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Equipment Discussions >> Eyepieces

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great_bear
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Pushing the magnification
      #5851243 - 05/10/13 01:24 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!...)


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buddyjesus
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Re: Pushing the magnification new [Re: great_bear]
      #5851286 - 05/10/13 02:29 AM

http://www.telescope-optics.net/telescope_magnification.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.


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leviathan
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Re: Pushing the magnification new [Re: buddyjesus]
      #5851300 - 05/10/13 02:54 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.


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David Knisely
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Re: Pushing the magnification new [Re: great_bear]
      #5851319 - 05/10/13 03:21 AM

Great Bear asked:

Quote:

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.


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great_bear
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Re: Pushing the magnification new [Re: leviathan]
      #5851340 - 05/10/13 04:14 AM

Quote:

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.


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Hermie
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Re: Pushing the magnification new [Re: great_bear]
      #5851365 - 05/10/13 05:11 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


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great_bear
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Re: Pushing the magnification new [Re: Hermie]
      #5851401 - 05/10/13 06:01 AM

Quote:

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.


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Hermie
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Re: Pushing the magnification new [Re: great_bear]
      #5851423 - 05/10/13 06:36 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

Edited by Hermie (05/10/13 07:10 AM)


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Jon Isaacs
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Re: Pushing the magnification new [Re: great_bear]
      #5851453 - 05/10/13 07:12 AM

Quote:

(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


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Illinois
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5851487 - 05/10/13 07:52 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.

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great_bear
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Re: Pushing the magnification new [Re: Hermie]
      #5851492 - 05/10/13 07:59 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)


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great_bear
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5851522 - 05/10/13 08:18 AM

Quote:

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?"


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csrlice12
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Re: Pushing the magnification new [Re: great_bear]
      #5851531 - 05/10/13 08:26 AM

"Can anyone shed any further light..."

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


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planet earth
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5851534 - 05/10/13 08:27 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


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great_bear
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Re: Pushing the magnification new [Re: planet earth]
      #5851542 - 05/10/13 08:33 AM

Quote:

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.


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Astrojensen
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Re: Pushing the magnification new [Re: great_bear]
      #5851553 - 05/10/13 08:39 AM

Quote:

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


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csrlice12
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Re: Pushing the magnification new [Re: Astrojensen]
      #5851589 - 05/10/13 09:04 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....

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Jon Isaacs
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Re: Pushing the magnification new [Re: great_bear]
      #5851615 - 05/10/13 09:20 AM

Quote:



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

Edited by Jon Isaacs (05/10/13 09:42 AM)


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Eddgie
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Re: Pushing the magnification new [Re: great_bear]
      #5851627 - 05/10/13 09:31 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.


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Starman81
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Re: Pushing the magnification new [Re: Eddgie]
      #5851771 - 05/10/13 10:54 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.

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

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

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

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

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

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

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

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

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

Pete

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


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

Quote:


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





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

dan


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

Quote:

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




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

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

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


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


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

Quote:


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


Well its concept in model but real in practice.

Pete


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

Quote:

Don,

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

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

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

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

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




OK, got it.

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

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


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

To all,

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

Mike


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

Pete,

Quote:

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




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

Quote:

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

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

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




I believe that shows in my avatar.

Mike


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

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

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


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

Quote:

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

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




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


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

Quote:

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

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



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

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

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

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


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

Quote:

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




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

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

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

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

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

Different objects, different observers/eyes, different magnifications.

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

Jon


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

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

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

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

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

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


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


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



Amen


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


Hello ,

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

Maryland has some very nice seeing at times .
Leonard


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

Quote:

To all,

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

Mike




Hi Mike,

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

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

Pete

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


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

Quote:

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

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




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

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

Jon


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

Quote:

To all,

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

Mike




Truer words were never said

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


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5854878 - 05/12/13 05:32 AM Attachment (6 downloads)

Quote:

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




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

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

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

I've now changed this to read:

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

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

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

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

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

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

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

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

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

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

Clear skies to you.

