Author Topic: Fields stops, focal point, and Dawes' limit... where is the "magic" happening.  (Read 391 times)

Michael Presley

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I'm hoping someone can help me gain a deeper understanding of what's actually happening when an eyepiece is reducing the FOV. I understand the math on FOV, Magnification, Exit Pupil, etc. But when I try to reconcile what is actually happening I end up with a conundrum regardless of how I slice it. As follows:

A short focal length EP is closer to the focal point, and this results in magnification and reducing the FOV, but where does the rest of the light gathered go? Is it cut off by the field stop, or is there some "magic" happening as the light expands from the focal point that results in a more narrow FOV the closer you are to the focal point?

If the latter, is there a word for the "magic" causing the FOV to get wider as you get further from the focal point?

If the former, and the field stop is cutting off the image, why doesn't that affect Dawes' limit (or does it but that's countered by higher magnification?) I know if I were to mask the outer edge on the front of the scope, say making a 5" a 4", then the resolution capability of the scope would be reduced. It seems the same thing should be happening at the back end with a field stop as does happen with the mask on the aperture. But we use high magnification to split double stars and get detail on planets so it appears "masking" at the back end via the field stop has a different effect than masking the aperture. So this is where I run into "magic" again. What happens at the focal point that makes it different on either side, does the light "mix" somehow?

I hope I've explained this well enough so someone can understand my conundrum. A reply with a word to google or link to something that explains this "magic" would be greatly appreciated. And sorry to keep using the word "magic", it's just the word I use for things that happen which I understand the output but don't have a full understanding of the inner workings.Thanks for reading and clear skies,

Bob



brascharnide

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I wish I could answer, but I just have no idea how all this works... I do understand the need to understand, I am just not there yet... I think that knowing will help you be a more informed, prepared observer/user of optics... like my other hobby, woodworking, knowing each tool's function/limitation helps me work wood better...

I will be interested in what the experts will say here...

Best regards!

CB

brentioscaraph

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I'm hoping someone can help me gain a deeper understanding of what's actually happening when an eyepiece is reducing the FOV. I understand the math on FOV, Magnification, Exit Pupil, etc. But when I try to reconcile what is actually happening I end up with a conundrum regardless of how I slice it. As follows:

A short focal length EP is closer to the focal point, and this results in magnification and reducing the FOV, but where does the rest of the light gathered go? Is it cut off by the field stop, or is there some "magic" happening as the light expands from the focal point that results in a more narrow FOV the closer you are to the focal point?

If the latter, is there a word for the "magic" causing the FOV to get wider as you get further from the focal point?

If the former, and the field stop is cutting off the image, why doesn't that affect Dawes' limit (or does it but that's countered by higher magnification?) I know if I were to mask the outer edge on the front of the scope, say making a 5" a 4", then the resolution capability of the scope would be reduced. It seems the same thing should be happening at the back end with a field stop as does happen with the mask on the aperture. But we use high magnification to split double stars and get detail on planets so it appears "masking" at the back end via the field stop has a different effect than masking the aperture. So this is where I run into "magic" again. What happens at the focal point that makes it different on either side, does the light "mix" somehow?

I hope I've explained this well enough so someone can understand my conundrum. A reply with a word to google or link to something that explains this "magic" would be greatly appreciated. And sorry to keep using the word "magic", it's just the word I use for things that happen which I understand the output but don't have a full understanding of the inner workings.Thanks for reading and clear skies,

Bob

An eyepiece has an apparent field of view and a focal length.

The focal length of an eyepiece determines how far away from the objective's focal surface (or focal plane) the eyepiece needs to be to put the telescope in focus. A 24mm eyepiece will be twice as far away as a 12mm eyepiece and reveal a field with twice the diameter as the 12mm, assuming that both eyepieces have the same AFOV.

Near-sighted people can get a small amount of extra FOV at the cost of a small amount of magnification, since they will probably focus the eyepiece a small amount farther from the focal plane.

SCTs and Maks might complicate this picture if the eyepieces are not parfocalized since such telescopes move the focal plane and change focal length when the focus is adjusted.

Ryan Fletcher

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Thank you for the replies CB and Caveman.
I'm good on fov, focal length and all that. My question is more about the how than the what. As CB was alluding, there really isn't any need to know the answer to what I'm asking to be proficient in understanding the mechanics and the output to the eye. It's just the specifics of how the mechanics actually work that I'm curious about. Was hoping there was a somewhat easy answer that resolves my conundrum.

tyrrcencifunc

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First, we need to get one thing straight: The eyepiece has NO influence WHATSOEVER on the image the objective lens or mirror projects at the focal plane. None, zero, nada.

The image at the focal plane is a fixed entity. Whether you observe it with the naked eye, a camera or the eyepiece doesn't matter, its properties are not going to change. Its appearance only depends on the aperture, focal length and accuracy of the objective and the latters inherent aberrations.

How the image APPEARS to the eye looking through the eyepiece will depend HEAVILY on the eyepiece.

All of the light that the objective gathers from a given object or any given point on the surface of that object ends up in the image of that object or point. No light is lost in the image you can see, even if the eyepiece can't take it all in, due to a narrow field of view.

How much of the image the eyepiece can see depends on the width of its field stop. This is called the True Field of View, TFOV for short.

How large this field appears to the eye depends on a combination of the width of the field stop and the eyepieces own magnification (because the field stop is also magnified). This is called Apparent Field of View, AFOV.Clear skies!
Thomas, Denmark

chirafepes

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Thank you for the replies CB and Caveman.
I'm good on fov, focal length and all that. My question is more about the how than the what. As CB was alluding, there really isn't any need to know the answer to what I'm asking to be proficient in understanding the mechanics and the output to the eye. It's just the specifics of how the mechanics actually work that I'm curious about. Was hoping there was a somewhat easy answer that resolves my conundrum.

