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 Köhler illumination: lamp, field diaphragm and condenser alignment Any number of websites will waste space pointing out how important it is, then provide miserable, incomplete and wrong instructions. Köhler alignment simply means getting filament focused on objective back focal plane, which will also be in focus at condenser aperture and defocused at object plane. Fortunately, Wikipedia does all right. Köhler 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 conjugate focal planes and minimizing stray light to optimize contrast. The "Hidden" Assumption of Köhler Illumination An illuminated ground glass can provide more planar and even illumination than can implementing Köhler with incandescent filaments. I did not have Köhler issues 20 years ago with my AO Series 10, because frosted illuminator and neither field nor condenser diaphragm. Filament alignment applet: Köhler Illumination: Interactive Tutorials: Lamp Filament Alignment Focusing a lamp filament (or LED equivalent) on the condenser iris (or conjugate objective back focal plane) consequently maximizes illumination ray parallelism in the image plane. Without a centering telescope or Bertrand lens, 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 Note: the above image oversimplifies Nikon illumination optics, which include a field lens between Aperture Diaphragm and Condenser. 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 AO 120 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. Köhler mystery  Optiphot field lens focuses Universal EPI illuminator LHS-H100P-1 lamp filament on ground glass,
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