71 replies to this topic

[syxbach]

Hello I am pretty interested in this new 140 mm triplet. Dual ED 140!!!  Any expert can get some information from this spot diagram?

 

http://www.sharpstar...series/232.html

 

Thanks!

 

Yuexiao

Attached Thumbnails

• 
• 
• 
• 

Edited by syxbach, 10 October 2019 - 09:10 AM.

[dscarpa]

 The SVX 140T uses FPL53 and Lanthanum. I wouldn't be surprised if the the Sharpstar does too. David 

[syxbach]

I contacted to their manager a lot. They did not tell me the combination of the glass used. I also got some information from some dealers in China that from 1 year before, Sharpstar's own refactors (such as 61EDPH, 76EDPH) started to used domestic ED glass rather than FPL53 and FCD100 from Japan. I spent some time to figure out what glass they are using. Finally, I found a glass with similar properties with FPL53 and FCD100. The product name is H-FK95N. This is new this year. 

 

http://www.cdgmgd.co...pdf/H-FK95N.pdf

 

I think because of using domestic products, they can afford to use two ED glasses on one 140 mm refractor. However, I still do not know the combination. I am quite excited about it, its price and performance. 

 

 

 The SVX 140T uses FPL53 and Lanthanum. I wouldn't be surprised if the the Sharpstar does too. David 

Edited by syxbach, 10 October 2019 - 06:33 PM.

[25585]

[junomike]

This isn't that new as I recall referencing it back when the ES 140 came out (same OTA IMO).

 

See Post #18

[syxbach]

They introduced it at the end of last year, but only recently they provided those specs and have stocks. More importantly, I think ES 140 uses FPL53 glass (if that is rebranded, although FL and aperture are the same) while sharpstar's own version will use dual ED. Will see how the new scope behave. I already put it on my list

 

 

This isn't that new as I recall referencing it back when the ES 140 came out (same OTA IMO).

 

See Post #18

[Alan French]

I don't see any reason there should be an advantage to using two ED elements, and there are certainly some disadvantages, beginning with cost. 

 

Ohara FPL55, for example, is 15.5 times the cost of BSL7. So if a BSL7 blank costs $100, a FPL55 blank would be $1550. Using two FPL55 blanks instead of one would make a large price differential in raw material.

 

But you know marketing - if one ED element is good, then two must be great! Convince the consumer it's true, and you're golden. 

 

Clear skies, Alan

[dscarpa]

 Somehow I doubt Tak would put two ED elements in the TOAs  just  for marketing purposes.  It puts them at disadvantage cost wise vs brands that  use one. The dual ED doublets have been getting rave reviews for quite some time. I'll soon be able to weigh in on that score as I've got a Vixen 130ED SS dual ED doublet on the way! David

[Vla]

TOA is not a usual PNP triplet. The reason they went with a double ED is that higher order spherical can be practically eliminated only using a wide first gap, with the front lens being positive (i.e. forming converging beam). That's what I found out when trying to create TOA-like objective.

[Suavi]

I don't see any reason there should be an advantage to using two ED elements, and there are certainly some disadvantages, beginning with cost. 

 

Ohara FPL55, for example, is 15.5 times the cost of BSL7. So if a BSL7 blank costs $100, a FPL55 blank would be $1550. Using two FPL55 blanks instead of one would make a large price differential in raw material.

 

But you know marketing - if one ED element is good, then two must be great! Convince the consumer it's true, and you're golden. 

 

Clear skies, Alan

Why settle just on 2 ED elements - get rid of the middle inferior glass and make a pure 3 ED elements triplet instead - that would have to be the best refractor of them all, until someone would make a 6 ED elements sextuplet that is 

[Alan French]

TOA is not a usual PNP triplet. The reason they went with a double ED is that higher order spherical can be practically eliminated only using a wide first gap, with the front lens being positive (i.e. forming converging beam). That's what I found out when trying to create TOA-like objective.

Interesting, but the wide air gap is a bit different than the design being asked about here.

