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Field stop?

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#1 bamastar

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Posted 18 January 2017 - 10:01 AM

I'm sure this has been covered many times but in looking I'm not seeing a explanation at the moment. What is a eyepiece's field stop and should I consider this in purchasing a ep? There's some technical terminology that I don't understand but feel silly asking. It was only 6 months ago before I knew what OTA was. Lol



#2 Alex McConahay

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Posted 18 January 2017 - 10:10 AM

The field stop is a ring placed near the focus point of an eyepiece. It defines the outer edge of the visible area.

 

Imagine you are looking through a round window. The window is fully illuminated edge to edge. But you notice that out on the edges, the window is dirty or fuzzy, while in the middle it is sharp. You decide you do not need the edge so much because the dirty, distorted edge distracts from the pretty, sharp, clean middle. SO, you tape a circular "donut" of cardboard or something around the edge so you get only the good central part coming through the window. You cannot see as widely as you saw before, but what you get is nice and clean.

 

This "donut" is the field stop.

 

Controlling the size of the donut helps control the field of view, and eliminates the errors out at the edge.

 

Note that there is not necessarily an actual spearate "Donut" in an eyepiece. The job can be done by constructing the eyepeice itself so that light is blocked from the edges of the lens.

 

Alex


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#3 caveman_astronomer

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Posted 18 January 2017 - 10:11 AM

I'm sure this has been covered many times but in looking I'm not seeing a explanation at the moment. What is a eyepiece's field stop and should I consider this in purchasing a ep? There's some technical terminology that I don't understand but feel silly asking. It was only 6 months ago before I knew what OTA was. Lol

It's a circular opening placed at the eyepiece's focal plane in order to provide a clean looking edge to the field of view, or to restrict the field of view to the acceptably sharp part of the focal plane, however one might define acceptably sharp.

 

I would simply look at the eyepiece's advertised apparent field of view when making choices.

 

And I have an eyepiece or two that don't seem to have field stops.


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#4 bamastar

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Posted 18 January 2017 - 10:31 AM

So the blocking of the "dirty view" is blocked by the field stop, that an internal feature inside the ep and the size of the field stop is how much of the view it's blocking? Is that correct?



#5 Joe1950

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Posted 18 January 2017 - 10:43 AM

Or how much it is letting through. If I may butt in.

 

Eyepiece design limits the Apparent Field of View each design is good to. A Plossl, for example, can be designed to give a relatively clean, sharp view of about 50 to 52o. The field stop aperture will limit the field of view of the eyepiece to that value.

 

If the field stop were not there, you would see more of a field, but it wouldn't be sharp and clean of other aberrations, and you wouldn't have any use for the wider area.



#6 caveman_astronomer

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Posted 18 January 2017 - 10:43 AM

So the blocking of the "dirty view" is blocked by the field stop, that an internal feature inside the ep and the size of the field stop is how much of the view it's blocking? Is that correct?

I think you've got it.

 

One of my finders has a homemade Ramsden eyepiece, a type that usually has a field stop that allows 40 degrees or less.  But since the idea of a finder is to provide a wide field of view, I made the field stop opening much wider and get a field closer to 45 degrees.  I don't care how bad the edge of the field is, even if purists might cringe at the thought.



#7 bamastar

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Posted 18 January 2017 - 11:13 AM

So, when a ep is say 68° or 82°, whatever it maybe, is the field stop part of the said fov or is it taking away from said fov?



#8 Alex McConahay

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Posted 18 January 2017 - 11:15 AM

>>>>>the size of the field stop is how much of the view it's blocking

 

The size of the field stop is how much is getting through, not blocking.

 

Another note: I have an 82 degree FOV eyepiece that is a bit weird (misshapen stars, etc.) out at the edges. I paid about $100 for it. The manufacturer should have made the field stop smaller, maybe 74degrees. And then he would have had a well corrected eyepiece out to the edge with a 74 degree FOV. However, he did not because 82 degree FOV eyepieces were the rage. SO, he gave it an 82 degree sized field stop. A more reputable manufacturer like Tele-Vue puts their field stop just about where the errors start to show. My manufacturer made it oversized so he could say he had 82 degrees FOV, even though only 74 or so were well corrected.

 

If you were to take a well designed eyepiece like some 82 degree FOV eyepieces, and remove the field stop, you might get a 96 or something degree FOV eyepiece--but the outer edge would be poopy. (Note, that you cannot necessarily just remove a "Field Stop" because the Field Stop may in fact be part of the mechanical assembly holding the eyepiece together.) 

 

>>>>>>>So, when a ep is say 68° or 82°, whatever it maybe, is the field stop part of the said fov or is it taking away from said fov?

