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Home / “Collimating” Binoculars
by Ed Zarenski 04/17/03 | Email Author

The following article describes what I think binocular owners should know before they attempt simple collimation using the prism tilt adjustment screws. I have been using astronomical equipment including several binoculars for over 30 years. I now own about a dozen different pieces of equipment, including 6 pair of binoculars. For the last three years I have pursued learning what I can and teaching others what I learn.

Cloudy Nights has been my primary conduit for sharing astronomical information. I am pleased to be a member of the Cloudy Nights community in that they have encouraged and supported my endeavors to share my knowledge of astronomy with kids and the astronomical community at large. I encourage you all to join with Cloudy Nights in its various attempts to reach out to children and spread some astronomical knowledge. And if you intend to do it with binoculars, then by all means, read this article and get adjusted. Thank you.
Clear skies, and if not, Cloudy Nights.
EdZ.

Is Collimation simple?

How many of you have pried loose the rubber armor grip cover of your Oberwerk (or similar brand) binoculars to gain access to the prism tilt “collimation” screw. A quick turn of the screws and the alignment appears much better than it was before. The star images have been brought together and you consider your binoculars collimated. Well, maybe the images appear merged, but are they aligned on the centerline, or the optical axis of the binocular? I am guilty of this simple procedure. So please allow me to share my lessons learned with you.

Merged and Collimated

I just recently bought a used pair of Oberwerk 20x80’s on Astromart. The seller confirmed that he had to collimate the binoculars when he bought them. The binoculars were observed to have the little protective glue spot still in place over both screws on the left prism housing, but the screws on the right housing had been accessed. These binoculars may have been adjusted to merge the images, but they where by no means collimated. Properly collimated binoculars will show merged star images AND will show nice round exit pupils. These binoculars had nearly merged images, but the light in the exit pupil of the right eyepiece was so cut off it looked like a cat’s eye. I would estimate by the shape of the exit pupil that there was a 30% to 40% image loss.

My Exit Pupils Look Like Cat’s Eyes

What I want you to do is step back from your newly collimated binoculars and take a good hard look at the exit pupils. Are they still perfectly round? Or does one or both of your exit pupils now look something like a cat’s eye in daytime, sort of elliptical but with points at both ends. Improperly adjusting the prism tilt “collimation” screws can tilt the axis of one prism so badly, in relation to the other prism of that eyepiece, that the end result is a vignetted light path in the eyepiece. This can get to the point where you may be able to visually see the cut off in the exit pupil image. If you see this pointed elliptical image, you should start all over.

What Does this Screw Do?

To see what I’m talking about, do the following test in daylight. Caution! This test will modify the collimation of your binoculars. But, if they are already out of collimation, you have nothing to lose by proceeding. Set your binoculars on a table or mount them on a tripod facing out the window towards bright daylight. This will allow you to easily see the exit pupils. Now, with the jeweler’s screwdriver set in the prism tilt screw and ready to turn, back your eyes away from the eyepieces maybe about a foot or so. Turn the screw back and forth slightly and watch what happens to the shape of the exit pupil. Not only does the exit pupil move off center in your eyepiece, but it also changes shape. You will see that over-adjustment of the prism tilt screw results in an unacceptable change in the shape of the exit pupil image. Round images indicates the full light path is passing thru both prisms and out thru the eyepiece. Cat’s eye shaped images indicates the prisms are tilted out of alignment and you are losing light that is not passing through the prisms to the eyepiece. If you do not maintain near perfectly round exit pupil images, you have gone to far!

What Effect Does All of This Have?

Try this. Do you have a circle template handy? Use it to draw a perfect circle on a piece of paper. Now slid the template over just a little, say one quarter of the distance across the first circle you’ve drawn. Draw another circle. You now have drawn two circles that partly overlap. Color in the two crescent moon sections that do not overlap the other circle. The area of the central overlap now stands out. You can see you drew a picture that represents an exit pupil image that is cutoff as if two circles of light do not completely overlap. This is the same as the image produced when the light passing thru the two binocular prisms is not completely passed from one prism thru the other. This is what happens when you tilt the prisms in your binoculars.

