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Can you adjust out failure to merge?

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#1 dcoyle

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Posted 31 December 2012 - 01:51 PM

The most general form of the question I'm trying to ask, is, if the two eyes are otherwise "normal", can failure to merge, say because of a tendancy to cross the eyes in the horizontal plane, called "prism" in an eye prescription, be adjusted out by manipulating the bino viewer adjustments?

I understand this would render the binoviewer useless to others, not an issue here. I don't want to experiment on my aligned bino's to answer the question, either.

Any one familiar with eye exercises to strengthen the eyes against in prism?

Thanks

Dan

#2 Mike B

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Posted 31 December 2012 - 02:01 PM

I don't want to experiment on my aligned bino's to answer the question, either.

Any one familiar with eye exercises to strengthen the eyes against in prism?


Don't know on either count- but is an excellent question!

If someone had an inexpensive BVer & wanted to fiddle with it... or perhaps one already "out-of-whack", needing professional collimation, which further fiddling wouldn't "hurt" anyway...?
:shrug:

#3 GlennLeDrew

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Posted 31 December 2012 - 06:17 PM

The real question is, does this 'prism' inherently cause a failure to merge the images of a distant scene when viewed by the eyes alone? If one is sufficiently cross- or wall-eyed, or there is vertical displacement which requires eyeglasses to compensate for, it might require to maladjusted one of the prisms.

But any such adjustment would be valid for one eyepiece focal length only, or over a quite small range of focal length. Here's where a Power Switch or some such arrangement would be more than nice to have.

#4 dcoyle

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Posted 01 January 2013 - 03:52 AM

Glenn, thanks for responding, and would you explain this statement a little further, please?

"But any such adjustment would be valid for one eyepiece focal length only, or over a quite small range of focal length. "

The eyes are focused at their infinity distance, and are as parallel as they get. Why would the focal length of the EP matter on an afocal system?

Thanks,

Dan

#5 Eddgie

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Posted 01 January 2013 - 12:46 PM

Yes, prism can cause the inability to merge.

People with this condition may not be able to merge the images their eyes form even without binoviewers if the condition is severe enough.

They will have to turn their face to the side to a degree to compensate for this.

As I understand the condition, it is caused by the muscles of one eye being a bit short or not quite attached to the right spot in the eye, so the eye usually cannot point exactly forward when the individual's eyes are in a plane parallel to the target. We see these people as being "Cross-eyed", and most commonly, one eye points slightly in.

They will turn their face slightly to the side of the inward aimed eye so that the other eye can turn to align to the inward facing eye.

If the condition is mild, most people compenstate for it automatically and most other people are unaware that they have this condition, but for using binoviewers, it may not be possible to merge the images.

They could parhaps do the same thing as when using normal eyesight which would be to slightly turn the head so that one eye is closer to the eyepiece than the other eye to its eyepeice so they can get the angle right, but this is impractical if the prism is more than a few degrees.

Today, surgery is used to correct this condition in childern if training glasses or eyepatches don't work. 30 minutes by a good doctor... They go into the eye socket between the eye and the socket wall and adjust the muscle.

This is my understanding of the condigtion (assuming it is the same thing you are talking about).

#6 dcoyle

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Posted 01 January 2013 - 02:15 PM

I had never experienced any indication of inward prism in my use of binocculars or any other time, so I was very suprised when it came up with binoviewers.

For the first few moments, the view is fine, but after a bit the axis of the eyes seems to over converge. If I look away from the bino, at an object in the distance, I can look back in the bino for a bit before it reconverges.

I guess I'm "lucky", in that it is just in the horizontal plane, for me, so it seems that it could be adjusted out.

Dan

#7 Eddgie

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Posted 01 January 2013 - 04:48 PM

Well, the situation with binoviewers is a bit more complex because the powers are generally far higher so any misalignemt will tend to be aggrevated a bit.

But are you sure that your binoviewers are in good alignment? This could simply be what it sounds like, which is an uncollimated bino head... There are some tests somewhere on the web I think that tell you how to check this. I have not owned a pair of Binoviewrs that had an issue with merging, so have never learned to do the test.

#8 GlennLeDrew

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Posted 01 January 2013 - 06:19 PM

Dan,
With a binoviewer, miscollimation originates within *it*, by directing the separated optical axes offset relative to the other. For instance, if an on-axis star is placed dead center in the left eyepiece, it will be removed some distance from the center in the right eyepiece. If you replaced the eyepieces with a pair having a longer focal length, the left eyepiece image will still be centered, while the offset in the right side will be reduced. Now swap out to a shorter focal length pair. Again, on the left the star will still be centered, but on the right the offset will be the largest yet.

