36 replies to this topic

[SandyHouTex]

So I have heard, but cannot verify that the TSA uses FPL-53 sandwiched between two IDENTICAL flints.  In the case of the TOA objective, I’ve heard it’s two pieces of FPL-53 on the outside with a flint on the inside.

 

So now let’s go to “Telescope Optics”, by Rutten and van Venverooij, page 312.  The figure there, and the text states, that to design the best doublet possible, you need to pick elements as far apart as possible, and as close to a horizontal line possible.  However the two designs above are triplets, so let’s flip to page 324 and Fig. 21.21.  Here and in the text it says you need to pick elements such that a triangle is created, with the largest area, for the best possible correction.

 

Now if the TSA and TOA have elements as stated above, they will not create a triangle at all, but only a line, like a doublet.  Therefore, they cannot have any better correction than a doublet, and in that case, it makes NO SENSE to make a triplet at all.

 

And just for the record, of all of the triplet designs I’ve seen, they used three DIFFERENT glasses for the objective.

 

A long time ago, I was proficient in a DOS ray tracing program, and actually traced a 16 inch Buchroeder Tri-Schiefspiegler and an FPL-53 doublet.  Currently, I don’t have the time or inclination to learn OSLO or ZEMAX.  So what am I missing, if anything.  This conundrum is driving me crazy.

Edited by SandyHouTex, 21 February 2021 - 02:39 PM.

[Astrojensen]

 

Therefore, they cannot have any better correction than a doublet, and in that case, it makes NO SENSE to make a triplet at all.

Wrong, it can make perfect sense. In a triplet, you share the amount of curvature you need to make on more lenses, which drastically reduces spherochromatism, by making the curves much shallower.

 

 

Clear skies!

Thomas, Denmark

[SandyHouTex]

Wrong, it can make perfect sense. In a triplet, you share the amount of curvature you need to make on more lenses, which drastically reduces spherochromatism, by making the curves much shallower.

 

 

Clear skies!

Thomas, Denmark

So Rutten and van Venrooij are wrong?

[payner]

Yes, the TOA have virtually zero spherochromatism (most critical aberration to control along with spherical aberration) on paper and none visually. These are very wide spaced triplet, akin to a modified Cooke design.

 

When one looks through a TOA there truly is a clinical, cold appearance to objects. It is because those common aberrations are so well controlled, there's no "warm" tone. This also requires the utmost of precision optical figure--very expensive in itself.

[Alan French]

Using two elements greatly limits the designers glass choices, adding a third element increases glass choices and adds more degrees of freedom to correct aberrations. 

 

Back when ED (Extra-dispersion) glasses were more limited and the Vd difference between them and standard crown glasses was smaller, triplet designs sometimes used a different approach. Essentially two flints, one lying well off the Abbe line, like KzFSN-4, were used to create a flint element that was a good match to a single crown element. An example is a BaK1/KzFSN4/BaFN10 triplet. That is the approach Rutten and van Venrooij are talking about.  

 

Rutten and van Venrooij is a bit dated in its coverage of apochromats. 

 

Clear skies, Alan 

Edited by Alan French, 21 February 2021 - 06:47 PM.

[Astrojensen]

So Rutten and van Venrooij are wrong?

No. The color correction in a triplet with only two glass types isn't better than in a doublet with the same glass types, but by making it a triplet, it becomes much easier (or indeed possible at all) to make and have good control over spherochromatism. 

 

 

Clear skies!

Thomas, Denmark

Edited by Astrojensen, 21 February 2021 - 03:17 PM.

[SandyHouTex]

No. The color correction in a triplet with only two glass types isn't better than in a doublet with the same glass types, but by making it a triplet, it becomes much easier (or indeed possible at all) to make and have good control over spherochromatism. 

 

 

Clear skies!

Thomas, Denmark

So I have seen spotplots for a FS-102 and a TSA-102, and chromatic aberration is better controlled in the TSA-102:

 

https://astromart.co...kahashi-tsa-120

[Astrojensen]

So I have seen spotplots for a FS-102 and a TSA-102, and chromatic aberration is better controlled in the TSA-102:

 

https://astromart.co...kahashi-tsa-120

And?

 

 

Clear skies!

Thomas, Denmark

[Kunama]

I guess someone forgot to tell Mr Yuyama at Takahashi Seisakusho that it can't be done.......

 

I don't think Takahashi have ever disclosed the actual mating glasses to their ED or CaF2 designs. 

The TSA has one FPL53 element between two unknown elements, 

The original TOA130 had an FPL53 at the front, unknown in the middle and FPL52 at G3,

The later TOA130 and the TOA150 have FPL53 at G1 and G3 and another 'mystery' glass in between..... 

 

The only doublet that comes close is the FOA60.

Edited by Kunama, 22 February 2021 - 02:31 AM.

[zirkel 2]

Takahashi TOA : FPL-53/BSL-7/FPL-53 

Source : https://www.telescop...po_examples.htm

Edited by zirkel 2, 22 February 2021 - 03:46 AM.

