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astroneil
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Re: Stranger than fiction! new [Re: Mauro Da Lio]
      #5006874 - 01/08/12 09:07 AM

Thank you for summarising that article.

I have decided to reveal the details of another study I was preparing which concludes that clasical achromats serve up the most stable images of all refractors in the field.

The details revealed in "Stranger than Fiction" only applied to optical systems that have already attained thermal equilibrium with their environments. It did not however, consider instruments that are in the process of acclimating or indeed reacting to temperatures that are changing. And as I have shown in other works, refractors larger than 5 inches take much longer to acclimate in comparison to smaller instruments.

So let us now consider a long focus achromat in the process of acclimating and the factors that stabilise the image in comparison to a shorter focus refractor with ED glass.


Optical glass varies considerably in its ability to expand and contract when experiencing a temperature change. Indeed the coefficient of thermal expansion of these glasses ranges from between 4 and 19 x 10^-6/K. Dr. Juergen Schmoll, an astronomer and instrument scientist based at the Centre for Advanced Instrumentation, Netpark, Durham, UK, informed me that the thermal expansion of low dispersion glasses is significantly higher than either of those used in a classical achromat:

F2: 8.2 * 10^-6 /K

F5: 8.0 * 10^-6 /K

N-BK7: 7.1 * 10^-6 /K

N-BAK4: 6.99 * 10^-6 /K

Now compare these values to modern low dispersion glasses;

S-FPL51: 13.1 x 10^-6 /K

S-FPL53: 14.5 x 10^-6 /K

Fluorite: 18.9^-6/K

The higher the CTE, the more the glass is likely to change shape while acclimating which in turn affects the definition of the image. For example, a lens that morphs as it cools will be more difficult to focus accurately as it will introduce aberrations similar to spherical aberration into the optical train. As you can see, the new, synthetic fluorite glasses have CTEs that are ~ 1.5x to 2x higher than the old glasses, with fluorite itself exhibiting even higher values (~2.5x). This is the reason that oil spacing had been invented for lenses such as the as the legendary Zeiss APQ series (now sadly discontinued) and those more recently offered by TEC (USA) and GPU (Hungary).

This is a very significant revelation, as plate glass is well known to change shape while cooling. We can conclude, with absolute certainty, that modern low dispersion glasses will undergo significant changes in shape as they struggle to acclimate to the outside air, and indeed will continue to change shape as temperatures fall during a typical night’s observing. Curiously, the classical achromat, with its continued use of traditional glasses (crown & flint) fairs considerably better in this regard.
.
An analogous situation has already been widely discussed and acted upon by makers of Newtonian mirrors. Originally plate glass was used but was gradually replaced by Pyrex owing to its lower CTE (4 x 10^-6 /K compared to 9 x 10^-6 /K for plate glass). Curiously the difference between these CTE values is of the same magnitude as that between traditional crown flint glass and the contemporary ED glasses and fluorite.


This goes quite some way to explaining reports made by observers (yours truly included). For example, the Czech particle physicist and keen amateur astronomer, Dr. Alexander Kupco, posted his findings comparing an older, long focus (f/15) Zeiss AS 80 and a modern Stellarvue SV80S f/6 triplet apochromat.

Source: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4741850/page/0/view/collapsed/sb/5/o/all/fpart/2/vc/1

Lens thickness and cooling rates
The focal length of a simple lens can be determined from the lens maker's formula:

http://en.wikipedia.org/wiki/Lens_%28optics%29

where
f is the focal length of the lens,
n is the refractive index of the lens material,
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source, and
d is the thickness of the lens (the distance along the lens axis between the two surfaces.
It is evident from this equation that the thickness of the lens d is inversely proportional to f, the focal length. Thus, lenses with long focal length can be made (and generally are) made more thinly than their shorter focal length counterparts. The equation also shows that the focal length scales directly as the radius of curvature of the lens, implying that as R increases so too does focal length.

