardance, wherein the control element is configured to select a target retardance for the variable retarder element, and adjust the first retardance to the target retardance, based on a selected operating temperature range and the thermally dependent characteristic of the second retardance, the target retardance for the variable retarder element being selected so as to impose a compensating shift in the variable retarder element to compensate for thermal drift in the second retardance of the fixed retarder, such that the net retardance produced by first and second retardance is independent of temperature over the selected operating temperature range and does not change, for a certain wavelength λ, the order N(λ) over the selected operating temperature range, the order N(λ) being an integer counting the number of full wavelength periods of wavelength λ of the net retardance, so as to maintain a bandwidth B(λ) of the tunable filter stage over the selected operating temperature range.