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Collimation Oberwerk 20x90

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#1 Guest_**DONOTDELETE**_*

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Posted 28 February 2004 - 09:44 AM

Trying them out for the first time in daylight I had to make a small adjustment to get perfect horizontal alignment On the first clear night my first impression was one of delight.Saturn very bright and clear separation of the rings,Jupiter I could make out the banding with averted vision and all the Galilean moons.The moon was awesome,tried Pleiades and found that no matter how much I tried I was unable to get many of the stars to a pinpoint that includes some lower magnitude ones,today the sun was reflecting of a window about a mile away I got it in focus and stepped back a bit and saw that the two images did not merge together is this a collimation problem and if so would attempting to adjust the screws nearest to the main focusing knob cure the problem.

#2 EdZ

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Posted 28 February 2004 - 10:20 AM

Keep in mind that adjusting the screws to correct alignment 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, 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.

edz



#3 Diego

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Posted 01 March 2004 - 06:05 AM

Hi Jeff,

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:

Attached Thumbnails

  • 63506-Burgess+light+path+from+obj.jpg


#4 Diego

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Posted 01 March 2004 - 06:09 AM

Here's a pic for comparison, the first one is a Burgess and the second Miyauchi :



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  • 63507-Miyauchi+light+path+from+obj.jpg


#5 EdZ

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Posted 01 March 2004 - 06:40 AM

In this image above here you see the perfect example of prism light cutoff.

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,

or

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.

edz

#6 Diego

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Posted 01 March 2004 - 06:56 AM

Ed, as always you're right all the way. It confirms what I said about us newbies. The image from above obviously shows prism cut off, probably prisms too small if you ask me (budget binos indeed). Will you actually see oval or cat eyes, looking from the objective side?

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...

#7 EdZ

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Posted 01 March 2004 - 08:51 AM

Diego,

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.

edz

#8 KennyJ

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Posted 01 March 2004 - 12:47 PM

From what I can see of the first image ( Burgess ?)it looks to me like an undersized Bk7 prism.

Regards -- Kenny.

#9 EdZ

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Posted 01 March 2004 - 01:14 PM

Not necessarily Bak 7, just grossly undersized prisms to the point where the edges of the prism are showing. If you look closely down the objective end of all your binoculars, you are bound to find a few that have prism edges showing that are not covered by the nice round field stop of the prism shelf. or alternatively, you might see the back or sides of the prism covers sticking right out into the light path. It causes complete light cutoff, even worse than bak7 prisms that cause only light falloff towards the edges, but still have light.

edz

#10 BarrySimon615

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Posted 02 March 2004 - 10:43 AM

I don't mean to be overly picky, being so new to this group, BUT, the two photos used in this thread (and posted by Diego)are photos taken by me of equipment lent to me for testing (Burgess 20x100 binoculars) or my own 20x100 Miyauchi binoculars on my Light Speed Wagon mount. I really have no problem with anyone using any image that is mine as long as either permission is asked and/or credit is given. BUT do not do it without permission and/or credit. Both photos used were taken from the Photos or Files section of the BinocularAstronomy Yahoo Group.

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.

Barry Simon

#11 Diego

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Posted 02 March 2004 - 10:57 AM

Dear Barry,

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.

Regards,

Diego

#12 BarrySimon615

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Posted 02 March 2004 - 11:20 AM

No don't delete the post. I fully accept your apology. I am also glad that the photos I took helped to illustrate the point being made.

Best regards,

Barry Simon

#13 EdZ

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Posted 02 March 2004 - 11:27 AM

My apologies Barry for not recognizing your work. Both of these pictures which I have seen before.

edz

#14 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 11:35 AM

Does anyone have any photographs of the actual binocular prism housing interior showing the two mechanical reasons (stated previously by EdZ) by which prism light can be cutoff? I can sort of picture this in my head, but I think an actual photograph(s) would really cement it. Seeing a cross sectional drawing is one thing, but to see a photo would really help. But I would take a drawing too if no photos are available.

#15 EdZ

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Posted 02 March 2004 - 11:49 AM

I do not have photos, but I have seen it many times.

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.

edz

#16 BarrySimon615

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Posted 02 March 2004 - 12:25 PM

I just yesterday helped Cory Suddarth (BinoFixer) with some shots of prisms he sent me of various prism types and laser projection thru them. We put his photos on one blackscreen and added the labeling. The photo can be found in the Files Section of BinocularAstronomy (Yahoo Group). I will put the link to the photo here and hope it works:

http://f5.grp.yahoof...Comparisons.jpg

Barry Simon

#17 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 03:37 PM

Sorry Barry! The link didn't work as of 3:40pm EST.

#18 BarrySimon615

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Posted 02 March 2004 - 03:58 PM

Works for me, but I am a member of this Yahoo Group. You have to be a member to access the photo. I will post it here. Photo credits to Cory Suddarth, photo layout and text courtesy of me.

Barry Simon

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  • 64334-Laser Projection Comparisons.jpg


#19 EdZ

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Posted 02 March 2004 - 04:16 PM

didn't work for me either Barry, but you've solved the problem. thaks for that.

edz

#20 Guest_**DONOTDELETE**_*

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Posted 02 March 2004 - 05:37 PM

Thanks for posting the pics Barry.


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