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


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

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

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

Quote:

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





This isn't really the way it works.

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

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

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

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

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

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

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

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

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


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

Quote:




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

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




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

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


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

Quote:

Quote:




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

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




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




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

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

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

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


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great_bear
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Re: Pushing the magnification new [Re: Eddgie]
      #5855362 - 05/12/13 12:13 PM

Quote:

This isn't really the way it works.




Yes it is.

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

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

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


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

Don,

Quote:

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




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


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

Quote:


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




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


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

Post deleted by leonard

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


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MRNUTTY
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Re: Pushing the magnification new [Re: leonard]
      #5855754 - 05/12/13 03:28 PM

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

Where's that popcorn emoticon?


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

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

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

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


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great_bear
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Re: Pushing the magnification new [Re: leonard]
      #5855861 - 05/12/13 04:37 PM

Hi Leonard.

It's not as mysterious as it sounds.

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

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

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

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

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

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

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

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


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

Quote:

Hi Leonard.

It's not as mysterious as it sounds.

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

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



Correct.
Quote:


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



Correct again.
Quote:


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



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

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


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

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

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

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




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


Hello ,

great bear ,

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

Leonard


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5856289 - 05/12/13 08:38 PM

Hi Don,


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

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

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

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

Quote:

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




Then look through my Mak and make me a liar!

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


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


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

Quote:


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




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

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

Jon

Jon Isaacs


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

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


William Optics 66SD and Orion Short Tube 80

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

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

Best,


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

Quote:

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




Boy howdy


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

I'm curious...

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

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

What am I missing?


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

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

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great_bear
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Re: Pushing the magnification new [Re: Jon Isaacs]
      #5856751 - 05/13/13 01:34 AM

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

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


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

Quote:

Hi Don,

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




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

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

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

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

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


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

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

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

Clears,


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5856848 - 05/13/13 04:36 AM Attachment (16 downloads)

Don,

Much of the unwanted light from the vicinity of the primary does not lie outside of the field stop. Here is another diagram to illustrate this more clearly. In the short-focus telescope shown, you can see some of the many rays of light (from various sources in the image) which come from the primary. These rays completely span the field-stop, go through the eyepiece and become part of the final image - just as you might expect.

If we move on to the long focus version of the same telescope (with the same eyepiece) you can see that those exact same paths of light come from unwanted, reflected light coming from the shiny interior of the extended tube.

Since the eyepiece is not a dynamic device it cannot behave differently in these two scenarios.

You are familiar with the issue of "unused light" in overly-large exit-pupils at low magnification I believe? Well, these unwanted reflections - lying outside of the circumference of the primary - form part of that "unused light" at low magnifications, resulting in better contrast at low powers compared to when you are pushing the magnification.

In all of this, it's important to remember that the exit-pupil-thick beam of light entering the eye from any specific angle does not contain a representation of the entire stellar image. It is a spread-out representation of a single point; an exit-pupil-thick beam of light either bright, or dark - or whatever is needed to recreate that spot that it will be focused back into on the retina by the human eye-lens.

Instead of countering with perfectly correct arguments against things I've never said, it would perhaps be more instructive to CN readers if you state what it is in the diagram that you have issues with.

Clear skies to you.


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Sarkikos
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Re: Pushing the magnification new [Re: ibase]
      #5856902 - 05/13/13 07:16 AM

Hernando,

Quote:

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

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




Not surprising, since the ST80 is an f/5 achromat. It is a nice rich-field wide-field for dark sites, especially since I've upgraded the focuser to a 2" Crayford. But an ST80 is not exactly the best scope for planet/lunar viewing. The image falls apart rapidly with higher magnification due to CA. My A70LF - a 70mm f/12.9 refractor - is much better for observing planets and the Moon, despite the smaller aperture. The CA ratio for the ST80 is 1.59, that for the A70LF is 4.69.

