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Equipment Discussions >> Eyepieces

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azure1961p
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Re: Pushing the magnification new [Re: Starman81]
      #5853418 - 05/11/13 09:17 AM

What I've found with the 1x per millimeter rule of thumb is (for me -and that's key) is that it can allow detection at this magnification but higher sais it better. I've done the reductionary thing with doubles and such where at 364x Id drop down to see how low I can go and still see the companion and such and 200x-240x is about where it becomes threshold, again, for me. The trouble Ive felt with adhering to this 1x rule is that while its true its too severe and fringey. Diffraction patterns are so much easier to study at 40x to 60x per inch and and that threshold fringe becomes much plainer. Now, no NEW detail becomes evident but what's there is just far easier to perceive. I'd reserve Dawes-notch hunting for 2x per inch though and couldn't call my observations at 1x conclusive. Some folks go as high as 4x per millimeter just to get a fat enough pattern scale to examine.

Jupiter is often mentioned as being a 25x per inch target and this is pretty well online with 1x per millimeter but Ive found exceptions to this...

Jupiters OVERALL contrast is nicely shown at 1x per millimeter but per feature detail some things stand far higher magnification even if the rest of the view goes to hell. Ill give last years apparition as an example...

Following the GRS a lot of folks may recall the chambered look of the SEB where in there appeared this chambered look of light and dark. It was a section where dark festoon like branches in sweeping arcs *framed* this succession of lighter areas like a ladder layed on its side. Well at 200x -240x these dark festoon like features were neat contrasty and well shown in the better 7/10 moments. It was even textured looking. They were dark and emanating south at uniform angles.

When increased magnification to 312x then 364x my drawing had to be redrawn here...

The festoon like dark branches emanating from the northern edge of the SEB were WASP WAISTED! They emerged tapered in slightly than broadened out again a d diffused the farther they went south in there arcing angle. The rest of Jupiter was overly large (though not bad actually) but this wasp assisting of these things was a new feature. The problem at 200x was the waist reduction was so slight (maybe .20" of an arc sec.) that my eye couldn't pick it out. It may have actually been there to see at 200x but the festoon like arcs were so very small seeing them as merely contrasty successive lines was the end of it for me. I needed more image scale.

Another time - and Ill be brief, that same apparition had thstmicronspot between the GRS and the small GRS . It was rather spot like and intermittently visible at 240x. A point if you will. But going past 300x again yield not just a mere point but true surface area and featuring color/toning that wasn't available at lower mags.

The whole of Jupiter suffered but at times specific details within Jupiter particularly at the arc second or sub arc second level be edit for me a nice 1.5x per millimeter.

Pete

Edited by azure1961p (05/11/13 09:37 AM)


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dan_h
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Re: Pushing the magnification new [Re: great_bear]
      #5853436 - 05/11/13 09:33 AM

Quote:


The light from the primary isn't "falling on the focal plane", since the focal plane is a virtual, not real, image.





The image at the focal plane is a real image, it is not a virtual image. This image can be displayed on a screen, captured by a camera and even viewed without an eyepiece. It is very much a real image.

dan


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great_bear
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Re: Pushing the magnification new [Re: dan_h]
      #5853470 - 05/11/13 09:53 AM

Quote:

The image at the focal plane is a real image, it is not a virtual image.




Apologies, I did completely misuse the term "virtual".

What I meant to say is that the light doesn't fall "on" the plane, as it does on (say) a cinema screen, but falls through it - there's a big difference.

I've updated this now to read: "The light from the primary isn't "falling on the focal plane", it's falling through it."


Edited by great_bear (05/12/13 06:49 AM)


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azure1961p
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Re: Pushing the magnification new [Re: dan_h]
      #5853489 - 05/11/13 10:03 AM

Quote:


The light from the primary isn't "falling on the focal plane", since the focal plane is a virtual, not real, image.


Well its concept in model but real in practice.

Pete


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Starman1
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Re: Pushing the magnification new [Re: great_bear]
      #5853616 - 05/11/13 11:33 AM

Quote:

Don,

Please go back and look at the picture again. I'm afraid you've completely misunderstood what the illustration is showing (and the accompanying text) if you believe that I am suggesting anything remotely as ridiculous as glare "smearing" a star.

I'm showing a factual drawing of what you actually see if you look into the OTA in the absence of an eyepiece - when that OTA is pointed at a bright object, and (independently) light is being cast onto the inner surface of the Mak baffle-tube from the moon, or a poorly-positioned street lamp or similar light source (light from someone's bedroom window etc.) from an oblique angle outside of the field-of-view.

