Telescope Reviews: Need Binocular Collimation or Repair?

For most binoculars with a minor need for an alignment adjustment, turning the prism tilt screws is the primary means available to the end user and is an appropriate adjustment, called conditional alignment, but commonly referred to as collimation. True collimation can only be accomplished by a qualified repair service with the proper instruments.

Repair Services
links to binocular repair and/or collimation services
Binos need Cleaning or Repair or Adjustment about a dozen different links
Suggestions Please / Repairs
Binocular Repair

COLLIMATION
Collimating binoculars takes on two different meanings. The first and most common that everybody knows is to get the images from one barrel to merge with the other. But the second and much more difficult problem is the alignment of the optical elements along the optical axis in each tube separately. The first is really conditional alignment while the second is true collimation.

See this article in CN reviews dealing with conditional alignment
“Collimating” Binoculars

And see this article here in the forum dealing with collimation
Binocular Collimation and conditional alignment by Bill Cook
2) Alignment that is within the original JTII standards can be considered to function as “perfect” in that the eyes can easily accommodate the error without introducing noticeable eyestrain. For example, an 8 power instrument should have errors equal to or less than 4’ Step, 10’ Divergence and 6’ convergence, with a 16 power having 2’, 6’ and 3’ respectively.

or
JTII @ 4.5 power
8 ' and under for step (dipvergence)
20 ' and under for divergence
15 ' and under for convergence

Cheers,
Bill

and see this for some ALLOWABLE TOLLERANCES
Test for Convergence or Divergence

also see this
A lengthy discussion of collimation tollerances
as posted on different websites and by different manufacturers.
You'd never know by the thread topic that this is 90% of the content in this thread.

MERGING THE IMAGES
I'd recommend you assess the degree of miscollimation before attempting any adjustments.

Do you normally wear glasses? Do you have astigmatism? If no, OK, proceed. If you do have astigmatism, do not attempt to even visually test for miscollimation without your glasses.

Are you familiar with some double stars, test collimation on a double star of known separation. This gives you a benchmark separation from which you can determine the severity of miscollimation.

There are allowable tollerances. Standards seem to be more than I personally can tollerate, but you could easily have 2 to 3 arcmin of miscollimation and need no adjustment at all. Standards would indicate greater deviation is allowable. The tollerances get much tighter as magnification increases. A 7x or 8x binocular with 2 to 3 arcmin miscollimation won't look bad at all. However, in a 25x binocular with even 50 to 60 arcseconds of miscollimation it may be difficult to merge images.

An excellent double to test collimation is Nu (v) Draco. It has a separation of 62". It's like having a 1 arcmin measuring device in your image. If your low power binocular collimation is within 2 or 3 times the separation of Nu Draco, you may want to just leave it alone.

Miscollimation would produce a well lit exit pupil with black edges all around. It may or may not be round, ...but it should have dark all the way around it.

If it is determined as necessary to adjust the binocs, the amount of turn on these screws is very very small. You might turn each screw less than a quarter of a turn to reach adjustment.

Keep in mind that adjusting the screws to merge the images is being done without the use of any lab equipment. The eyes can tell when images are not merged, but they cannot tell which of the four prisms are the ones causing the misalignment.

The general trend is to stick with adjusting the two screws closest to the eyepiece end of the tube. However you have no way of knowing that they are the proper screws to adjust.

Sometimes you see or hear discussion of problems related to binoculars that seem to be in alignment (merged), but the exit pupils are not round. One possible cause of this condition is adjusting the wrong screws. If the front prism is slightly tilted and is causing misalignment but the back prism is adjusted to reset alignment, what you now have is two prisms that are both set incorrectly. The result may be an image that appears centered and aligned with the opposite barrel, but it would not be surprising to see oval exit pupils, out of round diffraction pattern or field of view not matching the other barrel.

In some cases turning prism screws can make the situation worse

A root cause of this condition is an objective lens that is not seated exactly centered and perpendicular to the optical axis. This may be the underlying condition that is forcing the adjustment of the prism screws to get the images merged.

In a truely collimated and aligned binocular, prism surfaces are perfectly perpendicular to the incoming light path and prism pairs are also perfectly parallel to each other. Tilting prisms to accomodate any deviation of either of these above conditions to result in merged images is done to compensate for some other misalignment in the light path and may result in one of the exit pupil or field of view conditions mentioned above.

