In article ,
"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote:

Jon Elson fired this volley in
:

It may have had the optical grease between the glass elements
dry out and separate from the glass.

There are also (what do you call them?) "restrictor plates"... orifaces
designed to allow only the center of the light column to enter. If one has
been removed, that might also be the problem.

I don't remember what they're called, but they were essential to making
telescopes with spherical lenses operate with a clear image.

They are called "stops", and do exactly what you describe.

But what Gunner describes is probably not a missing stop. I'd bet on
creeping grease or mold.

Joe Gwinn