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Should Telescopes be Nulled in Red?

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#76 peleuba

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Posted 24 July 2024 - 09:55 AM

Not sure if its been mentioned in the pages of science homework here, but Stellarvue already did their homework on this.

 

As Jim says its not about a single scope, nor single manufacturer.  But, we need to be accurate when describing this.  My sense is that things were not as you say - "StellarVue already doing the homework".  Rather, they scrambled to stop the bleeding when presented with deficiency in design of their "flagship" telescope.  In other words, it was damage control.

 

 

From their website:

In the summer of 2023 based on customer requests we implemented a slight change in our process to move the final optical correction toward the middle of the visual spectrum. During the spring of 2023 we tested this process extensively to ensure that photographic performance would not be compromised due to this change. Both red and green figured, high-Strehl SVX objectives delivered the same excellent color correction free of the dreaded "blue bloat." Since most of our customers are imagers, it was important that we took the time to test the results before we implemented this change. Based on our findings, we modified our final spherical correction slightly to ensure that our objectives are now most accurate in green light.

 

This is totally marketing.   The verbiage on the website has changed perhaps a dozen (or more) times on this very subject.  At one point, it was a cornucopia of buzz words strung together that made little sense to anyone with even elementary knowledge of optics and light.     Its unlikely they (StellarVue) performed "extensive testing" to insure that nulling in green would not cause any untoward issues.  Some of the world's best refracting telescopes used as astrographs from Takahashi and Astro-Physics have been best corrected in the green and green-yellow for decades.  Nulling in green is a well known tenet in optics that leads to best performance in both visual and photography.  If StellarVue would have user-tested this scope visually on objects a large APO is meant to resolve - the planets - this would have been obvious even to the novice when comparing it to a scope more neutrally corrected.  

 

From a personal perspective, Guys/Gals, we need to stop romanticizing this.  It was not a natural evolution of the StellarVue design.  It was a forced change the market demanded after amateur testing uncovered the issue.   This might not sound good on a company website but that's precisely what happened.  An evolution of design is a positive and iterative process of self-improvement using information gleaned from internal R/D where incremental changes make the product better.  This is not what happened here.  Rather, its somewhat analogous to the Ford Pinto and it was made worse by the manufacturer taking a tough stance toward the client and the test and attempting to discredit the tester.  Then the usual StellarVue apologists piled on.

 

At the end of the day until we, collectively, stop glamorizing what was an operational screw-up, we'll continue to have these arguments and I'll politely continue to call out folks who try to impart their version of reality into the discussion - and this includes StellarVue.  Often, the best thing to do is admit it, fix it, and move on.  Not sugar-coat what was, largely, a self-inflicted issue with flowery gobbly-gook type of language.

 

 

EDIT: I also recall someone mentioning last time this came up that most places use red lasers simply because they are much cheaper than green in the quality level and power needed for the process.

 

That was me, HERE and HERE.  And its a good point in which to re-visit.

 

Red lasers are less expensive overall and work better in interferometry as their light is coherent over longer distances and their wavelength fluctuation is less when compared to green lasers.  Red is the standard in business and industry as the most surfaces being measured are reflective and the results can be scaled to any wavelength (color) using Algebraic expressions.   When measuring lenses, there is no easy way to scale the results between different wavelengths.  Red is also most common in the ZYGO brand of interferometers that have become the standard by which others are measured.


Edited by peleuba, 24 July 2024 - 01:45 PM.

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#77 AZStarGuy

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Posted 24 July 2024 - 10:09 AM

I don't read Japanese but you don't need multiple lines to show at what part of the spectrum the lens is best corrected. I don't believe that chart is specific enough. 

 

Notice the Astro-Physics Strehl chart: one line with the wave length and Strehl easily called out. One can see in what part of the spectrum the lens is best corrected. And you really don't need a chart at all, as on the left the wavelength and associated Strehl are numerically indicated.  A spot diagram will not show that.

