How do I interpret Takahashi diagrams?

[JeremyS]

They say there are no stupid questions. So here goes.

I see diagrams such as the below posted everywhere. In this case I get it that the curves for the different wavelengths are tighter in the DZ, so presumably better. But what is the diagram actually showing? What are the X and Y axes and what are the units?

[omo]

Will be very interesting to hear the answers to this - its a great question 👍

[JeremyS]

And while we are about it, similar q for the Technosky diagram John Handleby has posted:

 

This time the X axis is labelled mm, but mm of, or from, what? And the Y axis? And what is pupil radius 62.5 mm?

[John]

I've been interested in getting to the bottom of such diagrams for some time so I will follow this thread with interest

This is the one for the LZOS 130 F/9 triplet:

I think that the number of "crossings" and where they occur is part of the performance picture but how that all adds up I'm rather in the dark about

 

[KP82]

At 0.750 on the Y axis (3/4 out from the centre of the lens, 0 on the Y axis is the centre. 3/4 accounts for most of the area of the lens surface), the lines are more closely bundled together in the DZ than DL. Also in the centre of the lens, the lines in the DZ are tighter together than the DL. Both point to superior colour correction of the DZ.

The X axis measures the focus deviation in mm. 0 means perfect focus. Each of the coloured lines represents a different visible wavelength. You want the 546nm to be as close to 0 and vertical to the Y axis as possible.

According to those 4 graphs, the theoretical performance are: 130 f/9.2 > 100DZ > 100DL > 125ED

Edited February 13, 2021 by KP82

[John]

2 minutes ago, KP82 said:

According to those 4 graphs, the theoretical performance are: 130 f/9.2 > 100DZ > 100DL > 125ED

That's a good point - these are theoretical charts for the lens design. Individual units might vary I guess ?

[KP82]

Just now, John said:

That's a good point - these are theoretical charts for the lens design. Individual units might vary I guess ?

Yep. Doublets are easier to make than triplets plus 100mm is considerably smaller than 130mm, so I wouldn't be surprised to see a good sample of 100DZ beating a poor sample of TMB 130 f/9.2.

[festoon]

I don't know if this makes a difference, but does not image scale and focal length make a difference when comparing these charts. The DL will be relatively more magnified than the DZ, or are these scales normalised for focal length?

[JeremyS]

18 minutes ago, KP82 said:

At 0.750 on the Y axis (3/4 out from the centre of the lens, 0 on the Y axis is the centre. 3/4 accounts for most of the area of the lens surface), the lines are more closely bundled together in the DZ than DL. Also in the centre of the lens, the lines in the DZ are tighter together than the DL. Both point to superior colour correction of the DZ.

The X axis measures the focus deviation in mm. 0 means perfect focus. Each of the coloured lines represents a different visible wavelength. You want the 546nm to be as close to 0 and vertical to the Y axis as possible.

According to those 4 graphs, the theoretical performance are: 130 f/9.2 > 100DZ > 100DL > 125ED

Thanks. Why is there less deviation from focus (curves closest together) at 75% from the centre of the lens, than at the centre. I would have anticipated better correction at lens centre.

And what is pupil radius 62.5 mm? I see it in both LZOS and Technoski - may be it’s mentioned in Japanese on the Tak diagram.

And are these just models with no measurements? Or are they examples of real measurements?

Edited February 13, 2021 by JeremyS

[John]

20 minutes ago, KP82 said:

Yep. Doublets are easier to make than triplets plus 100mm is considerably smaller than 130mm, so I wouldn't be surprised to see a good sample of 100DZ beating a poor sample of TMB 130 f/9.2.

I think Takahashi maintain very consistent standards. I've not heard of any poor LZOS objectives. I think APM has quite stringent minimum specs for LZOS to meet:

https://www.apm-telescopes.de/media/files_public/uguxbbhkgbx/APMApo-Linsen-Spezifikationen.pdf

There will of course be the odd one that slips through the net.

I guess it is the level of quality control and consistency that one pays for with these premium priced items.