I'm not displaying a stellar image - I'm showing the insides of the telescope. The practical upshot of this is a general increase in the field glow - exactly as you describe.

Hope that's clearer - since what I'm showing is not open to debate - it's a factual representation of what's going on, and you'd get the same thing if the OTA was a refractor with no baffle stops - which is precisely why those baffle-stops are put there.

In the same way as an overly-large exit pupil (when masked by the observer's iris) becomes increasingly dominated by the shadow of the secondary (in a newt/sct), conversely, the area surrounding smaller exit pupils become increasingly dominated by the inside surfaces of the OTA - be they shiny (in the case of a poor telescope) or inky-black (in the case of a flocked/well-baffled one). That's what leads to the loss of contrast at higher magnifications where the exit pupil is appreciably smaller than the dilated iris of the observer.




OK, got it.

However, the image in the exit pupil in both cases you show is equally damaged by the intrusion of light into the exit pupil. Light outside the exit pupil but which enters the pupil of the eye is essentially peripheral light, and can be blocked. You won't see it superimposed on the exit pupil, as annoying as it may be. It won't come through the eyepiece because it is outside the field.

Now one exception to what I just said is when a bright star is outside the viewable field of view but still inside the field of view of the telescope. Before the brightness of the focal plane of the scope drops to zero, there is some field that is outside the eyepiece's field of view. Contemplate a 2" focal plane and an eyepiece only looking at the center 1" of that focal plane. A really bright star can be on the telescope's focal plane but outside the field stop of the eyepiece.
If the telescope has great contrast, the spikes from the bright star will still be visible in the field of the eyepiece looking at the center 1" of focal plane. I see this in my 12.5" all the time when the bright star is in the field of the scope. If I move the scope until the bright star exits the field of view of the telescope, the spike visible in the field of view I'm looking at disappears like a light switch turning off.


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Sarkikos
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Re: Pushing the magnification new [Re: azure1961p]
      #5853630 - 05/11/13 11:42 AM

To all,

IME it's often more instructive to relate real world observational techniques, adaptation strategies and equipment enhancements that improve the image. Questions about exactly how and why these procedures are effective can be worked out later. Much of this is trial and error in the field, though theory, prior experience and common sense can give us some hints on what might work before we proceed.

Mike


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Sarkikos
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Re: Pushing the magnification new [Re: azure1961p]
      #5853655 - 05/11/13 11:57 AM

Pete,

Quote:

Jupiter is often mentioned as being a 25x per inch target and this is pretty well online with 1x per millimeter but Ive found exceptions to this...




Yes, I find this to be true, also. Jupiter is low-contrast for the most part, but there are surface features which have high-contrast qualities. These include festoons, belt edges and the various point-like objects. The high-contrast features benefit from higher magnification, as long as the seeing allows.

Quote:

Following the GRS a lot of folks may recall the chambered look of the SEB where in there appeared this chambered look of light and dark. It was a section where dark festoon like branches in sweeping arcs *framed* this succession of lighter areas like a ladder layed on its side. Well at 200x -240x these dark festoon like features were neat contrasty and well shown in the better 7/10 moments. It was even textured looking. They were dark and emanating south at uniform angles.

When increased magnification to 312x then 364x my drawing had to be redrawn here...

The festoon like dark branches emanating from the northern edge of the SEB were WASP WAISTED! They emerged tapered in slightly than broadened out again a d diffused the farther they went south in there arcing angle. The rest of Jupiter was overly large (though not bad actually) but this wasp assisting of these things was a new feature. The problem at 200x was the waist reduction was so slight (maybe .20" of an arc sec.) that my eye couldn't pick it out. It may have actually been there to see at 200x but the festoon like arcs were so very small seeing them as merely contrasty successive lines was the end of it for me. I needed more image scale.




I believe that shows in my avatar.

Mike


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Geo31
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Re: Pushing the magnification new [Re: Sarkikos]
      #5853735 - 05/11/13 12:32 PM

Interesting conversation. When I got into this 40 years ago the rule was 2x mm or 50x inches.

I guess telescopes and eyepieces have gotten worse in the last 40 years.


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David Knisely
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Re: Pushing the magnification new [Re: Geo31]
      #5853845 - 05/11/13 01:50 PM

Quote:

Interesting conversation. When I got into this 40 years ago the rule was 2x mm or 50x inches.

I guess telescopes and eyepieces have gotten worse in the last 40 years.