 

The second chart is from Agema optics. That chart shows "7 different telescopes" including some from Takahashi. ALL are nulled in the green. Again, one line for each scope.

 

Bob

This is correct - 5 *'s to Bob.  

 

I think that he previous post above Bob's showing the Tak diagrams may be interpreted incorrectly?  I have always looked at those and thought that you go from the point of focus which is further up the chart where lines converge the closest horizontally.  In that instance the Taks are both nulled somewhere towards green but the resolution of the charts Tak provides leaves some wiggle room to interpretation.  I could be wrong here so good to hear someone else's thoughts with a background in this stuff.  

 

Pretty interesting thread!!  



#78 Polyphemos

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Posted 24 July 2024 - 10:20 AM

With Paul’s succinct description of the evolution of StellarVue’s revision of their marketing, testing, and figuring out of the way, and with Paul’s spot on warning not to glamorize or romanticize, which I will second while expanding it to including everything related to optics, let’s move away from topics not related to the physics of refraction and leave those to other threads more suited to the purpose.



#79 Polyphemos

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Posted 24 July 2024 - 10:36 AM

This is correct - 5 *'s to Bob.  

 

I think that he previous post above Bob's showing the Tak diagrams may be interpreted incorrectly?  I have always looked at those and thought that you go from the point of focus which is further up the chart where lines converge the closest horizontally.  In that instance the Taks are both nulled somewhere towards green but the resolution of the charts Tak provides leaves some wiggle room to interpretation.  I could be wrong here so good to hear someone else's thoughts with a background in this stuff.  

 

Pretty interesting thread!!  

I could be mistaken but I believe that the base of the line chart is center of focus, and as you move up the y-axis you are moving in the direction of the perimeter of the lens. It seems that many designers put the point of best convergence of RGB near the area of the lens 50% to 75% of the distance from the center of the lens to the perimeter. I think that helps to minimize the errors we would see at the edge of the field of view and it’s within the zone where most observation occurs.

 

 

IMG_2860.jpeg

 

In the example above the value of zero on the y-axis represents conditions at the center of the lens while the value 1.0 represents conditions at the perimeter. Here best convergence of RGB is around 65% of the distance from the center to the perimeter, while remaining good throughout. If best convergence was at the center of the lens things might have gotten ugly near the perimeter or before.


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#80 hendric

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Posted 24 July 2024 - 11:01 AM

Doesn't it have to do with the amount of area represented by each part of the lens? The area from 0 - .25 lens radius is only 6% of the total area.  The area from .5 to .75 is 31% of the total area. Half of the lens area 0 - .707, the other half .707 to 1.

 

All parts of the lens contribute to all parts of the FoV, as I understand it.



#81 Polyphemos

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Posted 24 July 2024 - 11:15 AM

Doesn't it have to do with the amount of area represented by each part of the lens? The area from 0 - .25 lens radius is only 6% of the total area.  The area from .5 to .75 is 31% of the total area. Half of the lens area 0 - .707, the other half .707 to 1.

 

All parts of the lens contribute to all parts of the FoV, as I understand it.

I think you’re right on with regard to area, though there’s more to it than that. While the area fro 0 - 0.25 lens radius is only 6.25% of the field area, it’s probably 90% of where we spend our time looking when observing small objects like double stars, globular clusters, and the like. If we include half of the lens area we can include most open clusters, wide views of the moon, and so on. Certainly all parts of the lens contribute to the FofV as I understand it also, but the first 75% of the diameter I think is the largest contributor from a practical point of view.

 

I’ll use this as an opening to bring up the topic of aspheric lenses. The potential benefits are excellent correction in RGB across the entirety of the optical area, but they’re very difficult to make and figure. Hopefully improved technology will bring them to amateur astronomy telescopes at a price that’s palatable. They’re already widely used in more expensive applications. Can anyone comment on where things stand with aspheric lenses?