 

The Field Stop always takes away from the field of view. The 68 degree might have 75 or so actual degrees if you just count the glass. But that edge (past 68) is not good. So, the manufacturer stops it down to 68. That 68 degrees is what is left.

 

Alex


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#9 caveman_astronomer

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Posted 18 January 2017 - 11:38 AM

So, when a ep is say 68° or 82°, whatever it maybe, is the field stop part of the said fov or is it taking away from said fov?

If you look through the eye end of 82 degree eyepiece at a white illuminated wall, you will see a white circular region that is 82 degrees in diameter.  The black or dark area immediately surrounding that is, or is caused by, the field stop.

 

Replace that field stop with one that has a smaller hole and you will get a narrower field of view.  Replacing it with a field stop with a larger hole, might or might not give you a larger field of view, depending on the lenses in the eyepiece.


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#10 bamastar

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Posted 18 January 2017 - 12:37 PM

Ok. Now I think I'm on the same page! Now when I hear talk of "field stop" I know what everyone is talking about. I just purchased a ES 82 8.8mm and in the market for a few lower powered panoptic or ES68. Through researchin ep's that phrase has kept coming up and looking at different sites, the FS diameter is always in the specs. Figured I'd ask what it was bc I could be overlooking something important.


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#11 Joe1950

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Posted 18 January 2017 - 12:54 PM

Good question! The ES 82 8.8mm is a very fine eyepiece. I like their products and their customer service.

 

They will soon have a line of 62o eyepieces that run about $100. That's plenty of field for me, but everyone has favorite brands and styles. Plenty of good choices for all wallet sizes.

 

Enjoy!



#12 bamastar

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Posted 18 January 2017 - 01:53 PM

I have the 82° 11mm and love it! Wanted to get a higher powered ep in 82° so I purchased the 8.8mm. Still waiting for it to arrive. I have a 22mm panoptic and love that one as well. I've used all the panoptics in my scope besides the 41mm and they preformed great. Wasn't to impressed with the 15mm though.



#13 macdonjh

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Posted 18 January 2017 - 01:54 PM

A few eye pieces don't have an actual ring that makes the field stop.  I think my old Orion Optiluxes are "sans field stop".

 

Some times the field stop is inside the eye piece, between some of the lenses.

 

One situation where field stop diameter can be important is if you're matching a small scope to a long focal length, wide field eye piece.  In extreme cases, the baffle tube of a small SCT is smaller than the field stop of a large eye piece.  If that's the case, the scope won't pass a light cone large enough in diameter to fully illuminate the eye piece.  That's one form of vignetting.



#14 Tony Flanders

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Posted 18 January 2017 - 02:06 PM

A few eye pieces don't have an actual ring that makes the field stop.  I think my old Orion Optiluxes are "sans field stop".

 

Some times the field stop is inside the eye piece, between some of the lenses.

 

All eyepieces have some kind of field stop. In many designs -- almost all the new, complex designs that have more than four individual elements -- the field stop is between the lenses, which obviously makes it hard to measure.

 

In eyepieces that maximize the field of view possible for their barrel size, such as 32-mm Plossls, the field stop may simply be the end of the barrel.

 

Sometimes the field stop isn't placed precisely at the focal plane. You can see that when using the eyepiece by the fact that the view fades out rapidly near the edge, which is fuzzy rather than sharp.

 

It's a manufacturer's choice how wide to make the apparent field of view. Some people argue that a wider field of view is always better. It is still useful for star-hopping and for framing the stuff that you care about, even if it's ugly.



#15 Jon Isaacs

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Posted 18 January 2017 - 02:28 PM

I'm sure this has been covered many times but in looking I'm not seeing a explanation at the moment. What is a eyepiece's field stop and should I consider this in purchasing a ep? There's some technical terminology that I don't understand but feel silly asking. It was only 6 months ago before I knew what OTA was. Lol

 

The field stop is a ring at the focal plane (hopefully) of the eyepiece that defines how much of the telescope's focal plane will be visible.  When you look through an eyepiece, you see it as the edge of the field of view, hopefully sharp and clean.

 

If you take an eyepiece like a Plossl and turn it around and look down the barrel, you can see the field stop, it will be some sort of a ring.  You can often measure the diameter of the field stop... 

 

The actual size of the true field of view depends on the field stop diameter, the bigger the field stop the greater the TFoV and the focal length of the scope, the longer the focal length, the greater the image scale and so the smaller the field of view.  

 

If you want to accurately determine the TFoV,  if one knows the field stop diameter and the focal length of the scope, then a simple triangle whose height is the focal length of the scope and whose base is the field stop diameter suffices.  These angles are relatively small so one can use the small angle approximation so that:

 

TFoV = (180degrees/Pi radians) x Fieldstop/focal length scope = 57.3 degree/radian x FS/FLscope.

