So... I think I have it now. here is a picture of it and how it relates (in principle) to what I am doing in case you are curious... The octagonal thing holds the 12" diameter telescope mirror. [image: 9DRX_rockerbase.jpg] First, I drew the outline and used the offset tool to draw 1/2" in (on a separate copy) I drew the first large hole bottom dead center. then I measured 3 degrees to the left and estimated the center, trial and error. Then I did the same thing on the right for the remaining holes. The crescent attaches to the mirror box with four 5/16" bolts. The box, the crescent and the tine on the far side rotate in altitude about the hole in the tine. The crescent rests on two teflon bearings. The whole thing rotates about the azimuth in the center of the base plate. I gotta say, that after making the crescent, (took perhaps 3 hours) the rest of it went together in about 45 minutes from scratch. Did I ever mention how much I love Sketchup?

@billwheaton said: Thanks Martin. And I read those and they were a great help, but I didn't find much there about what the 'inverse' method does. I've never had occasion to use the inverse() method. I just use my Chapter 16 classes and the Transformation class stays out of my life.