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Blurred field stop - an easy fix


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I've recently gained an interest in using Orthoscopics again. In my eyepiece case I have two circle Ts: a 9mm and a 7mm, both mostly unused in favour of more wide field eyepieces. I'll be adding to my collection having just purchased a 25mm yesterday. A 12.5mm is my next priority.

The 7mm is fine, but the 9mm has always had a horribly blurred field stop. Today I decided to take it to bits to see what was wrong.

The answer couldn't be more simple. The field stop is a separate fitting to the optical fitting. It  screws out. I assume the previous owner had screwed it in tight. I screwed the field stop back in until it was sharp. 
Job's a good 'un as they say :smile:

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Indeed, field stop needs to be positioned at focus plane - then it is in focus. Blurred field stop just means it is a bit out of focus and needs to be moved to exact focus to be sharp.

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If I'm not mistaken, some folks with limited focus accommodation who don't observe while wearing glasses can also perceive a blurry field stop because they have to move the eyepiece off of the scope's focal plane to compensate for their near or far sightedness.  Thus, the field stop is no longer positioned at the focal plane on purpose by the user.  This was certainly not the case for the OP's ortho, but I've read of many folks complaining about blurry field stops while not wearing their glasses without realizing the connection.

That's also why I always focus my eyepieces with glasses on before taking my FOV images.  This is particularly difficult to do with eyepieces having less than 5mm of eye relief, but I manage.

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Glad you got it sorted Michael :smiley:

I find blurry field stops quite annoying. They don't really impact the performance of the eyepiece but I still find them annoying. The Baader Classic Ortho 6mm had this issue but I could not sort it out because the field stop on that eyepiece is machined into the lens retaining ring. Loosen that to put the field stop at the correct (sharp) position and the lens elements rattle :rolleyes2:

 

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I asked Al Nagler once about field stop position, and he opined that placing the field stop at infinity focus was not usually preferred by observers because when they examined an object for details, most people found a closer in focus preferable.

So the optimum placement of the field stop would be a little closer focus than infinity.

The Delite eyepieces have a user-adjustable field stop, and TeleVue gave me a list of the placements of the field stops in the eyepieces after the controversy over field stop placements in their first production of 18.2, 11, and 7mm.

TeleVue corrected it on all the eyepieces, but I noticed I did not have a problem with where they were in the first place.

I "corrected" the field stops to the location TV recommended and found that they were fine there, too, pointing out that my eyes, even at nearly 70, still have some accommodation left.

Along the way, however, I discovered there is a fairly wide range of field stop position that works fine, so if a field stop is really badly out of focus for an observer with good vision, it might be very far from its optimum position.

If, on the other hand, the observer is a nearsighted person and viewing without glasses, expecting the field stop to be in focus might be expecting a bit too much.

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One thing I do like to see in eyepieces is a crisp edge to the field stop.

In many eyepieces though the field stop is internal. Not much chance of adjusting those without great difficulty. Some are part of the optical assembly and not adjustable at all.

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On 28/11/2020 at 23:41, John said:

Glad you got it sorted Michael :smiley:

I find blurry field stops quite annoying. They don't really impact the performance of the eyepiece but I still find them annoying. The Baader Classic Ortho 6mm had this issue but I could not sort it out because the field stop on that eyepiece is machined into the lens retaining ring. Loosen that to put the field stop at the correct (sharp) position and the lens elements rattle :rolleyes2:

 

Maybe that was the main reason for the design change John? The 32mm BC plossl has a threaded field stop so you can shift it to your desire but it’s quite loose and rattles- could do with some threadlock.

Mark

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12 hours ago, markse68 said:

Maybe that was the main reason for the design change John? The 32mm BC plossl has a threaded field stop so you can shift it to your desire but it’s quite loose and rattles- could do with some threadlock.

Mark

With short focal length plossls and orthos I've often seen the field stop incorporated into the lens retaining ring. I guess having these small field stops as a separate screw in element would be a more complex way to make the eyepiece.

 

 

 

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I have a Celestron RACI 9x50 and I need to screw out the top of the eyepiece a considerable way to get the reticule in focus. One the other hand my new iOptron 9x50 doesn't need any adjustment.

4 hours ago, John said:

With short focal length plossls and orthos I've often seen the field stop incorporated into the lens retaining ring. I guess having these small field stops as a separate screw in element would be a more complex way to make the eyepiece.

Yes, but gives it a more quality feel. These Circle T orthos have a surprising good build quality.

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