4mm is getting a bit thin for the edge of a 112mm lens. You might want to consider making a nice Schupmann from the glass tho as it's quite probable that most, if not all, of the glass is from the same melt which is one of the requirements for the Mangin mirror in the Schupmann design. Bob May http://nav.to/bobmay bobmay@nethere.com NEW! http://bobmay.astronomy.net