For any binocular with a very minor need for adjustment, turning the prism tilt screws is the primary means available to the end user and is an appropriate adjustment. However, True collimation can only be accomplished by a qualified repair service with the proper instruments.

Conditional alignment vs. true collimation

Visual alignment of the field of view can be checked in daylight. However it is not recommended that any sort of prism adjustment be made based on a daylight condition. For adjustment, the light source should produce such a small point of light, it is generally thought that stars are the best source for this visual test.

WHERE TO FIND THE PRISM ADJUSTMENT SCREWS
Back (Eyepiece end) prism screw
Front (Objective end) prism screw

MISALIGNMENT OF THE OPTICAL AXIS
If you adjusted the wrong prism or if you were seeing an out of alignment optical tube, similar to a refractor tube that is not collimated, you might see an oval exit pupil. You would not see an oval shape in the exit pupil from the objective end. It must be viewed in the exit pupil at the eyepiece end. From the objective end, you might see that it would force you to view off center to see the full circle of light rather than viewing down the center of the axis of the light path.

Look here
Cat's Eyes

Oval or cat's eye shape in the exit pupil would be a clear indication that the light path in that side of the binocular is not on the optical axis. Could be several things causing it. All have to do with the alignment of the optical path in that barrel. They all end in the result of reducing the light output, which can be significant.

Here is an excellent pictoral of the view thru one barrel of a binocular with interchangable eyepieces. in place of the eyepiece is a Cheshire eyepiece. You can easily see when the optical alignment is off in one barrel.
Using a Cheshire Eyepiece to check an interchangable eyepiece binocular

Generally there are three collimation errors to be concerned about. For all misalignments the allowable error is smaller as magnification increases. Actually this results in the same apparent visual error, the angular error multiplied by the magnification.

Vertical Alignment, a serious error, is when one image is higher than the other image. The eyes have no muscles to accommodate for vertical error. The allowable divergence is only 4 arcmin at 7x to 10x, only 3 arcmin at 12x to 15x and 2 arcminutes at 15x to 20x. Personally, I cannot tollerate even 2 to 3 arcminutes of vertical error even at magnifications of 10x and only 1 arcmin at 16x.

Horizontal Convergence, the least problematic error, is when the image in the right eyepiece is to the left of the image in the left eyepiece. You might think of this as cross-eyed. The most separation can be tolerated in horizontal convergence, 10 arcmin at 7x to 10x, 8 arcmin at 12x to 15x and 6 arcmin at 15x to 20x. Most eyes can readily accommodate this error, but these limits I find a little too wide. I could not tollerate even half that much.

Horizontal Divergence, another serious error, is when the image in the right eyepiece is to the right of the image in the left eyepiece. The images are spread apart, an error that the eyes cannot accommodate without strain. The allowable error for horizontal divergence is 6 arcmin at 7x to 10x and only 4 arcmin at 12x to 15x and 3 arcmin at 15x to 20x.

Keep in mind, the magnification of your binocular has a significant influence on how you see this error. Your eyes see the apparent error, or the angular error multiplied by the magnification. For instance a 3 arcminute error in a 10x binocular will appear to have a magnified apparent size of 30 arcminutes. That same error in a 15x binocular will have an unacceptable apparent size of 45 arcminutes and in a 20x binocular will appear as 60 arcminutes apparent error.

For reference 1 inch at 100 yards is just slightly less than 1 arcminute.

Personally I cannot accomodate my eyes to these tollerances. I would recommend cutting all of these in half. HD is the error I see almost all the time. Because of the way the prisms rotate, there is always some vertical alignment error with it. I can accomodate 2-3 arcmin error at 10x-12x. I begin to find it difficult to accomodate a 2 arcmin error at 15x-16x. I could not tolerate 2 arcmin error at 20x. In large binoculars at 25x or higher, I find any collimation error of more than 1 arcminute unacceptable. I have my BT100 at 44x merged to about 20 arcsec.

Rotational Error, a fourth collimation error, is when one image is rotated in relation to the other image. I have never seen it in any binocular.

Suspect your binoculars are out of collimation and you wantto know by how much? Observe a known diuble star such as nu Draco (62 arcseconds) and allow your eyes to relax so you can observe the error. Use the spread of the double to compare to the spread of the collimation error.

Does anyone know if it is possible to build/buy a binocular collimator for something less than $100.

A comment on collimating binoculars (or how you might screw up the collimation of your binoculars)

Edited by EdZ (11/05/07 04:57 PM)

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