It's important to get the best - or at least a better - tool for the job. That goes for telescopes as well as eyepieces.

Equivalent Chromatic Aberration of Achromatic Refractors

Mike


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dan_h
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Re: Pushing the magnification new [Re: Geo31]
      #5857013 - 05/13/13 08:57 AM

Quote:

I'm curious...

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

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

What am I missing?




I don't think you are missing anything. If lens are properly sized and proper stops are in place, there should not be any possibility of light striking the edges of the lenses.

However this is not a popular point of view. It seems that there are a number of magical properties that have been given to edge blackening.

Yes, there can be stray light as a result of defects in the surface polishes or even in the glass itself but the amount of light that is scattered and actually hits the edges of any lens has to be incredibily tiny. The amount that gets reflected back into the image is much smaller again.

dan


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Re: Pushing the magnification new [Re: Sarkikos]
      #5857067 - 05/13/13 09:33 AM

Quote:

Hernando,

Quote:

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

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




Not surprising, since the ST80 is an f/5 achromat. It is a nice rich-field wide-field for dark sites, especially since I've upgraded the focuser to a 2" Crayford. But an ST80 is not exactly the best scope for planet/lunar viewing. The image falls apart rapidly with higher magnification due to CA. My A70LF - a 70mm f/12.9 refractor - is much better for observing planets and the Moon, despite the smaller aperture. The CA ratio for the ST80 is 1.59, that for the A70LF is 4.69.

It's important to get the best - or at least a better - tool for the job. That goes for telescopes as well as eyepieces.

Equivalent Chromatic Aberration of Achromatic Refractors

Mike




Mike, well said, different eyepieces/telescopes for different jobs.

Really like the ST80, as you said, for the rich wide starfields, although the WO66 has become a sort of rapid deployment scope for lunar/planets, the Delos 6mm is current favorite on it - really enjoyed using this scope when Jupiter was at opposition and missing a stripe, seeing the GRS distinctly and following moon transits. For closer looks though, will bring out the WO 102ED or 6" Antares refractors.

PS. Nice CA chart, thanks!

Best,


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Re: Pushing the magnification new [Re: great_bear]
      #5857208 - 05/13/13 10:59 AM Attachment (5 downloads)

Quote:



If we move on to the long focus version of the same telescope (with the same eyepiece) you can see that those exact same paths of light come from unwanted, reflected light coming from the shiny interior of the extended tube.





<< shiny interior of the extended tube >> ?

Seems the appropriate thing to do is eliminate the problem at the source. Get rid of the shiny interior and add a baffle or two. And don't forget to add an appropriate glare/dew shield to block the off axis light from entering the lens. Problem solved.


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great_bear
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Re: Pushing the magnification new [Re: dan_h]
      #5857225 - 05/13/13 11:08 AM

- yes for sure - no doubt about that


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Re: Pushing the magnification new [Re: great_bear]
      #5857299 - 05/13/13 11:47 AM

And note that the unwanted light from outside the field of view gets into the field of view by reflection. I was never arguing that wasn't the case.

But the unwanted light that does get into the field by reflection gets into the field of view by passing through a part of the field that is inside the field stop.

Light that is outside the field stop (think of a ray reflected from the side of the tube that does not get through the eyepiece's field stop) does not make it to the eye. You can think of the field stop as the final baffle in the system.

Ergo, though unwanted light from outside the field of the scope or eyepiece CAN get into the field of the eyepiece, all that light will be IN the field of the eyepiece and inside the exit pupil for the eyepiece. Light outside the exit pupil is outside the field stop of the eyepiece and does not make it through the eyepiece.

In the two-refractor example, you essentially prove that. You also show that long ratio or short, unwanted light can get through without proper addressing of the issue of scattered light.