Of course, this is first principle optics and it can be modified to accommodate two or three lens elements, but the broad result is the same. After all, an objective is designed so that all the elements behave as one, or as closely as possible anyway.
Data supporting the Lens maker’s formula, particularly, the notion that the larger the radius of curvature of the lens the less massive it is, was difficult to come by but I did find one curious correlation for a series of 6 inch refractor objectives;
f/5: 2.65kg
f/8 : 2.6kg
f/10: 2.5kg
f/12: 2.2kg
Source: http://www.istar-optical.com/istar_017.htm

In other online discussions, I recall one amateur being astonished at the weight of the triplet objectives found in large (8-inch) triplet apochromats compared to his 9” classical Clark objective. Indeed, the mass difference was over 50%! Such an enormous mass differential will have significant results in the field, with the latter achieving thermal equilibrium considerably faster under typical observing conditions.


The ‘unwarping’ of the lens as it struggles to equilibrate with ambient air temperature manifests itself as a number of aberrations in the image, including spherical aberration and defocus. I came across this paper authored by J.H.Burge at the University of Arizona.
See: http://www.optics.arizona.edu/optomech/Fall09/Notes/dfdt.pdf
Specifically, the rate of change of focus with respect to temperature scales proportionally to F ratio and also depends on the CTE of the objective glass. Thus, in an idealised system, an F/5 optic will take 3 times longer (all other things being equal) to serve up diffraction limited images than its F/15 counterpart and thus will suffer from apparent poor seeing for longer. OSLO analysis appears to confirm this generalised idea. The F ratio connexion is also alluded to by J.B Sidgwick in his book, The Amateur Astronomer’s Handbook, (pp 191).
As a case in point, I have often taken my fine, yet inexpensive 80mm f/11 doublet achromat out from a warm indoor setting to the cool of the night air. The temperature delta can vary between 10K and 30K. My aims were to show how quickly I could resolve Sigma Cass (3 arc second separation) and the Lyra double double (~2.5 arc seconds). I was astonished to discover that, under a temperature change of 10-15K, I could begin to fully resolve both these pairs WITHIN 5 MINUTES using the 5mm orthoscopic ocular yielding 180x (seeing permitting). I have repeated the process many times, i.e. dismounting the scope, bringing it indoors to warm up before bringing it back out again. I obtained results that varied little from each other:- 4-5 minutes in each case.

Indeed, I have already described a comparison of the 80mm f/11 achromat with a fine 76mm f/6.3 doublet apochromat.
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4618465/page/22/view/collapsed/sb/5/o/all/fpart/1

As I stated in that thread, the simpler doublet achromat was considerably more efficacious at resolving tricky double stars in comparison to the short tube apochromat, especially in cold weather (large delta T between inside and outside).


I believe the connexion between focal length, lens curvature and thickness is one of the keys to unlocking the mysteries of classical achromats and it has been entirely overlooked by modern telescope makers. It is almost certainly responsible for a good part of their magic in addition to that which has already been highlighted. Apochromatic lenses, on the other hand, are usually thicker than achromats. The former are usually triplets, or doublets with a strong radius inside. You can see that when you just look into a 2-lens ED refractor – the steep curvature between the two lenses is quite striking.

According to Dr. Schmoll, this should affect cooling in ED/fluorite refractors in two ways: once, as the lenses must be thicker to accommodate the steeper radii, so that it takes longer to cool down. On the other hand, the steepness itself means that the difference between the thickest and the thinnest point of the lens is larger, giving rise to a larger dimensional difference during cool-off, and this becomes visible owing to the strong refractive power of the steep lens surfaces. During the acclimation process, refractors exhibit marked under-correction and so the process of cool down can be monitored using the star test. If the intra/extra focal images look as symmetric as possible, then you can be fairly sure the telescope has reached thermal equilibrium.

