ED doublet refractors. Objective lens type & quality.

[Guest]

Just thinking about refractor that I have owned over the years. I currently own a Skywatcher Eqinox 80mm it has one lens which is FLP 53 glass and a Schott glass mating element. For visual it gives excellent views. ED 100mm F9 with FLP 53 glass have given me very good views. As did a Skywatcher ED 120mm. One of the best views of the Double Cluster was given by an ST 120 mm in a dark sky setting. This was an chromatic doublet. Not an exotic ED lens.

I did own a Vixen for a short while. Nice views.

I understand that Takahashi refractors are excellent. As are Vixens and others.

But I have not had the opportunity to observe through  Taks. Perhaps one day.

Interestingly there are now available 102 mm ED refractors with F7 focal ratios.

Several similar looking models with different brands. One company states that one objective element is FLP 53 without saying what type of glass that the mating element is. Another states the objective as being FLP 53 with a Lanthanum mating element. Would this refractor be better than the other? Is it hit and miss buying refractors with regards to performance for visual observing?

 

Edited June 2, 2020 by Guest

[johninderby]

The StellaMira 80mm f/10 doublet has thoroughly impressed me. Has 1x ED APO FPL-53 lens and 1x Lanthanum lens (both Japanese Ohara). Amazingly sharp and contrasty optics and absolutely zero CA. Far better than a SW80ED but then it costs nearly 2 1/2 times as much. William Optics level of build quality. 

 

[GazOC]

It's also a lot longer than the ED80, there'd be something seriously amiss if the Stellamira wasnt better corrected for colour than the ED80 lanthanum or no lanthanum.

[johninderby]

It’s not just colour correction its the increased sharpness and contrast. Just higher grade optics.and a big step up in performance.

Edited June 2, 2020 by johninderby

[Stu]

4 minutes ago, johninderby said:

It’s not just colour correction its the increased sharpness and contrast. Just higher grade optics.and a big step up in performance.

Have you done a side by side comparison John? I would expect improvements but the ED80 is a good scope so would expect incremental gains rather than a big step up.

[GazOC]

It'll be interesting when the tests start showing up for those scopes for sure

[johninderby]

I did have a Celestron ED80 which is the same optics as the SWED80 so am familiar with the performance. The StellaMira is in a different class and don’t forget it is a lot more expensive so not surprising it outperforms the ED80. I was surprised at the improvement over the ED80.

The ED80 is still good value for money though so a good alternative if you don’t want to spend more.

[michael.h.f.wilkinson]

21 minutes ago, johninderby said:

The StellaMira 80mm f/10 doublet has thoroughly impressed me. Has 1x ED APO FPL-53 lens and 1x Lanthanum lens (both Japanese Ohara). Amazingly sharp and contrasty optics and absolutely zero CA. Far better than a SW80ED but then it costs nearly 2 1/2 times as much. William Optics level of build quality. 

 

Absolute zero chromatic aberration doesn't exist for a doublet, ED glass or not ("Ye cannae change the laws of physics capt'n" as Scotty would say ). Being F/10, all aberrations will be reduced, possibly to the level of being invisible. It will also be much kinder on the EPs, resulting in sharper views with far less astigmatism towards the edge of the FOV

[Stu]

Just now, johninderby said:

I did have a Celestron ED80 which is the same optics as the SWED80 so am familiar with the performance. The StellaMira is in a different class and don’t forget it is a lot more expensive so not surprising it outperforms the ED80. I was surprised at the improvement over the ED80.

The ED80 is still good value for money though so a good alternative if you don’t want to spend more.

Understood, although I think it difficult to compare scopes from memory as that big variable ‘sky conditions’ can have a huge impact on perceived performance on different nights.

Again, I’m not saying the StellarMira isn’t better, it should be, it’s just quantifying how much that is the challenge and that really needs a side by side on the same night.

[Guest]

Mine is the 80 mm Equinox at F 6.2. The 80 mm ED is F 7.5. So I accept that there will be colour visible. A great little performer. My first refractor was an 80 mm F 16 Towa achromat. Due to it''s tube length have very sharp contrasty views of the moon and planets.

