This 1.25" collimator is a tool to help align the primary and secondary mirrors in a Newtonian style telescope. The laser used is a pen style pointer. http://www.gyrobot.co.uk http://www.facebook.com/gyrobotukPrint without support. Add 6 x M2.5 nuts and screws. Insert a pen style laser pointer and centralise with the two sets of M2.5 alignment screws. Insert collimator into telescope and adjust primary and secondary mirrors to get laser to bounceback to desired accuracy. Tip: Keep the laser on/off push button allways on with sticky tape. Loosen and tighten the rear battery screwable cover to switch laser on and off. You can attach to telescope with my "Self Centering Bush" http://www.thingiverse.com/thing:91390 Crosshair grooves also printed for the addition of wire or thread. Since printing the prototype,I've modified the sight hole in the .stl to be a teardrop shape to aid printing without support. Here's a video showing how to use a commercial version: http://youtu.be/N1asF3m2Mh0