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from psneeley@xmission.com's AO (American Optical) Spencer Microscope Manuals and Catalogs

ANALYZER TURRET REFERENCE MANUAL

AO 635 Camera, 1057 Lens & Shutter REFERENCE MANUAL

DIY direct projection photo tube


Reference Manual

Series One-Twenty

MICROSTAR Microscopes

Kohler Illumination

Kohler as described most places is NOT about even illumination;
using the condenser to focus and center the field aperture,
then tweaking condenser iris in the objective back focal plane,
is about minimizing stray light to optimize contrast.
The "Hidden" Assumption of Kohler Illumination
Using an illuminated ground glass can provide a more planar illumination source and consequently lead to more even illumination than any implementation of true Kohler is able to provide.

I did not have Kohler issues 20 years ago with my AO Series 10,
because frosted illuminator and neither field nor condenser diaphragm.
Nice Java applet no longer runs in Chrome browser:
Kohler Illumination: Interactive Tutorials: Lamp Filament Alignment

Even illumination is about focusing the lamp filament (or LED equivalent)
on the condenser iris (or equivalent objective back focal plane),
which consequently maximizes illumination defocus in the image plane.
Practically, my old eyes can better focus that lamp filament
on a ground glass over the field lens port, NOT on condenser iris.
For illumination port ground glass and condenser iris to be parfocal
seemingly implies parallel rays between the field lens and condenser... NOT:

https://zeiss-campus.magnet.fsu.edu/articles/lightsources/lightsourcefundamentals.html

For my Nikon Optiphot with the swing-out 0.90 condenser,
field aperture cannot be focused on the image plane
when my epi stage plate is in place...
The ELWD 0.3 condenser has no problem focusing the field aperture
with epi stage plate spacing slides 4mm higher from it...

Perhaps the Optiphot-66 stage and swing-out condenser.will be parfocal.

AO Series 120 has appropriate apertures and removable lamp diffuser:
.. but weirdly labels filament focus knob as "Condenser"
  because illumination condenser, which most others call "collector".

Focusing the filament using ground glass on the field illumination port is easy,
but on AO120 corresponds neither with apparent focus at object back plane
nor smoothest illumination,
which two do seem to better correspond, but change among objectives.
Condenser top swing is wanted for AO objectives less than 10x.
Objective back focal plane filament focus issues were discussed here at MicrobeHunter.com.
The solution, as prompted by viktor j nilsson, was to disassemble and clean its Bertrand insert.

A centering telescope is want for the Optiphots, particularly considering that one condenser lacks an aperture
and wants dialing in a phase turret repurposed for dark field.