Celestron Counterweight is there to balance weight of the optical tube (OTA) and accessories on the other side of the Right Accession (RA Axis). If you add accessories to your telescope setup (ie camera, guider, etc), you may need to add additional counterweights to off-set the weight. This counterweight comes with a sturdy knurled knob locks to safely hold in place on the counterweight shaft. The counterweight is designed to fit 19mm counterweight shafts.