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These 90mm i.d. Astro-Tech split and hinged optical tube mounting rings are made of solid aluminum, anodized black for long life. They are felt-lined and have a large hand-tighten knob to lock them on your optical tube. A heavy nylon washer prevents the knob from marring the anodized finish. The rings have a 90mm internal diameter to fit refractors with a 90mm OD tube, such as the Astro-Tech AT80 and all William Optics 80mm telescopes (all versions of the Megrez 80 and ZenithStar 80, for example), plus any other scope with a 90mm OD tube. There are five mounting holes on bottom of the rings for mounting on most equatorial mount dovetail plates. The center three holes are 6mm metric threaded, while the outer two are 1/4”-20 thread. One 6mm hole is centered in the flat mounting boss in the bottom of each ring. It is flanked by a pair of 6mm holes on 60mm centers, and a second wider pair of 1/4”-20 holes on 80mm centers. An identical set of five holes on the top of each ring allow installing a second dovetail plate to carry accessories. Two 16mm long x 6mm hex head metric bolts are supplied with each ring for mounting the ring on a dovetail plate or equatorial mount using the center hole or two flanking holes.
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