124 replies to this topic

[StarAlert]

Actually the FOA60, without the Q, is superior also.  I find this fascinating.  It is optically superior to a FC100 and to a TSA120.  

Seriously? Please explain your rationale.  Takahashi states that the FC100DZ has a minimum Strehl of "97.6% across the entire visible range", while the FOA60 has a Strehl "in excess of 96%". 

[NC Startrekker]

Actually the FOA60, without the Q, is superior also.  I find this fascinating.  It is optically superior to a FC100 and to a TSA120.  

Tom, no disputing that the FOA represents a phenomenal achievement by Takahashi for fluorite doublet optics, but it still will not "best" the equally well figured and executed triplet designs in the TOA and TSA lines.  Its simple physics.  I will let Takahashi's own literature on the TOA and TSA lenses speak for itself:

 

"The TSA design reduces color aberration to 1/3 that of fluorite apochromats."

 

"The Strehl ratio of a telescope is a numerical value that represents the percentage of the light of a star's image that actually falls into the Airy disk, compared to the theoretical maximum possible. A Strehl ratio of 0.95 is within 95% of perfection and is generally considered excellent. It equates to a 1/8th wave system accuracy. A Strehl ratio of 0.978 equates to a 1/12th wave accuracy. The Strehl ratio of the Takahashi triplet design is 0.992. This means that the Takahashi TOA and TSA objectives are within 99.2% of perfection. This compares with a Strehl ratio of 0.946 for a best-selling fluorite doublet system that has long been considered one of the very best telescopes available." [This quote predates the FOA which has a similar Strehl ratio to the TOA/TSA]

 

"The Takahashi triplets reduce the residual deviation from a flat line response over the blue to green portion of the visible spectrum of previous apo designs (even fluorite systems) by a third. The maximum deviation from all colors coming to a focus in precisely the same plane is no more than +/- 0.01mm from the blue end of the spectrum (436nm) to the H-alpha line at 656nm. The violet halo of chromatic aberration vanishes, and the tiny residual blue halation around bright stars at high powers essentially disappears. Stellar images are tight, with stars in the 12~20µm range, even at the very edges of the fully-illuminated image circle. CCD images are crisp and realistic, and visual observing is unparalleled in its clarity. Quite simply put, the Takahashi TOA and TSA optics have no equal."

Edited by NC Startrekker, 23 February 2021 - 01:15 PM.

[mikeDnight]

The fact that the FOA is so good really is a remarkable achievement. And as Alan points out, the quote he made from Takahashi predates the FOA, and is likely referring to the FS series, but it also predated the advances Tak have made with the latest incarnation of the FC100D series, the DZ. All the FC100D's are undeniably outstanding visual instruments,  but now with the DZ it appears Takahashi have actually produced a refractor doublet that is tickling the performance capability of the TSA. Plus it is lighter and will cool quicker too. 

[dweller25]

DZ v TSA - It’s only a light tickle 

 

Edited by dweller25, 23 February 2021 - 03:08 PM.

[25585]

The fact that the FOA is so good really is a remarkable achievement. And as Alan points out, the quote he made from Takahashi predates the FOA, and is likely referring to the FS series, but it also predated the advances Tak have made with the latest incarnation of the FC100D series, the DZ. All the FC100D's are undeniably outstanding visual instruments,  but now with the DZ it appears Takahashi have actually produced a refractor doublet that is tickling the performance capability of the TSA. Plus it is lighter and will cool quicker too. 

TSA-102 possibly. 120 has the larger aperture advantage. At sub-5" apertures, is there really that much difference in cooling down times?

[mikeDnight]

It can be as much as a 30 minute difference  in cool down between a triplet and a doublet, depending on the temperature. So if a doublet takes 15 mins then a triplet can take 45 mins.

[Ihtegla Sar]

It can be as much as a 30 minute difference  in cool down between a triplet and a doublet, depending on the temperature. So if a doublet takes 15 mins then a triplet can take 45 mins.

That difference in cool down can be huge for people who live in bad weather areas.  Last night, for instance was the first clear skies I have had since January 23, and the skies were only partially clear and only for about an hour.  That was long enough for me to realize that there was some partial clearing of the clouds, set up my FC100DL, wait ten minutes for it to cool and spend about half an hour viewing M42, M45 and the first quarter moon before the skies clouded over again. 

 

Seeing wasn't great and the moon was close to Orion, but I could still make out the E star in the Trap quite clearly, but couldn't quite make out the F star.  Also got a nice but quick view of the Moon and Pleiades.

 

If I had a triplet that took half hour or more to cook, I really wouldn't have had any time to observe with a thermally stable telescope last night.

[daquad]

Is Trap F achievable in a 4" ?  I would be surprised if the 100mm DZ could pull it off.  Maybe the TSA-120 at least.

I was able to see E & F in my C9.25 last October here in the NE when we had pretty steady seeing and good transparency.

