The 66297 Flange Step Down Adapter has a 3 Inch Series Female flange on one end and a 2 Inch Series male flange on the other. It allows you to take advantage of our wider selection of 1.5 Inch Series accessories. Keep in mind that there will be power loss from a collimated beam due to the reduction in clear aperture from 3.5 to 2 inch. If the beam is focused, the step down can be made without power loss. You can then re-collimate the beam with a 1.5 Inch Series secondary focusing lens assembly.