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Clive Gibbons
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Re: Stranger than fiction! new [Re: Alan French]
      #4080384 - 09/29/10 11:03 AM





So ends another bizarre episode of "Stranger Than Fiction!"



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astroneil
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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4080406 - 09/29/10 11:15 AM

Methinks someone's sulking

Too bad


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Clive Gibbons
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Re: Stranger than fiction! new [Re: astroneil]
      #4080410 - 09/29/10 11:18 AM

Hi Neil.

No sulking.
Just kiddin' around.

Sorry for the thread hijack...


OK, back to discussing your excellent report.


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astroneil
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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4080446 - 09/29/10 11:37 AM

Clive,

No worries. My comments were not directed at you. You just got there before me, that's all.

Cheers,

Neil.


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Alan French
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Re: Stranger than fiction! new [Re: astroneil]
      #4080515 - 09/29/10 12:14 PM

Quote:

Methinks someone's sulking

Too bad




Not me. I've always liked achromats, much preferably the traditional f/15 flavor.

But I do enjoy the portability and wider field abilities of their descendants.

Clear skies, Alan


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astroneil
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Re: Stranger than fiction! new [Re: Alan French]
      #4080784 - 09/29/10 02:21 PM

Eye,

horses for courses.


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Jared
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Re: Stranger than fiction! new [Re: Olivier Biot]
      #4082182 - 09/30/10 12:59 AM

Neil and Vla:

Thanks so much for the fine article. It shows a lot of hard work and dedication.

I have no problem believing that elevating an objective lens can improve seeing. I have no problem believing that fast ED doublets require much tighter tolerances to avoid spherical aberration. I am surprised that a typical quality 100mm f/15 achromat would have a polychromatic Strehl that is virtually identical to a typical quality f/6 ED doublet, but it's credible... Based on Vla's analysis, it appears that the greater spherical aberration and spherochromatism in the (typical) fast scope and the greater chromatic aberration in the longer focus achromat are essentially offsetting each other in terms of their effect on weighted, polychromatic Strehl.

By far the most interesting section from my perspective is the information contained in figure 5. If I am reading the graph correctly (which I am trying to because I think there may be a typo or two in the descriptive text--referencing the wrong points on the graph?), if you assume a 5% focus error is made (5% of peak contrast, that is) in either scope while focusing during bad seeing, then during moments of good seeing the fast ED doublet could have a polychromatic Strehl anywhere between 0.72 and 0.78 depending on which side of focus the mistake was made. The long focus scope does not have this asymmetry, and so would consistently have a value of 0.78 Strehl during the moments of good seeing. This, coupled with the benefits from being higher off the ground, is the first plausible explanation I have seen for why a longer focus scope would be able to "beat the seeing" when compared to a similar quality, faster scope.

At the end of the article, Neil mentions that he is worried about the classical achromat of simple crown/flint being relegated to history. I am curious, though... Most of the advantages ascribed to the long focus achromat in the article appear to be benefits to "long focus" more than benefits to "achromat". That is true for the higher objective. It is true for the "finicky" requirements for figure accuracy in fast scopes. I suspect it is true for the asymmetry around the E-line when the scope is mis-focused (perhaps Vla can comment here since he has presumably run other designs through OSLO, not just the two scopes covered in the report). If anything, this article appears to be calling out for the creation of slower apochromats whose correction is centered on the peak sensitivity of mesopic vision. Would not a long focus apochromat provide the best of both worlds? At least for a visual observer who wasn't worried about portability?


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Astrojensen
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Re: Stranger than fiction! new [Re: Jared]
      #4083274 - 09/30/10 02:38 PM

Quote:

Would not a long focus apochromat provide the best of both worlds?




Hi Jared

Indeed it would! And I have such a beast, a 85mm f/19 Zeiss doublet apo, so I have first-hand experience with one. It does indeed have the image stability of a long-focus refractor coupled with the color correction and contrast of an apochromat. Neil has told me he once had the privilege to observe with a 4" f/18 Cooke triplet apochromat and the visual performance was stunning.

