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Olivier Biot
Amused
*****

Reged: 04/25/05

Loc: 51°N (Belgium)
Stranger than fiction!
      #4071952 - 09/25/10 07:00 AM

Stranger than fiction!

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jrbarnett
Eyepiece Hooligan
*****

Reged: 02/28/06

Loc: Petaluma, CA
Re: Stranger than fiction! new [Re: Olivier Biot]
      #4072182 - 09/25/10 10:53 AM

Did you hear that?

What was that sound?

That was the sound of two or more decades of misconceptions shrieking as they are thrown out of the window.

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.

Exceptional and well done!

Regards,

Jim


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J_D_Metzger
Carpal Tunnel
*****

Reged: 05/13/04

Loc: Tucson, AZ
Re: Stranger than fiction! new [Re: jrbarnett]
      #4072343 - 09/25/10 12:02 PM

Quote:

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.




Anyone considering a refractor for visual use, that is...


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City Kid
Carpal Tunnel
*****

Reged: 05/06/09

Loc: Northern Indiana
Re: Stranger than fiction! new [Re: jrbarnett]
      #4072345 - 09/25/10 12:04 PM

I just finished reading this article and all I can say is wow. Extremely well written and very interesting. I will read it again to make sure I am taking in all of the information because it is packed with more info than my humble brain can digest in one reading. My current refractor is a 101mm f5.4 APO. I have been thinking about a larger refractor and only gave cosideration to another APO. This article has caused me to consider a long focus achro as a possibility.

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DaveTinning
super member


Reged: 01/03/09

Loc: Central England
Re: Stranger than fiction! new [Re: City Kid]
      #4072541 - 09/25/10 01:48 PM

I take my hat off to Neil and his valued co-researcher. Being only a layman with a passion for visual astronomy and traditional (and some new) refractors, this is a clearly well researched, well written and very informative piece of work.

It deserves serious study and constructive responses from those academics with a far deeper understanding of the science of optics than I have.

For my own part, I am content to be pleased that my "gut feeling" about long focus refractors being in some way better able to present stable, clear images compared to their very short focus new exotic counterparts, appears to be founded in good science.

Neil, you deserve some kind of public recognition for this piece of work, as does your colleague. Well done!

very best wishes
Dave


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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
Re: Stranger than fiction! new [Re: jrbarnett]
      #4072591 - 09/25/10 02:21 PM

Quote:

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.




Neil,

An exceptional effort for sure! It will stand many enjoyable readings and re-readings ... a wealth of both finidngs and implications that I'm sure will bear much fruit. Destined to be a classic reference for the community in the years to come. Hats off to Vlad for the outstanding technical support as well. Quite deserves the coining of the "Sacek Effect" !!



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7331Peg
Sirius Observer
*****

Reged: 09/01/08

Loc: North coast of Oregon
Re: Stranger than fiction! new [Re: jrbarnett]
      #4072614 - 09/25/10 02:34 PM

Quote:

Did you hear that?

What was that sound?

That was the sound of two or more decades of misconceptions shrieking as they are thrown out of the window.

Truly a modern "magnum opus" and must read for anyone owning or considering a refractor.

Exceptional and well done!

Regards,

Jim





And I thought it was a jet!

Yes, very well written and, I might add, very even-handed in it's treatment of achromats and APO's. Neil has established a very solid foundation for future research - although, I'm not sure at this point what else can be added.

In one word - Superb!


John


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Astrojensen
Post Laureate
*****

Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Stranger than fiction! new [Re: Olivier Biot]
      #4072679 - 09/25/10 03:11 PM

So it finally got published!

Excellent! A fine piece of work, Neil. Now, on to the next project!


Clear skies!
Thomas, Denmark


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Ciprian
Vendor (SkyMap)


Reged: 02/25/10

Loc: Seattle, WA
Re: Stranger than fiction! new [Re: Astrojensen]
      #4072688 - 09/25/10 03:15 PM

Great article. Well done!

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Doug76
Long Achro Junkie
*****

Reged: 12/05/07

Loc: Refractor Heaven
Re: Stranger than fiction! new [Re: Ciprian]
      #4073030 - 09/25/10 06:19 PM

I am not surprised by this!
Nice Neil.

Doug


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Achroman
member


Reged: 08/30/10

Loc: Finger Lakes Region, New York ...
Re: Stranger than fiction! new [Re: Doug76]
      #4073299 - 09/25/10 08:53 PM

Excellent article, very well written, and makes the complex science very understandable. I have always had a suspicion that this might be the case, but now there is solid science to explain it. Well done!

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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Stranger than fiction! new [Re: Olivier Biot]
      #4073378 - 09/25/10 09:40 PM

Long live long focus telescopes!

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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: Olivier Biot]
      #4073547 - 09/25/10 11:35 PM

An impressive read.

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy... and how horse drawn transportation is actually far better than motorized vehicles.

I've used long focus achromats and compared them to all manner of doublet and triplet apo optics.

IMO, long achros show less sensitivity to seeing conditions, but that's the only inherent advantage they offer.

If somebody has a vested interest in championing the alleged superiority of a particular design, then it's not surprising that they will continue to promote it. Present lots of convincing looking charts, graphs and statistics, which apparently prove their point.

Can somebody point me to the 4" f/6.3 doublet apo mentioned in this article? I'd like to *actually* compare it to my 4" f/15 achromat.

Then again, I have observed with a 4.3" f/5.95 ED doublet stopped down to 4" F/6.3 and can report that it outperforms the 4" f/15 achro in all respects, except for sensitivity to seeing.


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BillP
Postmaster
*****

Reged: 11/26/06

Loc: Vienna, VA
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4073625 - 09/26/10 12:25 AM

Quote:

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...




As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine. A carb’s pressure differential atomizes the gas a lot better than spraying fuel through an orifice. And as professional racer Myron Cottrell puts it, if you're looking to drive the family car about 25k miles a year, get EFI; if you are looking to race, get a carb. With every advantage, comes a disadvantage...but this is off topic


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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: Stranger than fiction! new [Re: BillP]
      #4073666 - 09/26/10 01:10 AM

Quote:

Quote:

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...




As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine. A carb’s pressure differential atomizes the gas a lot better than spraying fuel through an orifice. And as professional racer Myron Cottrell puts it, if you're looking to drive the family car about 25k miles a year, get EFI; if you are looking to race, get a carb. With every advantage, comes a disadvantage...but this is off topic




Moral of the story: Pick your analogies carefully, CN members have diverse and varied experience


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Bob Myler
Carpal Tunnel
*****

Reged: 05/18/06

Loc: St Louis, MO
Re: Stranger than fiction! new [Re: Jeff Morgan]
      #4074068 - 09/26/10 10:04 AM

Printing it - then laminating it.

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Dr Morbius
Post Laureate
*****

Reged: 02/06/07

Loc: ManorvilleNY-but not for long
Re: Stranger than fiction! new [Re: Bob Myler]
      #4074575 - 09/26/10 02:39 PM

Don't forget to frame it also!

