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Equipment Discussions >> Eyepieces

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howard929
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Re: Pushing the magnification new [Re: Eddgie]
      #5855268 - 05/12/13 11:29 AM

Quote:




Any light falling anywhere on the focal plane does the exact same amount of damage to the image at any point in the focal plane where it falls regardless of the eyepeiece field stop size.

If it is not within the diameter of the field stop, you just don't see it.




Pardon. I "think" Great Bear is trying to make a subtly different point. That the light passing through the field stop is the total of all the light, wanted and unwanted that has entered the instrument. Further, that the unwanted light, magnified by high power eyepieces is the cause of contrast loss.

Edited by howard929 (05/12/13 11:42 AM)


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Starman1
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Re: Pushing the magnification new [Re: howard929]
      #5855354 - 05/12/13 12:07 PM

Quote:

Quote:




Any light falling anywhere on the focal plane does the exact same amount of damage to the image at any point in the focal plane where it falls regardless of the eyepeiece field stop size.

If it is not within the diameter of the field stop, you just don't see it.




Pardon. I "think" Great Bear is trying to make a subtly different point. That the light passing through the field stop is the total of all the light, wanted and unwanted that has entered the instrument. Further, that the unwanted light, magnified by high power eyepieces is the cause of contrast loss.




I don't buy it. High magnification or low magnification, if scattered light is present on the focal plane of the scope, the field of view of the eyepiece will see that scattered light and have its contrast reduced by it.
What's being argued here is that high magnifications display MORE of the scattered light than low magnifications, and that simply makes no sense.

Outside the field stop of the eyepieces is blackness and farther out, peripheral vision. Light from the scope outside the field stop of the eyepiece simply cannot get to the eye unless it is reducing contrast and scattering light over the field seen by the eyepiece.

Think of the field stop as a baffle. Light outside the field stop, if it is not affecting the field seen by the eyepiece, doesn't make it through to the eye.

Edited by Starman1 (05/12/13 12:11 PM)


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great_bear
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Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Eddgie]
      #5855362 - 05/12/13 12:13 PM

Quote:

This isn't really the way it works.




Yes it is.

I can draw an elementary ray-trace diagram if you like - or perhaps you could look one up in a book?

If there was a miniature translucent screen at the focal plane - like a back-projected cinema screen - then what you're saying would be true. However, in a telescope, light doesn't fall on to the focal plane, it travels straight through it. There's a big difference; the angle at which the light travels in straight lines through the focal plane counts for everything.

Are you really saying otherwise? - we're getting into pretty basic territory now...


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great_bear
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Reged: 07/05/09

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Re: Pushing the magnification new [Re: Starman1]
      #5855378 - 05/12/13 12:24 PM

Don,

Quote:

if scattered light is present on the focal plane of the scope...




Like Eddgie you're talking as if light actually lands "on" the focal plane. It does not. It goes straight through it.
In the absence of a surface to land on or reflect off, light travels in straight lines.


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howard929
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Re: Pushing the magnification new [Re: Starman1]
      #5855446 - 05/12/13 12:55 PM

Quote:


What's being argued here is that high magnifications display MORE of the scattered light than low magnifications.......




I may be mistaken here, I was under the impression that the point made is at higher magnifications, scattered light isn't increased, it's more noticeable.


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leonard
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Re: Pushing the magnification *DELETED* new [Re: great_bear]
      #5855505 - 05/12/13 01:39 PM

Post deleted by leonard

Edited by leonard (05/12/13 04:06 PM)


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MRNUTTY
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Reged: 11/22/11

Loc: Mendon, MA
Re: Pushing the magnification new [Re: leonard]
      #5855754 - 05/12/13 03:28 PM

How're our intrepid hero's going to get out of this one?!

Where's that popcorn emoticon?


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astro_baby
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Reged: 06/17/08

Loc: United Kingdom
Re: Pushing the magnification new [Re: Starman81]
      #5855850 - 05/12/13 04:08 PM

Unless I have missed it no one has raised atmosphere and seeing as an issue. In the UK seeing is most often restricted to a maximum of x235 and very ofetn lower than that which most often down to a very wobbly atmosphere and tough seeing conditions.

