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Equipment Discussions >> Eyepieces

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great_bear
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Re: Pushing the magnification new [Re: Eddgie]
      #5851829 - 05/10/13 11:24 AM

Thanks for this - some interesting points there.

One thing though:

Quote:

The image never breaks down wtih magnificiation.. The image always is exactly what it is. The only thing that changes it the power of the magnifiying glass you are using to inspect the image.




I'm not sure in practice whether this is entirely true - although I see your logic.

But in practice, what if - beyond the focal plane - there's a whole bunch of glare flooding is - as would be normal on a Mak along the inside of the baffle-tube when the moon is in the vicinity? Surely at higher magnification such glare and other unwanted light ingress becomes more objectionable (not due to magnification, but due to it's unchanged brightness relative to the darker image), and is thus perceived as causing a greater loss of contrast than it does at lower magnification? (otherwise there'd be no contrast benefit in long-focus Newtonians over their shorter cousins).

Just to be clear - I understand that what lies at the image plane remains unchanged - but the eyepieces relationship with it - and critically its position in relation to both image and to the inner-telescope environment is quite different at one extreme compared to the other.


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Starman1
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Re: Pushing the magnification new [Re: Starman81]
      #5851883 - 05/10/13 11:58 AM

What are the issues with the eye that might affect the magnifications we can use?

Well, at the low end it's pupil diameter and astigmatism of the eye. Nearly everyone has some "whole eye" astigmatism, so an exit pupil slightly smaller than the pupil of the eye, while not the optimum for brightness, might be the optimum for a combination of brightness and sharpness.

As the exit pupil shrinks and magnification goes up, there is a "eutectic" point of the balance between magnification and the eye's resolution that gets crossed. This seems to be, from all I've read, when the magnification yields an exit pupil of 1.8 to 2.5mm (and even larger, up to 4mm, with some individuals). My favorite eyepiece in each of the 23 scopes I've owned has always settled into the 1.8-2.5mm range.

When the exit pupil gets smaller, smaller details become apparent due to increasing size (lab tests show an effective size of 2 degrees is the point of maximum noticeability) but only up to the point where the Airy disc begins behaving like an extended object (i.e. has a visible size). After that, increased magnification will not yield improved resolution, because the brightness of point sources will star declining, but increased size may still yet improve noticeability.
This point does not have a set physiological limit, but most references say this occurs around a 1mm exit pupil with average vision (whatever average vision is--I see a spread in visual acuity that is a HUGE span, not a small difference), or slightly higher (maybe as small as a 0.8mm exit pupil.

The problem for humans is that this begins to be the point where the image begins to be bothered by floaters in the eye. Floaters are small protein agglomerations in the vitreous humor of the eye. They always seem to fall in front of the small detail on a planet or the moon when you are examining that detail at small exit pupils.

Due to huge variations in seeing conditions, the maximum limit is relatively undefined. I read 50X/inch often. In the old days it was 60X/inch. I've used 160X/inch (!) in a marvelous large refractor on an incredible night of seeing. Experimentation, though, showed the image of Saturn to be equally as detailed at 100X/inch, though all present preferred the image at that power to one at 65X/inch. It was easy to see details in the rings (did you know they were "grooved" like an old LP?) at those ridiculous magnifications that simply weren't seen at lower powers. It wasn't a matter of resolution, it was a matter of size of the details we sought to view.
[it was the first and only time I saw the ring features known as "spokes"]

The Sparrow Limit is about half the size of the Rayleigh limit in double star viewing. It is when a double star is overlapped and the image of the double is oval and may begin to have a slight pinched waist in the oval star image (normally, just oval). To see to that level requires incredibly good seeing and optics, but hardcore double star observers do do it. It's about 2.8"/aperture in inches (0.35" on an 8" scope). That is probably the physical (and physiological) limit for double star separation. For a person with decent visual acuity, that might be seen at 60X/inch--otherwise higher. That's probably where the 60x/inch I read in my youth came from.

Does it have any relevance for extended objects? Probably not if the detail being perceived is a dark feature next to a brighter detail. And it's probably too severe for equally-bright details side by side. And yet, I have heard and read of lunar observers seeing details at 100x/inch and higher that they could not see at lower powers. The Moon's features yield very high contrasts--more so than on planets--so this should not surprise us.

I really don't think there is a maximum magnification that is not limited by:
1.seeing
2.optical quality
3.personal visual issues
4.personal taste

These days, I seldom go above 25X/inch, but it's not because of any factor other than #4. On one superb night, I used 58X/inch just to see if the image held up, and it was OK, though I far preferred the quality and brightness of the image at 36X/inch.

