Reduce your telescope's coma aberration using the Baader Planetarium Mark III Laser Collimator from Alpine Astronomical. This compact and lightweight device fits directly into the 1.25" eyepiece tube of your Newtonian or Schmidt-Cassegrain telescope and uses its 1.0mW laser to help you fine-tune your optics. It's body design features a cutaway side wall that exposes a screen perpendicularly oriented to the beam, with a hole for the passage of the laser beam and a grid of finely ruled etched crosses for viewing the returned beam. The pattern formed by the return beam provides the information you need to properly compensate for the distortion.