Its often been my experience that smaller refractors and maks can puch above this even on a night where my 8" newt is hampered. The best exalmpe a can give is where my 4" axhro can run to highe mags than my 8" newt. This was heavily disputed by some but my own eyeballs have seen it.
An article did crop up on here ages ago regarding the cell like structure of the atmopshere where adjoining cells are at different temperature which creates a higher degree of wobble to a view IF the telescopes field of view takes in more cells...mre cells in view equals greater degree of wobble and consequent worse seeing and lower magnification.
I am sure this is true in practice because my 8" F5 almost always never gets to run above x200 whereas my f9 frac and f15 mak can nearly always exceed that and the Mak has run up as high as around x300 on various targets. Unfortunately as a diletante rather than a scientist I dont keep logs on the targets observed and at what mags but ai have been up to x300 in the Mak on a big range of targets including Saturn, M13, M42 and is been fine, Andromeda, the dumbell nebula and some other fainter objects always seems go suffer from too much mag in all scopes I have so the max mag is clearly target specific as well.

Another limiter on mag seems to me to be scope specifoc as well with higher mags obtainable in fracs overall than in newts. How much this is down to quality of the glass is hard to say. My experience to date suggest is down more to the cells in the atmosphere and the scopes field of view but then I dont own a fast frac to know for sure.


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great_bear
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Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: leonard]
      #5855861 - 05/12/13 04:37 PM

Hi Leonard.

It's not as mysterious as it sounds.

What happens is this:
(this is factual by the way, not theory)

1. The near-parallel, but slightly divergent light paths from a distant point (such as a star) which reach a telescope's primary are refracted to converge at a corresponding point on the focal plane.

2. Now, being straight lines, they simply cross right over and become divergent at the other side of that focal plane.

3. These diverging paths then enter the eyepiece, which (assuming it's correctly focused) refracts them once more into a set of near-parallel paths which - as far as the eye is concerned - places them at apparently infinity focus, just as the original star was. If the eyepiece is not exactly focused, instead of being parallel, the paths will either be too divergent - creating the illusion the image is too close - or slightly convergent, creating the illusion that the image lies beyond infinity - the blurriness that short-sighted people are only too familliar with.

Now - and this is the key point - the eyepiece must be exactly in-line between the primary and the human eye in order for this to happen. If the focal plane is observed from an oblique angle, there's nothing to see.

There's nothing complex about this. It's no different to looking at a house in the distance through a magnifying glass at arms length. You will see an upside-down image of that house. If you look carefully you'll notice that this image appears to be not within the magnifying glass, but actually somewhere closer to you by a few inches.

Nonetheless, you still only see this closer image confined by the outline of the magnifying glass that lies ahead of it. The image cannot extend beyond this, nor can any light from any other source "fall upon it".

Finally, remember that what you've been holding in your hands is a tubeless, ~F2 telescope.


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Pushing the magnification new [Re: great_bear]
      #5855974 - 05/12/13 05:38 PM

Quote:

Hi Leonard.

It's not as mysterious as it sounds.

What happens is this:
(this is factual by the way, not theory)

1. The near-parallel, but slightly divergent light paths from a distant point (such as a star) which reach a telescope's primary are refracted to converge at a corresponding point on the focal plane.



Correct.
Quote:


2. Now, being straight lines, they simply cross right over and become divergent at the other side of that focal plane.



Correct again.
Quote:


3. These diverging paths then enter the eyepiece, which (assuming it's correctly focused) refracts them once more into a set of near-parallel paths which - as far as the eye is concerned - places them at apparently infinity focus, just as the original star was. If the eyepiece is not exactly focused, instead of being parallel, the paths will either be too divergent - creating the illusion the image is too close - or slightly convergent, creating the illusion that the image lies beyond infinity - the blurriness that short-sighted people are only too familliar with.



This is where your argument falls apart. The rays entering the eyepiece from angles more off-axis than the circle of focal plane seen by the eyepiece are essentially baffled out. And rays may diverge, or converge, but in no ray-trace diagram I've ever seen do rays from sharply off-axis suddenly become parallel to the eyepiece and travel through. And the field stop in the eyepiece stops rays more off axis from making it through the eyepiece. The eye does not see light rays from farther afield. And IF they managed to enter the bottom lens, and IF they managed to be reflected by the sides of the lenses or the internal walls of the eyepiece, they would be stopped dead by the field stop in the eyepiece (assuming the field stop was not prior to the entry of the field lens).
Light from outside the field stop simply doesn't get through for the pupil of the eye to somehow see that light outside the exit pupil of the eyepiece. Light from outside the FOV of the eyepiece can make it through, but only if it is reflected from some internal surface at an angle that allows the rays to pass through the field stop of the eyepiece. Your example of the scattered light in the tube can only happen if you are looking through the scope without an eyepiece. Once you add an eyepiece to the system, that light is gone (or reflects elsewhere).

