mawyatt wrote:These are called Crayford Focusers I believe.
This should be a viable approach.
The best place for such a device will be between objective and tube lens. In that position, the rear lens will remain focused at infinity. The effect of the focusing extension will be to adjust distance between subject and objective, just like the focusing stage of a microscope. Being located in an infinity section of the optics, the added extension will have little effect on image quality, other than increasing or introducing vignetting.
You could also stick the focusing extension between the tube lens and camera. In that position, its major effect will still be to adjust distance between subject and objective. However, by extending the rear lens by the minimum thickness of the focuser, it will also force the rear lens to focus closer than infinity. That will introduce a certain amount of spherical aberration by forcing the objective away from its design point. The same as with AF motor focusing, the added aberrations will be small with low NA objectives, then becoming significant with high NA.
Helicon now makes a device they call Helicon FB Tube
. It is an automated extension tube that comes in Canon and Nikon mounts. It's not clear to me whether the device has small enough focus steps for use with microscope objectives on the front, but if it does, that could be a handy device.