This eyepiece is used to collimate your newtonian / printonian telescope's primary and secondary mirrors.Inspired by this laser collimator, http://www.thingiverse.com/thing:12624 but here there is no need to collimate the collimator ;-) .Use: several tutorials online, I especially like this one: http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm check also this:http://www.atmsite.org/contrib/Carlin/collimation/kolli2.htm#mkcombinationandhttp://www.skyandtelescope.com/howto/diy/3306876.htmlThis should be a close fit in a 1.25" eyepiece adapter like this one: http://www.thingiverse.com/thing:19200Check your printer calibration with test_cheshire.stl (only 5 mm high), eventually modify the diameter in the openscad file and regenerate a stl.Print collimator_grid.pdf or dxf, cut, center and glue on the inner bevel. Enjoy!Also included a mask to mark the center of a 4.5" primary mirror.