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banatop
member


Reged: 05/06/07

Re: Stranger than fiction! new [Re: Bob Myler]
      #4074581 - 09/26/10 02:40 PM

Whilst I agree that long focus achromats can be excellent telescopes, one of the main reasons for buying a shorter focal length Apochromat is because of the optical quality, combined with its shorter, more manageable length. This would probably outweigh any slight optical differences for many owners. When I decided to go to a bigger refractor, I chose the 178ed because a similar achro ( probably a DG "8 f15), would have meant going round to knock on my neighbours door every time I wanted to remove the dew shield. I ask, in all humility,that this discovery be called: "the not waking up the neighbour at two in the morning effect".

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Preston Smith
The Travel Scope Guy
*****

Reged: 04/24/05

Loc: Eureka, Pa
Re: Stranger than fiction! new [Re: banatop]
      #4074701 - 09/26/10 03:41 PM Attachment (362 downloads)

The answer is simple folks.

Buy one of each.


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astroneil
Carpal Tunnel
*****

Reged: 07/28/09

Loc: res publica caledoniae
Re: Stranger than fiction! new [Re: Preston Smith]
      #4074776 - 09/26/10 04:32 PM

Hello everyone,

Thank you all for the fantastic feedback this article has so far generated. I am overwhelmed, really!

Some folk have kindly contacted me to let me know of a few little errors that have inadvertently crept into the text. Mea culpa! Hope to rectify them soon.

Jim: hope you didn't choke on your roasted chicory

Clive: I need to emphasise that the purpose of the article is to evaluate the performance of fast apos (can I call them that?) and long focus achromats primarily with respect to their perceived image stability. The intention was not to be adversarial but to point out how the respective instruments are likley to perform in the field. There are 4" F/6 doublets already in existence, so F/6.3 was not a fictional scenario in any way, shape or form.

Bill: The Sacek effect is something we all need to talk about sometime soon. It is fascinating to say the least!

Banatop:

"I ask, in all humility,that this discovery be called: "the not waking up the neighbour at two in the morning effect".


Preston: Too right! In fact, I'm out with my TV 76 F/6.3 doublet Apo (oooooh) tonight. Conditions are very calm, cool and stable.

Kind regards,

Neil.

Edited by astroneil (09/26/10 04:42 PM)


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: BillP]
      #4074784 - 09/26/10 04:36 PM

Quote:

Quote:

This reminds me of folks who swear ol' carbureted engines are somehow superior to the modern fuel-injected powerplants we now enjoy...




As profession drag racer Warren Johnson characterized it, properly tuned, carburetors make more peak power than EFI in a Pro Stock engine.




When it comes to image quality, talking peak HP isn't a good analogy. If folks want maximum visual "horse power", they should get a huge Newtonian.

But, that's veering ever further off topic.

Reviewing my comments about Neil's article I think they're rather blunt and/or unkind.
My apologies for that.

Neil has done a great job producing his report.
He's a masterful writer.
It obviously involved a lot of research on his part, with the assistance of Vlad.
Lots of interesting questions were raised and explored.

It just doesn't jibe with my experiences, which is why I have a problem with some of the findings.
Mathematical modeling, graphs and spot diagrams sometimes don't accurately describe what's going on. Or, they do but context is misinterpreted.

Case in point--
Some enthusiasts of a small, very well made Maksutov-Cass telescope are known to extoll it's virtues with great exuberance. That instrument's central obstruction has been argued (along with proof derived from optical modeling) to increase the scope's resolving power, compared to an unobstructed system. However, in real-world usage, it's obstructed optic doesn't perceptibly improve resolution and in the vast majority of situations, image quality is degraded. But that never seems to inhibit the diehard enthusiasts from revisiting the "central obstruction is good" viewpoint.

So, around and around we go.

Best wishes to Neil.
He is a great ambassador of the long focus achromat.


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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4074788 - 09/26/10 04:37 PM

Quote:

If somebody has a vested interest in championing the alleged superiority of a particular design, then it's not surprising that they will continue to promote it ad nauseum. Present lots of convincing looking charts, graphs and statistics, which apparently prove their point.




Another conspiracy theory, eh? The thing is, all the optics related graphs are reproducible with raytrace. Oh, well - didn't think of it before - OSLO had to be rigged too, just to support long focus achromat. Or were it those who created diffraction calculation itself? Oh, man...


Quote:

Can somebody point me to the 4" f/6.3 doublet apo mentioned in this article? I'd like to *actually* compare it to my 4" f/15 achromat.




Here are the specs:

100mm f/6.3 FPL53/KZFSN2
SRF RADIUS THICKNESS APERTURE RADIUS GLASS SPE NOTE
OBJ -- 1.3160e+20 1.1485e+18 AIR

AST 350.056000 13.160000 50.008000 AS O_S-FPL53 C
2 -184.230000 0.130000 49.513451 S AIR

3 -188.714400 6.580000 49.491049 S KZFSN2 C
4 -605.360000 618.462178 49.384613 S AIR

IMS -250.040000 -0.046529 5.478249 S

It is a top apo doublet, comparable to TMB 130*BLEEP* in color correction (when scaled up to 130mm and f/7). It does not
quite satisfy apo requierement, but it's close. The achromat is standard Fraunhofer.

Quote:

Then again, I have observed with a 4.3" f/5.95 ED doublet stopped down to 4" F/6.3 and can report that it outperforms the 4" f/15 achro in all respects, except for sensitivity to seeing.




Which proves that that particular apo was better than that particular achromat. The article considers very specific case of an apo with moderate amount of e-line correction error (0.050 wave RMS, equivalent of 1/6 wave p-v wavefront error of lower order spherical), and an achromat with an effective 1/8 wave p-v e-line error. That seems very realistic. You may have gotten luckier with your apo, and/or unlucky with the achromat...

Vla


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4074836 - 09/26/10 04:59 PM

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars.
This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo.
Only trouble was, the modeling didn't match what I observed.
So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors.
There was no conspiracy at work with the software he was using.
There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.


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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4074886 - 09/26/10 05:27 PM

Quote:

A few years ago, I reported to another web forum my observing experiences with a 4" f/9 ED doublet and how it displayed no blue/violet haloing around bright stars.
This prompted a very well respected apo lens designer and manufacturer to suggest that the c.a. was present, but for some reason I didn't notice it. He posted spot diagrams for a 4" f/9 ED doublet which, sure enough, showed a halo.
Only trouble was, the modeling didn't match what I observed.
So, it was suggested that my vision had a problem seeing defocused light at shorter wavelengths. That didn't jibe either, since I had no difficulty observing blue/violet defocus in other refractors.
There was no conspiracy at work with the software he was using.
There was an incorrect assumption made about the lens and it's color correction. Another case of the hypothetical not matching up with reality.




First off, switching the subject usually does not help
resolve an issue. And so doesn't not understanding (or misrepresenting?) optics and raytrace. The thing is, we do not look at the spot diagrams, and they have nothing to do with diffraction calculation.

Due to its nature (i.e. less exponential spread of energy toward farther off non-optimized wavelengths), sperochromatism does apear less colorful than secondary spectrum *at identical level of chromatic error*. Roland Christen goes as far to say that sperochrimatism is "white" (I would assume it refers to well balanced spherochromatism).

The point is, one cannot accurately judge the level of chromatic error based on the magnitude of color visible in an apo vs. achromat.

