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#1 great_bear

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Posted 10 May 2013 - 12:24 AM

It's always been a source of frustration for me (and no doubt a source of confusion for newbies) that even the most experienced and respected of commentators here cannot agree on what the maximum useful scope-relative magnification is for planetary observation.

Some say telescopes max out at 25x per inch of aperture and others say 30x per inch is closer the mark, however I remember EdZ saying that its possible to squeeze out more detail up to about 39x p.i. (0.65 Ex/Pupil)

From personal experience with my 7" Mak, the maximum magnification (on a good night) which still reveals more detail is 286x. In the absence of continuously variable eyepieces (I have no zoom, so my magnifications are at fixed points), I have been unable to pull this back to see what's the minimum magnification where I can still see these extra details (something I'd really like to know), but by definition it must be somewhere north of my next-lowest magnification (which is 231x)

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)

On my Mak, 286x not a place I like staying - since the view's not particularly pleasant up there, but I can't help but feel somewhere in the range shown above is the sweet spot where all possible detail is visible, with the view still remaining subjectively pleasing.

Can anyone shed any further light as to why there is so much variation of opinion on this subject, when there ought to be a specific value we can all agree on?

(instant disqualification for the first person who states "everyone's eyes are different" since I don't believe anyone's eyes are able to help them defy the laws of physics and it's scientific fact I am searching for here, since I want to drive subjectivity out of the issue once and for all!...)

#2 buddyjesus

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Posted 10 May 2013 - 01:29 AM

http://www.telescope...gnification.htm

i can't find it but saw something to the effect that at .7mm exit pupil you see something like 95% of theoretical resolution of the scope. Maybe someone else can be more accurate than I am at recollecting this.

#3 leviathan

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Posted 10 May 2013 - 01:54 AM

It depends on a planet I believe. Jupiter in my 8" SCT was sharp on 145x most of nights, where I've used 307x only on few nights of passed season. Saturn is another story, I can't just watch it so tiny on 145x and it seems to hold 307x most of nights and even 444x during several days last week with good seeing.

Generally, there's a formula of 1.4D (D - aperture in mm) and maximum useful magnification of 2D. Usually you don't get any new details after that limits, but image becomes dim.

#4 David Knisely

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Posted 10 May 2013 - 02:21 AM

Great Bear asked:

Can anyone shed any further light as to why there is so much variation of opinion on this subject, when there ought to be a specific value we can all agree on?


It varies mostly due to the fact that different objects "take" high power to different degrees. In general, for lunar and planetary work, 30x per inch of aperture to 35x per inch seems to kind of be an optimal overall high magnification level if the seeing supports it. Things like the moon or Saturn tend to take even higher power a little better than something like Jupiter, so again, it depends on the target. Going higher than 35x per inch can be useful for specific details like the trying for the Encke Division in Saturn's rings or detail on the Galilean moons of Jupiter, but in general, going significantly past 40x per inch results in image quality that starts to decline a little (the light intensity is lower and floaters can get a *lot* more noticeable). That does not mean that using more power can't be beneficial (or just plain fun sometimes). The first night I got my 14 inch Newtonian's mirror back from Lockwood Custom Optics, I used 836x on the moon for a really impressive view (nearly 60x per inch), although to be perfectly frank, I could still see most of this detail at somewhat lower magnifications (554x) with a little more pleasing contrast. In the end, there is not a real hard and fast limit on power, so basically it comes down to the experience of the individual observer. On a night with great stable seeing, keep cranking up the power until you don't like the image anymore, and that can be your "limit". Clear skies to you.

#5 great_bear

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Posted 10 May 2013 - 03:14 AM

Generally, there's a formula of...


But this is actually the problem I'm referring to.

The statement "there's a formula" sounds very authoritative; as if it's the definitive reference that everyone refers to. Newbies then take this as fact and start propagating it elsewhere: But there seems to be no such definitive formula which everyone agrees with. Other people state with great conviction that truly experienced observers ought to be able to see everything at 1D (i.e. 25x p.i. or 1mm exit pupil) - thus pressurising people into staying below that limit lest they appear "inexperienced" to expert observers.

#6 Hermie

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Posted 10 May 2013 - 04:11 AM

There are several factors that I can think of:

eye physiology - an exit pupil of about 0.8mm offers maximum resolution of lower contrast features. (There was an interesting thread within the last year, but I can't find it.) As with any body measurement there seems to be a range, and also higher contrast features can be seen at smaller exit pupils. Hence a lot of contention about the "limit".

quality of optic. Needless to say, a higher quality optic will allow more magnification before the image is adversely affected.

