Buckeye Duncan Mask for SCT Collimation

4. A camera works best, but you can also use a high magnification eyepiece.  Grossly defocus the star. You should see three arcs that correspond to the three cutouts.

5.  As you bring the star closer to focus the curves will change into lines pointing to a common center.    

6.  Use the lines to adjust the collimation screw opposite the line.  As you adjust the screw the line will move toward the center.  The goal is to form a perfect Y where the lines touch in the center.