How Much Light Is Lost?

For a binocular with a perfect round 5mm exit pupil, if the image is cutoff to represent a normally 5mm high but now only 4mm wide pointed ellipse, instead of a perfect 5mm circle, a mathematical computation will show the resultant area of the image has been reduced by approximately 30%. That’s the equivalent of taking one side of a 80mm binocular and masking the objective lens down to 66mm, or for 70mm binoculars, reducing the objective to 58mm. Remember we are dealing with the area of circles here. A 4mm circle, only 20% smaller in diameter than a 5mm circle, has only 64% of the area.

Starting Over!

For those of you who have already created the cat’s eye exit pupil, resetting the prism tilt screw to the original position may be as simple as remembering which way you turned the screws before and turning them back to where they were. You need to get the exit pupil images back to round. This will probably result in your binoculars once again being out of collimation, however you will at least have a fully illuminated optical light path thru each side of your binocular.

True Collimation

The first, and probably most important thing you need to know is that proper collimation of binoculars is achieved primarily by adjustment of the objective lens, not by tilting the prisms. TURNING THE PRISM TILT SCREWS IS THE LAST RESORT IN COLLIMATING BINOCULARS. But to get down to real life, many of us are not going to remove the objective lenses from our binoculars to re-center the object lens with the optical axis. And in many cases, the binoculars you own may not even allow for removal of the objective lens. For those of you who will attempt lens removal, for a thorough discussion of this procedure I refer you to “Choosing, Using and Repairing Binoculars”, by J.W.Seyfried, University Optics, Inc., 1995. Seyfried is founder and President of University Optics. This book is available through Edmund Scientifics Online at www.scientificsonline.com, item # cr30075-24, cost $19.95. Odd, but I did not see this book offered for sale on UO’s website.

There’s More Than One Screw

The second thing you need to know is there are two prisms in each eyepiece, and THERE ARE TWO PRISM TILT SCREWS ON THE OUTSIDE OF EACH EYEPIECE PRISM HOUSING. By this time, you have already found one of the screws, near the back end of the binoculars, on top of the prism housing, just under the rubber edge. The other screw is on the prism housing towards the front very nearly behind the binocular pivot bar. With the binoculars setting on a table, consider up to be 12 o’clock. Facing as you would if you were viewing thru the binoculars, for the left eyepiece, if the first screw you found is located at 11 o’clock near the back of the housing, then the other screw is exactly 90 degrees or at 2 o’clock and towards the front. Likewise, for the right eyepiece, if the first screw you found is located at 1 o’clock near the back then the other screw is exactly 90 degrees or at 10 o’clock and towards the front.

Do I Need Adjustment?

Back to the world of people who need a small adjustment, (that would be most of us!). No, no, no. I mean, people whose binoculars need a small adjustment. If your binocular collimation is out of adjustment by only the slightest little bit, you could probably get away with adjusting only one screw on each eyepiece with no drastically noticeable light cutoff. And if you can achieve merged images without noticeably altering the exit pupil image, then you can stop and consider yourself lucky. But if your collimation needs quite a bit of adjustment, the process should involve turning the screws on one eyepiece a little bit and then turning the screws on the other eyepiece a little bit. Screws should be turned in baby steps, sort of like collimating a Schmidt Casegrain Telescope.

Are My Images Merged?

So how do you determine if the images are not merged, indicating binoculars in need of collimation? If they are off by a lot, it’s easy. Everything you look at will look screwy. You will not see the same image in each side of the binoculars. If they are off by a moderate amount, your eyes may be working hard at pulling the images together, causing undue eyestrain. If they are off by a little, your eyes will do the work for you of pulling the images together and you may never notice it.

