Kim Coxon
Moderator
Hi Joe,
Can you post a pic of the rear group. From your original description, it didn't sound like a balsam fault to me. Normally, any separation of the doublet will affect only a small area of the group. It will grow over a period of time but my understanding is that it would be rare for the whole group to separate. All of the balsam faults I have seen appear to look more like an "air bubble" in the middle of the glass. I would have thought it likely that if the balsam had gone right across the surface, the 2 elements would have come apart. A doublet is usually made such that the 2 faces that meet should be identical in a "perfect" lens and make a single element. This is never achieved in practice and you are likely to get newton rings where the 2 elements meet. The purpose of the balsm is mainly to fill the very small imperfections and the "glueing" is more of a useful by product.
Looking at Bill's links, it should be fairly easy to tell if it dissolves in xylene. You should be able to get some at a hobby store. Floquil paints used to be xylene based and I am sure there must be others.
Kim
Can you post a pic of the rear group. From your original description, it didn't sound like a balsam fault to me. Normally, any separation of the doublet will affect only a small area of the group. It will grow over a period of time but my understanding is that it would be rare for the whole group to separate. All of the balsam faults I have seen appear to look more like an "air bubble" in the middle of the glass. I would have thought it likely that if the balsam had gone right across the surface, the 2 elements would have come apart. A doublet is usually made such that the 2 faces that meet should be identical in a "perfect" lens and make a single element. This is never achieved in practice and you are likely to get newton rings where the 2 elements meet. The purpose of the balsm is mainly to fill the very small imperfections and the "glueing" is more of a useful by product.
Looking at Bill's links, it should be fairly easy to tell if it dissolves in xylene. You should be able to get some at a hobby store. Floquil paints used to be xylene based and I am sure there must be others.
Kim
