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photonovore
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Re: Stranger than fiction! new [Re: wh48gs]
      #4077333 - 09/27/10 09:40 PM

Quote:

This means less energy in the rings, and better contrast transfer in the left side of an MTF graph. Since this includes all detail sizes from Airy disc up, it covers most of details observed.




Do you mean to say that the left side (half) of the MTF is where detail down to the size of the airy disk resides? Tell me you meant to say right side...

BTW, speaking of MTF's and contrast thresholds, what you have marked on your site (sec 4.8.2., fig 48 etc) does not jive with your ref for placment of those lines ((Rooten & van Venrooij). The placement of the photopic low contrast detail threshold is correct in their book--but you have evidently modified it to correlate the unmagnified focal plane (what the MTF represents) with the human contrast response. That would be fine--if we never magnified the focal plane during observation! When seeing allows us to exploit the detail resident in the image plane, we are using our 50x/inch on the frequencies within the imagplane represented by the lower left tail of the scope's MTF plot. Magnification translates the focal plane frequency at that point (much too high for us to sense low contrast within without benefit of magnification) to a much lower frequency (~7cyc/degree), where our eyes *can* access such low contrast photopic detail.

Of course the upshot of this is that the comparisons between those different scopes in your examples becomes much more competitive down the line where the fine detail resides...


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wh48gs
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Re: Stranger than fiction! new [Re: photonovore]
      #4077399 - 09/27/10 10:25 PM

Quote:

Quote:
--------------------------------------------------------------------------------

This means less energy in the rings, and better contrast transfer in the left side of an MTF graph. Since this includes all detail sizes from Airy disc up, it covers most of details observed.


--------------------------------------------------------------------------------



Do you mean to say that the left side (half) of the MTF is where detail down to the size of the airy disk resides? Tell me you meant to say right side...




Nope, it meant exactly what it says. The actual value of cutoff frequency normalized to 1 is Lambda/D in radians, which is about 0.41 the Airy disc diameter. This means that the line width corresponding to the Airy disc diameter is at the normalized frequency 0.41, which is on the left side of MTF graph. Vast majority of object and details in general observation is in the 0-0.4 range of the normalized spatial frequency.

Likewise, the MTF cutoff for low contast details on my site is identical to theirs, only normalized to 1.

Btw. it is Rutten, not "Rooten".

Vla


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photonovore
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Re: Stranger than fiction! new [Re: wh48gs]
      #4077474 - 09/27/10 11:05 PM

Well, that was quick! Thanks for the name correction. Tell me, does Rutten agree with your interpretation of this? Well, of course not! I don't know why you even bothered to ref him (as your representation has no relation to the -correct- representation in his book.)

Here's where high resolution lies:

from R.N. Clark: Dawes limit = 1/(Fw) in lines per mm, where

Rayleigh limit = 1/(1.22*Fw) in lines per mm, where

F = the focal ratio, and w = the wavelength of light in mm.

The Rayleigh limit is at a modulation transfer function (MTF) of about 9%.


Which, according to you, is in "no-see-um" territory on the mtf, below your cutoff line, waaaaay to the right of the "0-0.4 range of the normalized spatial frequency". In fact it lies at about 0.9 normalized spatial frequency.

Re; your page mentioned previously and the f/12 example--do the math. It results in the Rayleigh limit (which is by no means the maximum spatial resolution accessible to the eye or present within the focal plane) being placed at 136 lines/mm. What would be normalized as ~0.9 as 1 on that particular MTF diagram is 155 lines/mm. Now tell me: where is the 9% contrast/136 lines/mm point on the MTF curve located? Ans: in the lower right corner.

Time for some revisions, Vlad. What I am wondering is how someone like you could get so turned around about something this basic?


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Clive Gibbons
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Re: Stranger than fiction! new [Re: photonovore]
      #4077533 - 09/27/10 11:51 PM

More importantly, the admission that effective resolution is decreased by almost 4% is bound to stick in the craw of the most persnickity double star aficionado.
That 4" long focus achro is working at only 3.856" effective aperture.
Oh, the humanity!!...

A bitter pill to swallow.


