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Hi,

Thanks a lot for your suggestions!

I need this pinhole to be positioned such that it can be removed (e.g. for
other users doing normal epi-fluorescence).  The best way to do this without
re-engineering the whole field diaphragm unit, seems to be to replace one of
the ND filter sliders with a pinhole.  However, the plane of this filter is
about 5 mm distant from the field diaphragm.  I guess that means I'll have
to re-focus the lamp light onto the pinhole, instead of the field diaphragm?

The only focusable element in the light path is a 'collector' lens on the
lamphouse itself - I don't know if that will suffice to refocus the light. 
There are two lenses in the main epi-fl unit; one rear and one front.  It
looks like the front one (nearest the objective), while not 'focusable' per
se, can be translated by unscrewing a nut that holds it in place.

Any ideas on how best to refocus?  The lamp-house we use is mounted on the
rear end of the microscope and that leaves no room for inserting new optics
in the light path, so the only option is to replace one of the existing
lenses.  Is this feasible, e.g. replacing the lamphouse collector lens?

thanks again for your help!
best
badri