Install extension ring

 If you are using the 105 mm camera lens, you will need to be sure that the extension ring has been attached to the etalon/collimator section of the Fabry-Perot tube. This ring adds approximately an inch to the distance between the etalon and the camera lens. If this ring is not installed, there will be insufficient room in the Fabry-Perot tube for both the wedged etalon and the camera lens. The ring must be oriented properly. One of the two small holes on the outside of the ring should be centered over the rectangular slot through which the camera focus ring arm will protrude. The extension should be flipped to place this small hole beside the slot. (The hole is intended for a guide screw on the focus arm plate.) Then note that the camera lens plate has a small guide pin. This pin must be placed in the corresponding guide hole in the extension ring, such that the camera focus arm will then fit through the slot between the extension ring and the etalon/collimator section. It will take some thought to ensure that everything is oriented correctly. The extension ring should be secured with six screws of sufficient length to reach through the entire thickness of the ring.