De gustibus, non est disputandum.


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Sarkikos
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Re: Pushing the magnification new [Re: Eddgie]
      #5851898 - 05/10/13 12:12 PM

Eddgie,

Quote:

And with binoviwers, I find that I don't even need 1.3x. I am doing a lot more observations with less power and seeing detail more easily.




+1

Mike


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Sarkikos
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Re: Pushing the magnification new [Re: Starman1]
      #5851911 - 05/10/13 12:18 PM

Don,

Quote:

What are the issues with the eye that might affect the magnifications we can use?

Well, at the low end it's pupil diameter and astigmatism of the eye. Nearly everyone has some "whole eye" astigmatism, so an exit pupil slightly smaller than the pupil of the eye, while not the optimum for brightness, might be the optimum for a combination of brightness and sharpness.

As the exit pupil shrinks and magnification goes up, there is a "eutectic" point of the balance between magnification and the eye's resolution that gets crossed. This seems to be, from all I've read, when the magnification yields an exit pupil of 1.8 to 2.5mm (and even larger, up to 4mm, with some individuals). My favorite eyepiece in each of the 23 scopes I've owned has always settled into the 1.8-2.5mm range.

When the exit pupil gets smaller, smaller details become apparent due to increasing size (lab tests show an effective size of 2 degrees is the point of maximum noticeability) but only up to the point where the Airy disc begins behaving like an extended object (i.e. has a visible size). After that, increased magnification will not yield improved resolution, because the brightness of point sources will star declining, but increased size may still yet improve noticeability.
This point does not have a set physiological limit, but most references say this occurs around a 1mm exit pupil with average vision (whatever average vision is--I see a spread in visual acuity that is a HUGE span, not a small difference), or slightly higher (maybe as small as a 0.8mm exit pupil.

The problem for humans is that this begins to be the point where the image begins to be bothered by floaters in the eye. Floaters are small protein agglomerations in the vitreous humor of the eye. They always seem to fall in front of the small detail on a planet or the moon when you are examining that detail at small exit pupils.

Due to huge variations in seeing conditions, the maximum limit is relatively undefined. I read 50X/inch often. In the old days it was 60X/inch. I've used 160X/inch (!) in a marvelous large refractor on an incredible night of seeing. Experimentation, though, showed the image of Saturn to be equally as detailed at 100X/inch, though all present preferred the image at that power to one at 65X/inch. It was easy to see details in the rings (did you know they were "grooved" like an old LP?) at those ridiculous magnifications that simply weren't seen at lower powers. It wasn't a matter of resolution, it was a matter of size of the details we sought to view.
[it was the first and only time I saw the ring features known as "spokes"]

The Sparrow Limit is about half the size of the Rayleigh limit in double star viewing. It is when a double star is overlapped and the image of the double is oval and may begin to have a slight pinched waist in the oval star image (normally, just oval). To see to that level requires incredibly good seeing and optics, but hardcore double star observers do do it. It's about 2.8"/aperture in inches (0.35" on an 8" scope). That is probably the physical (and physiological) limit for double star separation. For a person with decent visual acuity, that might be seen at 60X/inch--otherwise higher. That's probably where the 60x/inch I read in my youth came from.

Does it have any relevance for extended objects? Probably not if the detail being perceived is a dark feature next to a brighter detail. And it's probably too severe for equally-bright details side by side. And yet, I have heard and read of lunar observers seeing details at 100x/inch and higher that they could not see at lower powers. The Moon's features yield very high contrasts--more so than on planets--so this should not surprise us.

I really don't think there is a maximum magnification that is not limited by:
1.seeing
2.optical quality
3.personal visual issues
4.personal taste

These days, I seldom go above 25X/inch, but it's not because of any factor other than #4. On one superb night, I used 58X/inch just to see if the image held up, and it was OK, though I far preferred the quality and brightness of the image at 36X/inch.

De gustibus, non est disputandum.




Plenty of good and useful information here. Thanks.

Mike


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Bill Boublitz
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Reged: 05/04/13

Re: Pushing the magnification new [Re: Starman81]
      #5851988 - 05/10/13 01:10 PM

There are many excellent points made in these posts. Enough to demonstrate that the whole idea of optimum magnification is continuously variable and cannot be separated from a.) the instrument in use, b.) the atmosphere at time of observation and... c.) the eyes doing the looking. I've always considered the magnification "rules" to be more guidelines than physics.

When it comes to optical law, it is generally recognized that 1 arc minute is the smallest detail the typical human eye can 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. Who spends an evening studying Jupiter or Saturn at x60? I typically use something like x130 - x160.

