on 7/19/01 7:31 AM, Mark Dapoz at md@xxxxxxxxxxxx wrote:
> On Thu, 19 Jul 2001, O.C wrote:
>
>> The problem described that occurs with the foam and prism is that the foam
>> rots and becomes bound to the prism surface. When stuck on the glass it
>> alters the glass to air interface that is required for the total internal
>> reflection to occur. What you see instead is the foam through the glass.
>> Thats why cleaning the prism of the gunk and restoring the original glass
>> air interface will restore things.
>
> In theory what you describe is true, but I've found in practice that's not
> the case. I have a prism which is about 80 0e-silvered. I can get it
> to work reasonably well if the prism surface is perfectly clean. Unfortunatly
> problems arrise at the sharp edges of the prism (at the top and around the
> eyepiece). At these points you can clearly see a faint line in the
> viewfinder.
> No amount of cleaning will make these lines disappear. I suspect the silver
> backing which is applied to the prism somehow "fixes" this problem. A prism
> with no silver backing is still useable, it's just not as good as one with a
> silver backing.
> -mark
>
Could you pull a prism out and have it silvered at a mirror-reslivering
service? Astronomers resilver the big mirrors (even doing it in-place) on
the telescopes... seems to me this could be done for a price. Depending on
that price and how difficult replacement new prisms are to find it might be
worth it... and in the future it might be the only way to fix the problem.
--
Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney...
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