The advantages of depth of focus

The findings in my "Stranger than Fiction" essay alerted readers to the advantages of depth of focus, and its reciprocal, the defocus aberration, in combating the deleterious effects of thermal changes during an observing run. The slower ( higher f ratio) scope has a larger depth of focus over its faster (lower f ratio) counterpart and so enjoys a broader range of focus positions over which the Strehl is acceptably high when seeing error subsides. The faster scope enjoys less latitude in this capacity. One can readily see this effect by hooking up a high F ratio scope and a low F ratio instrument of the same aperture to a CCD camera. By focusing on the screen, it is easy to see that the high F ratio scope has a greater range of focus positions over which the image remains useable in comparison to its faster F ratio counterpart.
Cooling induced defocus, in an of itself, is nothing new. But how does an F/5 system differ from an F/15 instrument as it cools? To see what can happen, consider the depth of focus of the two scopes. In the absence of any spherical aberration, the diffraction limited defocus range is given by 4.13 x lamda x F^2 and that results in a defocus tolerance of +/- 0.028mm for the F/5 scope, whereas the F/15 instrument has nearly an order of magnitude more tolerance at +/- 0.247mm. Most telescope tubes are made from duralumin – an aluminium alloy with high tensile strength. Suppose you were to set up an F/5 and F/15 refractor at the same time and leave them cool off. Suppose further that after 15 minutes or so, you focus both scopes as accurately as you can and then leave to grab some coffee. When you returned a few minutes later would you notice a difference? Most certainly!
The CTE for aluminium is 2.3 x 10^-5/K, so the focus shift caused by a change in tube length for, say a 3K temperature differential would be 0.104mm for a 1.5 metre long tube, and 0.035mm for a 0.5 metre tube. This tube contraction would place the F/5 scope outside its allowed defocus latitude causing the observer to refocus. In contrast, the F/15 image would still be in focus!


Bearing in mind that it takes at least 50 minutes for even a modest 5” achromatic objective to reach the same temperature as its tube for a temperature change of just 15 K (converted from the graph sourced here http://www.cityastronomy.com/cooldown.htm ),
it is reasonable to conclude that larger apertures, (with their larger bulk glass mass) will take significantly longer to fully acclimate.



What is more, the cooling time is obviously accentuated still further by larger temperature gradients, which can often be experienced during the winter months in cold and temperate climates.

Many decades ago, the great French mirror maker, Jean Texerau, concluded that a temperature difference of less than one Kelvin within a telescope’s tube could degrade the optical wave-front enough to push the instrument outside its diffraction limit. This is not only true of reflective optics but refractive systems also.


Conclusions & Implications
Not all refractors are created equal. Air spaced triplet apochromats usually have their low dispersion element sandwiched between two other elements, which insulates the former and slows its acclimation.

Indeed, according to one source, a 10-inch air spaced triplet apochromat would be overkill:
Quote:
I have seen one 10" f/9 triplet from a commercial maker and it showed no visible color error. That is not to say, however, that the telescope gave a stellar performance. On the contrary, by introducing air gaps back into apochromatic lenses--which inevitably show strong internal curves--we bring back the old problems of the Zeiss B and Taylor triplets, namely their great sensitivity to temperature and to internal alignment. I was rather aghast to see the severe spherical aberration in the 10" lens, due to the falling temperature that night. Because of the great thickness and mass of the lens, as well as the fluoro-crown's very high coefficient of thermal expansion and its insulated position in the middle of the lens, this $40,000US extravagant objective never performed as well that night as a decent 10" Newtonian would. My impression is that the owner found this true on other nights as well and lamented that the lens could not keep up with the falling temperature. Other examples of this type of instrument also show the same problem I am told by my optical acquaintances. So while the smaller lenses of this type in the 160mm range may be fine, it would appear to me that the makers of the larger lenses have overreached the limits of what triplet apos are capable of--at least the air-spaced variety. It is a shame that oiling, the revolutionary technical advance introduced by Wolfgang Busch and Roland Christen almost 30 years ago, has been abandoned. Oiled lenses even of rather large thickness show much more moderate variation of spherical aberration during cool-down in my experience. Perhaps the large air-spaced beasts will work well on tropical islands where the diurnal temperature variation is minimal. But people who live in temperate climates may wish to be careful of large air-spaced ED lenses.”
End Quote
Source: http://rohr.aiax.de/BuschHAB-Chapter%204b.htm

Horses for climates


Doublet refractors fair much better and are apparently the instruments of choice, especially in larger formats. But, if the thermal data presented above is to be taken into consideration, then it is clearly the classical achromat that has the best thermal properties of all i.e. it has the lowest rate of change of Strehl, of all refracting telescopes and is thus best equipped to deal with changing temperatures in the field. These findings go some way to explaining why our telescopic ancestors did so well using the simple glass prescriptions of yore and why they continue to delight patient or experienced observers today. Q.E.D.