The 50x per inch "guideline" was created mostly for those who have never had a telescope and are exposed to the ridiculous power claims made by department store small telescope makers. It hopefully prevented new people from falling into the trap of getting one of those scopes and then being disappointed when they couldn't use those powers. The magnification used for a given object will depend highly on what object is being looked at, so I suppose there isn't really a maximum number for all scopes. For some high surface brightness planetary nebulae, I often will go as high as 72x per inch of aperture to help compensate for the low resolution of the eye at low light levels when using averted vision. I have gone even higher on some very tight double stars, but much of the time, my observing is done at powers under 40x per inch. Clear skies to you.


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Starman1
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Re: Pushing the magnification new [Re: Geo31]
      #5853863 - 05/11/13 01:58 PM

Quote:

Interesting conversation. When I got into this 40 years ago the rule was 2x mm or 50x inches.

I guess telescopes and eyepieces have gotten worse in the last 40 years.



When I was a kid, all the books I read said "max 60X/inch".
But, bear in mind that, then, an 8" newtonian would have been considered a really large scope. Most amateurs had 2" to 6" scopes, where 60x/inch would have meant 120x-360X. The lucky amateur with the big 8" could burn all the way to 480X on double stars. No book or article I read ever mentioned that high a magnification was essential for anything except splitting double stars.
[Double star splitting was a popular and important part of observing. Seeing faint galaxies was not, for aperture reasons.]

If we fast forward to today, where scopes of 10" through 32" are common at star parties (and occasionally larger), 60X/inch isn't going to be supported by the atmosphere because of seeing conditions. In another thread recently on CN, it was discussed that scopes of larger apertures are all going to be seeing-limited, not aperture-limited. I observe under pretty good skies, yet it is rare for the seeing to support more than 300-400X, no matter how big the scope is.

And, I would estimate there aren't as many amateurs trying to split super-close double stars any more (I think the Sparrow Limit was the reason for 60X/inch), so high-end limits are often described as 50X/inch now. In reality, if the scope is 12" or larger, the high power of 25X/in (1X/mm) will suffice because the magnification will be high enough to see just about anything Plus, really large scopes are seldom used for planetary viewing--maybe a few minutes over a night--and super-high powers are not that likely to be used for other kinds of objects. Well, except planetary nebulae, where high magnifications are great, but, again, the big scopes are still seeing-limited.

So I don't think scopes or eyepieces have gotten worse. What people observe has changed, and what constitutes a really high power for observing star clusters, nebulae, or galaxies is different than double stars and lunar viewing.


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Jon Isaacs
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Re: Pushing the magnification new [Re: Bill Boublitz]
      #5853966 - 05/11/13 02:57 PM

Quote:

When it comes to optical law, it is generally recognized that 1 arc minute is the smallestdetail thetypicalhuman eyecan resolve. If you're using a scope with a theoretical resolution limit of 1.0 arc seconds (like a 4" refractor), x60 is all that is required to magnify the angular size of 1.0 arc second so it appears as 1 arc minute in the eyepiece. In other words, at x60 a 4" refractor is already "showing" everything it is capable of resolving.




Consider your own experience. The double-double pairs are both separated by 2.3 arc-seconds. If your eye could resolve 1.0 arc-minute, you would be able to resolve the double-double at 26x. Can you resolve the double-double at 26 x, I cannot. (The Rayleigh criteria for a 4 inch is actual about 1.35 arc-seconds)

You actual visual acuity depends on a number of factors, as Mike pointed out, the dark adapted eye has far less resolving ability than the daylight adapted eye. This is because of the distribution of the cones and the way they are wired.

If you want to demonstrate this, I suggest viewing the quarter moon through a solar filter. This transforms the moon from a bright object that has a surface brightness of about 3 magnitudes per square arc second to a bright for a DSO surface brightness of about 15.5 MPSAS. The same level of detail exists in both images, the difference in what you see is the result of
the way you eye responds. If you haven't done this, I recommend it, it can be a definite mind opener. At the least, it should make one wonder what details could be seen in a DSO were it 13 magnitudes brighter.

The response of the human eye is complex, in the simplest terms the resolving power depends on both contrast and brightness. For a bright object, resolving 2 arc minutes, the double double at 52 x, is conceivable, for dim objects resolution might measured in degrees.

For any device with a complex non-linear response, trial and error, experimentation is required. Each of US had our own pair of sensors and we try to optimize the view for a given object by optimizing the magnifying glass we use.

Different objects, different observers/eyes, different magnifications.

As Eddgie says, the eyepiece, it's just a magnifying glass.

Jon


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Geo31
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5854136 - 05/11/13 05:08 PM

Don and David, hopefully you caught the " " in my response.