Vla

Edited by wh48gs (09/26/10 05:31 PM)


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4074965 - 09/26/10 06:08 PM

Quote:

You may have gotten luckier with your apo, and/or unlucky with the achromat...

Vla




I usually try to optimize the system, when it comes to ED doublets and triplets.
In the case of my apos, employing a prism diagonal (either BK7 or BAK4, depending on the objective lens characteristics) has often resulted in improved chromatic and spherical correction.
Perhaps that accounts for what I've experienced.

Question for Vla--
When formulating the 4" f/6.3 apo doublet, did you optimize correction for the yellow/green or blue/green?


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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4075242 - 09/26/10 08:52 PM

Quote:

I usually try to optimize the system, when it comes to ED doublets and triplets.
In the case of my apos, employing a prism diagonal (either BK7 or BAK4, depending on the objective lens characteristics) has often resulted in improved chromatic and spherical correction.




If apo correction is imperfect, a prism can be beneficial, or make it worse. With 1" prism (BK7) this particular 100mm f/6.3 apo went from 0.92 to 0.95 polychromatic Strehl. The prism improved both, red and blue, but worsened violet and deep red (the latter nearly don't matter for visual, so the overall visual Strehl went up; if it was photosensitivity, the two probably would mainly offset each other). That indicated that the color curves in the apo were slightly more separated than the optimum; increasing front radius by 3mm and increasing lens spacing by 0.06mm, the Strehl went to 0.943. The 2-3 ratio points differential is bellow the level of perception (comparable to 0.97-0.98 and 1 Strehl), but it is always better to be up than down.

If the apo went onto the graph with 0.943 Strehl, it would be down to 0.85, not 0.83 after adding 0.050 wave RMS e-line error. Not quite as the achromat at 0.86 Strehl, but practically identical.

However, adding 2" prism lowered the Strehl to 0.906; the curves got too close, and best foci of other colors were past the e-line focus plane.

So with one same apo the two different prism sizes had the opposite effect.

The other part of the equation is the achromat. While it is very hard to produce sufficient deviations in the radii or spacing to result in a significant error (frankly , it would require very sloppy fabrication/assembly), some other issues, sort of below the radar, can affect its quality: glass homogeneity, deviations of actual glass from design indici, surface figure integrity and smoothness, wedge control. As little as 0.01 degree wedge (little over 1/60 mm) in the 100mm f/15 achromat lowers its poly-Strehl by 3 ratio points.

Assuming that you've got this particular f/6.3 apo, and used it with 1" prism, you would have a 0.95 Strehl instrument. You could consider such scenario very lucky indeed. And being less lucky with the achromat could have it down to 0.80 Strehl or so. That is a possibility. In the article, an attempt was made to present what would be a reasonably likely average scenario with both instruments with high quality optics. Somewhat greater e-line error in the apo is simply to expect considering *much* tighter tolerances for the radii values and lens separation (in addition to other quality factors being as tight, or more so than in the achromat).

Quote:

When formulating the 4" f/6.3 apo doublet, did you optimize correction for the yellow/green or blue/green?




They are both (apo and achromat) optimized for minimum error in the e-line, with the red and blue ends roughly balanced. That makes them comparable.

Vla


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4075514 - 09/26/10 11:13 PM

Thanks for the information, Vla.
Much appreciated.


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roscoe
curmudgeon
*****

Reged: 02/04/09

Loc: NW Mass, inches from VT
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4075879 - 09/27/10 08:13 AM

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............

I've always been a long skinny tube guy, (although lately my 'main' scope is a short f/8), so I'm delighted to read Neil and Vladimir's work, and I'm sure
I'll read it a couple more times to really understand it all.
I feel a bit bad for all you folks with a gazillion dollars tied up in finely crafted APO's, beautiful 2-speed focusers, and gorgeous hand-grenade EP's, when it turns out that a couple pieces of glass and a simple focuser does indeed get the job done........... but then again, those APO's actually fit in an auto, and a decent mount for a 12-foot-long yard cannon might just cost as much as a decent used car.........
I am a bit dissapointed to find out, though, that my beloved skinny-tubes don't win out in the faint fuzzy department..........ah well.......
Thanks, Neil and Vlad, for your impressive research!
Russ


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Alan French
Night Owl
*****

Reged: 01/28/05

Loc: Upstate NY
Re: Stranger than fiction! new [Re: roscoe]
      #4075910 - 09/27/10 08:37 AM

Quote:

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............
(snip)

Russ




Russ,

I think having an objective well above the ground and well away from the observer is a significant advantage. It may actually be the single biggest advantage.

Clear skies, Alan


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Clive Gibbons
Mostly Harmless
*****

Reged: 05/26/05

Loc: Oort Cloud
Re: Stranger than fiction! new [Re: wh48gs]
      #4075913 - 09/27/10 08:40 AM

What puzzles me somewhat is the polychromatic Strehl value of 0.9 for a 4" f/15 achromat.
That seems remarkably high.
IIRC, a value closer to 0.81 has been mentioned in the past for an achro of that size and f/ratio.
In a posting earlier this year, Vla refers to a 4" f/30 achro as having a polychromatic Strehl at just a tick over 0.9, so getting that from a 4" f/15 is intriguing.

Vla's website elaborates about it:
http://www.telescope-optics.net/polychromatic_psf.htm

"The numbers for achromats may look somewhat optimistic, but that is what the raytracing results imply."

Indeed.

P.s.,
more rereading and trying to understand the "Sacek Effect"...
it's mentioned that this results in a "slightly enlarged central maxima". Does this mean that the Airy disk of an achromat is larger than what's normally calculated for a scope of a given aperture?

Edited by Clive Gibbons (09/27/10 10:52 AM)


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Doug76
Long Achro Junkie
*****

Reged: 12/05/07

Loc: Refractor Heaven
Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4076306 - 09/27/10 12:45 PM

I think all the emphasis on no CA in an apo vs some CA in a long achro ignores a very important point.
CA isn't the only noticeable aberration, and some are worse than CA, and most of those other aberrations are much more prevalent in a short scope.
I've said it before, I'll say it again.
For me, a long achro, always, for viewing. Apo's are best used for imaging.
Just my opinion, but it's the opinion I trust most.


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wh48gs
Carpal Tunnel
*****

Reged: 03/02/07

Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4076471 - 09/27/10 02:12 PM

Quote:

What puzzles me somewhat is the polychromatic Strehl value of 0.9 for a 4" f/15 achromat.
That seems remarkably high.
IIRC, a value closer to 0.81 has been mentioned in the past for an achro of that size and f/ratio.
In a posting earlier this year, Vla refers to a 4" f/30 achro as having a polychromatic Strehl at just a tick over 0.9, so getting that from a 4" f/15 is intriguing.




The 0.91 polychromatic Strehl is for the e-line focus, for a e-line optimized 100mm f/30 achromat.

E-line Strehl for a 100mm f/15 achromat is 0.82. On the graph in the article, this is the Strehl value of the solid red line at the zero defocus line.