So the issue I have had with the discussion is that, all along, there was an implication that scattered light from outside the field gets to the part of the pupil of the eye that is outside the exit pupil of the eyepiece if the exit pupil of the eyepiece is smaller than the pupil diameter of the eye, and there is no way for that to happen.
[Excepting reflection from the cornea to the eyepiece and back]

I fully agree that light from outside the field of view can make its way into the exit pupil, for sure.

In the example of the 2 refractors, reducing the size of the eyepiece or field stop therein would also reduce the unwanted reflected light from entering the eyepiece or exit pupil. That implies that there would be LESS effect from scattered light at high powers, and this conforms to my experience, too.


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5857913 - 05/13/13 04:50 PM Attachment (8 downloads)

Don,

Quote:

So the issue I have had with the discussion is that, all along, there was an implication that scattered light from outside the field gets to the part of the pupil of the eye that is outside the exit pupil of the eyepiece if the exit pupil of the eyepiece is smaller than the pupil diameter of the eye, and there is no way for that to happen.




Yes there is, explained many times in my previous posts, but let's see if another picture helps


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5858007 - 05/13/13 05:21 PM Attachment (5 downloads)

Don,

Quote:

Light outside the exit pupil is outside the field stop of the eyepiece and does not make it through the eyepiece.




This is not correct - as demonstrated in Fig 1 in my previous post above.

The field-stop cannot mask off any of the area outside the exit pupil. That would be geometrically impossible. All it can do is restrict the maximum angle that a light bundle can travel through the exit-pupil.

Again, an illustration is in order:

Edited by great_bear (05/13/13 05:36 PM)


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dan_h
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Re: Pushing the magnification new [Re: great_bear]
      #5858064 - 05/13/13 05:35 PM Attachment (6 downloads)

But junk light is not focused as your drawing shows. Junk light washes everything it can reach. The attached drawing show this and it doesn't really matter what eyepiece you use. Longer eyepieces use wider field stops and bigger lenses so they are washed in a greater amount of junk light.

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great_bear
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Re: Pushing the magnification new [Re: dan_h]
      #5858148 - 05/13/13 05:56 PM

Hi dan_h

Quote:

But junk light is not focused...




Agreed.

Quote:

...as your drawing shows.




That's not what it shows. It's not actually showing the junk light focused if you think about it.

Look again - Fig 1 in my diagram (not your version) is showing two light paths from unrelated areas of the junk light being refracted onto the same point. This happens because all light paths - related or otherwise - which cross a specific point on the focal plane will meet again at a specific single point on the retina.

In the case of junk light, different light paths from the same part of the junk get scattered and refracted all over place. But then you know that, since that's exactly what you've drawn.

Quote:

Longer eyepieces use wider field stops and bigger lenses so they are washed in a greater amount of junk light.




Depends on the angle - look again at fig 2 of my diagram that you reference. It's pretty hard to draw a straight light path from the junk light to the eyepiece lens - and if the focal length was even longer, it would be impossible.


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GeneT
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Re: Pushing the magnification new [Re: great_bear]
      #5858193 - 05/13/13 06:09 PM

Quote:

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)




Here's how I came up with my working answer/solution to your question. For my F5, 12.5 inch Dob, I consider high power when moving to a 10 mm focal length eyepiece, and smaller. When moving into the 'high power' range, I have eyepieces going in smaller, 1 mm increments to my 4 mm Radian. In other words, I have eyepieces ranging from 10, 9, 8, 7, 6, 5 and 4 mm. Some nights, due to seeing, or other issues, I can't go lower than my 12 Nagler. When viewing the planets, I keep my working my way down to smaller and smaller focal length eyepieces. At some point I hit the wall, i.e. the view will not support more magnification. Rarely can I use my 4 mm, and it does not give me any better views of the planets than my 5mm Pextax XW and XO eyepieces. You might consider buying one of the better zooms. Many people report that they do great on the planets, and would save you from having to buy several eyepieces.


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great_bear
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Re: Pushing the magnification new [Re: GeneT]
      #5858204 - 05/13/13 06:14 PM

I did consider it yes - but I use binoviewers and didn't like the idea of two zooms getting out of sync.