By the way a telescope forms an image on the focal plane. Put some translucent scotch tape across the empty focuser and point the scope at the Moon. You can easily focus the moon on the tape and that position of the tape when the Moon's image is in focus corresponds to the focal plane of the scope.
Quote:


Now - and this is the key point - the eyepiece must be exactly in-line between the primary and the human eye in order for this to happen. If the focal plane is observed from an oblique angle, there's nothing to see.

There's nothing complex about this. It's no different to looking at a house in the distance through a magnifying glass at arms length. You will see an upside-down image of that house. If you look carefully you'll notice that this image appears to be not within the magnifying glass, but actually somewhere closer to you by a few inches.

Nonetheless, you still only see this closer image confined by the outline of the magnifying glass that lies ahead of it. The image cannot extend beyond this, nor can any light from any other source "fall upon it".

Finally, remember that what you've been holding in your hands is a tubeless, ~F2 telescope.




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leonard
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Reged: 10/19/07

Loc: West Virginia
Re: Pushing the magnification *DELETED* new [Re: leonard]
      #5855987 - 05/12/13 05:44 PM


Hello ,

great bear ,

I have deleated my last post as it makes little sense even to me . I apologize for my lack of comprehension of this thread and my foolish speculation .

Leonard


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great_bear
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Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5856289 - 05/12/13 08:38 PM

Hi Don,


Your refutations are unfortunately always based on misinterpretations of what I'm saying, which is unhelpful for those trying to follow the thread.

When I was discussing parallel relationships, I was discussing a bundle of parallel light rays - not rays parallel to the eyepiece. All objects at (effectively) infinity focus emit such bundles of parallel light rays. Every individual star in a stellar image will leave any correctly-focused telescope's eyepiece as an exit-pupil-thick bundle of parallel rays pointing through your iris in the direction of where that star will end up on your retina after your eye-lens has converged the bundle back to a single point. The field-stop constrains the angular extent that these bundles can point to (thus defining the edge-of-field), but it does not control each bundle's diameter; I'm sure that you are perfectly aware that's a function of aperture; as per the aperture/exit-pupil relationship we all know so well.

You don't need to be following this discussion too closely to see that if the aperture is bigger, the exit pupil leaving the eyepiece is bigger. If the aperture is smaller and surrounded by horrible reflective surfaces, the exit pupil leaving the eyepiece is smaller and surrounded by horrible reflective surfaces too. That's why quality telescopes are made with carefully-placed baffle-stops or very good flocking - you want everything outside of the border of the primary to be as black as possible, because it all fits through your iris at smaller exit-pupils when the magnification is high.

The demonstrations I described earlier clearly show this, so I don't understand why it merits further discussion.

Quote:

Your example of the scattered light in the tube can only happen if you are looking through the scope without an eyepiece.




Then look through my Mak and make me a liar!

In all seriousness though, I am not wrong on this issue; I've merely been unable to explain it to you successfully. Perhaps someone else will be more successful in future.


Clear skies to you.
P.S. I had already mentioned that there are important differences when projecting onto a translucent tape or screen; "diffusion" would pretty-much cover it.


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Jon Isaacs
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Re: Pushing the magnification new [Re: great_bear]
      #5856461 - 05/12/13 10:24 PM

Quote:


Now - and this is the key point - the eyepiece must be exactly in-line between the primary and the human eye in order for this to happen. If the focal plane is observed from an oblique angle, there's nothing to see.




I am not sure what point you are trying to make... I haven't been following the part of the thread, it appears to be totally off-topic. I will say that the eyepiece can be rather severely misaligned and off-axis and it will show up as a tilt and offset. At high magnifications this is particularly true since the focal plane is much larger than the field stop.

But in any event, I am bewildered at how this thread has transformed itself from "my question might be better phrased (for the more experienced of understanding such as yourself) as "What's the minimum scope-relative magnification that will resolve for human eyes all possible detail?" to this current discussion.

Jon

Jon Isaacs


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ibase
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5856607 - 05/12/13 11:42 PM

Have seen the effect on refractors, like the two below:


William Optics 66SD and Orion Short Tube 80

Magnification can be pushed further on the WO66 without image breakdown, the 80mm ShortTube was outclassed. Probably better optics, better baffling, better Petzval design, either or all have contributed to the better views on the WO66 when pushing the magnification.

