Reader: Thanks for the previous reply. I have more detailed information for you this time, and another question. My supervisor tells me that our lenses which I have to test will each be mounted in its own metal cell. Our test needs to align them to a common optical axis and test them working as a system to produce an image of a test target. The target will be back-lit and use white light. The goal of this test is: determine the correct spacing between each lens which will result in the very sharpest image of that target. This has been calculated already using software, but we need to convince ourselves that the designer was correct! Finally, one of our other divisions has an alignment telescope which we can borrow from them to help with this testing. It is a Davidson Optronics D-275 Model Telescope and it’s in good calibration. How can I use this instrument to align those lenses and help make this test work? Thank you!
Tony: Yes, you can use that alignment scope for this test. First, you need to ask your lens designer if it is safe to assume that these lenses when mounted in their respective cells do not suffer from any “wedge” problem. If they do not have any significant “wedge” issues, then the optical axis of each lens will be co-linear with their mechanical axis. (Note that it is not a given that this will always be the case. I have seen purchased lenses which met all the procurement specs, but had bad wedge issues, because this had not been specified during procurement; a bad mistake. As a result, their optical axes were skewed with respect to their mechanical centerline axes.)
Next, make a cross-hair reticle to mount on each of the lens cells. This can be as fancy a reticle as you can afford to make. Or simple. You can have these made and mounted in your machine shop, if you have the time, and you need them to be perfect. Otherwise, make your own reticles with mono-filament nylon fishing line from a sporting goods store. Use a line which is around 10 mils thick. Mount two pieces of this mono-filament nylon line on each lens cell so that they cross at the mechanical center of each lens. These lines do not have to be at exactly 90 degrees to each other. The important thing is that their intersection (crossed-hairs) is as close to the exact center of the lens as possible. Mount them under some tension, so they do not flop or flex. This concept assumes that the lens does not exhibit any wedge! When they all have cross-hair reticles mounted, continue on.
Before you begin to mount these lenses on your test bench, first set up the D-275 Telescope behind the place where you expect the image plane to be, so that it’s looking directly at the center of your test target, and so that its line of sight is passing directly through the center of your image circle in the image plane. Mind you, this is not easy to do, it will take some effort and time, but it is critical. So do this part well! What this will accomplish is the line of sight of that D-275 Telescope will represent (take the place of) the actual optical centerline of the lens system you will be testing. (Remember, you can focus the Davidson D-275 scope anywhere in space from infinity down to 16 inches away from the front of the lens barrel; so take this into consideration when setting up the scope.) Note, from this point on, do not move the D-275 Alignment Telescope in any way! It is now your reference line of sight. When you are sure you have this condition set up, then continue on.
As you insert each lens and cell into your test bench, refocus the alignment scope as needed until you see the reticle on that lens cell in sharp focus. Move or remount each lens cell until that cross-hair intersection is located exactly on the center of the thin black internal reticle of your alignment scope. You may have to use a bright light source to illuminate the lens reticles to see them well. Repeat this step with each lens & cell until they are all mounted on your test bench.
When you have all the lenses aligned to the line of sight of the D-275 scope, you can be fairly certain that their optical axes and mechanical axes are co-linear and aligned to your test target and image circle. Note that when you have to remove the Alignment Telescope, (I assume you will have to remove it to proceed with your image quality evaluation), it would be ideal if you had it mounted in such a way that it could be replaced back on that test bench and returned to the exact position in space where it was before. That’s in case you need to repeat the alignment. Otherwise, you will have to realign your line of sight again! This may be tricky to do, but it’s well worth the effort to get it right the first time! (Experience speaking here!)
Of course, if your lenses need to be in very close proximity, so that the reticles interfere with lens surfaces, then you will have to cut them off prior to final adjustments of the lenses. This is where you would appreciate a “cleverly made” reticle which can be removed and reinstalled any time. We do not always have that luxury. You may need to remove them anyway, to prevent them from optically perturbing your image evaluation. It depends on the details.
Then, just move those lenses back and forth, but only in their axial direction, until you find their optimal spatial positions for the best image results. I am assuming that you have a mechanical means for precisely adjusting the axial position of each lens. You are going to need that! I have omitted some details in this overview. Since your company has done other optical testing, it’s fair to assume that you will have the ability to work through some of the detailed engineering steps, which you are likely to encounter in this work.
Good luck with your testing. I hope this information will help you. T.D.
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