Parallax Free Arcuate Astigmatic Keratotomy Marker, Thonrton Design. 360 Degree marker.6 millimeter inside diameter and 8 millimeter outside diameter ring with a measure outside line (outside diameter). Lowest profile with center pointer.The round handle is 12.2 centimeters long. Marks at every 10 degrees and features two wings to assist in marking the axis of astigmatism span style="color: #004fa3; font-family: verdana, Arial, Helvetica, sans-serif; font-size: 12px;