I've read on forums that some people seem to do it though...


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Re: Pushing the magnification new [Re: astro_baby]
      #5858243 - 05/13/13 06:22 PM

Quote:

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




Also in the UK, many observe from housing estates where - no matter where you point the scope - you're always aiming above the rooftop of a nearby house.

When buying our house in London my wife let me choose one that was well-positioned for astronomy. The gardens of our street go in a line running south-east/north-west, so that's a great house-free observing corridor in one direction.


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Starman1
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Re: Pushing the magnification new [Re: dan_h]
      #5858342 - 05/13/13 06:51 PM

Please refer to the following link's illustration as I explain a few things about exit pupil and how the eyepiece produces an image.
http://www.telescope-optics.net/eyepiece1.htm

1) Your drawings do not show the exit pupil, but only the center of the field of the instrument illustrated. The rays coming back away from the eyepiece start out the width of the eye lens and converge on a small flat plane known as the exit pupil. It is the narrowest point of convergence of the rays exiting the eye lens. That is why, as your eye moves away from the eyepiece the visible field narrows and why, when you are too close to the eyepiece, the iris of your eye cuts off the field. Even on eyepieces of small exit pupil, this is true, and you can hold your eye too close to allow all the rays from the eyepiece to enter the pupil of your eye. It is in focus at all distances from the eyepiece.
2) The objective has a field of view and it is illustrated in the diagram to which I link. The farther off axis, the farther toward the edge of the field stop do the rays become, until, at the field stop, a maximum off-axis field is defined. Note that rays can continue to diverge after the field stop, but the field stop defines the edge of the field visible by the objective for that eyepiece.
3) Rays from farther afield cannot make it to the exit pupil because they do not make it past the field stop.
Nowhere in this illustration does it preclude the possibility of the reflection of light off an internal surface that would make it through the field stop and into the exit pupil behind the eyepiece.
4) the exit pupil contains all the light that makes it through the eyepiece. It utilizes all the light that makes it through the field stop of the eyepiece, unwanted or not. Light that does not make it past the field stop (at whatever angle) is not part of the exit pupil. Seems simple enough. If the field of the eyepiece is 50 degrees, you are seeing no light from 30 degrees off axis.
5) Ergo, if the exit pupil of the eyepiece is smaller than the pupil of the eye, all the light passing through the exit pupil goes into the eye. This may occur, depending on the exit pupil size, at a small range of distances from the eyepiece.
6) The error in your first illustration is that it shows all rays leaving the eye lens of the eyepiece parallel (and what you mark as the exit pupil is only part of the exit pupil). They are not. They converge on the exit pupil and diverge thereafter.
7) Therefore, and I will word this as carefully as I can, light outside the diameter of the exit pupil (if that exit pupil is smaller than the pupil of the eye) but inside the diameter of the pupil of the eye and outside the exit pupil of the eyepiece, did not come through the eyepiece. It cannot have. It could have come from the field peripheral to the eyepiece as, for instance, the outside surface of the scope or the top surface of the eyepiece reflecting the sky or some extraneous light. But it did not come through the eyepiece. If it had it would be the equivalent of seeing a 60 degree field through a 50 degree eyepiece.


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5859173 - 05/14/13 05:44 AM Attachment (3 downloads)

Hi Don,

The diagram you refer to is dual-point, mine are all single-point (kinda obvious, I thought?)

No matter... It makes no difference. Whether the diagram is single-point, dual-point or even triple-point for good measure (as shown below), it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it.

You might not like this. You may even want to put your fingers in your ears and scream "no! no! no!", but the fact remains it's clearly illustrated in this diagram for all CN members to see - and anyone with a ruler can validate it.

Clear skies to you.

Fig 4:


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planet earth
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Re: Pushing the magnification new [Re: great_bear]
      #5859186 - 05/14/13 06:09 AM

"it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it."

