When getting the 135mm Rokinon, one downside to using it is how to mount it. If you were, for example, mounting on a full size equatorial mount, you need a dovetail. You could connect the dovetail to the body of the camera, but it might not be fully aligned with the mount. Add in a guide scope, and you need somewhere to mount that. This is where the Astrodymium Rings come in. The rings make sure you are perfectly aligned every time. And a bonus! It comes with a spot to mount a guide camera. Putting it together is fairly simple, please follow along with the video below to see what I mean!