What I found out when looking into achromats defocus is that its poly-Strehl improves going from e-line toward red/blue focus. This is actually very logical, and almost obvious consequence of all the wavelengths focusing farther than e-line, including those to which the eye is highly sensitive. The highest polychromatic Strehl for the achromat, 0.86, is shifted 0.13mm toward the red/blue focus (which is 0.84mm from the e-line focus). That is the top value on the solid red line.


The effect of slightly enlarged central maxima - another phenomenon directly related to the chromatic defocus - is that peak diffraction intensity does not as closely match (relative) encircled energy within the central maxima as it does with other common monochromatic aberrations of similar magnitude. This is something I knew before, and was described on my site for a couple of years. With spherical aberration, for instance, the encircled energy is nearly identical in value to the peak intensity: 1/4 wave p-v will result on 0.80 Strehl, and the central maxima is encircling 0.80 (or 80%) of the energy that is there with zero aberrations.

With the same Strehl level resulting from defocus (1/3.89 wave p-v), the central maxima encircles nearly 0.87 of the energy that is there with aberrations absent. 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. The price it pays for it in larger central maxima is negligible: it is 3.6% larger, linearly, corresponding to the central maxima size of 96.4mm aperture. Theoretically, it would result in slightly inferior resolution of near-equal doubles, but it hardly has any practical significance. As for the contrast/resolution of details smaller than the Airy disc size, it affects only bright contrasty objects, since this detail size range is beyond the limit of resolution for low contrast objects.

In short, at identical central diffraction intensity (Strehl), defocus error will result in more energy in the central maxima, and less in the rings. For the majority of objects, it will act as if it has higher effective Strehl. This effective Strehl for the achromat is plotted as the top red dashed line on the graph. Its highest spot is at 0.91 polychromatic Strehl. It is obtained by applying empirical approximation that fits the raytrace data to the best polychromatic Strehl (0.86)(F+3)/(F+2), where F is the achromat's focal ratio.

The 0.91 Strehl for the 100mm f/30 is its e-line Strehl. I didn't check its best Strehl value, but would expect it to be around 0.95. And its effective Strehl, due to higher encircled energy, is still somewhat higher.

Vla


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Alan French
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Re: Stranger than fiction! new [Re: wh48gs]
      #4076501 - 09/27/10 02:26 PM

One thing I've often wondered about - and this might have some bearing here - is what the eye perceives as "best focus." In relation to the chromatic focal variation, where do we set the focus? Does it vary among people? I would guess it would be somewhat toward the blue/red focus and inside the green focus, but it would be interesting to see some experiments along this line.

Clear skies, Alan


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Re: Stranger than fiction! new [Re: wh48gs]
      #4076512 - 09/27/10 02:30 PM

Thanks for the detailed explanation, Vlad.



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Re: Stranger than fiction! new [Re: astroneil]
      #4076926 - 09/27/10 05:54 PM

Well, now that you mention it Neil, I do have a bone to pick with your article.

Specifically, I note that it has not been peer reviewed by your mum and a cadre of garden gnomes.



- Jim


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Alan French
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Re: Stranger than fiction! new [Re: jrbarnett]
      #4076951 - 09/27/10 06:08 PM

Yeah, but those garden gnomes move so slowly you can't tell they're doing anything.

Clear skies, Alan


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astroneil
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Re: Stranger than fiction! new [Re: jrbarnett]
      #4076985 - 09/27/10 06:27 PM

Quote:

Well, now that you mention it Neil, I do have a bone to pick with your article.

Specifically, I note that it has not been peer reviewed by your mum and a cadre of garden gnomes.



- Jim




Unfortunately,
The gnomes were away at a conference and my mum rejected it outright, as there wasn't enough math in it!

Regards,

Neil.

Edited by astroneil (09/27/10 06:31 PM)


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Re: Stranger than fiction! new [Re: roscoe]
      #4077176 - 09/27/10 08:22 PM Attachment (57 downloads)

Quote:

Actually, the REAL reason that those long-focus refractors provide such nice images is that the objectives are half-way to space..............





The Federal Aviation Administration has requested that I put strobe lights on my dewcap. And of course, I can continue my observing programme (in honor of Neil) during rainy weather.


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Re: Stranger than fiction! new [Re: Alan French]
      #4077180 - 09/27/10 08:25 PM

Quote:

One thing I've often wondered about - and this might have some bearing here - is what the eye perceives as "best focus." In relation to the chromatic focal variation, where do we set the focus? Does it vary among people? I would guess it would be somewhat toward the blue/red focus and inside the green focus, but it would be interesting to see some experiments along this line.

Clear skies, Alan




It could also be that focus varies with age and eye accommodation. Eyepieces that are "parfocal" don't seem to be that way for me anymore. Maybe I am picky about focus.

Yes, I prefer that explanation.


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Re: Stranger than fiction! new [Re: Preston Smith]
      #4077247 - 09/27/10 08:55 PM

Quote:

The answer is simple folks.

Buy one of each.




Overcome. Adapt. Well done Preston!


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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Re: Stranger than fiction! new [Re: wh48gs]
      #4091919 - 10/04/10 12:46 PM

Quote:



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.

Vla




0.000512 is the light frequency I happened to use in the formula. If you use 0.000538 for the frequency, the results would agree with 0.82 normalized. (the average between 505nm and 555nm happens to be 530.5nm, so it appears that location of the Rayleigh limit at 0.82 normalized is predicted upon some sort of rather arbitrary mesoptic peak--as there is no consensus upon peak sensitivity for mesoptic vision.... )


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Astrojensen
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Re: Stranger than fiction! new [Re: BillP]
      #4092224 - 10/04/10 02:36 PM

Quote:

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.




Hi Bill

No, my 85/1600mm Zeiss is a true apochromat. According to the 1916 Zeiss "Astro 30" main catalogue, it has three times better color correction than a conventional air-spaced achromat of the same focal length. The f/ratio has to be kept at f/17 or longer, due to the strong internal curvatures of the lens. Mine is even longer than that, because it was designed as a 90mm, but due to a fracture in the edge of one of the lenses, it was ground down to a 85mm (this is at least the guess of the previous owner, who was extremely knowledgeable about the early history of Zeiss). There is still a small clamshell in it.

The color correction is really stunning. Visually, there is no hint of any color in or out of focus. It can be taken to ridiculous powers. I have used 800x on the sun, using my projection box, to observe the umbrae of large sunspots.

Here is a raw, unprocessed video of how it performs on the moon. Notice that it was taken with a color webcam.

http://www.youtube.com/watch?v=f9MKUE0WBis

It is truly a shame that no one is making telescopes such as this anymore. People that claim their f/7 apos to be just as good simply have not had a chance to observe with my 85mm f/19, so they don't know what they're talking about. Really. Stopped to 75mm it completely and utterly destroyed a 76mm TeleVue apo on Mars, Saturn and the moon.


Clear skies!
Thomas, Denmark


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Clive Gibbons
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Re: Stranger than fiction! new [Re: Astrojensen]
      #4092298 - 10/04/10 03:11 PM

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.


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jrbarnett
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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4092386 - 10/04/10 03:56 PM

"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."

True, but it's a pretty easy sell to the other 90% who have double-knit butts of large proportion and aging frames that love the dyna-ride float-o-matic suspension. There's no doubt that the boat...er...Buick is a better tool for certain automotive jobs than the Porsche 911. Coincidentally, it just so happens that the long focus achromat is a better tool for certain tasks than a fast achromat.