Let Your Eyes Relax

The easiest way I have found to see the divergence in the images is to stand back 6” to 12” from tripod mounted binoculars that are centered and focused on a very bright star. Try to select one that is not to high in the sky so you don’t need to crane your neck. As you back away, you need to keep your eyes on the images in the eyepieces. You need to let your eyes experience that feeling you did purposely when you where a kid, the one where you were looking at something and then let you eyes go loose as though you really weren’t looking at anything. What you will find when you let your eyes “relax” in this manner is that the two separate images you see before you will start to move together. Keep your eyes on the images. They will either stop at some point, showing you the divergence or separation in the images, or they will come completely together. Do this a few times until you are sure your eyes aren’t working to pull the images together the last little bit.

OK, So I Need To Collimate My Binoculars.

Once you have determined the need for collimation, you need to set up to do the collimation. I have found it rather easy to do the collimation adjustment at night using a bright star. No other object you can view will give you the pinpoint precision as the in focus image of a bright star. The same stance is used for the adjustment process, 6” to 12” back away from the binoculars. Once you acquire the ability to let your eyes relax and see the split images in the left and right eyepieces, you will easily be able to watch as you move the images with the adjusting screws.

A Little Bit Here, A Little Bit There.

You should test adjust each of the screws to see which way that screw moves the image. Move these screws very slowly. I tiny bit of a turn will move the image a significant amount in the eyepiece. If you try to make all the adjustment using one screw on one side of the binoculars you will very quickly distort your exit pupil into the shape of the cat’s eye. So, I recommend you make a small portion of the corrective adjustment with each screw on each side of the binocular, a little at a time.

What Kind Of Error Can Be Tolerated?

Convergence is seen when the image in the right eyepiece is to the left of the image in the left eyepiece. Slight horizontal image convergence can be tolerated. Divergence is seen when the image in the right eyepiece is to the right of the image in the left eyepiece. Horizontal image divergence should not be tolerated. Vertical divergence should not be tolerated. The eyes have no muscles to accommodate for these two visual errors. For a thorough discussion of convergence and divergence I once again refer you to Seyfried’s book noted above.

What Benefits Will Be Gained?

First and foremost, you will see a benefit in the improved images if you needed a large adjustment to collimate your binoculars. Your eyes will experience the view thru the binoculars with a lot less work. You will also note a significant improvement in the light throughput if you successfully corrected the shape of the exit pupil. I improved my 15x70 binoculars image to the point that I was able to clearly see a double of 16” and although not split, correctly identified the position angle in doubles of 13” and 10”. I improved my 20x80 binocular image shape to the point that the previously noticeable difference in light throughput between the two eyepieces became much more closely matched and a double 10” was split.

You Should Know How.

If you own binoculars, you should know how to make minor adjustments to your binoculars. I view this in the same light as a Schmidt Cassegrain Telescope or a Newtonian telescope. Anyone who owns a mirror type telescope must learn the process of collimating the mirror, as these telescopes demand a regular adjustment. Likewise, you should know how to determine if your binoculars need adjustment, and if they do, you should know how to proceed.

A Temporary Solution.

I am not advocating collimation of binoculars solely thru the adjustment of the prism tilt screws. However, I have five pair of binoculars and two of these seem to need significant adjustment to improve the collimation. I have not yet discovered how to remove the objective lenses of either of these two binoculars. In fact one seems to have the objective lenses substantially sealed in place with a glue-like product. This leaves me with the alternative of attempting alignment by adjusting the prism tilt screws. I view this above procedure as a temporary solution to a more involved process that needs to be fully learned and undertaken to solve the problem completely.

Collimating Binoculars Adendum 11-27-02

Soon after I wrote this article I initiated a discussion on the Astromart binocular forum relative to collimation. That post ultimately resulted in an exchange of relevant thoughts and comments between Kevin Busarow (bigbinoculars.com) and myself. I felt the outcome of that exchange of useful information was worth reprinting here, since this is the original article that prompted me to enter into that discussion. Although this reprint is slightly longer than the final post, (final post was limited to 3000 characters), it is true to form.