Al Nagler (Hey, what does he know?) wrote an article for Sky & Tel titled "Choosing Your Telescope's Magnification" which is still available on the Tele Vue website. It covers every aspect of the problem, both from an optical designer's point of view and that of the observer. Also specific advice regarding different types of objects. Double stars, small planetary nebula typically benefit from higher magnification to reveal their detail than do planets, nebula or galaxies. Highly recommended.

The short answer? The right magnification is the one that reveals the most detail. Period.


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Sarkikos
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Re: Pushing the magnification new [Re: Bill Boublitz]
      #5852058 - 05/10/13 01:40 PM

Another fly in the ointment is that at levels of visual adaptation below photopic, visual acuity is diminished for detail, contrast and color range. An eye observing Saturn at 60x in a 4" refractor at night is not the same eye observing a bright landscape at high noon. In order to discern fine surface detail when viewing a "bright" planet during the evening, it will probably be necessary to increase magnification just to compensate for the diminished visual acuity. (This is much more the case while observing planetary nebulae and galaxies when the eyes are deeply dark adapted.)

Though many amateurs complain that Jupiter or Saturn is too "bright" when seen in their telescopes, these objects only appear glaring because the observer's eyes are partially dark adapted. Observe the same planets at early twilight and they will have become magically much dimmer. Also, contrast, perception of surface detail and color range will improve. More magic! Well, not really. The eyes have just photopically adapted, improving their visual acuity. Some observers claim that this is a "contrast effect" or is caused by better seeing at twilight, but it makes more sense to me that it is largely due to a difference in adaptation of the eye.

Alternatives to bumping up the power when viewing planets at night are to (1) binoview and/or (2) look periodically at the reflection on a white piece of paper from a bright white-light flashlight. The binoviewer will allow use of both eyes, which enhances perception of surface detail. Exposure to bright white-light will temporarily bring the eyes closer to photopic, enhancing their visual acuity.

These solutions will not work so well for DSO. Binoviewing will dim the image enough to hamper detection and observation of the faintest deep sky objects. White light - bright or otherwise - is never a good idea when observing DSO. So we're left with bumping up the magnification to see finer structure in the object.

Mike


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BillP
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Re: Pushing the magnification new [Re: great_bear]
      #5852345 - 05/10/13 05:13 PM

Quote:

The above then, places my experience of maximum useful magnification as lying somewhere between:

33x - 40x p.i. (equivalent to exit pupil range 0.78mm - 0.63mm)




FWIW, this is EXACTLY my experience as well. Specifically for planetary, .60-.75mm exit pupils is where the optimum maximum is.

I find the maximums vary by target, so things like the Moon can take much more magnification successfully (and smaller exit pupils). Does more detail result? Yes...if you know where to look. So the more detail that pops out when one goes to hyper-magnifications where the exit pupil is less than .5mm on the Moon, is on very specific lunar structures. So it really does boil down to knowing where to look for the extra details when you leave the conventional wisdom of things. And this is the fun part of observing...there really are no rules. If you experiment, do it dilligently, then you will realize that the only rule is that there are none.

Edited by BillP (05/10/13 05:18 PM)


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Eddgie
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Re: Pushing the magnification new [Re: Starman81]
      #5852366 - 05/10/13 05:25 PM

The question I think is if this magnificationh showed you any new detail that was not visible at 100x?

You can use as much magnification as you like.

The scope itself though is exausted at about 1.1x per millimeter of aperture.

Increaseing the amount of magnificatino further does not increase the amount of detail in the view. It only changes the angular size. Now that might be good, and for a high contrast detail (black shadow in a crater, or shadow of a Jovian moon transit, or Cassini division), there is no limit on how much magnification you can use.

But you don't need more than 1.1x to see these features if your scope can resolve them at all, and chances are, the more you magnifiy the more low contrast detail is lost due to the way your eye responds to illumination falloff (the effect of a smaller exit pupil)

But that should not stop people from using as much power as they like.

It does not matter what I or anyone else says because there are not rules that forbid it.

I have been planetary observing for 40 years though, and my own opinion is that there is not much point in going beyond 1.3x per millimeter for the lowest contrast detail on most planetary and lunar targets.

Beyond this range, you can make the image bigger, but the result is that the most difficult detail often starts to fade out.

So, what good is making Triesnecker Rille bigger if the little craters that are nestled in some of the mesas on either side start to fade from view?

