Misaligned Lamp Filament?

[fastbilly]

I have been shooting images of subjects on prepared slides according to a grid so that I can then stitch them together. Though not noticeable to the eye in an individual image, when stitching two images (call them the “lower” and the “upper” images) I find that the bottom of the upper image is darker than the top of the lower image. When stitching multiple rows this pattern is repeated at each row. My stitching software (PTGui) can smooth for the exposure differences in each image, but in doing so it creates a smooth gradient running across the entire stitched image, from the bottom to the top, with a very dark exposure at the lowest image to a very light exposure at the uppermost image of the stitched grid (Photoshop and Autopano do the same thing.) The more rows included in the stitch, the more severe the exposure gradient becomes across the combined/stitched image.

I suspect a misaligned lamp filament in my microscope. I have an Olympus BHTU with trinocular head, 1.67 projection eyepiece, Canon DSLR and typically shoot with 10X objective. (I have been following all of the regular steps to set up my Kohler illumination.) As I have seen suggested in various places I have been trying to observe the filament to check its alignment but I see nothing but a uniform light when I remove the eyepiece or when I place a lens cleaning tissue over the fully opened field diaphragm; that is, I do not see the filament. (I am wondering if there is a diffuser built in to the light path between the bulb and the field diaphragm.)

I took a photograph of an empty (dusty) slide and then tested it in Photoshop. Indeed, the upper portions of the slide have brighter values than the lower portions when I test them with the eyedropper tool. I have enhanced the photo with a few adjustment layers to bring out the differences and you can see clearly how the upper portion differs from the lower.

I have included both the enhanced and unenhanced photos here.

My question: do you agree that it is likely the lamp is misaligned? And, how do I fix this? (My guess is this will require a technician.)

Any advice is much appreciated!

Bill