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wh48gs
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Re: Stranger than fiction! new [Re: photonovore]
      #4077762 - 09/28/10 05:15 AM

Quote:

Well, that was quick! Thanks for the name correction.




No problem - if anyone, I can help you.

Quote:

Tell me, does Rutten agree with your interpretation of this?




Interestingly, the only correction he suggested was his first name, which I had misspelled.

Quote:

The Rayleigh limit is at a modulation transfer function (MTF) of about 9%.

Which, according to you, is in "no-see-um" territory on the mtf, below your cutoff line, waaaaay to the right of the "0-0.4 range of the normalized spatial frequency". In fact it lies at about 0.9 normalized spatial frequency.




The Rayleigh limit for aberration-free contrast transfer is at the 8.9% contrast level; but its spatial frequency is 0.82, not 0.9 (spatial *frequency* is just that: the inverse of wavelength; that is why it is numerically smaller for larger details). Aberration-free contrast level corresponding to 0.9 spatial frequency is 0.038 (3.8%).

And yes, the Rayleigh limit is way to the right of the 0-0.4 range of spatial frequencies, which corresponds to detail sizes of Airy disc's (diameter) and larger, out of the resolution range for low-contrast details.

And this range is at the left side of MTF graph, as I said.

Quote:

Re; your page mentioned previously and the f/12 example--do the math. It results in the Rayleigh limit (which is by no means the maximum spatial resolution accessible to the eye or present within the focal plane) being placed at 136 lines/mm. What would be normalized as ~0.9 as 1 on that particular MTF diagram is 155 lines/mm. Now tell me: where is the 9% contrast/136 lines/mm point on the MTF curve located? Ans: in the lower right corner.





I've done my math, Mardy - it seems it's you who need to catch up. If we take 155 lines/mm as the cutoff frequency, the number corresponding to the Rayleigh limit is 127.1 lines/mm, not 136. It is to the right, but closer to 3/4 of the frequency range than to the left corner of MTF graph.

This, again, agrees with what I said: details of Airy disc size and up, which cover most of observable details, are on the left side of MTF graph. This directly implies that smaller details are (mainly) on the opposite, right side. Got that sorted out?


Quote:

Time for some revisions, Vlad. What I am wondering is how someone like you could get so turned around about something this basic?




As you can see, no need for revisions; not on my side, anyway.

Btw, sorry about your graph not been included in Neil's article. Turned out, the tube/lens thermals were not a factor significantly discriminating short apo vs. long achromat. It was up to Neil; blame him

Vla


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astroneil
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Re: Stranger than fiction! new [Re: wh48gs]
      #4077990 - 09/28/10 09:15 AM

Yes,

Sorry about that Mardi,; an earlier draft of the work, which did indeed feature your thermal cool down graph was ditched in favour of the current findings. What it did show is that as a telescope is cooling down, the long focus instrument is much more stable too (you don't need to refocus as often), but as Vla said, it is of little importance to a study which seeks to compare scopes that are properly acclimated. Astrophotographers should take note though!

Cheers,

Neil.


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Clive Gibbons
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Re: Stranger than fiction! new [Re: Alan French]
      #4077996 - 09/28/10 09:19 AM

Gotta admire Neil's sticktoitiveness.

Getting back to an automotive analogy, promoting the long focus achro vs. shorter apos reminds me somebody extolling the virtues of a big ol' Buick compared to modern sports cars having the latest in multi-valve engines. It's a tough sell to the auto enthusiast crowd. Well, most of 'em.
There's always a small contingent of traditionalists.
You can point out the land yacht's cushiony smooth ride, the engine's wide torque curve, how it's basically indestructible and hardly needs maintenance.

But it's still dad's ol' Buick.

Folks who are looking for more dash and excitement likely won't be convinced.

I'm certainly impressed by the latest sports cars... and enjoy a country drive in something more relaxing.

Once again, kudos to Neil for staying the course.


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photonovore
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Reged: 12/24/04

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Re: Stranger than fiction! new [Re: wh48gs]
      #4078335 - 09/28/10 12:34 PM

I used .000512 for the frequency (took that figure from Koren's webpage for convenience): thus my math was entirely correct using that figure. Using a longer wavelength doesn't change the fact that the Rayleigh limit lies on the far right side of the MTF, much nearer 0.9 normalized freq than your 0-0.4.