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#82 lylver

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Posted 24 July 2024 - 12:03 PM

Doesn't it have to do with the amount of area represented by each part of the lens? The area from 0 - .25 lens radius is only 6% of the total area.  The area from .5 to .75 is 31% of the total area. Half of the lens area 0 - .707, the other half .707 to 1.

 

All parts of the lens contribute to all parts of the FoV, as I understand it.

Beware of too fast conclusion about optic behaviour. That is OPD that counts in the formation of image.

OPD-LSA.png

 

Full aperture, green centered

spotALL.JPG

Half aperture contribution (0 to 0.5), same focus

spot-half-aperture.JPG

Note : green spot is too small and hidden behind blue contribution


Edited by lylver, 24 July 2024 - 12:49 PM.


#83 Polyphemos

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Posted 24 July 2024 - 12:12 PM

Beware of too fast conclusion about optic behaviour. That is OPD that counts in the formation of image.

attachicon.gif OPD-LSA.png

 

Full aperture, green centered

attachicon.gif spotALL.JPG

Half aperture contribution (0 to 0.5), same focus

attachicon.gif spot-half-aperture.JPG

Note : green spot is too small and hidden behind blue contribution.

 

Blue best focus : full pupil

attachicon.gif Blue-bestF.JPG

Iylver, can you please translate to something my limited brain can apprehend?



#84 lylver

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Posted 24 July 2024 - 12:17 PM

When you are close to the optical center, the contribution of longitudinal spherical aberration is not linear for r (0 to 1) portion of the pupil.

 

Edit : best explanation by Vla here

https://www.telescop...r_spherical.htm

 

primary_spherical_function.PNG spherical_aberration.PNG

Some interesting math properties :

 

For Longitudinal Spherical Aberration that are not too complicated, in fact those whose higher order can be ignored, they have a curve like a secondary degree curve with no inflexion point (parabola, ellipse...) from 0 to 1 increasing of the pupil.

 

- you can say that maximal error is at the optical center (paraxial) focus or at the marginal focus (the other extreme) and has a ratio of 1 of the Total Error. Any focus made inside this range (xm to xp) produces less relative error to the OPD° (seems natural indeed)

At the best focus, the error is 1/4th (25%) Rms : this is a fantastic optimisation. And the mastering of this (the crossing for the range of colors) makes you a excellent designer.

 

- If the Total Error is less than 1/4th of wave (aka diffraction limited xm-xp), A.E.Conrady (1929) said (after some calculations) that you can estimate accurately the best focus at √0.5 ~0.71 of the normalised pupil for a line from paraxial to marginal crossing this heigth. So don't split your hairs to guess the Chromatic Residue wink.gif (it would be rms but not geometric).

This easy to estimate the quality of an astrophotographic lens because they use MTF09 (<=> diffraction limited for the contrast).

 

Unfortunately, for visual, our eyes make some oversample (16 to 25 perhaps more cone aggregation). He needs more sharp LSA to be efficient at MTF02 or MTF03. As it is a differential, you can estimate the need for eye for the PtV to be √8 to √12 better

so the experimented observer says you need ~1/12th wave (makes ~0.98 strehl Rms for a small peak range).

(That what Vic Maris reported in the latest video at Denver, you can read also what are the criteria from Roland Christen or other for "premium" optics  see : DAS General Meeting - Jan 2024 | Vic Maris / Stellarvue The night in the Sierra experience of exceptional seeing.)

He dare to say officially what that makes the reason why premium exists. (I want to thank Mr Kubota from Kubota Optical who confirm this to me when we exchanged a bit about his babies the lens of Scopetech 80/1000 and 80/1200)

 

Best-focus.png

° Optical Path Difference.

 

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

Explaination of the image formation

LSA to SPOT

spot-LSA.jpg


Edited by lylver, 25 July 2024 - 06:36 AM.

