My 2-cents' worth:
Question: does your lamp housing have a concave mirror just behind the bulb? Either way the alignment is quite simple. It sounds complicated when I describe it below, but it is not.
The object is to get the filament image in focus at the back focal plane of the objective and filling (as far as possible) the aperture of the objectives (including the higher NA ones).
[Added in Edit]. Of course, this procedure is appropriate only for microscopes equipped with Koehler illumination.
1. If there is no mirror:
a. remove all diffusing filters (if any) from the light path.
b. focus the microscope on a slide using a 10x or 20x objective (stage micrometer is excellent for the purpose, but any slide will do). After you obtain focus, move the slide to a clear area.
c. Adjust the condenser so the edge of the field diaphragm is in focus, then open it all the way.
d. Turn down the lamp voltage so the filament just glows. Remove an eyepiece (or use a phase-centering telescope, or, if your microscope is equipped with one, use your Bertrand lens) and look at the back focal plane of the objective. If you are using a telescope or Bertrand lens, close the condenser iris partially so you can see its edge, then focus the telescope or Bertrand lens on the edge of the condenser iris.
e. Open the condenser iris fully, and, while looking at the back focal plane of the objective, adjust the lamp such that the filament is centered in the field of view and is in focus (filament is sharp).
f. Check the adjustment using higher NA dry objectives that you have installed. The image of the filament in the back focal plane of the objective should be centered, sharp, and should fill the aperture (you may not be able to to have the width of the filament fill the aperture if the lamphouse is not equipped with a mirror).
That is it.
2. If behind the bulb there is a mirror, then in step
e above, you look at the image of the filament
and the image-of-the-image of the filament thrown by the mirror and adjust the two images to be side-by-side and in focus. The idea is to better fill the back focal plane of the objective with the image of the [filament+its mirror image]. Otherwise the adjustment is the same as in
1 above.
If your lamp housing does
not incorporate a mirror, then, if your bulb is rated at 12 V and 50 W (or even 60 W) I think it would be worth using the Zeiss bulbs described in the link below (I bought several and they are excellent and the selling price is low)
viewtopic.php?f=16&t=147
The filament in those bulbs is flat and has a larger width-to-length ratio than the usual ones available, so their larger width fills more of the objective aperture, giving better resolution.
(I just checked, and the seller still lists them for sale). I think that they are worth using even if there is a mirror behind the bulb.
After I finished writing this I saw that lorez indicated that there is no mirror in this lamphouse, but I decided to keep the section concerning adjustment when there is a mirror just to be complete.
