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Binocular Optical Alignment and image merging

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#1 bcarter1234


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Posted 15 April 2019 - 06:06 PM

I'm hoping someone can provide the silver bullet, say "Do this..." and all will be well.


I have serious problems merging images in any binocular over about 4X. I own decent 10x50 and 20x80 and have almost exactly the same image offset in both so it is unlikely the equipment is to blame. When looking through either at the moon for instance I can see a merged image for about 10 seconds but can feel my eyes straining to maintain it. After this I see two moons, with overlap ranging from 1/3 to 1/2 the diameter. It could be a physical issue or perhaps my brain just prefers to double the pleasure. The Pleiades have 14 sisters for me. My right eye is much weaker than my left all my visual memories include the left side of my nose, except when looking through an eyepiece. 


I've built an 80mm asymmetrical binocular telescope and now have the parts to build a much nicer one with 127mm objectives.


Here is what has worked so far on my prototype 80mm BT.


1) The upper left tube is fixed. The mirror of the 1.25" diagonal on that tube can be titled up/down and left/right.

2) The right tube can be moved laterally to the left or right and set parallel to the left tube or not.

3) The right tube can be raised at the front and set parallel to the left tube or not.

3) The 2" diagonal on the left tube can be rotated so that the light path going to the eyepieces is parallel or not.


My best results so far come from these settings.This all assumes the left tube is horizontal just so we are visualizing the same thing.

Right tube

Sits on and adjustable sled that measures 410mm front to rear.

Angled in 5mm at the front, approximately 0.7 degrees.

Angled up 9mm at the front, approximately 1.25 degrees.

The vertical section composed of the 2" diagonal, a section of tube and helical focuser is rotated clockwise, away from the right vertical section 2 degrees clockwise.


Left tube

The vertical section is vertical 90 degrees to the horizon.

I use the adjustment screws on the 1.25" secondary to fine tune alignment as needed.


At 20X-67X I get a nicely merged image that I don't lose even after several minutes. When merging is achieved this way I can feel my eyes relax, similar to the feeling of putting on a pair of prescription glasses that are well corrected for you.


Does anyone have any suggestions for other combinations of alignment I should try?


At first I tried keeping both scopes on parallel axes, including the vertical sections. I could merge images using just the adjustable mirror on the 1.25" diagonal but the view showed two distinct overlapping circular field stops. The overlap was about 1/3 the diameter of one circle. The "hard" black outline was shaped like an eight on its side. The outline that passed through the other circle's field was more like a strong shadow than a hard edge field stop. I could see the merged image "through" them but it was like looking through haze. Looking through one eyepiece at a time at 67X the star centered in the left scope would be very low and very far to the left in the right scope. Adjusting only the mirror this way also resulted in several degrees of field rotation.


Next try was to move the right tube in relation to the fixed left. The results were similar but less pronounced in each instance. I don't recall any significant field rotation but that could be my memory.


Rotating the right diagonal and vertical section brought me the closest I've been so far to a circular field of view showing a roundish field stop. At 20X I'd say I have a circle, at 67X more of an oval but acceptable. I don't believe I have yet tried having the two scopes" horizontal axes parallel and trying to merge using only rotation of the right diagonal and vertical section. Nor have I tried tilting both vertical sections out by half the amount.  


Nicer components arrived on my doorstep Saturday so I'd like to head down the right track. The current plan is to put them together first as an asymmetrical telescope. I also intend to build something very close to Mr Bill's Bino Box to put them in so I can see which format works best for me. My hope is to use the 80mm scopes as a sandbox to work out the kinks so I don't have to build in so many adjustments for the larger BT.


Are commercial BTs and binoviewers off the table for me based on what you read here? I have no idea what provisions they have for alignment and assume my issues fall outside the bell curve. I've been tempted by both but don't want to find out the hard way that I can't use them.


Thanks in advance for any help.


Take care,



Feel free to move to Binoculars if that is a more appropriate location. 


#2 bcarter1234


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Posted 17 April 2019 - 04:38 PM

I'll add a few notes here from time to time just to have an online record. Someone with a similar issue may benefit someday from this trial and error approach.


It seems so far that the less I use the adjustable diagonal mirror to merge the image the better. It makes sense to my very uneducated understanding of optics. By tilting the diagonal one part of the mirror ends up farther from the objective than another so the image doesn't come to a sharp focus across the field of view. Flaring or what looks to me like coma are the result.


I get better results from adjusting the optical axis of the right hand scope by tilting it as a unit relative to the fixed left scope. The images merge and the stars remain sharp. I will acknowledge that it seems wrong to look through the left eyepiece at a centered star at 67X and find the same star far to the left of the field of view in the right eyepiece. Once I do this  though I can "center" the star in the "binocular field", which at high power is certainly more oval than round, and when I pull my eyes back the image stays merged. I estimate that for me the axes of the two scopes are converging at about 1.5 degrees.


Next experiment will be parallel optical axes, adjustable diagoanl in stock neutral postion and attempting to merge images primarily through rotation of the left scope. This will result in the two vertical portions converging at somewhere between 2 and 5 degrees. If this alone does not work, I'll endeavor to achieve alignment by converging the horizontal axes while leaving the adjustable diagonal in its stock position.


Take care,


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