**This is my third instructabe. It was originally written for a magazine but was never published. Rather than rewriting the whole thing, I am posting it as it was supposed to appear in the periodical**
So I'm at the Heart of America Star Party. I'm at the swap meet. I'm browsing along and here's this fella with quite a few pair of binoculars and other assorted optical instruments laid out on the table. I look at some lens assemblies and porro prisms, glance at the spotting scopes and find myself at the binoculars. I pick up a pair of 20x60s and look through them. I ask the gentleman how much, fully expecting not to be able to afford them. After about the third time, my mind finally gets the message that yes, they are free. I ask to take them outside for a better look.
Satisfied that the binoculars were usable, I informed the gentleman that I would be glad to take them off his hands for him. A short time after getting back to my campsite, the luster of getting such a good deal was quickly wearing off. I attached the binos on my mount and looked at a distant tree. I almost immediately noticed I was having trouble fusing the two images reaching my eyes. After attempting to adjust the inter-pupillary distance (IPD), and the focus for each eye individually with no improvement in the image, I realized that there was a deeper problem. Casually inspecting the binoculars showed that there was very thick and sticky grease around the right eyepiece housing and adjusting the IPD caused the eyepieces to twist vertically. Still, I was confident that cleaning the dirty spots and lubricating the sticky spots would yield a functional set of binoculars.
Jump to a few days later. I have the binoculars at the work bench. Using a piece of sheet steel, I am able to release the screw that holds the eyepieces to the focusing rod. This allowed the eyepieces to be individually removed from the binoculars. A little more work with a lot of paper towels and some solvent (I used acetone) nicely cleaned up the excess grease. I reassembled the eyepieces and took a look. I still had trouble fusing the images and the eyepieces were still floppy when adjusting the IPD. Its back to the work bench for me and my new binoculars.
A closer inspection of the binoculars revealed that the eyepiece flop was due to a looseness in the focusing mechanism. After removing the eyepieces, the focusing knob can be unscrewed and the entire focusing mechanism pulled out of the binocular body. After much thought on how to improve the fit of the mechanism, I decided to clean the rod and add foil tape to improve the fit into the body. Reassembly after some light lubrication revealed only a small improvement. The majority of the wiggle now seemed to be in the threads of the focuser. Reviewing my work so for, I have cleaned, tightened and lubricated most of the moving parts of the focuser. I am still having trouble fusing the images from each eye.
My eyes are good. Must be collimation.
If you are like me, the idea of collimating binoculars causes a brief shudder to crawl across you. But do not fear! Now that I have done it, I can say that it is not that bad. Perhaps the trickiest part of collimation is finding the collimation mechanisms. I can say that the two pair of binos I now own appear quite different on the surface. However their basic concepts are the same. Provided the binoculars are not severely out of collimation, the collimation is accomplished using the porro prisms housed just in front of the eyepieces. The prisms are held onto a mounting plate with a metal clip . The arrowed screws in the images are the collimation screws that pass to the outside through the housing. They work by forcing the prisms to tilt within the optical path thereby shifting the view. Binocular designers like to hide these screws so they may not always be obvious. The 20x60s that I am working with have putty in the holes that matches the grip coating on the housing. My 7x50 Nikons that already work wonderfully have the housing covered in a rubber cloak that has to be pried up to reach the screws. Once the collimation screws are located the collimation can begin in earnest.
Before I began, I referred to the book, "Basic Optics and Optical Instruments" which is the civilian reprint of, "Opticalman 3&2" originally printed by the U.S. Navy. It is a wonderful resource for binoculars and basic optical concepts. In it, it describes the navy's method for optical procedures. It describes the use of the Mk I 3x auxiliary telescope to assist in collimation of binoculars. Not having this specific scope is not a problem as any good, small finder scope will work. I used a 6x30 finder with a cross-hair reticle with excellent results.
The goal of collimation is to have both sides of the binoculars pointing in exactly parallel directions. The book I used declares that alignment vertically should be better that 2 minutes of arc (one side pointing higher than the other). The alignment left to right should be better than 2 minutes of arc inward (think of being cross-eyed) and 4 minutes of arc outward (the condition opposite of being cross-eyed).
With this information in hand I was ready to collimate! First thing first, I mounted the binos in a solid mount. I took the setup outside and sighted a distant tower. Placing the finder scope between the eyepiece and the eye does two things, first, it magnifies the image by the product of the two devices (6x finder and a 20x binocular = 120x) Second, as with any increase in magnification, it restricts the field of view. This has the affect of forcing one to see only the center of the binoculars FOV. With the finder scope I aligned the left side of the binoculars on a tower in the distance. Next I sighted through the right side of the binoculars. The place I saw is indicated as point "R1" with point "L" being the place the left half of the binocular was pointed to. I selected one of the collimation screws and gave a small turn (less than 1/8 of a rotation) which caused the image to shift laterally. Trying the other collimation screw caused the image to shift vertically. To make a long story short, I pointed the right half of my binoculars to the point indicated by "R2". During the process, I repeatedly checked both the right and left optics to verify my progress. With collimation completed I filled the collimation screw holes with plumbers putty.
My free binoculars are now working like a champ. I hope this instructables helps those of you with troublesome binoculars to get them up and running. Good luck and clear skies!