[edif300]

Takahashi TOA : FPL-53/BSL-7/FPL-53 

Source : https://www.telescop...po_examples.htm

I think that is not the TOA's design. Is an example of an Ortho-APO design... Spacing between G2 and G3 in the TOA seems to me far wider than in the example.

Edited by edif300, 22 February 2021 - 05:54 AM.

[zirkel 2]

You have to look at the table, not the drawings which are examples, # 20 being an ortho apo lens indeed.

 

- Takahashi America Said :

 

The TOA series is a member of Takahashi’s line of triplet refractors. In the TOA design, three lens elements are used, with the front and rear constructed from ED glass.

Source : https://www.takahash...-refractor.html

 

Sure FPL-53/BSL-7/FPL-53 optical design TOA series 😉

Edited by zirkel 2, 22 February 2021 - 07:43 AM.

[edif300]

What I mean is that the air gap between G2 and G3 seems wider than in the example. So other than diameter and F the example cited is an example with no real correlation with the TOA's design (IMO).

 

Perfectly known that TOA-130 uses a FPL53 and FPL52.  TOA-130N uses two FPL-53 as well as the TOA-150. Mating BSL7 is good choice too...

I spent some years here in CN dedicated to this task ...

Edited by edif300, 22 February 2021 - 08:30 AM.

[25585]

I guess someone forgot to tell Mr Yuyama at Takahashi Seisakusho that it can't be done.......

 

I don't think Takahashi have ever disclosed the actual mating glasses to their ED or CaF2 designs. 

The TSA has one FPL53 element between two unknown elements, 

The original TOA130 had an FPL53 at the front, unknown in the middle and FPL52 at G3,

The later TOA130 and the TOA150 have FPL53 at G1 and G3 and another 'mystery' glass in between..... 

 

The only doublet that comes close is the FOA60.

xOA class is their flagship range, not surprising.

 

The TSA-120 is an excellent scope, it really is. Not being xOA does not make it less. I would buy a TSA-130 or 140 in a heartbeat.

[peleuba]

The TSA has one FPL53 element between two unknown elements, 

 

 

Hi Matt - Yes the elements are not disclosed; but its pretty easy to figure it out.  There are really precious few choices (you can count them on one hand) that would give you the desired  correction/be a good match with FPL53; is high quality (homogeneity); can be readily worked (polished and figured); and is available.  

 

If looking at the glass catalogs and Relative Partial Dispersion diagrams found on the web, one would think that there a bunch of choices in which to mate FPL53.  The issue becomes availability, quality and workability.  Few glasses have all three.

 

Anyway, my guess is that the TSA120 is two lenses of N-BK7 surrounding the single piece of S-FPL53.  All elements separated by an air gap.

 

Coincidently, the SkyWatcher 120 (a doublet) is a single piece of N-BK7 mated with a single piece of S-FPL53 separated by an air gap.

Edited by peleuba, 22 February 2021 - 10:23 AM.

[n2068dd]

 Recent telescope objective designs are different from old fashioned classic design that of 'Rutten and van Venverooij'.

 Mr. Yuyama say " A wide air gap design in FOA is thoroughly different than SKY-90, though It may looks similar. I like to design telescope objective with 'air lens'. A wide air gap itself do not the work for spherical correction, but with adding some material and curve selection it will do the work for higher order cancelling function. Maybe, Agema and CFF also uses this method. Despite seeing their design and product, Agema looks a little hype for me. CFF, I'm concern with their design.'

 In my astronomy club member Mr. Abe he is a Pentax SDP and eyepiece XP designer say, ' I never do that type of TOA design. It's too radical design that major optic company merely do. It'll needs so many precision control in lens, air gap and lens cell. He must adjust so many narrow spot to complete.' and ' I thought my Pentax SDP will be durable in hundred years. Despite TOA have shock hazarded design and construction. Their TOA wide air gap will needs some precision of a micron order. If someone opened for cleaning, he never can rebuild as it should. the air is the lens.'

 At a glance, we can't know of the modern method from old fashioned method, a professional designer can asses in optic design. I suspects.

 

note: corrected the mistake SDF to SDP

Edited by n2068dd, 22 February 2021 - 11:43 AM.

[SandyHouTex]

I found the glass prescriptions.  The TOA uses FPL-53, BSL-7, FPL-53.  It’s analyzed here, thanks to Vla, as objective #20:

 

https://www.telescop...po_examples.htm

 

The TSA is BSL-7, FPL-53, BSL-7.  The prescription is in post 59 here (thanks to Gus):

 

https://www.cloudyni...i-optics/page-3

 

I did some reading and it appears there is only so far ED glass can go with correcting chromatic aberration.  At some point spherochromaticism rears it’s ugly head, and begins being a bigger problem.  It then requires either aspherization of one of the elements, or if they’re left spherical, another element, as Tak has done here with the TSA and TOA.

