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Question About Diopters

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#1 vdog

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Posted 07 December 2019 - 10:58 AM

For context, I'm using an unbranded Arcturus clone binoviewer.  My EP pair are Meade HD-60 25mms.  Oh, and I've used a binoviewer twice so far, so I am a BV newbie for sure.

 

I did some digging on this question but found no clear answer.  Are there optimal diopter settings for a given observer that will work on all targets?  If so, and I find them on a given night, would it work to mark them and then just use those settings every time?

 

Does anyone do this, or is this just something you have to adjust every time you use the binoviewer?



#2 sg6

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Posted 07 December 2019 - 11:31 AM

Someone had better explain "diopter" to me.

A diopter has always been the number/value of 1/f where f is the focal length in meters.

 

So a 2 diopter lens is one with a focal length of 0.5 meter.

 

It was a simple means for opticians to add the lens they tested your eyes with, they could imply add the diopter values and obtain an answer. Otherwise it is a case of adding the focal length recipricols and not so obvious.

 

So what are your diopters?



#3 vdog

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Posted 07 December 2019 - 11:53 AM

Apologies if my terminology is not quite correct.  

 

The binoviewer has compression rings that lock the EPs in place.  Underneath these are the rings that adjust the placement of the EPs inward or outward.  These have to be aligned with each other in order to merge the images, and these are what I meant by "diopters."

 

If there's a different term I should be using, I would appreciate knowing what that is. 

 

Thanks.



#4 Eddgie

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Posted 07 December 2019 - 03:39 PM

The diopter adjustment does not move anything by the eyepiece. It is like a little microfocuser that lets you adjust for slight focus differences between your right and left eye, and for slight differences between the exact location of the field stop or point of best focus in different eyepieces.

 

If all of your eyepiece pairs had exactly the same field stop position, in theory, you should be able to adjust your diopters once for your particular eyesight and never touch them again.

 

In fact, there really is not much need for two diopter adjustments. 

 

For example, if completely lower and lock your diopters and focus on a star, if both eyepeices are in perfect focus, then you never have to change them.  But suppose your left eye is in focus but your right eye is not in focus.  Now you could turn the focuser so that the focuser tube goes in and see if your right eye gets better.  If it does, then bring the telescope to best focus for your right eye.  Because you had to rack the focuser in, if you now unlock the left diopter and screw it up (like racking the focuser tube out) you can bring that eye into focus. Now, once set, you should not have to ever touch them again.

 

But that is not the way it works out.   Eyepieces are not always exactly parfocal and this means that as you change eyepieces, it might be necessary to tweak the position of the left diopter (if you are like the example above).  You always leave the right diopter locked down and fully insert the right eyepiece, and bring the scope to focus, then you would use your left diopter to adjsut for any slight differences in the non-parfocal eyepieces.  (This would of course be reversed if your left eye was the one that required the most inward focuser travel).


So, once you know which eye requires the most inward travel of your telescope focuser (with the diopters locked down) you can simply leave the diopter for that eye locked down and use the other diopter to raise (rack out) that eyepiece between different eyepiece pairs (this assumes that one eye has a much different infinity focus than the other). Again, you would bring the scope to focus for the eye that requires the most inward focuser travel and then just use the diopter on the other side to correct the focus for that eye.

 

So, while in theory you could adjust your diopters once and leave them, in practice, because the field stop of even the same kind of eyepiece or the focal lenght of the eyepieces might vary very slightly, it is pretty common to have to adjust for one side or the other, but the above method means that you should only have to focus the scope with the "short" eye, then rack out the other eyepiece.


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#5 vdog

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Posted 07 December 2019 - 05:06 PM

The diopter adjustment does not move anything by the eyepiece. It is like a little microfocuser that lets you adjust for slight focus differences between your right and left eye, and for slight differences between the exact location of the field stop or point of best focus in different eyepieces.

 

If all of your eyepiece pairs had exactly the same field stop position, in theory, you should be able to adjust your diopters once for your particular eyesight and never touch them again.

 

In fact, there really is not much need for two diopter adjustments. 

 

For example, if completely lower and lock your diopters and focus on a star, if both eyepeices are in perfect focus, then you never have to change them.  But suppose your left eye is in focus but your right eye is not in focus.  Now you could turn the focuser so that the focuser tube goes in and see if your right eye gets better.  If it does, then bring the telescope to best focus for your right eye.  Because you had to rack the focuser in, if you now unlock the left diopter and screw it up (like racking the focuser tube out) you can bring that eye into focus. Now, once set, you should not have to ever touch them again.

 

But that is not the way it works out.   Eyepieces are not always exactly parfocal and this means that as you change eyepieces, it might be necessary to tweak the position of the left diopter (if you are like the example above).  You always leave the right diopter locked down and fully insert the right eyepiece, and bring the scope to focus, then you would use your left diopter to adjsut for any slight differences in the non-parfocal eyepieces.  (This would of course be reversed if your left eye was the one that required the most inward focuser travel).

So, once you know which eye requires the most inward travel of your telescope focuser (with the diopters locked down) you can simply leave the diopter for that eye locked down and use the other diopter to raise (rack out) that eyepiece between different eyepiece pairs (this assumes that one eye has a much different infinity focus than the other). Again, you would bring the scope to focus for the eye that requires the most inward focuser travel and then just use the diopter on the other side to correct the focus for that eye.

 

So, while in theory you could adjust your diopters once and leave them, in practice, because the field stop of even the same kind of eyepiece or the focal lenght of the eyepieces might vary very slightly, it is pretty common to have to adjust for one side or the other, but the above method means that you should only have to focus the scope with the "short" eye, then rack out the other eyepiece.

Exactly the info I was looking for.  Thank you so much, Eddgie.  waytogo.gif




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