So someone who owns the ST80 will say that magnification can only be pushed to x factor, while the WO66 owner will say a different higher figure. One reason maybe why everybody have divergent views on max. magnification.

Best,


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Geo31
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Re: Pushing the magnification new [Re: Bill Boublitz]
      #5856653 - 05/13/13 12:17 AM

Quote:

40 or 50 years ago, we were all likely under darker skies, too. One more thing to throw into the mix. (Smiles.)




Boy howdy


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Geo31
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Re: Pushing the magnification new [Re: Geo31]
      #5856724 - 05/13/13 01:12 AM

I'm curious...

If stray light that won't reach the image are blocked by the field stop, why does it matter than elements have their edges blackened?

It would seem to me that if imaging light is all that reaches the eye, then blackening the edges would be a red herring?

What am I missing?


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Lamb0
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Re: Pushing the magnification new [Re: Geo31]
      #5856739 - 05/13/13 01:20 AM

Theory is nice; but choices are better! Start low, then boost the power to taste, or beyond then back off. Though detail peaks for me ~25x/inch, 30x/inch is easy for me and details are easier to see. Though it's more complex in my scope with it's 30% obstruction 35x/inch has about the best contrast, but 43x/inch is better for quick looks, while 61x/inch helps dim the Moon w/o a filter for a great eyeball slap when seeing is at it's best. In other words, try a bunch! YMWV

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great_bear
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Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Jon Isaacs]
      #5856751 - 05/13/13 01:34 AM

> I am not sure what point you are trying to make...
- that particular point being that the focal plane is not something that light falls "onto" as had been described. It can only be projected "through"

> I am bewildered at how this thread has transformed itself [...] to this current discussion.
It had been incorrectly stated that pushing the magnification does not reduce contrast. The title of this thread is "Pushing the magnification".


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Starman1
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Re: Pushing the magnification new [Re: great_bear]
      #5856772 - 05/13/13 02:08 AM

Quote:

Hi Don,

You don't need to be following this discussion too closely to see that if the aperture is bigger, the exit pupil leaving the eyepiece is bigger. If the aperture is smaller and surrounded by horrible reflective surfaces, the exit pupil leaving the eyepiece is smaller and surrounded by horrible reflective surfaces too. That's why quality telescopes are made with carefully-placed baffle-stops or very good flocking - you want everything outside of the border of the primary to be as black as possible, because it all fits through your iris at smaller exit-pupils when the magnification is high.




The light reflected from bright, shiny, surfaces outside the field stop of the eyepiece are visible only insofar as they modify the image that is WITHIN the field stop.

The prevention here is trying to keep light that is outside the field from altering what is seen in the field of view.

If that light reflects to somewhere outside the field of view, it is not seen.
That is why some excellent eyepieces, barlows, adapters, and the like have beveled bottom surfaces to reflect extraneous light away from the field.

It matters not what light is outside the field of view, only what effect that light has on what is IN the field of view. It can alter the background brightness, or even modify the star images or reflect spikes into the field of view. What it cannot do is get around the entire field of view of the eyepiece without entering the field of view seen and somehow show up in the eye. If you see scattered light in your eye and it is not from something peripheral to the eyepiece, as a streetlight or the like, then something outside the field of view reflected or scattered light somehow into the field of view.

This has to do with the angle of incidence, which explains flocking and baffle stops. It has nothing to do with magnification. It could, however, be inversely related to field stop size.


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Shneor
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Re: Pushing the magnification new [Re: Starman1]
      #5856790 - 05/13/13 02:34 AM

The best view by far I have ever had of Jupiter was with the 18" f/4.5 I once owned. I could see whorls, bands with lots of detail and sub-bands, at over 1000X, using at least one barlow (it was in the mid to late 90s at Blue Canyon). The seeing for those moments was perfect. I believe a fellow observer who had a view decided then and there to get an 18" dob. With that same telescope I once had a great view of M57 at about 950X with trememdous detail, even though it was just a few degrees over the horizon. That was at a star party in southern Arizona, before the border lights went up.

So for that telescope, 50X per inch is 900X. I'm tempted to think that aside from seeing making a huge difference, collimation and mirror (or lens) quality; that 18" mirror was correct to 1/16 wave.

Clears,


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