Sincerely,

Neil English

This study is dedicated to my father, John J. English (1923-2012)


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astroneil
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Re: Stranger than fiction! new [Re: astroneil]
      #5006881 - 01/08/12 09:11 AM Attachment (67 downloads)

In the high-noon of the Apochromatic Age, who will speak for the humble crown & flint refractor? Who will defend my culture?

Sincerely,

Neil English.

Edited by astroneil (01/08/12 09:38 AM)


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johninderby
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Reged: 11/10/06

Re: Stranger than fiction! new [Re: astroneil]
      #5006997 - 01/08/12 10:21 AM

Hi Neil

Interesting and obviously well researched post on the subject. Real "Cat among the pigeons" stuff. I can hardly wait for some of the replies.


I have indeed noticed how quickly my newly completed 4" f/13 refractor delivers excellent views after setting it up outside. Much quicker than my FLT98 ever did. Wonder how well a project based on the Istar 6" f/15 doublet objective would confirm your findings?

John


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astroneil
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Re: Stranger than fiction! new [Re: johninderby]
      #5007072 - 01/08/12 11:03 AM

The heart is deceitful above all things, and it is exceedingly corrupt: who can know it?
Jeremiah 17:9

“Now, what I want is, Facts. Teach these boys and girls nothing but Facts. Facts alone are wanted in life. Plant nothing else, and root out everything else. You can only form the minds of reasoning animals upon Facts; nothing else will ever be of any service to them.”
Mr. Gradgrind
From Charles Dickens’ Hard Times


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jrbarnett
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Re: Stranger than fiction! new [Re: astroneil]
      #5027739 - 01/19/12 11:04 PM

That's quite intriguing.

I'm looking forward to the whole enchilada.

My longer focal length refractors certainly appear to settle down more quickly and maintain steadier images during the session than do my faster refractors. It's nice to read some ideas as to why that is the case.

Regards,

Jim


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Sgt
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Re: Stranger than fiction! new [Re: jrbarnett]
      #5028139 - 01/20/12 07:43 AM

Neil, my condolences regarding the passing of your father.

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astroneil
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Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: Sgt]
      #5028559 - 01/20/12 12:30 PM

Sgt,

Thank you. He had a good innings.

Nothing good lasts forever.

Best Wishes,

Neil.


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ukcanuck
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Re: Stranger than fiction! new [Re: astroneil]
      #5028577 - 01/20/12 12:41 PM

Neil, I have nothing but admiration for the immense amount of time and work you have put into researching the achromat refractor. While I can build a rather nice one, I would never have been able to put into words (and with scientific justification) why they captivate people the way they do.

Thank you for taking the time, and I wish you much better times ahead for 2012.


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astroneil
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Re: Stranger than fiction! new [Re: jrbarnett]
      #5028878 - 01/20/12 03:17 PM

Quote:

That's quite intriguing.

I'm looking forward to the whole enchilada.

My longer focal length refractors certainly appear to settle down more quickly and maintain steadier images during the session than do my faster refractors. It's nice to read some ideas as to why that is the case.

Regards,

Jim




Hello Jim,

Yes, it was a rather impulsive posting, that's for sure.

Rest assured though, I will weave that work into my up-and-coming book, a chapter of which will be entitled, "What the Classical Achromat has done for us".

With best wishes,

Neil.

Edited by astroneil (01/20/12 03:26 PM)


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astroneil
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Re: Stranger than fiction! new [Re: ukcanuck]
      #5028890 - 01/20/12 03:25 PM

Quote:

Neil, I have nothing but admiration for the immense amount of time and work you have put into researching the achromat refractor. While I can build a rather nice one, I would never have been able to put into words (and with scientific justification) why they captivate people the way they do.

Thank you for taking the time, and I wish you much better times ahead for 2012.




Richard,

It was a pleasure, really. Had you not possessed the vision you had, none of this would ever have happened and we'd be none the wiser.

The War is over.

I'm free now to recount the illustrious history of the classical achromat in all its glory.

