An optical spectrum monitor includes a tunable optical detector that receives optical channel monitoring signals from an optical channel and receives a control signal that selects a frequency of an optical channel monitoring signal for detection. The tunable optical detector detects the optical channel monitoring signal selected by the control signal and generates an electrical signal related to a power of the detected optical channel monitoring signal. A processor analyzes a plurality of differences between selected optical channel monitoring frequencies and their corresponding ITU frequencies and then generates the control signal at the output that corrects for at least one of systematic band offset and systematic band tilt in the optical channel monitoring frequency.