Summary

This is used for lining up the mirrors on a Newtonian Telescope in the daytime

Looking through you should see the focuser tube end. the secondary mirror outside edge and the reflection on the secondary mirror, generally with the focuser all the way in you want to adjust the tilt screws and center screw on the secondary mirror to get them all concentric,

The line on the side of the tube is just to improve printing by giving somewhere for the seam to be placed, its not functional,