Clear skies, and if not, Cloudy Nights.
Ed Zarenski.

Hi Kevin, Thanks again for your response. The number of people joining the ranks with moderately priced binoculars continues to grow every day because such products are now made available in the marketplace, and that's a good thing. People are purchasing binoculars that are adjustable and they need all the information they can get to understand what they are doing. I hope this correspondence helps serve that purpose.

I’d like to summarize several of the main points for the community at large and eliminate wrong assumptions. Since we both agree with some of what each of us said, I have combined parts of what we both said into one process flow that I think could be considered a good direction to follow. Your concurrence would be beneficial. (Kevin did respond positively.)

Collimation is the alignment of all the optical elements along the binocular optical axis. Originally collimation is set by adjusting the objective lens and then by making the final critical adjustments with the prism tilt screws. The most readily available method for the common user to adjust collimation is by the prism tilt screws. Pause for a second and understand what is happening when you take this action. Understand the process completely before you proceed and you will be able to perform this adjustment when it’s needed.It seems most reasonable to assume that if a binocular arrived in good collimation and then has been knocked out of alignment, the most likely component in the optical path that shifted would be the prism as it's the only part that's not locked into position. If a prism were knocked out of alignment due to an unfortunate jolt, the user should see not only images that are not merged, but also the oval appearance of the exit pupil, although possibly very minor and difficult to see. The oval appearance of the exit pupil would identify which barrel needs adjustment, but it doesn’t give a clue as to which prism in that barrel needs adjustment. If the images are not merged but the exit pupils still appear round, its possible that collimation is only so slightly off as to need a very minor adjustment. Follow the procedure outlined trying to maintain round exit pupils.

So a user should attempt to adjust the prisms from the tilted position back to a true (perpendicular to each other) position using the prism tilt screws. While adjusting, observe the merging of the images AND observe the trueness of the exit pupil. It may even help to give a gentle rap with the knuckles on the binocular body to help "settle" the prism between the setscrew and spring hold-down, otherwise it often shifts slightly some time later. If either the images do not merge or the exit pupils do not appear round the user has failed to collimate the binoculars properly.

Some binoculars provide adjustment screws inside the housing for the tilting of the prism shelf and do not allow for the tilting of the front prism in respect to the back prism. This method addresses the fact that the axis of the prisms must remain true. Any tilting of one prism with respect to the other prism in a pair effectively reduces the light pass to less than 100%.

Once the prism is adjusted properly, the images should appear merged and the exit pupil should be perfectly round again. If merged images are achieved and round exit pupils are seen, you have succeeded. If exit pupils are not round, then it is likely that the prism that shifted is not the one that was adjusted in the attempt at collimation. The result of two misaligned prisms is going to be a less-than-round exit pupil. If this occurs, the prism that was adjusted should be turned back to where it was when you started. The other prism (on the same side of course) would then be adjusted to attempt collimation, following the same procedure.

This is not as difficult as it may seem. If the wrong prism was adjusted, the images can be merged but the shape of the exit pupil will be less round than before. Setting that prism back to where it was only requires making a mental note (or a sketch) of where the images were before collimation was attempted. Turn the screw to reset the prism back to that out-of-collimation image. Then attempt collimation again, but this time turning the other prism screw on that same barrel.

So once again, for the majority of collimation problems, adjusting the collimation screws is the BEST method to achieve perfect collimation AND round exit pupils. If collimation is still way off and the exit pupils appear round it may indicate the problem resides within some other component of the binocular. If this is the case, any further movement to the prisms is causing a greater misalignment of the optical system. Of course, it's possible that the binocular was not properly aligned at the factory, and/or may have a flawed component. In this case, we basically have to start from scratch. But this is the exception, and not the rule. At this point, most people should probably send the binocular back for warranty repair or replacement.

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