The edge of the field stop is the boundary between the illuminated area seen through an eyepiece and what we might call the "inside of the barrel" of the eyepiece.

Todd Topcic

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Thank you for the replies CB and Caveman.
I'm good on fov, focal length and all that. My question is more about the how than the what. As CB was alluding, there really isn't any need to know the answer to what I'm asking to be proficient in understanding the mechanics and the output to the eye. It's just the specifics of how the mechanics actually work that I'm curious about. Was hoping there was a somewhat easy answer that resolves my conundrum.

Cut two circles in a piece of cardboard, one circle twice the diameter of other. Then hold the cardboard a few inches from your eye and compare what you can see through each. The edge of those circles is analogous to a field stop.

Michael Thompson

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Thanks, Tomas. I understand the majority of what you said, but this bit certainly pertains to my question: "No light is lost in the image you can see, even if the eyepiece can't take it all in, due to a narrow field of view." How can "no light be lost" if "the eyepiece can't take it all in"? It seems that both can't be true.

Bob

brodsandbacksosp

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First, we need to get one thing straight: The eyepiece has NO influence WHATSOEVER on the image the objective lens or mirror projects at the focal plane. None, zero, nada.

The image at the focal plane is a fixed entity. Whether you observe it with the naked eye, a camera or the eyepiece doesn't matter, its properties are not going to change. Its appearance only depends on the aperture, focal length and accuracy of the objective and the latters inherent aberrations.

How the image APPEARS to the eye looking through the eyepiece will depend HEAVILY on the eyepiece.

All of the light that the objective gathers from a given object or any given point on the surface of that object ends up in the image of that object or point. No light is lost in the image you can see, even if the eyepiece can't take it all in, due to a narrow field of view.

How much of the image the eyepiece can see depends on the width of its field stop. This is called the True Field of View, TFOV for short.

How large this field appears to the eye depends on a combination of the width of the field stop and the eyepieces own magnification (because the field stop is also magnified). This is called Apparent Field of View, AFOV.Clear skies!
Thomas, Denmark

The eyepiece has an apparent field of view, the telescope with an eyepiece has a true field of view.

Jeremy Swaine

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Thanks again, Caveman, but I seem to be having trouble making my point. I know exactly what a field stop is and it's affect on what I see, my question is the next level deeper. Is the field stop cutting off the light around the edges, and if so, how does that relate to Dawes limit since a "field stop" or mask on the aperture would affect it.

mellidonde

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To add what Thomas wrote, it is worth understanding at little more about the eyepiece.

The field stop is a metal ring that you see as the edge of the field of view. With many eyepieces you can turn it around , look down the barrel and see the field stop.

The image exists at the focal plane. The role of the eyepiece is to allow you to bring your eye closer to the image. To view the image, you bring the eyepiece to focus at the focal plane. That's an image and each point is complete, receives the full light from mirror, there is no more information to be gained.

The field stop merely blocks the light at the edge of the field.. Imagine looking through a 10 foot tube at a TV screen. The diameter of the tube determines how much of the image you will see, that's the field stop. The length of the tube determines how much much your eye can resolve, that's the focal length. If you cut the tube in half, you would resolve more because you r eye would be closer.

But the diameter of the tube does not effect the brightness and resolution ofwhat you see..

jon

Brenton Crosby

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Thank you Jon! This "the role of the eyepiece is to allow you to bring your eye closer to the image" has helped me a great deal. I was picturing it more as a one way flow, but picturing it as the eyepiece magnifying the light as it is at the focal point, rather than the light expanding out of the focal point to the EP, has certainly flicked on a light for me...

Chris Mancia

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Thanks again, Caveman, but I seem to be having trouble making my point. I know exactly what a field stop is and it's affect on what I see, my question is the next level deeper. Is the field stop cutting off the light around the edges, and if so, how does that relate to Dawes limit since a "field stop" or mask on the aperture would affect it.

Dawes limit is not involved, the field stop does not mask the aperture, only the real image that exists at the location of the field stop. You can make a field stop using some of that cardboard to which I alluded earlier. You could even make a square field stop if you wanted.

Cut a circle small enough to fit in the barrel of an eyepiece, cut a square hole that is smaller than the existing field stop and push the cardboard up against the eyepiece's existing field stop. Then look through a telescope using that eyepiece.

Or think about how cross hairs block portions of the field of view.

specconcheckre

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I appreciate the replies Caveman, I think I'm on the right track now after reading Jon's reply. It seems I didn't fully understand how the EP related to the focal point, and thus, I was thinking the field stop was doing more than it is actually doing.

tenpaseper

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To add what Thomas wrote, it is worth understanding at little more about the eyepiece.

The field stop is a metal ring that you see as the edge of the field of view. With many eyepieces you can turn it around , look down the barrel and see the field stop.

The image exists at the focal plane. The role of the eyepiece is to allow you to bring your eye closer to the image. To view the image, you bring the eyepiece to focus at the focal plane. That's an image and each point is complete, receives the full light from mirror, there is no more information to be gained.

The field stop merely blocks the light at the edge of the field.. Imagine looking through a 10 foot tube at a TV screen. The diameter of the tube determines how much of the image you will see, that's the field stop. The length of the tube determines how much much your eye can resolve, that's the focal length. If you cut the tube in half, you would resolve more because you r eye would be closer.

But the diameter of the tube does not effect the brightness and resolution ofwhat you see..

jon

If I cut the tube in half I would get a wider field of view, not a closer view, unless I also moved to a place twice as close to the screen.