 

Mystery solved.

[edif300]

Recent telescope objective designs are different from old fashioned classic design that of 'Rutten and van Venverooij'.
Mr. Yuyama say " A wide air gap design in FOA is thoroughly different than SKY-90, though It may looks similar. I like to design telescope objective with 'air lens'. A wide air gap itself do not the work for spherical correction, but with adding some material and curve selection it will do the work for higher order cancelling function. Maybe, Agema and CFF also uses this method. Despite seeing their design and product, Agema looks a little hype for me. CFF, I'm concern with their design.'
In my astronomy club member Mr. Abe he is a Pentax SDF and eyepiece XP designer say, ' I never do that type of TOA design. It's too radical design that major optic company merely do. It'll needs so many precision control in lens, air gap and lens cell. He must adjust so many narrow spot to complete.' and ' I thought my Pentax SDP will be durable in hundred years. Despite TOA have shock hazarded design and construction. Their TOA wide air gap will needs some precision of a micron order. If someone opened for cleaning, he never can rebuild as it should. the air is the lens.'
At a glance, we can't know of the modern method from old fashioned method, a professional designer can asses in optic design. I suspects.

And nearby 20 years from TOA introduction. Awesome.

[n2068dd]

Yes, If you hope to build a commercial 'APO' telescope, you must control zero, 4th, 6th, 8th order aberrations for the total minimum spherochromatism using curve differences, air travel differences and lens selections. The ED elements only do for the secondary spectrum. The classic method of the triplet is setting the crossing point to the 0.7 from the center in seven colors and set each curve as straight as it can. If it was turned sharply around the edge, it can't balanced in-out error in the aperture surface. Takahashi FCT and TSA was designed in this method. FC-DZ was designed as this with only two elements, though less perfect. That may be the reason Mr.Yuyama did not say it as FOA-100.

[peleuba]

 

Mystery solved.

 

 

Thanks for posting these links, Sandy.

 

Referring to my post above to Matt (#15) I surmised that the mating element on the TSA series was BK7 as its time tested, readily available in uber high quality, and mates well with FPL53. 

 

I was suprised in reading your post that in fact, the TSA mate is BSL-7.  I was wondering how I could have gotten this wrong as I was not at all familiar with BSL-7 as a mating element to FPL-53.  I did some additional research combing through some glass catalogs archived on my home server and found this Optical Glass Equivalents Table.  It can also be found here: 

 

https://www.edmundop.../optical-glass/

 

At least I can sleep easy tonight knowing that I was not completely off-base...   BK7 is the Schott equivalent to Ohara BSL7. 

Attached Thumbnails

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Edited by peleuba, 22 February 2021 - 11:54 AM.

[n2068dd]

A question ' Why Takahashi returned FS design to FC design ? '

 

Mr. Yuyama replied ' It was only my interesting, ha,ha,ha.' but seriously add ' For the owner of their FC. In new FC-D design,  the fluorite element is identical of the old FC. so, they can replace frozen mated element in years.' and ' I find FC style have further potential for the better correction. I am searching.' it was 5 years ago with him.

[edif300]

So the true mystery has been solved at the end... ha ha !!! the question regarding to returning to FC design has been written many times. Interesting to know why Mr. Yuyama choose it.

[n2068dd]

And nearby 20 years from TOA introduction. Awesome.

Hi,

 

though, I recently needs for the lite weight and less price 5 inch not as TOA or TSA.

not concern doublet or triplet, just handy for grab and go.

Astro-TEC looks reasonable for me.

before this, I should build my Makstov system with perfect motorized style, auto focuser, field rotator... and so on.

very heavy both on weight and budget....

 

pardon for off-thread.

 

Regards

Hiromu

Edited by n2068dd, 22 February 2021 - 12:40 PM.

[edif300]

Hi Hiromu,

 

Will be happy to see designs during next years. Maybe there will be a FOA-100 and FOA-140, TOA-110 and 170. The FSQ-106ED has some years too (2007) so ...

 

Best regards

Iñaki

Edited by edif300, 22 February 2021 - 12:22 PM.

[Simoes Pedro]

A very important detail:

 

the practical difference between TOA and TSA triplets is that the TOA is airspaced while the TSA is a contract triplet. Although both have 6 curves to correct for aberrations, the lens spacing in the TOA is also a design spec. In theory the TOA can be better corrected. 

 

Note how both TOA and TSA are superior to cemented/oil-spaced triplet. These systems only have 4 surfaces as degrees of freedom. Nowadays, there is no motivation to use 3 different glasses to correct secondary spectrum. The function of the 3rd element is to provided extra curves to correct sphero-chromatism.

 

Glass types is a non-issue. FPL53/FPL55 mated to BK7 or ZK7 are virtually color perfect. In practice, color aberration must be induced to counter sphero-chromatism. This also applies to less "premium glass" like PK52A/FPL51/FCD100 mated to a lanthanum Crown.