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#85 Jeff B

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Posted 24 July 2024 - 02:22 PM

I've been taking a more "pedestrian" and simplistic view of things like lens center zones.  And I might well be wrong, but here I go:

 

The light coming in is a collimated beam.  The area towards the center is small compared to the full aperture.  The surface of the glass towards the center is almost normal to the incoming light so it does little work (bending) on the beam (dead center, there is no bending done).  If I mask the outer 70% of the lens, I have a smaller aperture with a much larger focal ratio.  Over the reduced aperture, there is less aberration, like "CA". 

 

Jeff


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#86 RichA

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Posted 24 July 2024 - 04:07 PM

You would only want it nulled in red IF you are only going to image, and take pictures of red (Ha) nebula.

And why even bother?  Nebula's don't yield up details like planets so absolute correction isn't needed.



#87 RichA

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Posted 24 July 2024 - 04:13 PM

I've been taking a more "pedestrian" and simplistic view of things like lens center zones.  And I might well be wrong, but here I go:

 

The light coming in is a collimated beam.  The area towards the center is small compared to the full aperture.  The surface of the glass towards the center is almost normal to the incoming light so it does little work (bending) on the beam (dead center, there is no bending done).  If I mask the outer 70% of the lens, I have a smaller aperture with a much larger focal ratio.  Over the reduced aperture, there is less aberration, like "CA". 

 

Jeff

Light is only collimated as much as the entrance angle of acceptance of the light is.  So, while an f/15 scope is technically visualizing only a more or less collimated beam, an f6 scope is imaging more angled rays.  Look at a 10mm camera lens and it is imaging even more angled rays.  This is why it's a lot harder (dealing with the peripheral, angled light rays) in a fast scope than a slow one.  In a camera lens, a wide one, it is impossible for it to be diffraction-limited across the field of view, even sometimes when it is stopped down considerably.


Edited by RichA, 24 July 2024 - 04:15 PM.

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#88 Jeff B

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Posted 24 July 2024 - 05:07 PM

Light is only collimated as much as the entrance angle of acceptance of the light is.  So, while an f/15 scope is technically visualizing only a more or less collimated beam, an f6 scope is imaging more angled rays.  Look at a 10mm camera lens and it is imaging even more angled rays.  This is why it's a lot harder (dealing with the peripheral, angled light rays) in a fast scope than a slow one.  In a camera lens, a wide one, it is impossible for it to be diffraction-limited across the field of view, even sometimes when it is stopped down considerably.

Which is "another way of looking at it."  grin.gif



#89 RichA

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Posted 24 July 2024 - 07:52 PM

Which is "another way of looking at it."  grin.gif

When it comes to curves, the shallower, the better.  Things came to an extreme in the early 1900s when they created lenses with centres less than 1mm thick!


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#90 Polyphemos

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Posted 24 July 2024 - 08:58 PM

When it comes to curves, the shallower, the better.  Things came to an extreme in the early 1900s when they created lenses with centres less than 1mm thick!

Wow! I wonder what kind of focal ratios they were figured to?



#91 lylver

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Posted 25 July 2024 - 07:07 AM

Wow! I wonder what kind of focal ratios they were figured to?

At this time, f/15 to f/20 for high magnification. Lens bigger than 3" maybe around 1880.


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#92 Gert

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Posted 25 July 2024 - 07:54 AM

Hi,

 

You would only want it nulled in red IF you are only going to image, and take pictures of red (Ha) nebula.

Do Lunt and Coronado 'null' their lenses for Solar H-A scopes in such way? Or do they just '3rd party source' some achromats and hope that aberrations in the targeted single wavelength of H-A 'aren't going to be too bad for folks to complain'?

 

Clear Skies,

Gert



#93 lwbehney

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Posted 25 July 2024 - 09:36 AM

I think you’re right on with regard to area, though there’s more to it than that. While the area fro 0 - 0.25 lens radius is only 6.25% of the field area, it’s probably 90% of where we spend our time looking when observing small objects like double stars, globular clusters, and the like. If we include half of the lens area we can include most open clusters, wide views of the moon, and so on. Certainly all parts of the lens contribute to the FofV as I understand it also, but the first 75% of the diameter I think is the largest contributor from a practical point of view.

