ED vs APO - in layman's terms

[MaJiC79]

So I'm perusing astrogear for sale (pick a site, there are many) and I see people listing ED Doublets as Apochromatic. As I understand it, the doublet brings two wavelengths to a common focus, and a triplet converges three. Can an ED doublet really focus three wavelengths?

[wulfrun]

I think it's worth pointing out that the number of objective lenses doesn't correspond to the number of wavelengths correctly brought to focus.  A doublet doesn't bring two wavelengths to correct focus and a triplet three. Worth reading up:

https://en.wikipedia.org/wiki/Chromatic_aberration

Can't answer your actual question though.

[MaJiC79]

That's the thing, I have read that article (and several related ones) which point to a common Achromatic design using a doublet, and a common Apochromatic design using a triplet. And yes, I know there is more than one way to skin this mouse, and that's the reason for my question. Can an ED doublet truly offer apochromatic colour correction, or is it snake oil marketing?

[Louis D]

Have you read this webpage?

Semi-apo and apo lens objective examples

Here's an instructive set of diagrams of various doublets and triplets:

The glass types are explained in the table above it on that webpage.

My takeaway from this diagram is that there is no one best solution.  Refractors will always be a compromise when it comes to bringing all wavelengths to a common focus across the entire field.

[tompato]

Aren't the ones that are doublets that use ED glass usually called semi-apochromatic? I think because the glass (or coating) used is such a high quality it significantly reduces chromatic aberration.

I guess some places maybe just 'forget' the semi part and label them as apochromatic? Not sure. 

But yes I believe a true APO should be a triplet. I could be wrong though.

[johninderby]

The old definition of what is a APO doesn’t really work nowadays due to advances in optics and modern coatings.

A good modern ED doublet is probably better than a cheap triplet APO. The quality of the optics is the important bit and a good modern ED doublet can certainly be an APO in reality even if it doesn’t meet the old definition. 

Semi-APO is an old term that refered to an achro doublet that was better than a regular achro and shouldn’t be used to describle a modern ED doublet. 

So it’s the quality of the scope that matters rather than a strict definition. Yes a high end ££££££ triplet is the best but most triplets aren’t high end. 

 

Edited January 9, 2021 by johninderby

[MaJiC79]

24 minutes ago, Louis D said:

Have you read this webpage?

Semi-apo and apo lens objective examples

Here's an instructive set of diagrams of various doublets and triplets:

The glass types are explained in the table above it on that webpage.

My takeaway from this diagram is that there is no one best solution.  Refractors will always be a compromise when it comes to bringing all wavelengths to a common focus across the entire field.

Thanks, that's a really interesting article - it's a lot more detailed than I was expecting!

Ok, so I'm more reassured now than I was. As always, it's buyer beware - so I will need to know a bit more about what I'm buying into before I commit some significant cash into anything

[tompato]

15 minutes ago, johninderby said:

The old definition of what is a APO doesn’t really work nowadays due to advances in optics and modern coatings.

A good modern ED doublet is probably better than a cheap triplet APO. The quality of the optics is the important bit and a good modern ED doublet can certainly be an APO in reality even if it doesn’t meet the old definition. 

Semi-APO is an old term that refered to an achro doublet that was better than a regular achro and shouldn’t be used to describle a modern ED doublet. 

So it’s the quality of the scope that matters rather than a strict definition. Yes a high end ££££££ triplet is the best but most triplets aren’t high end. 

 

Ah that's interesting, it makes sense tho as I guess it's the end result that's important.

[MaJiC79]

25 minutes ago, tompato said:

Aren't the ones that are doublets that use ED glass usually called semi-apochromatic? I think because the glass (or coating) used is such a high quality it significantly reduces chromatic aberration.

I guess some places maybe just 'forget' the semi part and label them as apochromatic? Not sure. 

But yes I believe a true APO should be a triplet. I could be wrong though.