So many things have changed SO much in the last 40 years it's not funny. Back then a 20" scope was almost unheard of in the hands of an amateur and was still considered a research sized instrument. I used to go out my back door in the city (Rochester, NY) and be able to easily see 5th MAG and a little below, occasionally seeing the Milky Way. Light pollution has become worse than I ever could have imagined. Anyway...

Totally with you comments about seeing being the limiting factor. Magnifying an atmosphere fuzzed image just makes a larger fuzzy image.

I do get a kick out of some of the conversations. I feel like I've been stuck in a cave somewhere for 35 years. Some things have changed and some have not. Instrument size has changed a LOT. Eyepieces have changed a lot. And sometimes not so much. When I left for the sidelines 35 years ago, Orthos were king and Possls were just becoming popular. It's funny that many of those Orthos from back then are now very desirable.

Anyway, I was just making a tongue-in-cheek comment. Some people were saying that you could only use half of what was discussed back 40 years ago. As you point out correctly, instrument size changes that equation.


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leonard
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5854188 - 05/11/13 05:43 PM


>>>> As Eddgie says, the eyepiece, it's just a magnifying glass. <<<<



Amen


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leonard
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5854211 - 05/11/13 06:07 PM


Hello ,

Roland of Astro-physics in his early ads would advocate using up to 100X per inch for the Moon/planets and double stars ," If the seeing was excellent ". He called his lens " high resolution lens " which over time has proven true . I have never used anything like 100 X per inch when I had my 4 inch A-P scope as the seeing would come into play . On one wonderfull night , I used my 7 inch on Saturn in Maryland skys and to my surprise was able to get a rock steady image at 355X , the detail was wonderfull and crisp. This is well above the magnification of 1.4X per mm . I'm not suggesting the the loss of fine low contrast detail above 1.4X per mm is wrong , just that what I saw may have loss some very low contrast detail but that image was just outstanding . The seeing has Pickering 9/10 , something not seen every day . Seeing counts a whole lot .

Maryland has some very nice seeing at times .
Leonard


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azure1961p
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Re: Pushing the magnification new [Re: Sarkikos]
      #5854265 - 05/11/13 06:56 PM

Quote:

To all,

IME it's often more instructive to relate real world observational techniques, adaptation strategies and equipment enhancements that improve the image. Questions about exactly how and why these procedures are effective can be worked out later. Much of this is trial and error in the field, though theory, prior experience and common sense can give us some hints on what might work before we proceed.

Mike




Hi Mike,

That night (or week?) the chambered look was extending away from the GRS further and the waist was more subtle but yes that's exactly what I was referring too! I've always liked that avatar. I'm guessing the waist deviated from a straight line by about .20 of a sec it was this slender difference . Had I not had 7/10 Ida missed it.

Should be gettin my mirrors back this week from Paul Z in Florida this week- can't wait for Saturn!!!! Though I'm doubting ill see a chambered belt

Pete

Edited by azure1961p (05/11/13 06:58 PM)


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Jon Isaacs
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Re: Pushing the magnification new [Re: Geo31]
      #5854284 - 05/11/13 07:15 PM

Quote:

Interesting conversation. When I got into this 40 years ago the rule was 2x mm or 50x inches.

I guess telescopes and eyepieces have gotten worse in the last 40 years.




I know you are having a bit of fun with this but I think Sidgwick's Handbook was written more that 50 years ago and if I recall he discussed 25X/in as analytical number but 50X/in as a practical number and that for doubles even higher.

I agree with Mike. Understanding the issues, the way the eye works, the way a scope works, how it all fits together can help point us in the right direction but it is out under the night sky where we can determine for ourselves just what works best just by careful observation.

Jon


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BillP
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Re: Pushing the magnification new [Re: Sarkikos]
      #5854469 - 05/11/13 10:04 PM

Quote:

To all,

IME it's often more instructive to relate real world observational techniques, adaptation strategies and equipment enhancements that improve the image. Questions about exactly how and why these procedures are effective can be worked out later. Much of this is trial and error in the field, though theory, prior experience and common sense can give us some hints on what might work before we proceed.

Mike




Truer words were never said

Regardless of the theories at work, and how accurately those theories may represent the facts, skill is the critical element in the chain. As with any activity that involves human skill, results vary by both the skill and technique used by the observer. This being the case there is much room for variation, not achieving the general rules of thumb or theory, and exceeding those rules.