Not to be rude but:
If my Newtonian get's superb planetary views, does it matter if this unwanted light exists?
To me it's really a non issue.
Sam


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great_bear
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Reged: 07/05/09

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Re: Pushing the magnification new [Re: planet earth]
      #5859199 - 05/14/13 06:40 AM

No, clearly not an issue in your case - but it's a problem in some telescopes unfortunately.
So your question's not rude; it's a perfectly reasonable one.


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planet earth
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Re: Pushing the magnification new [Re: great_bear]
      #5859202 - 05/14/13 06:45 AM

Just to note:
I do find this thread very interesting, and educational.
Even though it went a bit OT.
Sam


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Sarkikos
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Re: Pushing the magnification new [Re: planet earth]
      #5859217 - 05/14/13 07:05 AM

For Newts, it's pretty easy to prevent "junk light" ...

1) Fully flock the interior of the OTA.
2) Fully flock the focuser.
3) Make or buy a light shield about the same length as the aperture of the telescope, up to a foot long or so.
4) Fully flock the interior of the light shield.
5) Clean the mirrors.
6) Clean the eyepieces.
7) Use eyepieces with high light transmission, low scatter, good baffling and good coatings.

Then don't worry about it.


Mike


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Illinois
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Reged: 12/18/06

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Re: Pushing the magnification new [Re: Sarkikos]
      #5859224 - 05/14/13 07:09 AM

Interesting posts! Field stop make sense and what about extra long dew that would block any unwanted light?

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great_bear
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Re: Pushing the magnification new [Re: Sarkikos]
      #5859232 - 05/14/13 07:17 AM

True, but I have a Mak

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great_bear
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Re: Pushing the magnification new [Re: Illinois]
      #5859239 - 05/14/13 07:20 AM

Quote:

what about extra long dew that would block any unwanted light?




That only gets you so far. When observing the terminator of the moon, OTA reflections in the baffle-tube can really kill the contrast at high mags on a Mak.


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Sarkikos
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Re: Pushing the magnification new [Re: Illinois]
      #5859291 - 05/14/13 08:06 AM

An extra long dew/light shield is important for Newts/Dobs, Cats and refractors. They prevent dewing on the secondary mirror, the corrector and the objective. They block ambient glare. And they are easy to make.

A shield about the same length as the diameter of the OTA, up to about a foot, is sufficient. I make them quite a bit longer than the diameter for small refractors, such as my 50mm and 70mm finders. If the observer is concerned about vignetting their Newt, they can plug the numbers into NewtWin or a similar program and see what would be the maximum length of a shield for their telescope. Flocking the interior with strips of ProtoStar is a good idea.

Mike


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Sarkikos
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Re: Pushing the magnification new [Re: great_bear]
      #5859295 - 05/14/13 08:09 AM

Quote:

Quote:

what about extra long dew that would block any unwanted light?




That only gets you so far. When observing the terminator of the moon, OTA reflections in the baffle-tube can really kill the contrast at high mags on a Mak.




Yes, I've noticed that in my Maks and SCTs. But a shield can help. They might be even more useful in Newts. But at any rate, shields will prevent - or at least slow down - dewing on the corrector for Cats. And IME, shields will prevent entirely any dewing inside a solid tube Newt.

Mike


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great_bear
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Re: Pushing the magnification new [Re: Sarkikos]
      #5859310 - 05/14/13 08:22 AM

I agree - I made a shield from a cardboard bag from the gift-shop at a star-party once, and three-years on it's still going strong (upgraded with a thick black card lining). It not only reduces glare but does - as you say - prevent dewing very effectively - in my case, until everything else is so wet you'll have to (or at least want to) pack up for the night anyway.

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Scanning4Comets
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Re: Pushing the magnification new [Re: great_bear]
      #5859725 - 05/14/13 12:04 PM

I have a shield as well, fully flocked etc. I have tried using my scope without it and the difference is minimal for glare unless I am right out in my front driveway and the glare from the street lights are blazing. At the darker site I go to, I have forgotten to bring it, and it makes no difference out there when using it or not.