The degree of miscollimation depends solely on eyepiece focal length, scaling linearly with decreasing focal length. And this holds true always, no matter to which scope the rig is attached. Whether it be an 80mm f/5 refractor or a 14" f/10 SCT, the same set of eyepieces will deliver the same angular miscollimation as seen on your retina.

The reason for this is that the origin of the miscollimation is inside the BV, and the distance from this point to the focus is *fixed*--well, not quite; more below. Such a fixed distance means that at the focus, a given angular miscollimation results in a fixed *linear* offset. And so the angular offset on the retina depends on the magnifying power of the eyepiece.

There is an additional complication, in that the focal surface does not always lie at the same place inside the BV's eyepiece barrels. It can if all eyepieces in use are parfocal. Otherwise, the telescope is refocused, which moves the focal surface to closer to or farther from the BV's rear prism face. This causes a slight alteration in the relative linear offsets of the emerging optical axes.

The extent of change depends on the range of field stop location with respect to the eyepiece's shoulder. For example, some long focal length eyepieces may have the field stop right down at the bottom end of the barrel, which places the focus close to the BV's prisms. And some eyepieces have their field stops some distance above the shoulder, which locates the focus above the top of the BV's eyepiece barrels. This range is expected to result in a variance in the relative linear offset of the optical axes of some 20%.

#9 faackanders2

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Posted 01 January 2013 - 07:43 PM

Doesn't the JMI reverse binoculars allow adjusting eyepiece colimation/position both horizontaly and vertically. I believe they want you to do horizontal first, since doing vertical second is easier and you don't go cross-eyed and get headaches (which may happen if you do the procedure reversed).

#10 GlennLeDrew

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Posted 01 January 2013 - 09:50 PM

A binocular and a binoviewer are two different beasts.

#11 dcoyle

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Posted 06 January 2013 - 12:11 PM

Thanks, Glenn, for taking the time to explain, further.

Nuts. I think I am spending too much time at the computer with my eyes closefocused/crossed.

Thanks, again, Glenn.

Dan

#12 dcoyle

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Posted 06 January 2013 - 12:14 PM

Eddgie asked:

"are you sure that your binoviewers are in good alignment?"

They are new MaxBright's, so I assume so.

Thanks,

Dan

#13 orion61

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Posted 12 January 2013 - 01:10 PM

The prisms inside of a Binoviewer can be adjusted,
A buddy of mine dropped his and severely impacted the alignment. The mfgr wanted more than he wanted to spend so
he was going to throw them away. I told him I would take a crack at them with nothing to really lose.
I opened it up and decided to attempt alignment useing 1 side only, the side which was dropped (on Concrete) looked to be the side to try.
I focused in on an Artificial star. the prisms have a set of double locking screws, once loosened, Pulling the prism out moved the image in and moving it up moved the image down.
I put the screws to where they were touching but not tight.
I could move the prisms without having them slide around.
once aligned as close as I could by naked eye, I simply tightened the screws.
It worked out ok for him.
I am sure it is not perfectly aligned, especially when they
are opend and closed for different interpupillary distances.
This is not suggested to do, and not for the faint of heart.
These werent expensive ones, but they are still in use after 10 years.

#14 howard929

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Posted 17 January 2013 - 01:55 PM

Sorry to hear about your issues with your BV and I hope you can get past it.

But Hmmm.. I see the adventure never ends.

I bought a MaxBright in the hope that both my wife and I can enjoy the views. She has a medical condition that causes one eye to wander a bit depending on the severity of that condition at that particular time. Sometimes it's not too noticeable though I always manage to see it, sometimes it's so severe she's ready to toss out here trifocals because they aren't working.

I'm not going to mention this to her so she's not swayed one way or the other but this is going to be fun.

#15 dcoyle

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Posted 31 January 2013 - 12:03 PM

A follow up:

I can merge.

I tried a couple of things.

I have been using a Maxbright with three screws to lock each eyepiece in. I really must say that the three screw system stinks. I realized I had no way to ensure that the collimation was the same in both eyepieces.

To correct that, I rotated the individual diopter correctors to find the place where both eyes were in focus. Then I used only the screw that was at the 3 oclock position on each eyepiece holder. (Harry Seibert informed me of that technique for dealing with 3 screw holders.)

The second thing I did was use a chesire to check the alignment of my SkyWatcher 100 ED. It was spot on, litteraly, three concentric disks of pale blue and green. I then checked with the 1.25 star diagonal in place. Nope, displaced in the direction to the rear of the scope, greater than 90 degrees. I had been viewing from the side of the scope with the diagonal in place, so I was getting a skewed alignment. I set the mount height to allow direct viewing with out the diagonal.

I could merge down to my 7mm orthos with a 1.6 GPC,

#16 Mariner@sg

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Posted 03 February 2013 - 07:56 PM

Can a misaligned WO binoviewer be repaired or realigned?






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