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Pushing the magnification

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#26 Sarkikos

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Posted 10 May 2013 - 12:40 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

#27 BillP

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Posted 10 May 2013 - 04:13 PM

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.

#28 Eddgie

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Posted 10 May 2013 - 04:25 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.

#29 Eddgie

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Posted 10 May 2013 - 05:01 PM

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.

#30 Starman1

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Posted 10 May 2013 - 06:02 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.

#31 Sarkikos

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Posted 10 May 2013 - 07:50 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 :thinking:

Mike

#32 Sarkikos

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Posted 10 May 2013 - 07:56 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 :thinking:

:grin:
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#33 Sarkikos

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Posted 10 May 2013 - 08:02 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

#34 Bill Boublitz

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Posted 10 May 2013 - 09:03 PM

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

Keep Looking! ~ Bill

#35 great_bear

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Posted 10 May 2013 - 10:08 PM

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:

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#36 Starman1

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Posted 11 May 2013 - 12:31 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.

#37 great_bear

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Posted 11 May 2013 - 03:08 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.

#38 Sarkikos

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Posted 11 May 2013 - 07:04 AM

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

#39 Sarkikos

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Posted 11 May 2013 - 07:27 AM

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

#40 Eddgie

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Posted 11 May 2013 - 07:52 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.

#41 azure1961p

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Posted 11 May 2013 - 08:17 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

#42 dan_h

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Posted 11 May 2013 - 08:33 AM

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.

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#43 great_bear

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Posted 11 May 2013 - 08:53 AM

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."


#44 azure1961p

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Posted 11 May 2013 - 09:03 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

#45 Starman1

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Posted 11 May 2013 - 10:33 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.


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.

#46 Sarkikos

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Posted 11 May 2013 - 10:42 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

#47 Sarkikos

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Posted 11 May 2013 - 10:57 AM

Pete,

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.

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

#48 Geo31

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Posted 11 May 2013 - 11:32 AM

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. ;)

#49 David Knisely

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Posted 11 May 2013 - 12:50 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. ;)


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.

#50 Starman1

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Posted 11 May 2013 - 12:58 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. ;)

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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