 

[GazOC]

Those diagrams show why the claim "I could see no false colour either in or out of focus" should be treated with extreme caution 

[KP82]

2 hours ago, festoon said:

I don't know if this makes a difference, but does not image scale and focal length make a difference when comparing these charts. The DL will be relatively more magnified than the DZ, or are these scales normalised for focal length?

These are normalised I believe.

[KP82]

2 hours ago, JeremyS said:

Thanks. Why is there less deviation from focus (curves closest together) at 75% from the centre of the lens, than at the centre. I would have anticipated better correction at lens centre.

And what is pupil radius 62.5 mm? I see it in both LZOS and Technoski - may be it’s mentioned in Japanese on the Tak diagram.

And are these just models with no measurements? Or are they examples of real measurements?

Because as you move away from the centre, you cover more surface area of the lens. And you want better colour correction for that majority area than the single point at the centre.

The pupil radius is the radius of the maximum image circle formed at the focal plane.

They represent the theoretical maximum performance of these particular optical designs, not real measurements of specimens.

Edited February 13, 2021 by KP82

[jetstream]

@JeremyS do you have the TSA102 and 120's?

[jetstream]

4 hours ago, John said:

I've not heard of any poor LZOS objectives.

Me neither. LZOS used to supply lenses for Zeiss- this says it all IMHO.

[F15Rules]

5 hours ago, JeremyS said:

And while we are about it, similar q for the Technosky diagram John Handleby has posted:

 

I'd have thought John Handleby would have a handle on the diagrams??

Dave

[andrew s]

The plots show the results of ray tracing the optical design. The x axis is  distance across the pupil (often the lens)  and the y axis a measure of the deviation of whatever aberration is being plotted. 

If you look just at the spot diagram it is best to compare it to the size of the diffraction limit circle. For the Tak 100 f7.4 it has a diameter of about 9.4 microns if the spot fits in it's diffraction limited. 

Regards  Andrew 

Looking again at @JeremyS diagram it looks like the x and y are the other way round. 

Edited February 13, 2021 by andrew s

[JeremyS]

On 13/02/2021 at 16:51, jetstream said:

@JeremyS do you have the TSA102 and 120's?

There’s a diagram on page 4 of the TSA 120 module, Gerry: https://teleskop-austria.at/information/pdf/TSA120_Manual.pdf

And pg 5 of the TSA 102: https://teleskop-austria.at/information/pdf/archiv-20190524170515-TSA102_Manual.pdf

I note it says the TSA design reduces CA to one third of that in a fluorite apo. But who’d use fluorite in a lens anyway? 🤣

Edited February 14, 2021 by JeremyS

[John]

So why did Takahashi drop the TSA102 model ?

 

[JeremyS]

11 minutes ago, John said:

So why did Takahashi drop the TSA102 model ?

 

No idea, but I hear a used one will be hitting the market soon 🤣👍🏻

[jetstream]

47 minutes ago, JeremyS said:

I note it says the TSA design reduces CA to one third of that in a fluorite apo. But who’d use fluorite in a lens anyway?

Holy cow!

yes better not put these diagrams up here...🤣 I can't believe a non fluorite scope could produce this level of colour correction!!

I'm off to find my manual! still in its hermetically sealed bag.

[JeremyS]

1 minute ago, jetstream said:

 

I'm off to find my manual! still in its hermetically sealed bag.

Along with that Tak sticker they always seem to include. I’ve never understood that.

[dweller25]

1 hour ago, JeremyS said:

No idea, but I hear a used one will be hitting the market soon 🤣👍🏻

Change of plan as it seems the TSA102 is x3 better than a fluorite scope like for example an FS102 😉

[Peter Drew]

Open a bag of salt, take a large pinch ...........etc,etc.        😀

[andrew s]

 

1 hour ago, dweller25 said:

Change of plan as it seems the TSA102 is x3 better than a fluorite scope like for example an FS102 😉

Not withstanding the salt, remember these are geometric optics traces and exclude diffraction.  Not even Takahashi can alter the fact that the size of the Airy disk is a function of wavelength. 

Regards Andrew 

Edited February 14, 2021 by andrew s