Kind regards,

Neil.


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astroneil
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Re: Stranger than fiction! new [Re: astroneil]
      #5034620 - 01/23/12 09:14 PM

Hello Jim,

Yes, it was a rather impulsive posting, that's for sure.

Rest assured though, I will weave that work into my up-and-coming book, a chapter of which will be entitled, "What the Classical Achromat has done for us".

With best wishes,

Neil.




Actually, I will re-entitle that Chapter. It shall henceforth be called "The Magic Flute," precisely as it had been at its inception, some months back.

Edited by astroneil (01/23/12 09:16 PM)


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philjay
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Reged: 12/02/09

Loc: UK Derby
Re: Stranger than fiction! new [Re: astroneil]
      #5048280 - 01/31/12 04:09 PM

Ah found you Neil.
Yet again another fascinating insight into the long focus refractors behaviour.
To we few dedicated band of individuals fighting for the long focus achromat in the APO age what you say just strikes that chord - this is stuff I have noticed subconciously over all the nights I have been using these scopes, now its all been explained its obvious.
So thats why the image is so stable, so thats why it takes less time to get a decent image than my 5" poodle

Thanks for lifting the veil and giving us insight into why these scopes do the things they are so good at, oh and thanks for providing some factual ammo

Good luck and best wishes with the new book, looking forward to it already

Phil


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Glassthrower
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Re: Stranger than fiction! new [Re: Olivier Biot]
      #5048531 - 01/31/12 06:21 PM Attachment (23 downloads)

I've always been a fan of long-focus achromats. Other factors aside, they just look COOLER than a short tube.

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astroneil
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Re: Stranger than fiction! new [Re: Glassthrower]
      #5048834 - 01/31/12 09:31 PM

Elementary my dear Phil. It just took lots of clever people and over two years of detective work to flesh out all the details. Most of all, it is confirmed by hallowed and sober experience and, to that end, I have a lot of people to thank for taking that leap of faith.

Mike, too right! They are monuments to human genius, as fundamental to our civilization as good literature is.

Alas, the sad reality is that all too often they sit in great, domed Cathedrals that are slowly crumbling away because of lack of interest and/or funding.

Many others have been dismantled for parts and will probably never see the light of night again.

Regards,

Neil.

Edited by astroneil (01/31/12 09:39 PM)


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sixela
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Re: Stranger than fiction! new [Re: jrbarnett]
      #5061790 - 02/08/12 09:38 AM

Quote:

In fact, there's a very well-known article on a vendor site claiming precisely the contrary; that focal length/ratio has no bearing on image stability.

http://www.fpi-protostar.com/bgreer/seeing.htm

"Conclusion

Telescopes of equal aperture are affected the same by atmospheric turbulence, regardless of focal ratio. "

Given that this article contradicts that one




They don't contradict each other. They make different assumptions about how well a user can focus a scope in average seeing.

Bryan Greer's article also holds for all types of scopes --given what his company sells and with what instruments he observes, I don't think peculiarities of refractor designs were first and foremost on his mind-- whereas Neil's article is specifically about short APO vs. long achromat refractors (the particular effects that make a little defocus better for the long achromat do not apply to reflectors, who don't have different focus for different colour and spherochromaticism).

In other words: if someone thinks the articles contradict each other, it's useful to reread to understand exactly why the articles seem to contradict each other, to know exactly how the conclusions are qualified (and to understand why there's no paradox).

Neil's article is indeed in itself a very good illustration of why simplified 'conclusions' can be wrong if extrapolated to scenarios where they no longer hold.


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jrbarnett
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Re: Stranger than fiction! new [Re: sixela]
      #5062054 - 02/08/12 11:56 AM

You're correct Alexis. I subsequently discussed this point with Neil. I just never bothered to correct a misstatement I made in October, 2010.

Thanks correcting that. I shudder to think what might have happened had your vigilance slipped.

- Jim


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sixela
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Re: Stranger than fiction! new [Re: jrbarnett]
      #5062777 - 02/08/12 06:57 PM

Sorry about that -- I didn't bother to read the dates on the contributions. Ah well, at least it's on the record now.

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