I've not come across semi-apo, but I have seen super-apo. I'm wondering how much this can be modeled and measured. Yet another project (black hole) I may dive into

[johninderby]

It’s real world vs theory. Judge each scope on it’s merits not a strict definition. 

A rather fine doublet.

https://www.firstlightoptics.com/takahashi-fc-100-series-refractor-telescopes/tak_tfk10310.html

[MaJiC79]

4 minutes ago, johninderby said:

It’s real world vs theory. Judge each scope on it’s merits not a strict definition. 

A rather fine doublet.

https://www.firstlightoptics.com/takahashi-fc-100-series-refractor-telescopes/tak_tfk10310.html

One of the attractions for me is that this is a science - every aspect and adjustment can be measured. Judging a scope on its merits means having some quantification of those merits. That very nice looking scope does publish those merits, and I think when I buy an APO (ED or triplet) I will be looking for that same information so I can make the judgement

[johninderby]

Best to ask questions about any scope to get the real world experiences of actual users here on SGL and what they are using the scope for.

[Stu]

1 hour ago, johninderby said:

It’s real world vs theory. Judge each scope on it’s merits not a strict definition. 

A rather fine doublet.

https://www.firstlightoptics.com/takahashi-fc-100-series-refractor-telescopes/tak_tfk10310.html

It is indeed! I think for visual it is hard to beat a fluorite doublet; lovely contrast, lightweight and fast cool down time.

[mikeDnight]

1 hour ago, MaJiC79 said:

I've not come across semi-apo, but I have seen super-apo. I'm wondering how much this can be modeled and measured. Yet another project (black hole) I may dive into

I think there's quite a bit of marketing hype out there that can be tricky to navigate. 

[johninderby]

There used to be many heated discussions years ago about the term semi-apo. 😁

[johninderby]

8 minutes ago, Stu said:

It is indeed! I think for visual it is hard to beat a fluorite doublet; lovely contrast, lightweight and fast cool down time.

I really like a doublet for visual. Allways impressed with my StellaMira 80 f/10 every time I use it. No noticeable CA at all and will do me until I can afford to replace it with an FC-100DZ. 🙂

[paulastro]

16 hours ago, johninderby said:

The old definition of what is a APO doesn’t really work nowadays due to advances in optics and modern coatings.

A good modern ED doublet is probably better than a cheap triplet APO. The quality of the optics is the important bit and a good modern ED doublet can certainly be an APO in reality even if it doesn’t meet the old definition. 

Semi-APO is an old term that refered to an achro doublet that was better than a regular achro and shouldn’t be used to describle a modern ED doublet. 

So it’s the quality of the scope that matters rather than a strict definition. Yes a high end ££££££ triplet is the best but most triplets aren’t high end. 

 

I think this is a very good summary of the situation John.  What's written on the side of the dew shield, and if the lens has two, three or more elements, isnt necessarily  a good guide to performance.

As youself and Stu have said, a good doublet refractor is hard to beat visually, and is my weapon of choice.  The current range of Takahashi doublets are the best in my view,, but the good news is that there are a few other makes that have a high % of the performance of a Tak for a fraction of the price. 

In a similar theme, though FPL53 is considered the choice of glass after fluorite, I've owned a couple of non-FPL53 doublets that I'd certainly buy again - in fact I did in one case!   

If you are an experienced refractor user, you will know a good refractor when you look through it, and if you're not, SGL is a good place to point you in the right direction.  In the end of course, it's always best to try before you buy.

 

Edited January 10, 2021 by paulastro

[Mr Thingy]

6 hours ago, paulastro said:

The current range of Takahashi doublets are the best in my view,, but the good news is that there are a few other makes that have a high % of the performance of a Tak for a fraction of the price. 

I'd be interested in a few examples, if you could. I don't have any experience of high performance refractors, so tips on some good ones that are below Takahashi price would be very useful.