And that is what happens to me when I over-magnify. I can make the big, high contrast detail bigger, but it is at the expense of the "Richness" offered by the huge amount of very fine, low contrast detail that accompanies the easy stuff..
But that is me. Everyone decided for themselves what is best.


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Eddgie
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Re: Pushing the magnification new [Re: great_bear]
      #5852417 - 05/10/13 06:01 PM

Quote:

the eyepieces relationship with it - and critically its position in relation to both image and to the inner-telescope environment is quite different at one extreme compared to the other.





Not really.

The eyepeice is a magnifying glass. That is all it does.

The feild stop of the eyepeice simply frames an area of the focal plane and magnifies it to a given angular size.

The eyepiece always acts at exactly the same focal ratio as the telescope it is used in.

Any defect in the image that is within the border of the field stop will have the same effect on the image regardles of the magnification used, as long as that defect is present with the circumference of the field stop of the eyepeice being used.

In other words, if there was some light falling on the focal plane when using an eyepecie with a 30mm field stop, then the detail that is in the position in the focal plane where the light can reach it will have the contrast lowered.

As long as the eyepiece being used is framing that same area of the focal plane, the amount of damage that we see for that specific detail would remain exactly the same.

The eyepeice is just a magnifying glass. It's only role is to allow the observer to inspect a part of the image on the focal plane at a given scale.

Any defect in the image caused by any source will be present regardless of the magnification you use to inspect the image.

Once you have magnified the image sufficently well to resolve the space between two stars in a Dawes split, the scope is pretty much showing you all of the detail that it can. A camera though will quickly prove that there is more detail present. That is because cameras have better contrast sensitivity than the human eye and modern software allows us to extract as much of that contrast as possible.

But for the observer, changing the magnification past this point only changes the way your eye perceives the image.


I would like to recommend a book to you. It is called "Telscope Optics."

In chapter 18, the authors go into a fair amount of detail (but presented in a nice, easy to digest way) to explain not only how contrast transfer works, but how the eye works with different size and contrast details.

This is the most important point to remember in these conversations.

There is no limit to the amount of power you can use for high contrast details.

It is the sublte, low contrast detail that presents the real problem for observers. This would be the faint ovals on Jupiter, or the shallow rilles around Triesndecer on the moon.

These features start with very low contrast and this is where the richness of the view comes in. You can make the main rilles as large as you want because they are visible in even moderatly sized scopes.

But the small craters aroud the major rilles, and the multitide of tiny feeder rilles are to me what make the area so intereseing.

Many of these features start with very low contrast. Even if you start with a C14, if you over-magnify them, they start to become harder to see, not easier to see.


And this should be the way you select the optimal power.

Start with about 1x per millimeter of apeture.

Find the hardest possible, detail on the target.

Magnify until that detail goes away. Your optimal magnification is somehere between 1x per millimeter and the point at where the lowest contrast, most difficult to see details star to become impossible to see.

My bet is that most observers will say that the hardest detail to see goes away at about 1.4x per millimeter.

You can make the detail that is still visible as big as you want, but the bigger you go, the more low contrast detail you will loose.

And what is the point of making the big, high contrast detail bigger if you loose the richness of fine, low contrast detail that it contains or that is around it.


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Starman1
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Re: Pushing the magnification new [Re: Eddgie]
      #5852549 - 05/10/13 07:02 PM

Because sometimes it takes magnification to see the detail. When the detail is too small for the angular resolution of your eye to see it, though the contrast is good, you don't see the detail. When you make it bigger, you do.
I've done some tests recently on large, bright, local galaxies.
I see more detail in the cores at powers above 25X/inch, though I see a greater extent of the outer arms at less than 25X/inch.
That says to me that separating the details from one another in the core requires magnification, while seeing the outer spiral arms requires a larger exit pupil to make them brighter.
True, I didn't go beyond your 1.3X/millimeter limit.

In the Saturn example I made earlier, I suspect the same thing held true--we could see the details once they were made large enough to see them, and that was at almost 4X/mm. At 2.4x/mm most of us couldn't make out the small details in the rings any more. The details overlapped and blurred because of their small size and the poor resolution of the observers' eyes.

Normally, I can see everything I'm capable of seeing at 25x/in (1x/mm), but there are exceptions.