 

Without utilizing a wide air gap to remove most of the higher order spherical aberration, is there any advantage to using two ED elements?

 

Thanks, and clear skies, Alan 

[syxbach]

I think this is just a demonstration. I do not know the exact spacing for sure:) Two EDs can help reduce CA, IMO. The reason because they found a replacement for FPL53 or FCD100. 

 

Yuexiao

Interesting, but the wide air gap is a bit different than the design being asked about here.

 

Without utilizing a wide air gap to remove most of the higher order spherical aberration, is there any advantage to using two ED elements?

 

Thanks, and clear skies, Alan 

[junomike]

They introduced it at the end of last year, but only recently they provided those specs and have stocks. More importantly, I think ES 140 uses FPL53 glass (if that is rebranded, although FL and aperture are the same) while sharpstar's own version will use dual ED. Will see how the new scope behave. I already put it on my list

Good point.  I don't think however that this Sharpstar is using dual FPL-53 (like Tak does).  Interesting however.

[Tyson M]

Looks like a promising scope! 

[Alan French]

With permission, here is what Roland Christen says about this...

 

"There is zero advantage to using 2 ED elements with mate in middle versus 2 mates with ED in middle. ZERO!"

 

Roland pointed out that a triplet is simply two doublets combined. With the middle elements of the same glass, they can just be merged into a single element, creating a triplet.

 

So you can have a Crown-ED + ED-Crown becoming a Crown-EDED-Crown and thus Crown-ED-Crown (Steinheil + Fraunhofer).

 

Or an ED-Crown + Crown-ED becoming an ED-CrownCrown-ED and thus ED-Crown-ED (Fraunhofer + Steinheil).

 

Two f/15 doublets, with the same glasses adjoining and allowed to fuse together, form an f/7.5 triplet.

 

Whether you create an Crown-ED-Crown triplet or a ED-Crown-ED triplet. "the result is almost exactly the same - no change in color correction or any of the Seidel aberrations." - Roland Christen

 

Clear skies, Alan

 

 

[Vla]

Without utilizing a wide air gap to remove most of the higher order spherical aberration, is there any advantage to using two ED elements?

I wouldn't draw general conclusions, since it can vary with configurations and glass used, except that differences should be small. Here's an example of three triplets with the same glass combo: standard PNP (top), standard NPN (middle) and TOA-like PNP. All three 140mm f/6.5,so fully comparable. The first has little lower spherochromatism than the second one, but inferior central line correction. TOA-like has less spherochromatism (I should have said it has eliminated spherochromatism, not just higher order spherical)  than either, excellent central line correction, but somewhat more astigmatism.

Attached Thumbnails

• 

Edited by Vla, 11 October 2019 - 09:15 PM.

[Suavi]

One practical long term advantage of a Crown-ED-Crown lens cell over ED-Crown-Ed lens cell is that Crown is significantly harder than ED, so it scratches less easily. Properly cleaning and ED element is therefore somehow less forgiving. From memory, crown glass might also be more chemically resistant than ED, which may or may not matter.

[Alan French]

I wouldn't draw general conclusions, since it can vary with configurations and glass used, except that differences should be small. Here's an example of three triplets with the same glass combo: standard PNP (top), standard NPN (middle) and TOA-like PNP. All three 140mm f/6.5,so fully comparable. The first has little lower spherochromatism than the second one, but inferior central line correction. TOA-like has less spherochromatism (I should have said it has eliminated spherochromatism, not just higher order spherical)  than either, excellent central line correction, but somewhat more astigmatism.

Thank you very much! I appreciate it.

 

Clear skies, Alan

[syxbach]

Hi Alan

 

Much appreciate your input. Definitely Roland is an expert on this. This is the first time for me to learn how to read a spot diagram and judge an APO if it is good or not. I am new to this field, but love to own a 140 mm APO for imaging. Here TEC140 is a standard but much more expensive and difficult to get. I guess the price for this new scope will be more accessible for me. As long as the specs are close to those of TEC140, I will be satisfied with it. Actually, I do not care how they made and selected the glass, as long as I can use it for imaging, sharp results, flat field and round stars, but I would love to learn how to read the diagram and judge the scope. 