People who hasn't tried a truly long-focus apochromat have no idea how well they perform. Aperture for aperture, it may well be the finest telescope ever designed for visual planetary observing. Before someone else states the obvious, I'd say that I am *perfectly* well aware that for the same outlay you could get a much bigger newtonian. But aperture for aperture, I'd say that there is no finer telescope for visual lunar-planetary than a f/15 to f/20 apochromat. And they *really* come into their own for solar observing.


Clear skies!
Thomas, Denmark


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astroneil
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Re: Stranger than fiction! new [Re: Astrojensen]
      #4083700 - 09/30/10 05:33 PM

Hello Jared,

Nice to hear from you again, and thank you for your post.

Yes, I’ll try to remedy the typos this weekend, when I’ve got some spare time. Thanks for letting me know.

Now, to try to answer your question:

On a cold February evening, earlier this year, I had the immense good fortune to observe Mars through a 4” F/18 triplet apochromat designed by H. Dennis Taylor and built in the last decade of the 19th century; so it predates Thomas’ superlative Zeiss by about a decade or so. The 9ft tube was placed on a massive equatorial mount and driven by the gravitational potential energy of a falling weight.
The high power views were magisterial; rock steady in an ink black sky. Despite the absence of lens coatings, there was no sign of ghosting. It was certainly one of the most memorable views of Mars I have ever had in a telescope, period. I had to get used to seeing it the ‘wrong way’ round again – the Syrtis Major looked like it does in all the old planetary books I have. That's because I was viewing its 13 arc second orb in the traditional manner, without the use of a diagonal. It was a truly enchanting experience to say the least!
The view was clearly ‘different’ to those rendered by my 4” F/15 achromat – and it definitely had a tad more ‘punch’ to it. That said, I think Vlad’s significant discovery of the different ways the energy is distributed in both kinds of refractor might play out here too. The essay talked about double stars in particular, but it must surely have some effects on planetary images in kind.
So, to answer your question, I’m in no particular hurry to upgrade my objective to ED. For me, I see it as classic case of diminishing returns. Upgrade to apochromat, for how much gain (and financial loss)? A few measly points on the Strehl scale?
Besides, as I spoke of before, I have an irrational love of the faint violet haloes my long focus achromat imparts to bright double stars, and to my eyes, it is the achromat that delivers the more beautiful views. I know it is ‘avoiding’ the reality of the more objective view delivered by the apochromat, but that's a predilection my own eyes have acquired. It's an entrely personal perspective.
Others have commented on the limitations of long focus instruments in regard to their delivery of wide, rich field views. But surely the evolution of the eyepiece will help remedy that. We already have 110 degree FOV oculars in existence and if that trend is set to continue, then we'll have long focus refractors enjoying wider fields of view than ever before. How large a field is rich field anyway? Personally, I think it's about two angular degrees, since the majority of objects can be well framed within this area. Indeed, I can already get that with a 56mm Plossl in a 4" F/15. The future promises to give me still wider views with a new generation of super-ultra wide eyepieces should I desire them.

With best wishes,

Neil.


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Jared
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Re: Stranger than fiction! new [Re: astroneil]
      #4083744 - 09/30/10 05:51 PM

Thanks for the reply, Neil. Don't worry--I wouldn't suggest for a second that you should "upgrade" a 4" f/15 scope to which you have become attached. What I was actually asking, in a round about way, is which benefits you described in your paper are due to achromat vs. ED (apochromatic or otherwise) and which are due to long focus. It seems like most of the benefits are from the long focal ratio rather than from the materials used. The one possible exception amongst the things you highlighted in your paper is the asymmetric aspects polychromatic Strehl of the fast ED scope when mis-focused. I'm also unclear whether the concept of "encircled energy" that you and Vla described is as a result of the way achromats distribute energy, or whether it's a result of long focal ratios. I'd love to hear your thoughts.

Also, I am assuming since you didn't raise an objection that my interpretation of the meaning of figure 5 is accurate--that the same 5% mis-focus in either scope would give a consistent level of correction during moments of good seeing in the long focus achromat, but could give either very good or very not nearly as good correction in the short focus ED scope depending on which side of focus the error was made on. Is that what you were trying to get across?