Observer experience and preference. Different people have different tolerances for the dimness of the image at small exit pupil. Less experienced observers tend to chase a larger image. Older observers tend to have more floaters and other defects, so accept less magnification.

As David stated, different targets present differently. Mars looks better at the high end of the scale, but Jupiter is more pleasing at a lower magnification IMO.

When you throw in the actual seeing, there is no rule that applies. With experience with your scope and conditions you will develop preferences for particular magnification. And hopefully you will disregard those preferences when the conditions allow.

Hermie

#7 great_bear

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Posted 10 May 2013 - 05:01 AM

an exit pupil of about 0.8mm offers maximum resolution of lower contrast features.


That would be an oversimplification too far, because contrast resolution varies significantly depending on spacial frequency. You'd have to specify for what spacial frequency the 0.8 applies to, as 0.7 and 0.9 would be optimal for other spacial frequencies.

#8 Hermie

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Posted 10 May 2013 - 05:36 AM

Agreed. That is what was being discussed on the thread I'm thinking of. I wish I could find it.

I've re-read your question, and maybe I should shut up because I will be the first to tell you that everyone's eyes are different. Your technical understanding of the theory is probably better than mine, but I'm sure there is a difference between the sensitivity of your retina compared to mine and so on.

Also, the spatial frequency is different for different features, so again the maximum (or optimum) magnification will vary by the target - even on a single planet in perfect skies.

Hermie

#9 Jon Isaacs

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Posted 10 May 2013 - 06:12 AM

(instant disqualification for the first person who states "everyone's eyes are different" since I don't believe anyone's eyes are able to help them defy the laws of physics and it's scientific fact I am searching for here, since I want to drive subjectivity out of the issue once and for all!...)



Well...

There are certainly many factors that enter into choosing the optimal magnification but the human eye cannot be ignored, it is after all, the reason we need to magnify the image in first place. A few comments:

- Recognizing the fact that different people's eyes maybe different does not defy any law of physics. Consider choosing two people choosing the right eyepiece:

"Given the resolving power of my eye, and the resolving power of this telescope, I need to magnify the image X times for my eye to resolve the detail at the focal plane." Joe, given the same resolving power of the telescope, the same image at the focal plane, but a different resolving power of his eye, needs to magnify the image Y times to resolve the detail at the focal plane. The limit of what can be seen is simply the image that exists at the focal plane. The magnification chosen to view that image, that must depend on a number of factors including the response of the individual's eye.

People's eyes are different, people's eyes change, that's not subjective, that's an objective fact. My 65 year old eyes that have spent many years in the sun, they are certainly dimmer and have less resolving ability than they did when I was 20 years old when they were new, sharp and clear.

- A physicist/optical engineer looks at the entire system. In this system, there are three basic parts to this system, the objective, the eyepiece and the sensor, in this case the human eye. Were the sensor a camera, the eyepiece would be chosen on the basis of matching the image at the focal plane to the resolution and brightness sensitivity of the sensor. Different sensor characteristics, different eyepiece. Using a different eyepiece does not violate any optical principle. The eyepiece is a matching device.

- It is the resolution of the eye that determines the useful magnification. The image exists at the focal plane, the image exists in all it's detail in the exit pupil at any magnification, the question is, can your eye resolve it? Eyes are pretty similar but there is variation.

The appropriate magnification is about matching that image to your eye, nothing more. A good eye can see all there is to see at lower magnifications than a poor eye. If you look into the physiological basis for the limits on magnification, it is simply matching the resolution of the telescope to the resolution of the eye. Sidgwick discusses this. As an example, a 4 inch telescope produces an Airy disk that is 1.36 arc-seconds in diameter, 25X/inch is 100x, that means the eye must be able to resolve an 1.36 arc-seconds x 100 = 2.3 arc-minutes. That's basic optics. If you can cleanly split the double-double at 60x, then your eye can resolve 2.3 arc-minutes. Most will probably find that cleaner split is achieved at higher magnifications but eyes do differ.

- In reality, matching the magnification to the eye is more complicated than just considering the resolving power. The image brightness of an extended object decreases with increased magnification and since the eye can see more contrast in a brighter image, there is an optimization necessary which will be different for different eyes, different objects. When I was 20, with my sharper, brighter eyes, I could have been operating at lower magnifications with brighter images, the colors would have been more apparent, the details crisper.