- Jim


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Re: Stranger than fiction! new [Re: Clive Gibbons]
      #4092641 - 10/04/10 05:25 PM

Quote:

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.





I guess there's a number of ways to do that;

Option A: design an ED F/15 objective (FPL 51 would be nice ) http://www.aokswiss.ch/ scroll down page and click on red long focus ED.
Option B: design a chromacorr which corrects colour without introducing spherical aberration
Option C: Use a conventional achromatic doublet and fold the design for something like a 4-6" F/20 or some such. Not true Apo but near enough. It wouldn't look as nice though IMO

Neil.

Edited by astroneil (10/04/10 05:46 PM)


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Re: Stranger than fiction! new [Re: astroneil]
      #4092754 - 10/04/10 06:11 PM

Quote:

ist eines der besten Planetenteleskope seiner Grössenklasse.




(quote from that AOK ad- for the Schief)

Ah, HA! Like I said... Schief's a great planetery scope!!
All right.... I'll shut up now!
Wes


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Re: Stranger than fiction! new [Re: astroneil]
      #4093416 - 10/04/10 11:24 PM

Quote:

Quote:

Wow, that's a super image Thomas.
The Moon's lack of yellowish or greenish cast also indicates how well focused the colors are.

I agree.
Long focus apos (f/15 or longer) are exceptional performers.
While a long achro is really good, an f/15 (or slower) ED doublet or triplet would be phenomenal.
Hopefully a "niche" vendor will run with the idea.





I guess there's a number of ways to do that;

Option A: design an ED F/15 objective (FPL 51 would be nice ) http://www.aokswiss.ch/ scroll down page and click on red long focus ED.
Option B: design a chromacorr which corrects colour without introducing spherical aberration
Option C: Use a conventional achromatic doublet and fold the design for something like a 4-6" F/20 or some such. Not true Apo but near enough. It wouldn't look as nice though IMO

Neil.




Option D : Make a Schupmann refractor which has PERFECT color correction out of two piece of cheap BK-7 and enjoy one the best views possible. No false color, polychromatic Strehl of 0.99+, no coma, no astigmatism, no obstruction and it can easily corrector for atmospheric dispersion.

- Dave


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jrbarnett
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Re: Stranger than fiction! new [Re: DAVIDG]
      #4093548 - 10/05/10 12:29 AM

If the Schupmann design is so simple and wonderful, why in its 100+ year history has it never caught on commercially? Markets and competition are usually pretty efficient. If the Schupmann was offered competitive advantage over other designs, why are they so rare? There must be a catch. What is it?

- Jim


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Re: Stranger than fiction! new [Re: jrbarnett]
      #4093849 - 10/05/10 07:38 AM

I think that unusual telescope designs are a niche item even amongst a group of dedicated, die-hard astro enthuasiasts like we have here on C/N. It is fairly well accepted that off-axis telescopes offer the best of both worlds- better color correction with no obstruction. Yet Orion offered a 3.4" off axis scope- it failed commercially... For its aperture, it was a fairly large scope. DGM offers them, but I doubt if they sell a lot... AOK in Europe still offers Schief's, and they have a European following, other than a few enthuasiasts here, the more unusual designs just don't get their due. Not enough people are willing to accept them, even though they offer definite advantages. While your statement "Markets and competition are usually pretty efficient" is true, customers are pretty traditional. I think that even things such as appearance can put people off... if it doesn't look like a telescope, they tend to steer clear of it. And if people steer clear of it, manufactures won't stay in the market long if they can't sell them. The car market has had many revolutionary designs that offered many advantages, but for one reason or another, they failed. The Schief's and the Schupmann's definitely fall into the "unusual appearance" telescope category.
Just my humble opinion??
Wes

Edited by Wes James (10/05/10 07:45 AM)


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Re: Stranger than fiction! new [Re: jrbarnett]
      #4094027 - 10/05/10 09:43 AM

Quote:

If the Schupmann design is so simple and wonderful, why in its 100+ year history has it never caught on commercially? Markets and competition are usually pretty efficient. If the Schupmann was offered competitive advantage over other designs, why are they so rare? There must be a catch. What is it?

- Jim




If you read Jim Daley's book it gets into some of this. The largest Schupmann is 40" and used for solar research. http://en.wikipedia.org/wiki/Swedish_Solar_Telescope It has provides some of the very best ground based solar images taken so far. There is also a 13" at Stellafane. Having built three of them they are not difficult to make and actually much easier then achromat or APO. I'll be glad to provide spot diagrams that show how extremely well these telescopes are corrected. World Class ATM's like Gerry Logan, who has made just about every optical design known has stated that his Schupmanns give the best image of all his scopes. http://www.siderealtechnology.com/atm/Logan_Gerry/
All I can say is both theory and practice both bare out the fact that Schupmanns provide extremely good images with no chromatic abberation and they are made for the inexpensive optical glass.

- Dave


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Re: Stranger than fiction! new [Re: DAVIDG]
      #4094049 - 10/05/10 09:59 AM

You can also get a modest reduction (~18%) in secondary spectrum using Bak2-F1 glass at little additional cost to standard achromat optical glasses.

Some interesting reading here:

http://www.rfroyce.com/refractor%20spots.htm

Cheers,

Neil.


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Re: Stranger than fiction! new [Re: astroneil]
      #4094138 - 10/05/10 10:47 AM

Long focal length achromats have better color correction than short ones.

Long achromats have greater tolerances for mispacing on the lenses than short focal length triplet apochromatic refractors.

Long focal length achromatic refractors can outperform short apochromatic triplets in some situations.

Is this really news?

Unfortunately, long focal length achromatic refractors that have any aperture require large, heavy mounts that are very difficult to transport. And if they are not so mounted, they shimmy in the wind like the branches of a willow tree.
For people who live in the world of wind and weight, the 127mm f/7.5 triplet may outperform the 127mm f/15 achromat simply because it is portable and more immune to conditions of wind.

Having owned the long skinny scope before (a 4" f/15 Unitron), I doubt I will ever be tempted to go back to that long skinny scope on the high mount again. It was only stable when there was no wind, which was 5 days a year where I lived when I owned that scope.


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Re: Stranger than fiction! new [Re: Starman1]
      #4094147 - 10/05/10 10:53 AM



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astroneil
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Re: Stranger than fiction! new [Re: Starman1]
      #4094181 - 10/05/10 11:08 AM

Quote:

Long focal length achromats have better color correction than short ones.

Long achromats have greater tolerances for mispacing on the lenses than short focal length triplet apochromatic refractors.

Long focal length achromatic refractors can outperform short apochromatic triplets in some situations.

Is this really news?

Unquote:

Yes it is!:)

Quote:

Unfortunately, long focal length achromatic refractors that have any aperture require large, heavy mounts that are very difficult to transport. And if they are not so mounted, they shimmy in the wind like the branches of a willow tree.
For people who live in the world of wind and weight, the 127mm f/7.5 triplet may outperform the 127mm f/15 achromat simply because it is portable and more immune to conditions of wind.
Unquote

Not necessarily,

Lookie here:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4063058/page/3/view/collapsed/sb/5/o/all/fpart/1

As you can see, they needn't be heavy or expensive!