Just interested to here if anybody knows about the objective elements of the various 102mm F 7 ED's with different brands but very similar looking refractors.

Edited June 2, 2020 by Guest

[GazOC]

I had 80mm f15 achro years ago that was a very good planetary/ lunar scope but it fell into disuse due to its length when I got a ED100

Edited June 2, 2020 by GazOC
Typo

[johninderby]

Perhaps should have put have never seen any CA.

Yes difficult to compare from memory and difficult to quanitify it except that I never remeber the C80ED giving me the views that the StellaMira does. It’s the contrast and sharpness that impress.

It does seem that something new and improved comes out quite often nowadays. Hard to keep up. 😁

 

Edited June 2, 2020 by johninderby

[johninderby]

Oh yes FLO has shipped the ScopeTech 80 f/15 this aft so look forward to comparing the planetary views with the StellaMira. 🙂

[John]

I've tended to take the view that if a company uses FPL-53, they have selected a very expensive glass type so are likely:

a) to select a suitable and equal quality mating element.

b) to take some care over the figure, polish and coatings of the lenses.

I've owned 9 ED doublets over the years from a range of manufacturers and still own 3 of them. I can't say that I've come across one that has been less than a very good performer.

Remember that Lanthanum is a group of glasses, not just one glass type.

The figure and polish of the lenses is also a key part of the performance potential.

 

 

 

 

 

 

[Adam J]

11 minutes ago, John said:

I've tended to take the view that if a company uses FPL-53, they have selected a very expensive glass type so are likely:

a) to select a suitable and equal quality mating element.

b) to take some care over the figure, polish and coatings of the lenses.

I've owned 9 ED doublets over the years from a range of manufacturers and still own 3 of them. I can't say that I've come across one that has been less than a very good performer.

Remember that Lanthanum is a group of glasses, not just one glass type.

The figure and polish of the lenses is also a key part of the performance potential.

 

 

 

 

 

 

I thought Lanthanum was a material used as a coating and or doping to the flint element as opposed to the glass itself.  Basically acts as a intergrated minus violet filter.

Adam

Edited June 2, 2020 by Adam J

[John]

2 minutes ago, Adam J said:

I thought Lanthanum was a material used as a coating and or doping to the flint element as opposed to the glass itself.  Basically acts as a intergrated minus violet filter.

Adam

I think it is also the term used for glass which incorporates lanthanum oxide during the melt stage ?

 

[mikeDnight]

4 hours ago, Grumpy Martian said:

Just thinking about refractor that I have owned over the years. I currently own a Skywatcher Eqinox 80mm it has one lens which is FLP 53 glass and a Schott glass mating element. For visual it gives excellent views. ED 100mm F9 with FLP 53 glass have given me very good views. As did a Skywatcher ED 120mm. One of the best views of the Double Cluster was given by an ST 120 mm in a dark sky setting. This was an chromatic doublet. Not an exotic ED lens.

I did own a Vixen for a short while. Nice views.

I understand that Takahashi refractors are excellent. As are Vixens and others.

But I have not had the opportunity to observe through  Taks. Perhaps one day.

Interestingly there are now available 102 mm ED refractors with F7 focal ratios.

Several similar looking models with different brands. One company states that one objective element is FLP 53 without saying what type of glass that the mating element is. Another states the objective as being FLP 53 with a Lanthanum mating element. Would this refractor be better than the other? Is it hit and miss buying refractors with regards to performance for visual observing?

 

It's possible that the scope that doesn't claim a lanthanum element still uses lanthanum. I bought a Baader Zeiss BBHS prism from FLO, but there was no indication in the advert about its link to Zeiss. When it arrived, Zeiss was printed on the box. Also, I have used an old version (non-FPL53) of the F7, which I think is actually F6.9, and it was excellent. It played very nicely alongside my FC100DC at the time, and good quality all round. It was paulastro who owned it at the time. 

It's also interesting that Tak don't go into detail about their mating elements, which apparently differ between the DC/DF, DL and DZ, but all are superb scopes. I suspect whichever F7 you chose would still give wonderful views.