But that kind of weather comes very seldomly in these parts.

In good seeing I can see the E and F Traps with a 4" f/15.5 Jaegers achromat.  An APO is not required, just decent optics and good seeing.

 

Dom Q.

[payner]

In good seeing I can see the E and F Traps with a 4" f/15.5 Jaegers achromat.  An APO is not required, just decent optics and good seeing.

 

Dom Q.

This is the key, good seeing.

[Rollo]

This is the key, good seeing.

I agree.   90% of the time it's just the seeing conditions.

[edif300]

DZ v TSA - It’s only a light tickle 

1C3F1E1F-F3E0-4C23-8043-FB67E7E0DFF0.jpeg

TSA plot goes down to 422 nm while DZ, DL and FOA to 436 nm.

(TOA 392 nm)

[dweller25]

TSA plot goes down to 422 nm while DZ, DL and FOA to 436 nm.

(TOA 392 nm)

There’s always something better somewhere - on paper.

 

But I suspect in the real world at the eyepiece the difference is negligible and the views are the same until you change the aperture size. 

Edited by dweller25, Yesterday, 06:24 AM.

[NC Startrekker]

Truly an academic discussion.  TOA, TSA, FOA, DZ, if you stopped them all down to the same aperture and equalized all other components of the optical train, I would challenge anyone to consistently distinguish between them at the eyepiece.  Strehl, spot diagrams, transmission curves, etc. are useful tools to the opticians and lens designers but are commonly misused by the lay users to try and rank order real world performance.  And, perhaps even more misused as a marketing tool.

Edited by NC Startrekker, Yesterday, 09:19 AM.

[Daniel Mounsey]

Truly an academic discussion.  TOA, TSA, FOA, DZ, if you stopped them all down to the same aperture and equalized all other components of the optical train, I would challenge anyone to consistently distinguish between them at the eyepiece.  Strehl, spot diagrams, transmission curves, etc. are useful tools to the opticians and lens designers but are commonly misused by the lay users to try and rank order real world performance.  And, perhaps even more misused as a marketing tool.

[alan.dang]

Truly an academic discussion. TOA, TSA, FOA, DZ, if you stopped them all down to the same aperture and equalized all other components of the optical train, I would challenge anyone to consistently distinguish between them at the eyepiece. Strehl, spot diagrams, transmission curves, etc. are useful tools to the opticians and lens designers but are commonly misused by the lay users to try and rank order real world performance. And, perhaps even more misused as a marketing tool.

But we could also argue that cars with upgraded paint schemes are just a marketing tool.

I agree that you may not see a difference but one of the reasons the “behind the scenes at Takahashi” resonates with so many of us is that there is a pride in craftsmanship that many of us appreciate. I think the FOA-60Q and the DZ in contrast to the other FC-100D_ line also reflect that craftsmanship. Most of us wouldn’t be able to see the difference between the DZ/DL/DC/DF but we still look at the DZ as the visual flagship.

Nothing beats getting out and observing but on cloudy nights, it can be fun debating what the best doublet ever made is.

Edited by alan.dang, Yesterday, 10:42 PM.

[StarAlert]

Truly an academic discussion.  TOA, TSA, FOA, DZ, if you stopped them all down to the same aperture and equalized all other components of the optical train, I would challenge anyone to consistently distinguish between them at the eyepiece.  Strehl, spot diagrams, transmission curves, etc. are useful tools to the opticians and lens designers but are commonly misused by the lay users to try and rank order real world performance.  And, perhaps even more misused as a marketing tool.

I stopped my DZ down to 60mm some time ago to see what all the hype was about. I posted my findings on an ongoing thread somewhere here on CN. What I got was dimmer images. I remember quite vividly that I could hardly make out the C star in Iota Cass. I took the mask off and bam! There it was. 

I’m keeping my DZ at full aperture, thank you, because the views are far brighter (and better) to me than any 60mm scope is going to provide. 

[25585]

Truly an academic discussion.  TOA, TSA, FOA, DZ, if you stopped them all down to the same aperture and equalized all other components of the optical train, I would challenge anyone to consistently distinguish between them at the eyepiece.  Strehl, spot diagrams, transmission curves, etc. are useful tools to the opticians and lens designers but are commonly misused by the lay users to try and rank order real world performance.  And, perhaps even more misused as a marketing tool.

Conversely, if they were all "stretched" to 130mm, or 150mm, which would be best? 

 

Of the doublets, DZ or DL or FOA? 

[teashea]

Do the crown and flint glasses produced today allow for a better optical figure? I wouldn't think so. But I am sure modern glasses are more homogenous and have fewer inclusions, such as bubbles and/or striae. And of course, modern glass types allow for better color correction.

 

My post was only with regard to the Clarks' skill in figuring an objective- with the materials that were available at the time.

 

Too bad we can't go back in time and give Alvan Clark, Sr. an 8 inch fluorite blank and suitable glass for the negative element...