Quote:

he Rayleigh limit is way to the right of the 0-0.4 range of spatial frequencies, which corresponds to detail sizes of Airy disc's (diameter) and larger, out of the resolution range for low-contrast details.





Low contrast details are those which lie *at* the resolution limit of the telescope (Rayleigh, Dawes Abbe take your pick). Every time a close double is seen as split at the Rayleigh limit we are seeing what you say cannot be seen--9% contrast at a *native* focal plane frequency of between 0.8 and 0.9. We can access this part of the focal plane because we can *enlarge* it to a much lower frequency domain (7-12cyc/degree). Additionally, if you had spent any amount of time in high resolution observing of the Moon (the preeminent high resolution target in the sky) you would also know that your position in this is wildly offbase.

However, I know where your disconnect lies. You are mistakenly applying the human contrast response to the native, unmagnified focal plane image of the optic, which is what the MTF represents. That much is correct-- but completely irrelevant to practice and *that* is where the problem arises, as you attempt to apply it to actual visual practice and this is the relevant error. What you fail to consider is that the focal plane of the telescope is *never* observed by the eye without benefit of significant magnification. And what does magnification do? Magnification *changes* the frequency characteristics of the image by uniformly lowering all frequency domains within that image.

You need to approach a correct understanding of this issue by considering the telescopic image, as seen by the eye (that is with focal plane enlarged and as an *angularly* defined image) and then applying the human contrast sensitivity curve's frequencies to the magnified focal plane as the eye actually see it. You are instead applying a 60cycle resolution/contrast frequency domain (the human eye) to the native focal plane of a telescope which is in the 90cycle range, c/degree). So *of course* two thirds/half of the available detail in the focal plane would be invisible to the eye! But telescopes are not used this way. We use eyepieces with telescopes and for the same purpose as with using a magnifying loupe on a mineral specimen---to make detail otherwise inaccessible to the naked eye...visible, by enlargement.

Try reading a bit of R.N.Clark's work on contrast--he gets what I am trying to school you on. Maybe you can accept this information coming from someone else beside me.

I wouldn't have bothered to bring this gross error of yours up except that it significantly affects the conclusions vis a vis comparative high resolution between the apo and achromat examples used in the article, subject of this thread. The case for the achromat is much better than you paint it. As I have been trying to tell you for years now.

BTW, my name is Mardi not Marty. So I take it you prefer Vla to Vlad as a short for Vladimir?


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Wes James
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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4078397 - 09/28/10 01:06 PM

If anyone has access to the October '85 issue of Sky & Telescope- there is an article in there (Gleanings for ATM) by Roger W. Sinnott & Roland Christan which addresses this very topic of color correction vs. focal lengths, and indicates that with an objective diameter increase, for a given focal length- color correction deteriorates, and also for as focal length decreases, color correction also deteriorates- explaining why it is easy to have a good planetary telescope in the smaller sizes, but- due to degrading color correction as the size of a refractor objective increases, larger refractors become increasingly unsuitable for planetary use, as- according to James G. Baker, in his paper entitled "Planetary Telescopes" (Applied Optics, Feb 1963) states that for high resolution planetary work, basically- a telescope should have better than 1/10th wave color correction between the colors. Bigger refractors, poorer color correction= poorer planetary performance.
APO telescopes- through more expensive glass and additional elements over the typical doublet, makes it possible to have improved color correction in a shorter package more suitable for travel- with a wider FOV, but the basic laws of optics still apply.
The article is accompanied by a chart by Roland Christan graphically illustrating the direct relation of focal length/objective diameter/color correction in wavelength's that make it easy to both understand and predict this relationship in both doublets and triplets. To improve this relationship requires both more specialized, highly corrected designs as well as special glass materials.
This article goes on to describe one way around these issues, describing a telescope design- dubbed the tri-space refractor- uses a smaller triplet corrector, spaced well behind the larger main doublet objective. A telescope of this design was built- the 10" f/14 refractor at the Quarry Hill Observatory of the Rockford Amateur Astronomers group. This design meets the 1/10 wavelength goal for color correction- in fact, with a color corrector added to existing designs, it is even possible to greatly improve the color correction of scopes such as the 40" Yerkes refractor.