- Besides the response of the eye, there are obviously a number of other important factors in optimizing the magnification, the quality of the optics including the thermal aspects, the quality of the seeing, the contrast, detail and brightness of the object, the whole eye-brain training/experience issue...

The underlying assumption in your question is that there should be one optimal magnification that works for all observers for a given object in a given telescope regardless of any differences in an individual's eye. I believe this is a faulty assumption. My old eye requires about 800x to split a 0.5 arc-second double in a 10 inch telescope. I am sure there are others who can see a clean split at 500x or 600x.

Jon Isaacs

#10 Illinois

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Posted 10 May 2013 - 06:52 AM

I use 193, 245, 307 and 403 power to look at planets. I enjoy 193 and 245 power most of the time and almost never use 403 power. I use 6.7 mm eyepiece on my refractor and not often on 180mm Mak-Cass. Around 35 to 40 power per inch seem the best for me! I remember one good night to look at Mars in 2011 and I used all power. I can see details on Mars's surface at 245 power then use 307 and 403. Mars look the same but little bigger and little more blur. About 35 to 40 power seem to be great for planets.

#11 great_bear

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Posted 10 May 2013 - 06:59 AM

This PDF file that I found here <click> seems particularly relevant. I particularly like the chart with different circles ("Figure 4: Contrast-Resolution Test Target:")

(I can't reprint the chart from the article on CN, as it has a quite specific copyright notice regarding its use)

#12 great_bear

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Posted 10 May 2013 - 07:18 AM

The underlying assumption in your question is that there should be one optimal magnification that works for all observers for a given object in a given telescope regardless of any differences in an individual's eye.


On the contrary - I wanted to remove individual's eyesight inadequacy from the equation completely, without overcomplicating the question. I do of course understand that eyesight of <100% will naturally require greater magnification.

(the irony here being that another observer with an identical telescope, boasting of getting "more magnification" than oneself, is possibly inadvertantly admitting to worse eyesight than oneself :-) )

I'm also wishing to remove "seeing" from the equation.

In other words, I just want to establish a theoretical benchmark assuming 20/20 vision, from which real-life factors such as scope quality, seeing, eyesight etc. can be factored in on a per-case basis later.

Perhaps Jon, my question might be better phrased (for the more experienced of understanding such as yourself) as "What's the minimum scope-relative magnification that will resolve for human eyes all possible detail?"

#13 csrlice12

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Posted 10 May 2013 - 07:26 AM

"Can anyone shed any further light..."

I think I've found your problem..... :lol:

#14 planet earth

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Posted 10 May 2013 - 07:27 AM

"People's eyes are different, people's eyes change, that's My 65 year old eyes that have spent many years in the sun, they are certainly dimmer and have less resolving ability than they did when I was 20 years old when they were new, sharp and clear."



I don't know about that Jon?
I believe with years of observing planets and DSO's, your eyes probably percieve more detail then the when you were younger with possibly better vision and less eyepiece time.
Okay maybe a bit dimmer but that's about it.
Sam

#15 great_bear

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Posted 10 May 2013 - 07:33 AM

with years of observing planets and DSO's, your eyes probably percieve more detail then when you were younger with possibly better vision and less eyepiece time.


As an ex - Sound Engineer I can say that's certainly true for audio. My ageing ears are damaged from too much loudness (much permanent ringing now), but I can still hear the details that almost all younger people miss in a complex soundscape because of years of professional experience "unpicking" compositions and fault-finding (through listening) of complex digital audio signal chains.

Since short-term "visual memory" seems to play an important part in observing, I'd expect experience to play an important part in seeing just as it does in listening.

#16 Astrojensen

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Posted 10 May 2013 - 07:39 AM

I want to drive subjectivity out of the issue once and for all!



You can't, because the biggest factors here are observer experience and preferences, the most subjective factors of all. And that's why observers can't agree on a hard-and-fast rule of what magnifications are best for planetary observing.


Clear skies!
Thomas, Denmark

#17 csrlice12

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Posted 10 May 2013 - 08:04 AM

Agree, Different strokes for different folks. We are all individuals, like snowflakes, no one of us is completely identical to the other (not even twins). Especially as time and life situations take there toll. All we can deal with in this situation is generalities, there is no concrete answer, and there never will be....