Cheers,

Neil.



Edited by astroneil (10/05/10 12:46 PM)

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jrbarnett
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Re: Stranger than fiction! new [Re: DAVIDG]
      #4094268 - 10/05/10 11:55 AM

Thanks Dave. I caught the performance superiority claims from the initial post. What's missing is that if there's no "catch" (i.e., no downside with the design) then the lack of commercial examples makes no sense. Folks that build telescopes to make a living tend to be very quick to adopt better designs, methods and materials, and yet none of them have opted to make commercial Schupmanns. Why?

"Beats me" isn't a very satisfying (or credible) answer. Think of it this way. Back when Celestron Pacific was in its infancy and looking for a great design to produce commercially, and selected the Schmidt-Cassegrain, it faced several tough design problems foremost of which was how to cost-effectively figure the complex correctors. By investing time, resources and ingenuity into cracking the code for mass-production of correctors, Celestron was able to profitably mass produce the design (unchallenged) for decades.

There's got to be some very apparent, very practical reasons no commercial entrant has bothered with the Schupmann. I'm wondering what they are. I understand thet there are variations on the design, so not every comment regarding the design will apply to every derivative of the design, but here are some of the issues one source cites for the design:

"The final step in designing Schupmann is making the image accessible by tilting all three elements. Small concave mirror typically needs 4 to 5 degree tilt, catadioptric element about half as much, and front lens a fraction of it. Tilting the mirror doesn't induce appreciable aberrations, but tilting the CE does. This is why the front lens needs to be tilted as well, in the plane perpendicular to CE's tilt, in order to offset astigmatism induced by tilting the CE. If done correctly, tilting doesn't appreciably affect the mid-field aberration level. It does, however, increase off-axis astigmatism, and produces asymmetric, tilted image field typical for tilted systems. It also presents additional difficulty in achieving and maintaining proper system alignment ."

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

Could those highlighted issues partially explain the lack of commercial examples?

Regards,

Jim


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Re: Stranger than fiction! new [Re: Starman1]
      #4094301 - 10/05/10 12:07 PM

Sonoma county is very windy.

I've never had any wind-related problems with my 4" f/14.4 that weren't also problems for any of my other scopes. The scope actually has a smaller wind profile than a solid tube Dob, truss Dob with a shroud or large SCT. If well-mounted (in this case an Atlas is more than enough for the 10# 6-foot-long OTA, provided that a long dovetail is used and a second long dovetail joins the ring tops), a 4" f/15 is really not much different to mount or use than any other scope that could ride on the same mount.

I think a 4-inch long focus refractor isn't a bad choice scope for die-hard double star hunters. On the other hand, if your observing diet is more diverse, then it likely wouldn't be a satisfying "only scope".

Now if we're talking bigger such as a 5" f/15...yikes. That means carrying the OTA on a roof rack and not in the car and a G-11 class mount. 6" f/15? I wouldn't put it on less than an A-P 900 or better yet A-P 1200. Without folded designs or permanent installations, I think the audience for big long focus achromats (larger than 4") is pretty small.

Regards,

Jim


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Re: Stranger than fiction! new [Re: Starman1]
      #4094644 - 10/05/10 02:31 PM

Quote:

Is this really news?





This was basically my question. If the result is surprising or disagrees with anyone's experience - then theory alone has to be handled carefully. But if no one disagrees - and it's just a matter of the small, newer expensive thing not being as good as the long, simpler, old design thing - then it's more a matter of theory supporting the evidence. Separately there are issues of personal preference, usability, and ungainliness - but it sounds like the relative visual performance isn't a controversy.

Frank


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Re: Stranger than fiction! new [Re: jrbarnett]
      #4094703 - 10/05/10 02:53 PM Attachment (41 downloads)

Jim,
I have seen that quote, but I personally haven't had any problems keeping any of my Schupmanns aligned and they have been more robust then my Newtonians. If you look at the spot diagrams of many of them, you will see that the amount of off axis abberation are BELOW diffraction limited. My own 4" f/12 has over 1/2 degree field of view that is diffraction limited. If you compare this to many other designs, they are far worse.
I can't get explain why Schupmanns have not caught on commerically. The data shows that they are excellent.
One the other hand there are commerical optical design that fall far short, yet they are made.

- Dave


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Re: Stranger than fiction! new [Re: DAVIDG]
      #4094877 - 10/05/10 04:16 PM

The article has now been updated to correct for the aforementioned errors.

Cheers,

Neil.


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Re: Stranger than fiction! new [Re: freestar8n]
      #4094884 - 10/05/10 04:19 PM

That's just it. The premise of the report runs counter to the majoritarian position on the question of whether a long focus achromat produces steadier images than a faster apochromat. The majority of posters on multiple Refractor forum threads indicated that they believed there was no correlation between focal length and image stability. 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 and is also better documented and reasoned than that one, in my opinion it's timely, informative, valuable and important reading for anyone interested in the correlation between focal ratio and image stability in refractors.

It's also one of the best written, most transparently researched and documented articles on CN. In that regard it sets a new, higher standard for such articles.

Regards,

Jim


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freestar8n
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Re: Stranger than fiction! new [Re: jrbarnett]
      #4095041 - 10/05/10 05:36 PM

OK - I didn't know about that link and its theoretical assessment - but I'm focused on what actual humans claim about any differences they observe with real telescopes. If short apo people concede that their performance is inferior to long achro's - and long achro people also agree - then their conclusions are at odds with the theory at that link and the theory is suspect.

My point is that the only empirical data at hand come from human assessments at the eyepiece. Whenever you have a theoretical description of a very complex situation - it may be missing key elements that alter the result. I am not crazy about relying too much on MTF's, and I consider the way a human studies a scene in the presence of turbulence to be a very complex and dynamic perceptual process. So - if there are observations or opinions at odds with theory - they should be considered.

Frank


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Re: Stranger than fiction! new [Re: freestar8n]
      #4095966 - 10/06/10 02:02 AM

My take on the article after two readings was that the advantages of the achro are only apparent when the ED doublet APO is either (A) slightly out of focus, or (B) badly corrected.

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Re: Stranger than fiction! new [Re: astroneil]
      #4099776 - 10/07/10 07:04 PM

Thanks for the thoughtful article....though I spend most of my time in the far scotopic world observing dark nebulae with my refractors...binoculars, short focus achromats, and one lone apo.




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Re: Stranger than fiction! new [Re: johnnyha]
      #4100229 - 10/07/10 11:46 PM

Aaaaaaaaaauuuuuuuuuughhhhhhhhh!

No, no, no, no!

What the article says is that even a nigh on perfect f/6 apochromat will struggle to put up as steady an image under average seeing conditions as a mediocre long focus achromat, *and* one must nearly try deliberately to make a poor long focus optic.

Sell your Taks...quick...before news of this article spreads.

- Jim


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johnnyha
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Re: Stranger than fiction! new [Re: jrbarnett]
      #4100517 - 10/08/10 05:02 AM

Well I knew it said something like that...

