Collimation Oberwerk 20x90
Posted 28 February 2004 - 09:44 AM
Posted 28 February 2004 - 10:20 AM
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, 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 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 of either of these 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. True collimation can only be accomplished by a qualified repair service with the proper instruments.
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.
Posted 01 March 2004 - 06:05 AM
I agree with Ed that best way to collimate them is focusing on a star. This may not be a simple procedure for us newbies, being that we are not 100% sure what we're doing and being in the dark will make things worse. I had to collimate my 20x90 the first day I got them. I used a house roof as a guide to get the images to merge. They were slightly off vertically. I just picked a barrel and adjusted the screws until the image was merged. I couldn't to tell which barrel needed adjustment because the misalignment was not too great. Viewing the exit pupils from the EP side is a little misleading because you can see part of the prisms and the image is soo bright it's hard to tell what part is actually the pupil and which parts are the prisms. I think an easier way to get started is to set the binos on a steady surface like a table top or mount them on the tripod and point the EP to a bright source such as a window. Looking at the objective you will see a big image of the exit pupils. Full credits to Barry Simon for the following pictures:
Posted 01 March 2004 - 06:09 AM
Posted 01 March 2004 - 06:40 AM
This is not related to collimating binoculars. This is not what you would see if one side or both sides of the binocular were out of collimation within the tubes themselves. You would see ovals or cats eyes.
In this picture, either the prism housing infringes in the path or prisms themselves are so large that they infringe in the light path,
the prisms are to small to accept the entire light path from the objective and you see the edges of the prism cutting into the light path and cutting off the edges of the exit pupil.
At the positions 10oclock, 7oclock and 5oclock and 2oclock you can see, where there should be a perfectly circular image, there is a straight edge cutting across the circle.
Having seen this and measured it in a number of binoculars, I would estimate there is an 8% to 10% light loss in this particular barrel. All the light from the objective cannot passing thru the prism to the eye.
This barrel of this binocular is not operating to full potential, due to oversized internal housing parts or undersized prisms. I have never before seen a binocular that showed the prisms infringing to this degree and at all four positions. Usually only one or two positions are cut. This is the most prism light cutoff I have ever seen.
If you can take a photo of the exit pupil from the eyepiece end and post it with the model, after I can enlarge it, I will calculate the light cutoff and post results.
Posted 01 March 2004 - 06:56 AM
As a side note the binos in the picture are not mine. My 20x90 did not show either light cutoff or oval pupils, at least from the objective and the EP side, and the images merge. Even then, who knows if they are in perfect collimation...
Posted 01 March 2004 - 08:51 AM
You would not see the oval shape in the exit pupil from the objective end. It must be viewed in the exit pupil. 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.
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.
Neither of these issues relates to the merging of the image from the two barrels. That is what we commonly refer to as collimation. However, collimation in a refractor is the alignment of the optical path with the mechanical axis. When that is not done properly, we see cat's eyes.
Posted 01 March 2004 - 12:47 PM
Regards -- Kenny.
Posted 01 March 2004 - 01:14 PM
Posted 02 March 2004 - 10:43 AM
To answer the question about the possibility of BK 7 prisms, no, they are BAK4 in the Burgess. As Ed said, the prisms and their housing are just undersized. With BK 7, once again, as Ed said, you would see shadowing, but not complete cutoff of light.
Posted 02 March 2004 - 10:57 AM
Please accept my apologies. Yes indeed I got those pics from the Yahoo group. I really didn't mean to offend any one, just meant to show an example to all the forum members. The pictures show such a good and clear example of what was being discussed in this thread that I decided to post it, since it's sometimes harder to describe things words. I will delete the whole post if you want me to.
Posted 02 March 2004 - 11:20 AM
Posted 02 March 2004 - 11:27 AM
Posted 02 March 2004 - 11:35 AM
Posted 02 March 2004 - 11:49 AM
One condition happens when the prism itself is not wide enough to span fully across the circular opening in the prism shelf, the metal holder that acts as the field stop. You may be able to see this by looking into the objective end. You can verify it by observing a straight line in the exit pupil at the same location.
The second condition happens when the placement of the second prism that the light travels through, the prism towards the front (objective) end of the binocular, has some portion of the back of the prism or the casement enclosing the back of the prism, sticking into the light path between the objective and the circular opening in the prism shelf. This can always be seen by looking into the objective end of the binocular.
You may need to tilt the binocular back and forth a bit to see that a prism housing interferes with the light path. very similar to what occurs in a Newtonian when the focuser tube protrudes into the light path inside the telescope tube between the lightsource and the mirror. You won't see any perceptable difference in your view, but you will see the difference in the out-of-focus diffraction image, and if you look closely you will see it in the exit pupil.
Posted 02 March 2004 - 12:25 PM
Posted 02 March 2004 - 03:37 PM
Posted 02 March 2004 - 03:58 PM
Posted 02 March 2004 - 04:16 PM
Posted 02 March 2004 - 05:37 PM