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great_bear
scholastic sledgehammer


Reged: 07/05/09

Loc: Walthamstow, London, UK
Re: Pushing the magnification new [Re: Starman1]
      #5854878 - 05/12/13 05:32 AM Attachment (6 downloads)

Quote:

However, the image in the exit pupil in both cases you show is equally damaged by the intrusion of light into the exit pupil. Light outside the exit pupil but which enters the pupil of the eye is essentially peripheral light, and can be blocked. You won't see it superimposed on the exit pupil, as annoying as it may be. It won't come through the eyepiece because it is outside the field.




In my previous post I annotated the diagram with the statement:

"I have also shown the type of baffle-tube glare of a bright object just outside the field"

- which you might have you misinterpreted (quite understandably) as meaning the pool of light is outside the field.

I've now changed this to read:

"I have also shown that the moon - outside of the field-of-view - has unfortunately cast a bright pool of light along the inside of the telescope's baffle-tube (it's a Mak)"

- Hope that clarifies things (I've improved the annotation in the image itself too).

Anyway, as it is, the field-stop doesn't affect the contents of any of the bundles of light leaving the eyepiece (each bundle representing just a single point in the final image), the field-stop just restricts the range of angles that those bundles can come from.

The content of any particular light bundle from a specific direction is destined to become just a single point of the overall stellar image formed on your retina. In its spread-out form on leaving the eyepiece however, it takes the shape of an image of the entire inside of the telescope itself, dominated by the objective at the center.

Anyone - regardless of experience - whose knowledge of optics doesn't stretch this far, can easily satisfy themselves that this is the case by projecting this image onto card placed at the precise distance of the eyepiece's eye-relief specification whilst shining a torch (from some distance) into the front of the telescope. Easier still, just look down (from a few inches) at the top of an eyepiece inserted in your uncapped scope in daytime. There - for the world to see - is a little image of the inside of your telescope, hovering - at eye-relief distance - from the top of the eyepiece.

Now, when you approach the telescope and look into it for observing purposes, the lens of your eye, mates with this image so that the portion representing the primary can be refracted back into the stellar image being formed on your retina. If the size of the primary in this image is large enough that it completely spans your pupil - i.e. the magnification is low - then poor baffling in the telescope has no effect and you get a nice, high-contrast stellar image on your retina. If however, you are at a high magnification and the scope's exit-pupil is smaller, then not only the light from the primary - but also the reflective surfaces around it too - enters your eye - creating an additional, general background illumination on every point in the image, reducing overall contrast.

That is why - in less-than-excellent telescopes - contrast reduces as magnification increases.

I've attached another diagram. The first picture shows the inside of a poorly-baffled telescope (well actually it's a toilet-tissue roll, but it will do for now...). In the second picture (low magnification), the poorly-baffled part of the telescope's insides are outside the area of the observers pupil, and thus do not reduce contrast. In the final picture (high magnification), not just the light from the primary, but the light from the unbaffled tube lining also enters the eye, reducing overall contrast.

This is not open to debate - it is a demonstrable fact.

Anyway - I don't intend to drag this out further - just chat to Al Nagler, or Roland Christen, next time you see them, and ask them why the contrast drops at higher magnifications on poorly-designed telescopes. They'll tell you the same thing - although perhaps in a different way.

Clear skies to you.

Edited by great_bear (05/12/13 07:02 AM)


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Bill Boublitz
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Reged: 05/04/13

Re: Pushing the magnification new [Re: great_bear]
      #5855000 - 05/12/13 08:31 AM

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

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Eddgie
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Re: Pushing the magnification new [Re: great_bear]
      #5855129 - 05/12/13 10:03 AM

Quote:

the field-stop just restricts the range of angles that those bundles can come from.





This isn't really the way it works.

The field stop of an in focus eyepiece sits at the focal plane when the telescope is at best focus.

How could it restrict anything comeing from any angle when it sits at the point of best focus.

The field stop simply frames a portion of the focal plane.

The focal ratio of the eyepeices is always the same as the focal ratio of the telescope (assuming no barlow or compressor, in which case the eyepiece now works at that focal ratio).

The focal plane represents the point on the caustic cone of converging light where the light has come to best focus.

Any damage done by any light falling on the focal plane (and when you use an eyepeice with a field stop, which is coincident with the focal plane when the eyepeice is in focus) that lowers contrast for a detail that is still within the circle of the field stop will loose the same amount of contrast regardless of what the focal lenght or field stop of the eyepeice is.

That is what the field stop of an eyepeice does. It stops the field. It is at the focal plane and it only selects the area of the field to be observed and changes the angular size of the objects or details inside the field.

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.


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