#18 Jon Isaacs

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Posted 10 May 2013 - 08:20 AM

Perhaps Jon, my question might be better phrased (for the more experienced of understanding such as yourself) as "What's the minimum scope-relative magnification that will resolve for human eyes all possible detail?"


This was your original question:

"Can anyone shed any further light as to why there is so much variation of opinion on this subject, when there ought to be a specific value we can all agree on?"

I believe I documented why one should expect variation and that there should be no specific value we should all agree on. It is the eye that determines the optimal magnification. There are so many variables that one can only answer such a question based on experience and since one has a particular human eye the answer to this question must depend on the individual.

As far as your rephrased question:

Any in depth discussion of magnification is primarily physiological, the telescope and eyepiece optics are relatively straight forward, it's the characteristics of the eye that determine the optimal magnification. You cannot eliminate the eye from the table, in fact, whether you use an idealized eye or real eyes, it should be the focus of the discussion.

The eye is the sensor, what are it's characteristics, what does 20-20 vision mean in terms of resolution, how does this translate to exit pupil, magnification? Beyond resolution there is color perception, ability to resolve contrast as a function of image surface brightness? Further more, the eye-brain relationship cannot be ignored, it's critical.

It's probably worth pointing out that there is probably no one magnification of any object that allows all possible detail to be seen. Some detail may be more visible with a smaller brighter image, some may more easily with a dimmer, more magnified image.

I think this is an interesting topic but when I started looking into it, I realized it had less to do with the telescope optics and more to do with the eye.

I think a more fundamental question is asking why someone like David Knisely probably sees more than I do... the same image at the focal plane, the same eyepiece... it's not the optics.

As an experiment, if you want to see how important the response of the human eye is, look at the moon through a solar filter, the moon is visible through a solar filter. From an optical standpoint, the detail is there to be seen at the focal plane, in the exit pupil, but the eye cannot see it.

Jon

#19 Eddgie

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Posted 10 May 2013 - 08:31 AM

almost 100% of the detail that the scope can resolve in availabe at about 1x per millimeter of apeture.

Look at it this way.

The smallest detail that a telescope can resolve visually would be equal to the tiny black space that you see between a close double.

That is where the "Dawes Limit" comes from, yes? Now telescopes can resolve smaller than this, but this is about the limit of the human eye due to the inability for the dark adapted eye to resolve contrast differences less than 5% to 15%.

Now, how much power do you have to use in a telescope to split a close double?

If the smallest detail your scope can resolve is the space between a tight double, then it makes sense that once you have resolved this, further magnification will not resolve any new detail.

If you can resolve this at 1x per millimeter of apeture, then the scope is at its limit.

But our eyes differ and often detail is much larger than this. In fact, when the detail starts as very low in contrast, it can be quite large, and we might still struggle to see it.

For example, take the case of a pale oval on Jupiter.

Often these ovals are far bigger than the shadow of the moons when in transit, but we don't usually see them.

The reasons are that scopes loose contrast, but also, the eye struggles more when the contrast starts low.

For this reason, some people with a better contrast sensitivity threshold may report that some details improve past 1x per mm.

But past about 1x per mm of apeture, you are really already resolving almost all of the detail that the scope is capable of showing.

Think about this next time you resolve the tightes double you scope can show you. If the space between the components reprsents the smallest detail your scope can show, and you can see it at 1x per millimeter, then ask yourself what the point of going higher is?

From the stanpoint of being able to resolve new detail the scope is done.

From the standpoint of increasing the angular maghification to make the detail a more pleasing angular size, you have some room to go higher, but past about 1.3x per millimeter, and the image brightness falls off to the point that you are working agiants your own eyesite.

I often hear people say that the scope breaks down. 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 personally find about 1.3x per millimeter to be optimal, though I can usually see the gap in the closets of splits at less than this.

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.

#20 Starman81

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Posted 10 May 2013 - 09:54 AM

I recently posted about my experiences with pushing the magnifications to the extreme with a small refractor (80mm) in this thread. However, I was only observing the Moon on that night.

#21 great_bear

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Posted 10 May 2013 - 10:24 AM

Thanks for this - some interesting points there.

One thing though:

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.

#22 Starman1

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Posted 10 May 2013 - 10:58 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.

#23 Sarkikos

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Posted 10 May 2013 - 11:12 AM

Eddgie,

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

#24 Sarkikos

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Posted 10 May 2013 - 11:18 AM

Don,

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

#25 Bill Boublitz

Bill Boublitz

    Vostok 1

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Posted 10 May 2013 - 12:10 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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