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(AO 20 series)Aligning the light path

Posted: Tue May 12, 2020 4:45 am
by mnmyco
I wasn’t paying much attenion to what I was doing and loosed all three screws holding the nose piece in position on a AO series 20. Anyone have any advice on this. I wish I had a cross insert for the eye pieces. I have a cross Zeus slide, but even so in my opinion I should have a cross in the on the lens that the light exits the base from and in the condenser.

Is that many points of alignment actually needed? I welcome all opinions on the easiest method that produces proper alignment.

Re: (AO 20 series)Aligning the light path

Posted: Tue May 12, 2020 2:10 pm
by apochronaut
I can't recall, if I ever knew. Do you have a phase system and or a bertrand lens?

Re: (AO 20 series)Aligning the light path

Posted: Wed May 13, 2020 1:22 pm
by apochronaut
Having a phase system is useful in aligning because you can use the bertrand lens to align what are essentially a series of circular ghost images, concentric with an annulus in an objective. You see this , every time you engage the bertrand lens and the nosepiece can be aligned while viewing the annulus.
However , this calls in to question how concentric the rest of the objectives are? There isn't the possibility on those nosepieces to adjust each objective separately. What I do when I have an unknown microscope in my hands is, or one I suspect is not well centered is I rotate the objectives one by one around the nosepiece in the direction of rotation preference, and settle on the locations that are best for parcentering. You can do this by selecting a feature on a slide, then one by one check it's position with each objective. Then, swapping each objective forward one position you check the position of your chosen feature. Usually , one location in the rotation is better than the others and you can get to not much over 10 microns, 20 would be an outside variance. If there has to be one that is a bit off; in otherwords if one of the bores in the nosepiece is a bit off, I use it for a low power objective. I aim to have the 40X and 100X as close as possible for use and they can usually be relied upon to be within 10 microns, not much more. You are going to use the 10X objective to align visually, if you have no tools, so try to find a port for the 10X, where it is dead on the 40 and or 100X. That location is only used for the 10X , during alignment.

Next, check the location of the stop spring. These are adjustable and I have seen more than one that has gotten a bit off, one way or the other. This will affect the levelling of the objective. Aligning it is based on the stage being level, by lowering a flat bottom objective such as a 10X onto a flat surface stretched over the stage aperture. Adjust the spring, if need be so that the objective is perfectly flush on that surface. AO did not make it easy for that stop spring to be tampered with. The allen screws which fasten it are a very odd size. 1.75mm. It's probably a 1/16" but it fits a bit slack. I have found a Torx 6 to work best.

Once I have the objectives in the closest relative alignment and level, I remove the head of the microscope and looking down into the yoke, visually align the nosepiece in the dovetail by making the exposed threads of the three alignment screws the same length. I then replace the head and remove the condenser.

The only two elements in the microscope that need absolute alignment are the objective and the optical tube leading up to the exit pupil of the eyepiece. All else is adjusted relative to that but the illumination beam makes a useful reference to align the entire optical tube to. Without the aid of any pre-aligned visual tools such as crosshairs , visual alignment is usually good enough and will have to do. You need to make sure your illuminator is aligned well though. So, make sure your filament is centered and the illumination is even all the way around. You will be using the illuminator window as your alignment reference.

With the illuminator on, remove the eyepiece and look down the optical tube into the 10X objective. You will see the illuminator window reasonably centered and it will cast a glow on various physical features in the optical tube above the objective( adjust the illumination, low). The reflections on all those features should be even and condcentric all the way around. Adjust the nosepiece in order to obtain parcentering of whatever concentric rings of light you see. Pay particular attention to the glint on the lip of the optical tube. The illuminator window should be dead center, when the glint on the the lip of the optical tube is even all the way around. You may need a pair of reading glasses to get the illuminator window focused, well. You can get better than 95% using this methodology, probably closer to 98 or so. Condenser goes back in and gets adjusted to everything else.

Avoid any of this , if you can and are reasonably sure it is correct. I have made up a # 1076 10X objective with a cross hair at the focal point. It works but only on infinity corrected systems. I'm willing to lend it to anyone who wants to use it through the mail. Round trip would probably be about 25-30.00.