 

Jon


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#16 Starman1

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Posted 18 January 2017 - 02:58 PM

Every telescope has an image scale on its final focal plane.

Focusing an eyepiece is bringing the focal plane of the eyepiece to be coincident with the focal plane of the scope.

The amount of the telescope's focal plane you see is determined by the field stop of the eyepiece, i.e. the iris that restricts the field of view.

The eyepiece has an inherent magnification, so, in a very real sense, it is allowing you to use a magnifying glass to look at the focal plane

of the telescope.

 

You can know exactly how much true field of the telescope you will see by using the following formula:

 

TFOV = (EPFS/TFL) * 57.296

Where EPFS = eyepiece field stop, TFOV = true field of view, and TFL is Telescope Focal Length.

 

Note that this does not require knowing the AFOV (apparent field of view) of the eyepiece.

 

If you don't know the field stop of the eyepiece, you can use a less accurate formula:

 

TFOV = AFOV/M  where M = magnification.

 

You can derive an *approximate* field stop for an eyepiece with the following formula.  It is not perfectly accurate because of distortion in the eyepieces and the fact that

manufacturer's apparent field figures are usually also approximate.

 

EPFS = TFL * [AFOV / (TFL/EPFL)] / 57.296 where EPFL is eyepiece focal length


Edited by Starman1, 18 January 2017 - 03:07 PM.


#17 bamastar

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Posted 18 January 2017 - 03:42 PM

Every telescope has an image scale on its final focal plane.

Focusing an eyepiece is bringing the focal plane of the eyepiece to be coincident with the focal plane of the scope.

The amount of the telescope's focal plane you see is determined by the field stop of the eyepiece, i.e. the iris that restricts the field of view.

The eyepiece has an inherent magnification, so, in a very real sense, it is allowing you to use a magnifying glass to look at the focal plane

of the telescope.

 

You can know exactly how much true field of the telescope you will see by using the following formula:

 

TFOV = (EPFS/TFL) * 57.296

Where EPFS = eyepiece field stop, TFOV = true field of view, and TFL is Telescope Focal Length.

 

Note that this does not require knowing the AFOV (apparent field of view) of the eyepiece.

 

If you don't know the field stop of the eyepiece, you can use a less accurate formula:

 

TFOV = AFOV/M  where M = magnification.

 

You can derive an *approximate* field stop for an eyepiece with the following formula.  It is not perfectly accurate because of distortion in the eyepieces and the fact that

manufacturer's apparent field figures are usually also approximate.

 

EPFS = TFL * [AFOV / (TFL/EPFL)] / 57.296 where EPFL is eyepiece focal length

So for the ballpark TFOV for my ES82 11mm for a 1250mm scope would be 82÷114=0.72 of a degree of sky for TFOV? Did I do that correct? That formula comes in handy for I do wonder how much sky I'm truly seeing per ep.


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#18 Starman1

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Posted 18 January 2017 - 05:51 PM

You did it right.

 

The field stop in that eyepiece is 15.9mm.

Your focal length is 11x114=1254mm

15.9/1254*57.296 = 0.726°

 

The other way gets 0.719°.  Both methods are very close in this case off by only 0.4' of field.

Often, the two methods come out much more different.


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#19 macdonjh

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Posted 18 January 2017 - 07:57 PM

 

A few eye pieces don't have an actual ring that makes the field stop.  I think my old Orion Optiluxes are "sans field stop".

 

Some times the field stop is inside the eye piece, between some of the lenses.

 

All eyepieces have some kind of field stop. In many designs -- almost all the new, complex designs that have more than four individual elements -- the field stop is between the lenses, which obviously makes it hard to measure.

 

In eyepieces that maximize the field of view possible for their barrel size, such as 32-mm Plossls, the field stop may simply be the end of the barrel.

 

 

Well, OK, point taken.  But if you read my post, I was making a distinction about those few eye pieces that don't have a separate component that defines the field stop.  Some would call the end of the barrel the entrance pupil of the eye piece and that might be different than the field stop diameter.



#20 Starman1

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Posted 19 January 2017 - 02:02 AM

If the barrel defines the field stop, and the field stop is not at the exact end of the barrel, then the field stop will not be sharp or distinct.

I've had some eyepieces like this and added a small ring to make a field stop.



#21 Jon Isaacs

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Posted 19 January 2017 - 05:40 AM

If the barrel defines the field stop, and the field stop is not at the exact end of the barrel, then the field stop will not be sharp or distinct.

I've had some eyepieces like this and added a small ring to make a field stop.

 

Another small but significant point: The sharpness of the field stop depends to some extent on the observer's eye. Normally it will be placed at the focal plane of the eyepiece which assumes the observer's eye will be focused at infinity.  If the observer is nearsighted and unable to focus at infinity, the field stop will not be in focus though it may be close.  

 

Jon


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