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astroneil
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Re: Stranger than fiction! new [Re: Jared]
      #4083899 - 09/30/10 06:39 PM

Hi Jared,

I'm sure the grand master will chime in with more clarifications but here's my take on the situation;

As the article noted, it is the defocused nature of chromatic error in the achromat that places more energy in the central maxima for a given Strehl value and less in the rings area, especially the first bright ring. This effect would always be greater in an achromat compared to an apochromat of the same F ratio. How this plays out in more extended objects like planets, I'll leave to Vlad to expound upon.

You'll get all the relevant info here:

http://www.telescope-optics.net/polychromatic_psf.htm

Cheers,

Neil.

Edited by astroneil (10/03/10 07:42 AM)


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Loren Toole
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Loc: New Mexico USA
Re: Stranger than fiction! new [Re: astroneil]
      #4084701 - 09/30/10 11:40 PM

Neil
I also want to thank you making a great effort to address this controversy from a theoretical point of view.

My meager observational contribution from last winter did provide some motivation, I've continued to ponder different approaches that might give us some definitive evidence from the field. In July, I started to construct a chain of mirrors that could direct the extrafocal images from two scopes to a common digital camera. Still working on collimation issues but I think this will get used at some point in the future. Again, I am trying to prove the hypothesis (seeing improves with larger f/no) in the field.

The thermal issue was clearly a lingering issue for my observations, although I claimed at the time that thermals were minimized. Your paper provided real value there. Still needing to think through the apo vs achro analysis but my observation was based on two achros, no doubt of unequal spherical correction which is nicely discussed in your paper.

Again, kudos...


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astroneil
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Re: Stranger than fiction! new [Re: Loren Toole]
      #4085493 - 10/01/10 11:03 AM

Loren,

Good to hear from you!

Thank you for your comments. Much appreciated.

Cheers,

Neil.


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Steve Fisher

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Reged: 08/12/06

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Re: Stranger than fiction! new [Re: astroneil]
      #4086183 - 10/01/10 04:50 PM

Neil:

I keep looking at the view count and the replies for this "fire storm" topic. I'm thinking it might set a record.

Thanks again for your time, and your hard work.


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astroneil
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Re: Stranger than fiction! new [Re: Steve Fisher]
      #4089202 - 10/03/10 07:35 AM

Hi Steve,

Thanks for the compliments. I misspelled your surname. Corrections in the works.
Vlad worked on this like a Trojan too. I don't know about records, but I think he deserves the gold medal.

With best wishes,

Neil.


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BillP
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Re: Stranger than fiction! new [Re: Astrojensen]
      #4089561 - 10/03/10 11:59 AM

Quote:

Quote:

Would not a long focus apochromat provide the best of both worlds?



Indeed it would! And I have such a beast, a 85mm f/19 Zeiss doublet apo, so I have first-hand experience with one. It does indeed have the image stability of a long-focus refractor coupled with the color correction and contrast of an apochromat. ...




My presumption is that at the end of Neil's article by "achromat" he is referring to the traditional crown-flint air spaced doublet. In the example of your 85mm f/19 Zeiss isn't it still just a conventional crown-flint doublet? So an "acromat" of sorts even though at that focal length it's color blur should be low enough to classify it as near-apocromatic, if not fully visually apochromatic in performance.


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wh48gs
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Re: Stranger than fiction! new [Re: Jared]
      #4090551 - 10/03/10 08:28 PM

Hi Jared,

Sorry for the pause, I got sidetracked for a few days. I was in for a surprise myself, finding that long focus achromat reaches the level of a quality fast apochromat (this one is only little short of passing the requirements for a true apo for color correction at its design optimum).
What actually leads to it is that poly-Strehl increase going away from e-line focus. when I first saw the numbers I though OSLO (not me, mind you) is doing something wrong. But thinking about it for a minute, I realized it is not only logical, but unavoidable considering the lineup of color foci in an achromat.

The oother factor significant for contrast transfer is favorable energy distribution form, with relatively more energy within the central maxima.