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Sarkikos
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Re: Pushing the magnification new [Re: Starman1]
      #5852717 - 05/10/13 08:50 PM

The more dark adapted the eyes, the worse the visual acuity. Maybe this just might have something to do with how we perceive objects through eyepieces and telescopes. hmmmm

Mike


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Sarkikos
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Re: Pushing the magnification new [Re: Sarkikos]
      #5852722 - 05/10/13 08:56 PM

A good way to dial in the optimum perceived contrast, exit pupil and image scale to see the level of structure desired, is to use a good zoom eyepiece. I find this is even more effective for galaxies and other faint fuzzies than for planets and the Moon. hmmmmmm


Mike


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Sarkikos
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Re: Pushing the magnification new [Re: Sarkikos]
      #5852731 - 05/10/13 09:02 PM

We really shouldn't assume that every observer's eyes are equally well prepared to see what the optimum magnification and exit pupil should be able to show them in the object. This is more than simply a matter of experience. There is also knowledge of how best to prepare the eyes and the best observing techniques for each type of object.

It's not all about the optics.

Mike


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Bill Boublitz
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Re: Pushing the magnification new [Re: Sarkikos]
      #5852829 - 05/10/13 10:03 PM

Ever noticed: the more experienced the observer, the less magnification typically used? Says more than any rule or conceptual theory.

Keep Looking! ~ Bill


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great_bear
scholastic sledgehammer


Reged: 07/05/09

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Re: Pushing the magnification new [Re: Eddgie]
      #5852987 - 05/10/13 11:08 PM Attachment (16 downloads)

Eddgie,

Let me explain why magnification harms contrast.

The light from the primary isn't "falling on the focal plane", it's falling through it. This has a pretty big bearing on things.

In particular, the exit pupil from the eyepiece is an image of the primary, which gets focused back to each point in the image formed on your retina. However, it isn't *just* an image of the primary - it's an image of the primary plus whatever junk light is on the inner surfaces of the telescope.

Your iris screens off anything outside a certain distance from the primary. At large exit pupil sizes (low magnification) this means your iris becomes a very effective baffle-stop. At very small exit pupils however, *ALL* of the junk light on the surfaces surrounding the primary also gets focussed onto every point in the image, wrecking the contrast.

Perhaps a picture will demonstrate this more clearly. The brown circle represents the open area of your iris, the white circle represents the exit pupil - i.e. the image of the primary (shown with a central obstruction) - 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). This is a situation I frequently encounter:

Edited by great_bear (05/12/13 06:55 AM)


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Starman1
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Re: Pushing the magnification new [Re: great_bear]
      #5853155 - 05/11/13 01:31 AM

Glare doesn't smear the star--it adds extraneous light to the field of the telescope. Yes, it can be a spike into the field, but as often it is a general increase in the field glow.
In the case of exit pupil, glare would be dimmed by magnification just like the normal background field of the eyepiece experiencing no out-of-field light scatter.
So your analogy fails on several levels.


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great_bear
scholastic sledgehammer


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Re: Pushing the magnification [Re: Starman1]
      #5853228 - 05/11/13 04:08 AM

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.


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Sarkikos
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Re: Pushing the magnification new [Re: Bill Boublitz]
      #5853345 - 05/11/13 08:04 AM

Quote:

Ever noticed: the more experienced the observer, the less magnification typically used? Says more than any rule or conceptual theory.




There is some truth to that. But it isn't a hard and fast rule, either!

For instance, the optimum magnification can vary by the object. In my experience, a little higher magnification is more useful for Saturn and Mars than for Jupiter, probably because of the difference in image scale and the low-contrast features on Jupiter. Also, I have seen that the observer can often push the power with better results for a pointicular or linear object than for extended ones.

Mike


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Sarkikos
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Re: Pushing the magnification new [Re: great_bear]
      #5853361 - 05/11/13 08:27 AM

Quote:

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.




This problem is solved by flocking the interior OTA, attaching a flocked light shield to the end of the OTA and blocking ambient glare. Then whatever loss of contrast remains is caused by other factors, such as the central obstruction, turned down edge, spherical aberration, rough figure, astigmatism in the secondary, lack of thermal stabilization, bad collimation, etc.

Mike


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Eddgie
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Re: Pushing the magnification new [Re: great_bear]
      #5853387 - 05/11/13 08:52 AM

Light falling on to the focal plane damages the image everywhere it falls.

The effect is that it lowers the contrast of the detail that it falls on.

As long as that detail is in the field of view, you see the detail with reduced contrast.

It matters not what happens past the borders or inside the borders of the field stop.

If light is fallingn on the center of the field, and you are viewing Jupiter at the center of the field, it does not matter how much or how little magnification you use... The light that falls on Jupiter does the same amount of damage at the focal plane regardless of the magnificaiton.

The eyepeice simply presents that detail to you at different angular magnifications.

If the detail has less contrast because of a light leak, then that contrast is forever lost and no eyepeice will restore it regardless of the magnification.


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