 

I’ll use this as an opening to bring up the topic of aspheric lenses. The potential benefits are excellent correction in RGB across the entirety of the optical area, but they’re very difficult to make and figure. Hopefully improved technology will bring them to amateur astronomy telescopes at a price that’s palatable. They’re already widely used in more expensive applications. Can anyone comment on where things stand with aspheric lenses?

Agema Optics makes aspheric objectives. They are indeed time-consuming to make. A new run of their telescopes are going to be released this Autumn. 


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#94 Polyphemos

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Posted 25 July 2024 - 09:56 AM

Agema Optics makes aspheric objectives. They are indeed time-consuming to make. A new run of their telescopes are going to be released this Autumn. 

Thanks, Iwnehney, and I’ll look a bit more closely into what Agena is up to.

 

I believe other makers also, or at least have made, aspheric objectives. We don’t seem to here much about aspheric objectives, in part because they’re so difficult and expensive to make, and in part because when someone does endeavor to make one it’s a bear to final figure the objective to the degree desired. Perhaps some have tried and abandoned the project as uneconomical. By comparison a spheroid shaped lens is much easier to both make and figure and the results can be close enough to perfect to satisfy pretty much any amateur astronomer.

 

Still, and given their benefits, I predict that at some future time aspheric objectives will become a more widespread choice offered from the best makers. I believe Canon Optron makes aspherics for electronics manufacturing, so maybe Takahashi will take it up. I expect it will begin with the smaller objective diameters between 60mm and 100mm, and creep up from there.



#95 RichA

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Posted 25 July 2024 - 10:11 AM

Agema Optics makes aspheric objectives. They are indeed time-consuming to make. A new run of their telescopes are going to be released this Autumn. 

There is an interesting question;  is it possible to make an aspheric objective as highly accurate as a spherical one and if not, does the aspherizing alone allow the large lens to compensate and exceed the overall quality possible with a high-end spherical configuration?  My guess is, yes.  Since AP has been doing it for decades.


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#96 Jeff B

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Posted 25 July 2024 - 10:12 AM

When it comes to curves, the shallower, the better.  Things came to an extreme in the early 1900s when they created lenses with centres less than 1mm thick!

My understanding is that some of the early oil/glue/gel coupled AP triplets had very thin centers for the double concave center elements and great care was needed if the objective had to be disassembled for some reason.

 

Jeff


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#97 Polyphemos

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Posted 25 July 2024 - 03:14 PM

There is an interesting question;  is it possible to make an aspheric objective as highly accurate as a spherical one and if not, does the aspherizing alone allow the large lens to compensate and exceed the overall quality possible with a high-end spherical configuration?  My guess is, yes.  Since AP has been doing it for decades.

Rich, can you expand on AP’s work on aspherical objectives? Scopes, apertures, focal lengths, number of aspherical elements vs spheroidal, etc?



#98 RichA

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Posted 25 July 2024 - 06:06 PM

My understanding is that some of the early oil/glue/gel coupled AP triplets had very thin centers for the double concave center elements and great care was needed if the objective had to be disassembled for some reason.

 

Jeff

Yes, the capillary (?) action of the oil might stick enough to rip out the objective's thin center if the lens was oiled. 


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#99 RichA

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Posted 25 July 2024 - 06:14 PM

Rich, can you expand on AP’s work on aspherical objectives? Scopes, apertures, focal lengths, number of aspherical elements vs spheroidal, etc?

As far as I know, they aspherize one surface of some of their objectives as a way of refining the final product of the scopes, minimizing sphero-chromatism.  However, the recent GTX I believe doesn't need this.  The earlier 130mm and 155mm f7.0 scopes have an aspheric surface.  But,   there is a contention that Roland stopped making scopes oriented to visual back in the 1990s, and that since then, they've been optimized for imaging. 


Edited by RichA, 25 July 2024 - 06:28 PM.

