Tom, it is easy and inexpensive. I have been convinced by the thread here that dental stone is too expensive. Most every one suggest a cement that small backyard ponds are cast form. I think it is called hydrastone or plastic cement. Anyway it will cost about $20 for 100 lb sack. the dental stuff is $20 for 25 lb. Neither require sealing. I use a tile that has a cloth mesh backing and epoxy it to the tool. Make sure the tile is thick enough to equal the sagitta when you get to focal length.
Bob Madden <madden@netcom.com>