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astroneil
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Re: Stranger than fiction! new [Re: johnnyha]
      #4139867 - 10/24/10 09:50 PM

Some 2003 wisdom from a most respected man.

http://www.cloudynights.com/item.php?item_id=410

Thanks,

Neil.


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Re: Stranger than fiction! new [Re: astroneil]
      #4167481 - 11/06/10 12:17 PM

Hmmm..... the neverending debate on which telescope is best.

Though I have a big newtonian and a couple of large SCT's for imaging I have always had a hankering for refractors.

I have a 4inch f15 achromat in a brass tube that dates from around 1970. I chose to build a refractor back then as the images with my 3inch were so crisp compared to 6inch reflectors and I particularly wanted it for observing Venus. Looking at a Mars opposition with the 8inch Fry at Mill Hill Observatory convinced me that refractors were really the best choice for planetary detail as well as splitting close binaries. The original lens was a Wildey, whose OG's were well regarded at the time. Despite several returns to him either it wasn't sharp or had far too much colour. I was put off refractors for many years. The replacement was a hand chosen air spaced pair made by Emerson optical which transformed the scope. I watched the Venus transit with this instrument whilst the Apo's were automatically recording the event. Residual colour annoys me, and whilst this scope is now pretty good, and I still have it, the FS102 is far more pleasing as a visual instrument. Ah, I hear you say - but the Tak cost many times more. Nope - I paid £680 on fleabay for my Tak and heaven knows how much more over the years on my f15 achro.

I regularly use a 6inch f14 Cooke, and this instrument certainly delivers good images from a fairly mediochre lens. It does have the advantage of being on a mount that must weigh a ton or more, and that is probably the key. Having a second Cooke mount with nothing on it I tried a £200 6inch f8 Chinese achromat on it. With the Baader colour fringe reducer this performs adequately for open evenings though is not something I would recommend to any real observer. Even with a 48 inch focal length you need a really heavy mount to ensure the optics are limiting the image quality rather than the mount.

Visually comparing the Ed100 with the FS102 and f15 4inch the conclusion I have come to is that both the APO's have superior dark fields making nebulae and clusters stand out more than in the achro. The Tak has slightly more colour than the ED100 (damn) but it has a slighly more velvety dark field and so is marginally preferable as a visual instrument. The Tak is mainly used for imaging whereas this is not practical with the f15 achromat. If asked to suggest a small refractor then I don't hesitate to recommend the ED100 as this has by far the best performance to cost ratio of my refractors and can be used on a sensibly sized mount.

Just my two penneth. I would really like to believe an achro is as good as an apo - but I'm afraid I have yet to be convinced.

Edited by astronomer2002 (11/06/10 01:41 PM)


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Re: Stranger than fiction! new [Re: astroneil]
      #4167677 - 11/06/10 02:02 PM

Quote:

You can also get a modest reduction (~18%) in secondary spectrum using Bak2-F1 glass at little additional cost to standard achromat optical glasses.

Some interesting reading here:

http://www.rfroyce.com/refractor%20spots.htm

Cheers,

Neil.




Neil,

Your post reminded me that I'd been meaning to look at BaK1/F2 doublets compared to BK7/F2 (I'm assuming your BaK2/F1 is a typo).

For a C-F corrected BK2/F2 doublet I get a variation of focus of 1 part in 1788, which would translate into 1.279mm in a 6" f/15.

For a C-F corrected BaK1/F2 doublet I get a variation of focus of 1 part in 2002, which would be a variation of focus of 1.142mm in a 6" f/15.

Clear skies, Alan


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Re: Stranger than fiction! new [Re: Alan French]
      #4168306 - 11/06/10 08:12 PM

This may sound stupid. but what is the effect of the longer tube on long focal length refractors with regard to the column of air that they are looking through internally?

I read and understand that the defocus range is actually greater with the instrument that has the most focus range. I got that. So even if there were some defocusing caused by the air itself within the tube and not the atmosphere, it should be just as great with the longer tube.

Before anyone jumps on me, I am not attempting to refute any evidence within the article. Over the years I have gleaned a lot of knowledge from Vlad's postings here and elsewhere. His involvement in this at any level tells me that there is important data within. Just curious if anyone has ever done tests with regard to the effects of air stability as it relates to the length of a tube and how that impacts image quality, if at all.

Congrats to Neil. This was a large undertaking, hats off to you.

Edited by Donnie (11/06/10 08:17 PM)


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Re: Stranger than fiction! new [Re: Donnie]
      #4169955 - 11/07/10 04:43 PM

Alan,
Concerning your focus variation stuff, I think it would be better to talk to Bob Royce about that. Unfortunately, I do not have the means to verify your data.

Donnie, thank you very much for your comments. As ever greatly appreciated!
In response to your question; in earlier drafts of the work Vlad and I discussed more obscure tube-related effects relating specifically to close-tube instruments like refractors. One consequence of having a 'hermetically sealed' design is that the metal, which cools at a much faster rate than the enclosed air, causes the air layer close to it to cool down more quickly than the air in the middle of the tube. That would effectively form a very weak negative (concave) lens, with the outer rays travelling slower than those toward the centre of the tube. If the tube air temperature gradient changes so that a near-spherical wavefront shape forms, the effect could manifest itself as focus extension. However, this is a very minor effect IMO, but again, it causes defocus error approximately in proportion to the F-ratio.

With best wishes,

Neil.


Edited by astroneil (11/07/10 04:50 PM)


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Alan French
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Re: Stranger than fiction! new [Re: astroneil]
      #4170203 - 11/07/10 06:41 PM

Quote:

Alan,
Concerning your focus variation stuff, I think it would be better to talk to Bob Royce about that. Unfortunately, I do not have the means to verify your data.
[SNIP]

Neil.





Neil,

It's actually a simple calculation based on the data on the glasses from the catalog.

I also found I had designs using BK7/F2 and BaK1/F2. The first shows a focal variation from C to F (at the 70% zone) of 1.31mm, while for the second it is 1.15mm.

Clear skies, Alan

Edited by Alan French (11/07/10 07:00 PM)


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astroneil
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Re: Stranger than fiction! new [Re: Alan French]
      #4170226 - 11/07/10 07:00 PM

Alan,

Then, as I said, you need to talk to Bob Royce about that.

Cheers,

Neil.



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Re: Stranger than fiction! new [Re: astroneil]
      #4170286 - 11/07/10 07:27 PM

Neil,

About what?

Clear skies, Alan


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Re: Stranger than fiction! new [Re: astroneil]
      #4170531 - 11/07/10 09:34 PM

Thank you Neil. I was just curious as to the effects. I was not sure if they would have a correlation or not and even if they did if it was substantial or minor.

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Re: Stranger than fiction! new [Re: Donnie]
      #4174222 - 11/09/10 01:43 PM Attachment (58 downloads)

Great article and an informative read.

The age-old debate between the long-achro crowd and the short-apo crowd will never be settled to everyone's satisfaction. Both scope types excel in certain areas and have disadvantages in others.

I've owned both types, long achros and short apo. I liked both, and I would be hard-pressed to choose one over the other. Instead, in a perfect world, I would own both - a long achro for binary-splitting and planetary viewing, and a short-fast apo for richfield viewing.

