I have a an aspheric lens element exported in IGES format from ZEMAX. The import went fine; no errors to fix. But now I can't put a tangent relation to the aspheric surface. In fact, I can't do anything.

For example, I take the midplane of the lens, and sketch on it. I can select the silhouette of the aspheric surface, but if I try to "convert entities" it fails, and if I draw a line and try to make it tangent to that silhouette (which is what I actually need) it fails.

Anyone?