 

That is the reason why I sent these information here and ask for suggestions and comments. I also sent those diagrams to a CN member who can read these and got his response. That is for a faster scope like this (with reducer to F4.8), it should be very good for imaging.

 

If that is true, the quality control is another aspect. For the scope like this, no body wants to get it and find something wrong such as collimation. Sharpstar is making an effort to make better APOs with reasonable price. I hope the quality can be good enough.

 

Best

 

Yuexiao

 

 

With permission, here is what Roland Christen says about this...

 

"There is zero advantage to using 2 ED elements with mate in middle versus 2 mates with ED in middle. ZERO!"

 

Roland pointed out that a triplet is simply two doublets combined. With the middle elements of the same glass, they can just be merged into a single element, creating a triplet.

 

So you can have a Crown-ED + ED-Crown becoming a Crown-EDED-Crown and thus Crown-ED-Crown (Steinheil + Fraunhofer).

 

Or an ED-Crown + Crown-ED becoming an ED-CrownCrown-ED and thus ED-Crown-ED (Fraunhofer + Steinheil).

 

Two f/15 doublets, with the same glasses adjoining and allowed to fuse together, form an f/7.5 triplet.

 

Whether you create an Crown-ED-Crown triplet or a ED-Crown-ED triplet. "the result is almost exactly the same - no change in color correction or any of the Seidel aberrations." - Roland Christen

 

Clear skies, Alan

[Alan French]

One practical long term advantage of a Crown-ED-Crown lens cell over ED-Crown-Ed lens cell is that Crown is significantly harder than ED, so it scratches less easily. Properly cleaning and ED element is therefore somehow less forgiving. From memory, crown glass might also be more chemically resistant than ED, which may or may not matter.

 

FPL55, Knoop hardness, 340 (class 3). 

 

BSL7, Knoop hardness, 570 (class 6).

 

From the Ohara datasheets.

 

Clear skies, Alan

[noisejammer]

It's not my place to contradict Rolando, but the SA from the TOA arrangement is 5x smaller (note the different scales.) This suggests there is plenty of reason to use two slabs of FPL53, if it's done correctly.

[Alan French]

It's not my place to contradict Rolando, but the SA from the TOA arrangement is 5x smaller (note the different scales.) This suggests there is plenty of reason to use two slabs of FPL53, if it's done correctly.

But his point was that the same can be accomplished with a single element of FPL53 and two crowns. 

 

It's the use of a large air space that provides the improvement in the Tak.

 

Clear skies, Alan

[dscarpa]

 My WO ZS110 F7 with a Japanese made TMB designed triplet using single ED element of FPL51 is a excellent optic.  I use 85X per inch for L&P and doubles in it a lot and only CA it shows is sometimes a little on Venus which it shows shadings on. David

Edited by dscarpa, 13 October 2019 - 11:53 AM.

[jay.i]

But his point was that the same can be accomplished with a single element of FPL53 and two crowns. 

 

It's the use of a large air space that provides the improvement in the Tak.

 

Clear skies, Alan

My question is, then, why didn't Takahashi use a single FPL-53 element? It would be cheaper right? I feel like there is some bit of information we're missing, like the fact that it would be difficult to achieve the level of polish required, or you'd need a super robust cell for exact placement of the elements, or something like that. Because, again, why wouldn't you go with the cheaper option to get "the same" correction if it's possible to do so? There must be some caveats here.

[payner]

The TOA refractors are similar to Cooke designed wide spaced air gaps between lenses. They are, to me, the finest corrected refractors available. There must be a reason for the use of two FPL-53 elements or Takahashi designers would not have opted for that. Otherwise, it puts them at a distinct pricing disadvantage. Consider the correction being discussed is available in a 150-mm aperture at f/7.3. Pretty amazing. And while highly correcting for CA, they have virtually eliminated SA; that is impressive.