Downhole spectrometer tools are provided with two ways to increase the number of filters on an optical path. A first approach employs multiple filter wheels that rotate alternately in a common plane to intersect the optical path. Portions of the wheels are cut out to avoid mechanical interference between the wheels. A second approach drives the one or more filter wheels with a wobble that causes the filters to trace one or more hypocycloidal curves that each intersect the optical path.