Quote:

... I think there may be a typo or two in the descriptive text--referencing the wrong points on the graph?




Yes, I see it; where it says that "f/15 achromat would decrease to 0.79 Strehl, marked by point 3", should be point 4.

Another error is on graph 4, where the colors for achromat and apochromat are switched.

Quote:

Would not a long focus apochromat provide the best of both worlds? At least for a visual observer who wasn't worried about portability?




I would say so; with focal ratio nearly leveled, apo clearly pulls ahead (although not by as much as I thought not long ago).

Vla


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wh48gs
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Re: Stranger than fiction! new [Re: Jared]
      #4090621 - 10/03/10 09:02 PM

Quote:

The one possible exception amongst the things you highlighted in your paper is the asymmetric aspects polychromatic Strehl of the fast ED scope when mis-focused. I'm also unclear whether the concept of "encircled energy" that you and Vla described is as a result of the way achromats distribute energy, or whether it's a result of long focal ratios.





I would say that both are mainly due to the nature of color correction. An induced aberration affects all the wavelengths, but shorter more than the loger ones. In an achromat, they are both on the same side of green focus, tending to offset each other. In an apochromat they are usually on the oposite sides of green focus, which results in error imbalance producing slightly shifted best focus location (it is only a few of 100ths of a mm).

The encircled energy effect is entirely the result of error type - defocus in the case of an achromat. It is present at all focal ratios.

The 5% contrast error level is too optimistic for the conditions with significant seeing error. But even with 10% we are still within 100ths of a mm with an apo; only a few 10ths more and the defocus error in improved seeing doubles. This is not a major effect, but should be capable of producing an extra annoyance.

Vla


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wh48gs
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Re: Stranger than fiction! new [Re: photonovore]
      #4090700 - 10/03/10 09:39 PM

Quote:

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.




Thanks, but I still don't see where this 0.000512 wavelength comes from. The cutoff frequency is D/Lambda, normalized to 1. The frequency corresponding to Rayleigh limit is D/1.22Lambda, or 0.82 normalized.

Quote:

...you need only have at least mentioned somewhere that low inherent contrast is what you have been referring to from the getgo




Mea culpa. I tend to go with the logic, rather than convention. There is (inherent) contrast and contrast transfer. So when I use "contrast" alone it should be clear what it means, but it may not be from a different perspective.

Quote:

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...




Yes, let's not tell that to the poor guy. Seriously, it is of course worth knowing and investigating, but certainly not as important factor for the average observer as image quality produced by the objective.

Vla


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freestar8n
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Re: Stranger than fiction! new [Re: Olivier Biot]
      #4091299 - 10/04/10 08:02 AM

This is an interesting write up, but I'm not clear if the results are at odds with what people actually experience or not. The differences described seem pretty subtle, so there is a trade off between the bias that observers experience when they view through a telescope with certain expectations - and the limitations of applying theory to a complex situation involving optics, atmospherics, thermal equilibration, and the human visual system.

One thing missing is an empirical validation of the result - which seems possible since at the core is a claim that the Airy pattern is better defined in one case than another. If you can capture a cleaner Airy pattern in one 'scope than another, under similar viewing situations and in-situ - with tube currents and the atmosphere both playing a role - then that would close the loop between theory and measurement.

I wrote MetaGuide partly to help resolve issues like these - and to address the claims I would sometimes see of a new telescope showing, e.g. "6 clearly defined Airy rings." Refractors are much easier than SCT's to capture the Airy pattern because they tend to be smaller aperture, although the lack of secondary obstruction weakens the rings.

Here is a page describing a study of Airy pattern with altitude, showing the effect of atmospheric dispersion on the rings. In this case, it's a little known phenomenon that is contrary to what people describe when they see an Airy pattern as round and symmetrical - but it's backed both by emprical results and simulations.

So - if there are people who have experiences counter to the claims of a theoretical study - then there may be subtleties that were missed. Either way - if the root difference can be captured in the appearance of the Airy pattern itself - then direct images of it would provide experimental confirmation of the theory.

Frank


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