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wh48gs
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Re: Stranger than fiction! new [Re: photonovore]
      #4078505 - 09/28/10 01:56 PM

Quote:

I used .000512 for the frequency (took that figure from Koren's webpage for convenience): thus my math was entirely correct using that figure.




You are not showing it by writing down this unspecified number. You erred by stating that Rayleigh limit corresponds to the normalized spatial frequency ~0.9, instead of correct 0.82; can you specify how is the above number related to this?

Quote:

Low contrast details are those which lie *at* the resolution limit of the telescope (Rayleigh, Dawes Abbe take your pick).




Nope. When it comes to contrast transfer, we start with what is called *inherent (object) contrast*. Transfer efficiency of this inherent contrast depends on the angular size of a detail vs. angular size of diffraction energy spread, going from ner-full contrast transfer for relatively large details, to partial contrast transfer for smaller details, low contrast transfer for details whose angular size is somewhat larger than cutoff frequency, and no contrast transfer for smaller details.

Only details with high inherent contrast are resolvable at the Rayleigh limit. The fact that they have low contrast transfer is not to be confused with their inherent contrast level. Generally, these objects are limited to near equal doubles and, theoretically, high-contrast line-like patterns resembling MTF.

A single dark line on bright background can be resolved much beyond the MTF cutoff frequency, but that is because it is governed by Edge Spread Function, instead of MTF's Line Spread Function.

Quote:

Every time a close double is seen as split at the Rayleigh limit we are seeing what you say cannot be seen--9% contrast at a *native* focal plane frequency of between 0.8 and 0.9.




I haven't said it "cannot be seen"; rather than this class of object - near equal double stars - represents minority of the total of objects that can be seen in general observing.

Quote:

What you fail to consider is that the focal plane of the telescope is *never* observed by the eye without benefit of significant magnification. And what does magnification do? Magnification *changes* the frequency characteristics of the image by uniformly lowering all frequency domains within that image.





Magnification cannot show what is not already resolved in the objective's image. That makes objective's image the primary factor. You tend, as before, to turn this upside down, advocating that it is the eye that actually plays the main role. Eye contrast sensitivity varies with detail size on the retina, and it is simply optimized by adjusting magnification according to individual preference. Not worth writing a theory about.

Quote:

The case for the achromat is much better than you paint it. As I have been trying to tell you for years now.





Never satisfied. "Much better"? I'll volunteer a guess: I'd be on the right track if saying that a 100mm f/15 achromat is on par with best 100 mm f/10 apo there is?

Quote:

BTW, my name is Mardi not Marty. So I take it you prefer Vla to Vlad as a short for Vladimir?




I don't see Marty anywhere in my posts. Vladimir is just fine, except that is sort of long to type; a good man used to call me Vla, which comes as an extra to it being almost three times shorter.

Vla


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Steve Fisher

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Re: Stranger than fiction! new [Re: wh48gs]
      #4078994 - 09/28/10 06:02 PM Attachment (56 downloads)

I have never been the kind of person who names his scopes. However in light of what I have been reading I have decided to name my 8" f/12 Bob.

From this point forward it will be known as B-O-B, Big ol' Buick


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johnnyha
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Re: Stranger than fiction! new [Re: Steve Fisher]
      #4079476 - 09/28/10 10:00 PM

Looks more like a "Robert" to me...

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Jeff Morgan
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Re: Stranger than fiction! new [Re: Steve Fisher]
      #4079502 - 09/28/10 10:12 PM

Oh good, "Big Bertha" is still available ...

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Clive Gibbons
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Re: Stranger than fiction! new [Re: Steve Fisher]
      #4079648 - 09/28/10 11:34 PM

Quote:



From this point forward it will be known as B-O-B, Big ol' Buick




Big ol' Buicks are very cool.


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photonovore
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Reged: 12/24/04

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Re: Stranger than fiction! new [Re: wh48gs]
      #4079942 - 09/29/10 04:26 AM

Quote:



You are not showing it by writing down this unspecified number. You erred by stating that Rayleigh limit corresponds to the normalized spatial frequency ~0.9, instead of correct 0.82; can you specify how is the above number related to this?