[John]

Skywatcher now have a policy of not revealing the glass type of either the ED or the mating element for their latest ED doublets (the ED72 and ED150). Vixen and Tele Vue did not used to either. Their policies may have changed recently though ?

 

 

[Guest]

I believe that Skywated did use Schott glass. It was printed on the tube of some of their telescopes. They appeared to have stopped printing the Schott glass logo where they once did.

[johninderby]

4 minutes ago, Grumpy Martian said:

I believe that Skywated did use Schott glass. It was printed on the tube of some of their telescopes. They appeared to have stopped printing the Schott glass logo where they once did.

They are still using the Schott glass apparently but an agreement to put the Schott glass name on the scopes has run out. 

[John]

The ED element of the 80, 100 and 120 is Ohara FPL-53 glass and the mating element Schott BK7. With the 72 and 150, we don't know but they seem to work pretty well

I never quite know why Skywatcher didn't do a deal with Ohara to put a sticker on. That's the expensive stuff !

I guess Schott is perhaps a better known name for glass generally ?

[mikeDnight]

Schott is just the manufacturer though, not the glass type. Same as Takahashi using Cannon Optron, but they don't advertise it, and it doesn't indicate the type of mating element. When I first decided on buying an apo, many years ago, I gave no thought to the mating element. All I was concerned about was that it must contain fluorite. I had no idea why, other than I'd seen how good fluorite scopes could be. Later, a friend who couldn't afford a fluorite refractor, bought a Vixen ED. First he bought a second hand 102mm F9 which I thought was amazing. He later sold it and bought a 102mm F6.5 ED. I told him he was crazy for selling the longer ED but he went ahead anyway. When his short apo arrived I was yet again stunned by just how good it was. It seemed the more I learned the less I knew!

My friends Vixen 102mm F6.5 ED didn't state any particular glass types other than the general title of ED Apo, yet that scope floored my Televue NP101 IS on planetary. The 101 was sold the same week! Even today that old Vixen will give any modern apo, ED, Fluorite, doublet, triplet or quad a serious run for their money. The beauty of that old Vixen ED most likely lay in the quality of its optical figure rather than any particular glass type, and I think the same would apply to any of today's ED's.

[John]

I came across this post on another forum by Roland Christen of Astro Physics. It was dated 2004 though so things might have moved on in 16 years:

"FPL53 is the most advanced ED glass with the highest V number, which allows a lens of fastest focal ratio to be made. It is the most expensive of the lot. FPL52 is an intermediate ED glass and FPL51 is a first generation ED glass, which is least expensive. OK4 is the Russian equivalent to FPL52.

The overall color correction is dependent on the mating element, and all of these glasses can be made into lenses with any kind of color correction from poor to essentially perfect. The only difference would be that the glasses with the highest V number would have the lowest sphero-chromatism for any given design and F-ratio. Therefore, for equal performance, the ED with the lower V number would need to have a longer focal ratio for equal performance."

 

[Adam J]

10 hours ago, John said:

I think it is also the term used for glass which incorporates lanthanum oxide during the melt stage ?

 

Yes thats doping. So a typically rare earth nanoparticles suspended in the gaps between the polycrystaline matrix of the glass but not chemically part of the glass itself, hence if you padantic like me then Lanthanum is not a type of glass.  Some Lasers are made like that, hence things like Nd-Glass lasers where by the Neodymium is the active lasing element.

But you can also sputter deposit it onto the surface such as you would an AR coating and acheive the same effect.

Adam

Edited June 3, 2020 by Adam J

[John]

9 hours ago, Adam J said:

Yes thats doping. So a typically rare earth nanoparticles suspended in the gaps between the polycrystaline matrix of the glass but not chemically part of the glass itself, hence if you padantic like me then Lanthanum is not a type of glass.  Some Lasers are made like that, hence things like Nd-Glass lasers where by the Neodymium is the active lasing element.

But you can also sputter deposit it onto the surface such as you would an AR coating and acheive the same effect.

Adam

I'm not the pedantic type