TIA

Edited January 10, 2021 by Mr Thingy

[johninderby]

Love my StellaMira 80mm doublet for visual. A exceptional performer with great optics and no noticeable CA for visual. Have had a few 80mm doublets from Synta and WO and this StellaMira is in another class. However considering a SW 80ED is £379.00 and the StellaMira is £899.00 not very surprising that it is a better performer. The SW80ED is a great scope for the money though and good for imaging.

Would only get rid of it if I got a Tak doublet.

Edited January 10, 2021 by johninderby

[paulastro]

8 hours ago, Mr Thingy said:

I'd be interested in a few examples, if you could. I don't have too any experience of high performance refractors, so tips on some good ones that are below Takahashi price would be very useful.

TIA

I've, as John with the 80ED, found the Skywatcher series of Evostar pro ED refractors to be of excellent optical quality and probably by far the best value for money.  In this range I've owned several examples of the SW 80ED, the SW 100ED and SW 120ED and they have all been fine telescopes.

I have also recently bought a Tecnosky 102ED F7 FPL53 Doublet.  I bought it in preferance to the SW 100mm ED, which is F9, as I paticularly wanted an F7 focal ratio.  I've found this to be excellent both optic wise and in build quality - at least as good or better than SW in every way.

I will say I have no other experience of Tecnosky telescopes so I can't comment on any other telescope in the Tecnosky range.

TS and Altair Astro sell their own branded version of the 102mm Tecnosky I have, and people have said good things about them, but I've not used any of them.

 

 

Edited January 10, 2021 by paulastro

[John]

I currently have a couple of ED doublets (102mm and 120mm), a fluorite doublet (100mm) and a OK4 triplet (130mm) which is reputed to match the criteria for a "super apochromat".

Visually, the ED doublets only show false colour around the brightest objects and then only a small touch. The lunar limb is practically colour free with these. The fluorite doublet shows no false colour on anything that I can see, visually anyway, even Sirius. The OK4 triplet is the same as the fluorite doublet.

Compared to achromats of a similar specification it is remarkable how well a modern ED doublet can control false colour to my mind. Imagers would prefer triplets I'm sure because image capture will show up false colour that is not visible to the visual observer.

For me, it is the figure and polish of these objective lenses that is at least as important as their colour correction, particularly when it comes to maintaining excellent performance and high magnifications.

 

 

 

Edited January 10, 2021 by John

[MaJiC79]

21 hours ago, johninderby said:

There used to be many heated discussions years ago about the term semi-apo. 😁

It sounds like a minefield and I'm glad things have progressed since then

[MaJiC79]

6 hours ago, paulastro said:

If you are an experienced refractor user, you will know a good refractor when you look through it, and if you're not, SGL is a good place to point you in the right direction.  In the end of course, it's always best to try before you buy.

I'm the first to admit that I'm not an experienced user (enthusiastic amateur at the best) but I'm trying to learn and keep my mind open. The only stupid questions are the ones that aren't asked - I'll try to keep my questions sensible, and I thank everyone for their input so far. I'm really glad I joined

Edited January 10, 2021 by MaJiC79
Spelling

[MaJiC79]

2 hours ago, John said:

I currently have a couple of ED doublets (102mm and 120mm), a fluorite doublet (100mm) and a OK4 triplet (130mm) which is reputed to match the criteria for a "super apochromat".

As I understand, super keeps that aberration down over a much broader range. I start to question the need unless you are looking outside of the visible spectrum, although it occurs to me that if you shoot narrowband, can you refocus for the band you're shooting in?

Edited January 10, 2021 by MaJiC79
Clarification

[Louis D]

Optically and mechanically, Sharpstar Optics have been doing a good job of producing consistently excellent, premium refractors at affordable prices.  They're usually sold under various house brands around the world.  If you look at their focusers, you can usually distinguish their products from other Chinese optics houses.  I have a TS-Optics 90 mm f/6.6 FPL53 Triplet APO that has excellent optical correction and refined mechanicals.  It's pretty clearly an older Sharpstar.