 

Steve

 

P.S.- I can't remember if it was Roland Christen or if it was D&G optical that tested a particular Clark lens and found it to be better than 1/40th wave- and beyond the sensitivity of the equipment being used!

 

P.P.S.- If you have a "bad Clark lens" story, please don't post it and burst my bubble! 

A most excellent question.  

[teashea]

Tom, no disputing that the FOA represents a phenomenal achievement by Takahashi for fluorite doublet optics, but it still will not "best" the equally well figured and executed triplet designs in the TOA and TSA lines.  Its simple physics.  I will let Takahashi's own literature on the TOA and TSA lenses speak for itself:

 

"The TSA design reduces color aberration to 1/3 that of fluorite apochromats."

 

"The Strehl ratio of a telescope is a numerical value that represents the percentage of the light of a star's image that actually falls into the Airy disk, compared to the theoretical maximum possible. A Strehl ratio of 0.95 is within 95% of perfection and is generally considered excellent. It equates to a 1/8th wave system accuracy. A Strehl ratio of 0.978 equates to a 1/12th wave accuracy. The Strehl ratio of the Takahashi triplet design is 0.992. This means that the Takahashi TOA and TSA objectives are within 99.2% of perfection. This compares with a Strehl ratio of 0.946 for a best-selling fluorite doublet system that has long been considered one of the very best telescopes available." [This quote predates the FOA which has a similar Strehl ratio to the TOA/TSA]

 

"The Takahashi triplets reduce the residual deviation from a flat line response over the blue to green portion of the visible spectrum of previous apo designs (even fluorite systems) by a third. The maximum deviation from all colors coming to a focus in precisely the same plane is no more than +/- 0.01mm from the blue end of the spectrum (436nm) to the H-alpha line at 656nm. The violet halo of chromatic aberration vanishes, and the tiny residual blue halation around bright stars at high powers essentially disappears. Stellar images are tight, with stars in the 12~20µm range, even at the very edges of the fully-illuminated image circle. CCD images are crisp and realistic, and visual observing is unparalleled in its clarity. Quite simply put, the Takahashi TOA and TSA optics have no equal."

Excellent information.

[mikeDnight]

Is Trap F achievable in a 4" ?  I would be surprised if the 100mm DZ could pull it off.  Maybe the TSA-120 at least.

I was able to see E & F in my C9.25 last October here in the NE when we had pretty steady seeing and good transparency.

But that kind of weather comes very seldomly in these parts.

F is easily achievable in any good 4" refractor. My FC100DC plucked that out every time when the seeing was steady, and my FC100DZ is just as capable. So really, the six stars of the trapezium are a good indicator of the atmospheric seeing in a 4", with F being the most difficult, although its not actually that difficult, its just closer to its primary than the E star. 

[payner]

F is easily achievable in any good 4" refractor. My FC100DC plucked that out every time when the seeing was steady, and my FC100DZ is just as capable. So really, the six stars of the trapezium are a good indicator of the atmospheric seeing in a 4", with F being the most difficult, although its not actually that difficult, its just closer to its primary than the E star. 

Companion star F is more difficult to see because it is a bit closer (~0.1") compared to AE, but primarily due to the magnitude difference between C and F compared to A and E. That makes steady skies critical to separating CF.

[Alan French]

There are some Hartmann tests of some large achromats on page 268 of Louis Bell's "The Telescope" (Dover reprint). The tests include 3 Clark refractors, the 76.2cm Pulkown-Clark, the 61cm Lowell-Clark, and the 101.6cm at Yerkes.

 

Clear skies, Alan

[Daniel Mounsey]

I stopped my DZ down to 60mm some time ago to see what all the hype was about. I posted my findings on an ongoing thread somewhere here on CN. What I got was dimmer images. I remember quite vividly that I could hardly make out the C star in Iota Cass. I took the mask off and bam! There it was. 

I’m keeping my DZ at full aperture, thank you, because the views are far brighter (and better) to me than any 60mm scope is going to provide. 

 

This has more do do with being familiar with what double stars look nicer per a given aperture. The DZ's color correction can help when pushing the absolute limits but there's clearly a lot of misunderstandings others have interpreting the outcome of color correction. 

[Ihtegla Sar]

Companion star F is more difficult to see because it is a bit closer (~0.1") compared to AE, but primarily due to the magnitude difference between C and F compared to A and E. That makes steady skies critical to separating CF.

That makes sense. I live under the jet stream, so steady skies are rare, especially when Orion is high in the sky. I've never been able to see F in any of my scopes including my 20" Obsession. But I can usually see E in my FC100DF and my FC100DL. Couple nights ago, E was quite visible through a sucker hole in the DL but F was no where to be seen.

[gwlee]

All of my Taks were 100x per inch killers.

Which I suppose is important to those who enjoy looking through 0.25mm exit pupils. 

Edited by gwlee, Yesterday, 10:57 PM.