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Collimation and Interpupillary Distance

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#1 sftonkin

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Posted 25 February 2004 - 07:22 AM

[New poster here -- couldn't find anything else on this, but do point me to it if I've missed it]

Recent eye-test threw up something which, I suppose, ought to have been obvious, i.e. that my IPD varies by around 6mm depending on whether I am viewing near or distant objects. This got me thinking about miscollimated telescopes: Do we compensate for con/divergent miscollimation by con/diverging our eyes, hence changing our IPD (and necessitating different IPDs for different binoculars, depending on collimation)? If so, has anyone evaluated the relation between angle of miscollmation and IPD change?

Just a theoretical thought -- probably of limited practical use, since we set IPD by looking through the things, but...

#2 EdZ

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Posted 25 February 2004 - 12:44 PM

You will read in various places about "conditional alignment". Conditional alignment is where the end user has the ability to move prisms to adjust collimation. By doing so, it will only be aligned at the IPD for which the adjustment was made. It may not be to far off at other IPD's, but it is a conditional alignment.

True collimation, performed by a qualified shop, will adjust the collimation so that it stays collimated at every IPD. The end user has no way of performing this level of precision.

edz

#3 KennyJ

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Posted 25 February 2004 - 01:44 PM

I think this a very interesting question.

I think Stephen is correct about human eyes automatically compensating or "correcting" ( no pun or irony intended)
for any mis-collimation of convergence or divergence and is thus also correct to suspect it might be of little practical significance.

But I also suspect such a condition could be source of eyestrain.

I believe Ed is also spot -on with his distinction between "Conditional Alignment" and professional true collimation.

Since celestial objects are all more or less viewed at around the "infinity" end of the scale , I doubt if this is as potentially significant in astronomy viewing as it could be in close -range terrestrial work.

P.S - I bet Cory S. would know all about this !

Regards -- Kenny

#4 sftonkin

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Posted 25 February 2004 - 04:55 PM

Ta for the reply, EdZ.

Hmmm -- I obviously didn't phrase my question very well. What I intended to convey was not "conditional alignment" (isn't that a Bill Cook phrase?), but simple misalignment. Let me try again:

If, say, there is a 10 arcmin horizontal misalignment, do our eyes attempt to compensate for it by (e.g.) converging, or does the compensation (image melding) take place after the nerve impulses have zoomed down the optic nerve?

#5 EdZ

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Posted 25 February 2004 - 05:14 PM

The eyes will attempt to move to make up for the misalignment. This is what leads to eye strain.

Also, I didn't exactly answer your first question. What I described is the fact that the binocular alignment can change depending on how close together or far apart you move the barrels to acommodate various individuals.

When using binoculars, if focusing on a near object, the angle of each eye's line of sight is turned measurably inward.

If my eyes are 62mm apart and I focus on an object 10 meters away, I can measure the inward angle of my eyes. But the eyes don't just tilt to see the object 10 meters away, that inward angle is accentuated by a slight convergence of the eyes. At this focus distance, it is conceiveable that IPD may no longer be 62mm, but may be only 60mm or less. Your 6mm convergence seems quite large.

You may be able to watch this happen in a mirror. As you move closer to the mirror, watch to see if you can notice your IPD get slightluy closer.

edz

#6 sftonkin

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Posted 26 February 2004 - 02:15 AM

I suspected as much with respect to pupils attempting to compensate -- thanks for the confirmation.

I was, however, unaware that the pupils converge if we use binocs to view a near object. I'm trying to visualise why -- I'll probably have to resort to pencil and paper :-). I had always assumed that, when one refocused the bins to something near, the eyes still stayed focused at infinity and that their optical axes remained approximately parallel. I have certainly never noticed this convergence, which is surprising as my 10x42 have a near focus of about 2m and I don't notice any change in the relationship of my eyes to the FoV (which I do if I intentionally converge my eyes).

The 6mm convergence (actually, 6.5mm) was measured by my optician (67mm for infinity, 60.5mm for close reading).

#7 EdZ

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Posted 26 February 2004 - 06:35 AM

As long as your pupils stay within the bounds of the exit pupil, there is room for some movement without changing anything in the view. I suspect that maximum movement measured in your eyes does not occur until you attempt reading distances of 18" to 24", and probably you experience only little movement when focused at 2m to 10m.

When I move my eyes to focus from far to near, I can tell my eye pupils drop down closer to the bridge of my nose, but only for very close distances of 20" to 10". Iv'e never noticed it at any binocular close focus distances, even though there is a measurably different angle of convergence where the two eyes focus on the same spot.

edz


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