Never had a problem with dew on my secondary either, and my scope can get dripping wet with dew.


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Starman1
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Re: Pushing the magnification new [Re: great_bear]
      #5859734 - 05/14/13 12:07 PM

Quote:

Hi Don,

The diagram you refer to is dual-point, mine are all single-point (kinda obvious, I thought?)

No matter... It makes no difference. Whether the diagram is single-point, dual-point or even triple-point for good measure (as shown below), it's still the case that junk light is being transmitted outside of the exit-pupil, and there's nothing that the field-stop can do about it.

You might not like this. You may even want to put your fingers in your ears and scream "no! no! no!", but the fact remains it's clearly illustrated in this diagram for all CN members to see - and anyone with a ruler can validate it.

Clear skies to you.

Fig 4:




Well OK. I had to sit down and think about it. Could light from outside the field stop of the telescope actually make it through to the eye? And, of course, you are right. I was wrong in my thinking about it.

The simplest case I could think about was a dobsonian telescope where light could get into the bottom of the eyepiece from over the lip of the other side of the scope, directly from the sky. That light is 100% outside the field stop of the telescope, yet it can come through. AND, it could be anywhere in the pupil of the eye--inside or outside the exit pupil of the eyepiece. If bright enough it could even reflect from your cornea.

Of course, inside the exit pupil it would be thought of as scattered light that would cause a reduction in contrast as glare. Most likely, scattered light could be anywhere--inside AND outside the exit pupil.

Your explanation makes good sense, and I thank you for taking the time to get it through my thick-headedness on this topic.

It goes a long way toward explaining why baffling and flocking and paying attention to reflective surfaces are so important. And it explains how we can tell if a bright object is immediately outside the field of view.


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great_bear
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Re: Pushing the magnification new [Re: Starman1]
      #5859795 - 05/14/13 12:27 PM

Quote:

Your explanation makes good sense, and I thank you for taking the time to get it through my thick-headedness on this topic.




- and thank you for allowing me to do so :-)
(I'd never have got around to creating those diagrams otherwise!)


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Wol
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Loc: Perth Western Australia
Re: Pushing the magnification new [Re: great_bear]
      #5861582 - 05/15/13 07:39 AM

Yes thanks to all for the questions and answers and patience - I have been following this with interest and it has been educational

regards


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Illinois
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Re: Pushing the magnification new [Re: Wol]
      #5861787 - 05/15/13 10:03 AM

I see! Thanks! I wondered, anywhere that selling dew for my Orion 180mm Mak-Cass? Thanks!

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Hamsterdam
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Re: Pushing the magnification new [Re: Illinois]
      #5873110 - 05/20/13 05:32 AM

I wanted to do a short (me?) post on eye age/effectiveness.
While at almost 50, I have a slight amount of myopia at distance, when focusing on a small detail (and yes thru a lens), I think I have it over on many, for a specific reason. I started in the days of film scanning and wet etching, thru the 00's and today with Ps CS 5.5? (I'd have to look).

I had to be able to spot a 3-5% shift in percentage size of halftone dots, as they relate to the composition, registration and color balance of a CMYK image at 600dpi. I did this throughout my career, first in manual analog, and now, with a highly calibrated screen — matching the paper it will print on, plus the actual output color.

I since have had retirement thrust upon me, but my point is, I spent the greatest portion of my life, training my eyes to see not only obscure subtlety in color, but in detail, required in both positions I held. I continue to practice my Art, both on the computer and the canvas.

Our bodies, cell by cell, save for our neurons, are replaced every 7 years. Sadly, our repairman orders worse and worse replacement parts as we go, so I live under no illusion that our eyes do indeed fail in some respects. I also believe that both training and use, at the very least, extend that acuity, if not enhance it.

Best of views to all —davey

Edited by Hamsterdam (05/20/13 05:40 AM)


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