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#100 lylver

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Posted 26 July 2024 - 01:33 AM

Aspherizing was a basis during the 19th century, it was a common phase at the end of polishing.

You went back on the polishing tools if the visual sharpness was not at the level expected, the removing of matter is minimal so you can do it at this phase.

In fact quite all lens have a conic defect, making spherical need to pass on all the surface of the lens, the finger sensitivity of the master optician can make the difference. There is a particular feeling "silk" when you come near the perfect spherical curve, you can sense it with experience.

 

Assistant controlled the "whiteness" of the doublet by modifying slightly the curves and the master optician finished for the sharpness.

The tradition was continued by Zeiss and some european manufacturers like Leitz but it declines with the machining and the industrialisation.

 

------------ multiple SA tuning ------------

--- prism use and misuse ---

 

Zeiss controlled the results of their objective with back illuminated monochromatic target.

They did it in D-ray at start and then changed to e on purpose. It is often misunderstood and taken as the reference for visual.

 

They combinated lens in green and diagonal prism so you can have the two functions : photography directly on focal plane and the moving of the SA for the visual use.

For astro binocular use, the right compensation of the SA was done in the "dedicated barlow" called OCA.

For terrestrial binoculars, compensation it is done by the objective and the eyepiece.

There was a rush after WWII on military binocular Erfle that compensate a lot and where well adapted for deep sky wide field of vue.

 

Early lens D-ray tuned

 

The spherical deviations remain for the D lines (the brightest part of the spectrum) below 1/10 00° of the focal length

° : typo error it is 1/10 000 => 0.08 lambda PtV at f/15 (lambda/12), this means approximatively null SA from 500 to 700nm, it is at f/15 so, you have a good margin and it is easily verified on a simple star test.

ZEISS Euchrome.jpg

 

The working on prism teaches the heart of polishing

ZeissAprentices.JPG

 

Zeiss control of sharpness, e-ray

ZeissAC.jpg

 

Zeiss spatial optics design for photography, using their new color tuning (F'C' and high pitched SA around 532nm)

CZJ.jpg

 

----- Back to aspherisation and visual SA tuning -----

 

A could be old (KZFN2) Zeiss AS with slightly parabolized surface. It seems low tuned (chromatism), but the short Flint Kz glass permit this, in fact red is very shortened without causing excessive blue halo. If you take a classic BK7-F2, blue is more far away

chromatic_aberation_AS_vs_E.jpg

For the AS objective : e-F is 37 vs 64, e-C is 33 vs 45, this is the benefit of short flint. It shorten the dispersion and, comparing to the classic lens, permit to move the best sharpness nearer that is desirable. Shortening the f/D at the minimum you can is more for photographic lenses.

(Side effect : when you measure chromatic residue on an Zeiss AS lens (or any lens that incorporate a short flint and manage well the SA...), it is often no as good as you expect of the dispersion, but what a good visual feeling !)

Vixen SD series and Tak (for the FC100DF/DC/DL), use a short flint glass to move more easily chromatism and SA and make a dissymetry favorable to yellow-orange better precision.

 

For visual, by experience, an important color range to tune is to gather e to D focii as the table indicates (1/ 100 000 focal length). This with spherochromatism, leads to cross e and D focii to have their best focii at the same point. (Vixen 103SD, fl~ 800mm, delta e-D to be within ~ 8um). (Left is for the Vixen, right is the LSA of the aspherized AS lens, early period)

 

Best-visual-with Zeiss.jpg ST-AS.jpg

 

Edit : correcting those badly explained facts :

 

Use of aspherization can moves the SA null.

 

There is a benefit (for visual) to use spherochromatism to tune a high sharpness (but not the max) on a range of color.

 

You can adjust the level of spherochromatism by using aspherisation on more than one surface.

 

Completely aspherizing (doing on multiples surfaces) can lead to very low spherochromatism by cancelling high order terms of the SA, but then gathering the best focii is impaired and so ineficient.


Edited by lylver, 26 July 2024 - 07:41 AM.

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