This is a circle cutting jig that I made in order to cut & shape the various mdf molds that I used to cast the various fibreglass components for the telescope (eg. end rings, mirror cell). A threaded rod & nuts are used to adjust the distance of the rotation pin hole (drilled in the fibreglass slide that is held in a v-groove cut in the mdf) from the router. In this image the fibreglass slide is adjusted close to the router (small circle) - note the ruler set parallel to the slide which is used to set the radius, and the metal indicator that is screwed into the slide (this was later replaced with a clear plastic indicator with an inscribed line, as it was found difficult to get an accurate reading using the edge of the metal indicator).
I tried a commercial product first but found it to be too flimsy and did not cut perfect circles.