A method is provided to calibrate a detection array used for acquiring an image of an interferogram at an instrument. At least one of opposing faces of the base housing and the elastic wall member on the working air chamber side thereof has a plurality of rib portions which extend from an inner circumferential side thereof towards an outer circumferential side thereof. A sensing device is located at a distal end of the cannula for detecting changes in conditions at the distal end of the cannula.