In article , Creadick@nando.net says...

>I have read that the Nikkor 500 mirror needs a rear filter in place to
>complete the optical formula. Is this really true? Are they special
>filters or will any 39mm work? Thanks. Nowell

If you use any 39mm filter other than a Nikkor, MAKE SURE that
the rim thickness is similar, or the SLR mirror may strike it
if it is too thick. Optically, any (excellent) filter should
work, but filter quality is most important in long lenses.
The filter must be removed when using the TC14 converter,
and the combination (old 500mm f8 version) is excellent,
so my guess is that the filter is not needed, except
maybe for keeping the distance scale reasonably
accurate.
Hope This Helps