Sure. Rayleigh limit = 1/(1.22*Fw) in lines per mm, where

F = the focal ratio, and w = the wavelength of light in mm.

(12*0.000512)=0.006144*1.22= 0.0074956 1/0.0074956=133.41 lines/mm

the f/12 MTF range is 0-155 l/mm. 133 l/mm is therefore normalized to 0.8580645...for simplicity (and using accepted rounding convention) i rounded up to 0.9.

--------------

So. Although it was considerate of you to expound on simple contrast theory, you need only have at least mentioned somewhere that low inherent contrast is what you have been referring to from the getgo---that would have made for a much briefer exchange. Also i'd note that plotting low inherent contrast thresholds on a high contrast MTF is pretty unconventional. But at least I now know where that line came from, which was the question i wished answered.

To continue,

Quote:

Never satisfied. "Much better"? I'll volunteer a guess: I'd be on the right track if saying that a 100mm f/15 achromat is on par with best 100 mm f/10 apo there is?




What I am interested in is high spatial resolution observing. This class of resolution is only possible on bright high contrast targets at very high magnification: the Moon and double stars. And (seeing permitting access to the telescope's diffraction limit) occurs at roughly double the frequency domain you are focusing upon--line width of 1/2 the airy disk or the 0.8 frequency range -where there is indeed little to no difference in the resolution capability between these designs (by your own data). This is the context I have been basing my design equivalency arguments upon for years now. I don't know how popular this type of observing is compared to other types, but I do know that I am certainly not alone in my preference for it.

BTW, what on earth does that old cooldown graph of mine have to do with any of this discussion>? I knew from doing the thermal expansion coefficient calculations that tube expansion was irrelevant to this question long before Neil asked about the graph months ago. I would have been as surprised to see it in the article as I was to see my name in the acknowledgments of a piece I actually had nothing to do with in the first place. No apologies necessary or expected.

Oh, and I wouldn't tell R.N. Clark that magnification in visual observing isn't "worth writing a theory about" cause the poor guy wrote a whole book about just that...(hint: there are many telescopic topics worth investigating for which OSLO isn't the primary tool...just saying...)


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astroneil
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Re: Stranger than fiction! new [Re: photonovore]
      #4080001 - 09/29/10 06:13 AM

Hi Mardi,

I expect a few people will be surprised to see their names appearing on the paper. Most had little to do directly with the work, true enough. That said, I have learned a great deal from their collective contributions - including your own - in previous threads that I initiated.
As for the thermal stuff, I can forward you some earlier data on these curious effects. Anyone interested can PM me and I'll fire through some of the findings.

Cheers,

Neil.


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DaveTinning
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Reged: 01/03/09

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Re: Stranger than fiction! new [Re: astroneil]
      #4080192 - 09/29/10 09:29 AM Attachment (62 downloads)

Big Bertha is HERE!

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BillP
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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4080299 - 09/29/10 10:28 AM

Quote:

Getting back to an automotive analogy, promoting the long focus achro vs. shorter apos reminds me somebody extolling the virtues of a big ol' Buick compared to modern sports cars having the latest in multi-valve engines. It's a tough sell to the auto enthusiast crowd.




Nice try...but doesn't seem to be a tough sell to NASCAR, nor its 75 million fans!!! Wanna put up one of your mentioned "Park Avenue" consumer sports cars having the latest in multi-valve engines with this carbureted beauty ?? A nicely executed looong focal length achromatic refractor will bring more fans to the table than any stubby APO

Edited by BillP (09/29/10 10:34 AM)


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Clive Gibbons
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Re: Stranger than fiction! new [Re: BillP]
      #4080330 - 09/29/10 10:40 AM

How did we get NASCAR and refractor telescopes in the same posting?


OK, just checked the thread title.
Yup.


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Alan French
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Reged: 01/28/05

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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4080338 - 09/29/10 10:44 AM

NASCAR = Not Another Silly Chromatic Achromatic Refractor

Now you know.

Clear skies, Alan


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