Ultimately, to me, the debate is about true field size and not so much the lack of spurious color. Unless I am viewing planets, I'd prefer a wider TFOV that grabs more sky in a single eyepiece view.

But, I am attracted to the aesthetics of long achros, and prefer them whenever possible.

Best regards and clear dark skies,

MikeG


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astroneil
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Re: Stranger than fiction! new [Re: Glassthrower]
      #4181686 - 11/12/10 07:20 PM

Thanks Michael,

And a very happy birthday to you!

Best wishes,

Neil.


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Re: Stranger than fiction! new [Re: astroneil]
      #4212902 - 11/27/10 11:27 AM

Superb article! Thanks to the authors for all of their work on this.

I'm not sure if I'm going to regret stepping into this or not, but I always considered that the primary reasons that one bought short focal ratio refractors were not because of a perceived advantage in optical quality, but rather:

1. The shorter tube is easier to transport, handle, and mount; and
2. The short focal length allows for rich-field views.


If anything, the decision to choose a short focal ratio refractor comes with it the understanding that you will compromise optics in favour of these other parameters. Of course the slower scope will be superior optically, all else being held equal.

Many of the arguments here seem to be neglecting these considerations, or even assuming that optics are the only consideration when one chooses a telescope. They are not.


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Starman1
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Re: Stranger than fiction! new [Re: Mentor]
      #4212969 - 11/27/10 11:51 AM

Quote:

Superb article! Thanks to the authors for all of their work on this.

I'm not sure if I'm going to regret stepping into this or not, but I always considered that the primary reasons that one bought short focal ratio refractors were not because of a perceived advantage in optical quality, but rather:

1. The shorter tube is easier to transport, handle, and mount; and
2. The short focal length allows for rich-field views.


If anything, the decision to choose a short focal ratio refractor comes with it the understanding that you will compromise optics in favour of these other parameters. Of course the slower scope will be superior optically, all else being held equal.

Many of the arguments here seem to be neglecting these considerations, or even assuming that optics are the only consideration when one chooses a telescope. They are not.



True.
I have had a lot of fun with a 5" f/5 refractor (definitely a Jimi Hendrix "Purple Haze" scope) at low powers.
My 12.5" reflector, though, if a refractor, would be too long and too heavy to transport--even if (and I couldn't) I could afford one.
One of the reasons long focal lengths are optically superior (if/when they are, that is) is that the shallower curves require less time to make and are less likely to have serious errors in figure. It's the reason why telescope designs that use spheroidal surfaces (such as Maksutovs) can be optically excellent--the curves are easier to generate and polish than paraboloidal or hyperboloidal surfaces.
That's not to say that an experienced hand at making the optics won't be an important issue.

One thing, though, has appeared as I've gotten older--the small exit pupils that arise when using long f/ratios have become more problematical visually. And it's harder to get eyepieces that result in larger exit pupils when the f/ratios are long.


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Re: Stranger than fiction! new [Re: Starman1]
      #4213003 - 11/27/10 12:08 PM

Quote:

Quote:

Of course the slower scope will be superior optically, all else being held equal.



One of the reasons long focal lengths are optically superior (if/when they are, that is) is that the shallower curves require less time to make and are less likely to have serious errors in figure. It's the reason why telescope designs that use spheroidal surfaces (such as Maksutovs) can be optically excellent--the curves are easier to generate and polish than paraboloidal or hyperboloidal surfaces.
That's not to say that an experienced hand at making the optics won't be an important issue.




Agreed. The other advantage that I had in mind, and that the article touches on, is the depth of focus. The tight focus tolerance on a fast scope is a major issue. Not only does it lead one to install a micro-focuser, but it also makes the scope more sensitive to atmospheric aberrations.


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Re: Stranger than fiction! new [Re: astroneil]
      #5006765 - 01/08/12 07:10 AM

I read the article and found it interesting. This definitely puts the word "end" on the thesis that "seeing sensitivity" is caused by focal ratio per se.

I have some remarks. The first is that "linear" depth of focus is meaningless. You can stretch the x axis in figure 3 at your wish by using a different reduction ratio for the focuser. For focusing what matters is the ratio between turning the focuser knobs and the defocusing effect. A scope with half focus depth and doubled reduction will behave apparently the same way: producing the same amount of defocus per turn of the knobs. Of course shorter focus scopes need more reduction.

What is meaningful is the ratio between the ideal focus depth and the focus depth as reduced by spherochromatism. We know the shift in best focus caused by turbulence is locked to the focus depth of an IDEAL telescope. So perfect scopes behaves exactly the same with respect to seeing, regardless of their focus ratio. This is not a novelty: Fried already got this conclusion in his 1966 paper: Optical Resolution Through a Randomly Inhomogeneous Medium for Very Long and Very Short Exposures, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 56, NUMBER 10. So it is surprising that claims of long focus behaving different than short focus still survived.

However a focus with residual spherochromatism has reduced depth of focus. The ratio between this ACTUAL depth and the IDEAL depth can vary depending on the amount of residual aberrations.

This completely changes the perspectives, because it is now clear the the cause is not long focus but residual aberrations.
What you show is that telescopes with more spherochromatism are more sensitive to seeing. Incidentally short focus apos are more likely to have greater residual aberrations. But the causal link is with residual aberration not with length of focus. If a shorter focus scope had residual aberrations less than a longer one, the longer would be worse. Again, a Newtonian design, as well as any other scheme, perfectly made would have the same sensitivity to seeing as a IDEAL scope (all equal).

On the other hand it is not surprising that the cause is residual aberration. Seeing induced wavefront errors and residual aberrations stack together (see Suiter). Thus if a threshold is set for image quality, then this threshold is reached first if the scope already has some residual aberrations. In this form people (I would say mostly reflectors guys) know that bad figured scopes are less tolerant, because they already have "eaten" a fraction of the error budget.

To be clearer, we know that the expected value for the rms wavefront error induced by seeing is (roughness term only) http://www.telescope-optics.net/induced.htm

rsm_seeing = 0.058 (D/r0)^(5/6)

For a given aperture D, the rms_seeing increases with decreasing of the Fried scale r0.

However th quality of the image depends on the total rms wavefront error, which is the sum of the above and the rms error of the scope:

total_rms = rms_seeing + rms_scope.

Thus it is clear that to stay within the same total_rms budget, in case the scope is worse, we can tolerate less atmospheric seeing, which means that we get the same quality of the image only if r0 is smaller, or we get worse quality with the same r0. The greater the rms_scope the smaller the seeing that can be tolerated before exceeding a set image quality threshold. Should the scope be barely diffraction limited, it leaves no room for additional (environmental) wavefront error. Bad scopes looks like the seeing were worse.

In conclusions bad scopes are more sensitive to seeing that good ones. Whenever somebody thinks that his long focus scope is better than his short focus one, that only means that the shorter one is less corrected (or the focuser has not the proper reduction ratio).

Edited by Mauro Da Lio (01/08/12 07:20 AM)


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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 (71 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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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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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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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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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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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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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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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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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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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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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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Re: Stranger than fiction! new [Re: Olivier Biot]
      #5048531 - 01/31/12 06:21 PM Attachment (26 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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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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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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