ED glass in refractor...What are the advantages?

[Marmot]

Hi

Another question from a relative newcomer.

 

I have come across various references to ED glass being used in Refractors and I am just trying to work out exactly what the advantages are.

The obvious one, that comes up time and time again, is that it reduces CA.

But are there other advantages?

If the ED glass is better optically then will it provide clearer crisper views?

The other question that springs to mind is as follows: If ED glass is that much better than your standard refractor, then will it be capable of taking a higher magnification when compared to the same size standard achro?

 

In case it makes any difference to the answers, my interests at this time are purely observing and not AP.

 

thanks in advance.

[MarkAR]

ED glass is Extra Low Dispersion glass which means that chromatic aberrations are reduced. Combined with 2 or even 3 other lenses with various coatings and glass types, aberrations can be reduced even further to practically zero.

I think all new refractors use a variation of ED glass, reputed to be the best are LZOS, then FPL53, FPL51 etc, There are others with different nomenclature also. I've heard the term Ohara used.

 

There is one oddity in http://www.zerochromat.net

They used bog standard glass for the main lens. The image is reflected back up the tube at an offset angle and down again the much smaller ED lenses making it much cheaper to make large diameter refractors.

[AstronomyUkraine]

8 hours ago, Marmot said:

Hi

Another question from a relative newcomer.

 

I have come across various references to ED glass being used in Refractors and I am just trying to work out exactly what the advantages are.

The obvious one, that comes up time and time again, is that it reduces CA.

But are there other advantages?

If the ED glass is better optically then will it provide clearer crisper views?

The other question that springs to mind is as follows: If ED glass is that much better than your standard refractor, then will it be capable of taking a higher magnification when compared to the same size standard achro?

 

In case it makes any difference to the answers, my interests at this time are purely observing and not AP.

 

thanks in advance.

The other advantages to ED glass are more contrast, and sharper focusing of the target. For visual work an ED doublet will probably be good enough, most of the CA will be removed, but some will still be present. To remove CA completely rquires an ED triplet, and here is where the price starts to climb steeply, running from a couple of thousand for a 100mm triplet, up to several thousand for a 130mm with top quality glass. Once you go over the 130mm prices get astronomical, no pun intended. A 250mm could cost as much as $56,000.

 

If you purchase an ED doublet, as I mentioned it will still contain a certain amount of CA, therefore any magnification with eyepieces will also magnify the CA. Certainly an ED doublet will vastly improve the observational experience over an achromat, but a triplet will improve it further. It all depends how deep your pockets are.

[Nightspore]

I have three ED doublets with an Ohara FPL 53 flint, I've never seen any CA. ED doublets usually have better acuity, colour separation and contrast than triplets or quad's and are lighter with less cool down.

 

 

All of these will easily push 2.4x times their objective diameter (102mm, 80mm, 60mm). I've had 240x on Mars with the 80ED, which also has a Schott crown. I only tend to use my achromats for rich field, although I'm not so fussy about any CA for double star splitting.

 

 

The 72mm Evostar (above and below) can show some cyan hazing on a bright Venus, although I've never seen it on the Moon, even around 190x. I don't know what the flint is made of as Synta aren't saying. Possibly a CDGM FPL 51 equivalent. CDGM do manufacture these equivalents and they have the same Abbe numbers.

 

 

An old trick to ameliorate CA in achromats is to use a yellow or yellow-green filter.

 

 

A Baader Fringe Killer and Neodymium stack is popular. Although a Wratten #8 and a 'skyglow filter' will be similar. The yellow filter attenuates the cyan wavelengths and the skyglow/light pollution takes out the 589 nm sodium streetlight frequency. Giving a slightly more natural appearance. 

 

 

I have a few combinations of fringe killing stacks.

 

[Nightspore]

I have to add that I'm only visual though, for AP things are quite different. Triplets and quadruplets are probably better for cameras.

Edited May 14, 2021 by Nightspore

[Marmot]

Thanks to all of you. There’s many references to ED glass being better, but I just wanted to understand, as best I could, just how it was an improvement over a standard Achro.

 

If only there wasn’t so much stuff to spend your money on in this hobby🙂

 

 

[Marmot]

1 hour ago, Nightspore said:

I have to add that I'm only visual though, for AP things are quite different. Triplets and quadruplets are probably better for cameras.

Hi Nightspore

having looked at the pics you provided of your 3 differing scopes with ED glass, I believe the top one looks like an Altair Astro from the Starwave line up? Can I ask which one it is as I believe they do a couple of models using slightly different quality glass. FPL53 and then FPL51 (or similar) in the ascent series. The Altair’s scopes seem good value (Although they don’t come with as many accessories as the Skywatcher equivalents) and they appear to be of a good build with decent focuser. I would appreciate your views if you read this.

[Nightspore]

8 minutes ago, Marmot said:

Thanks to all of you. There’s many references to ED glass being better, but I just wanted to understand, as best I could, just how it was an improvement over a standard Achro.

 

If only there wasn’t so much stuff to spend your money on in this hobby🙂

 

 

 

You're welcome. Apochromat and achromat both mean 'without colour'. The first successful doublets and triplets 'without colour' were made by John Dollond (as in the later Dollond & Aitchison company) in England during the 18th century. The term 'achromat' was dreamt up by the marketing of the time. Dollond used doublets with English flint glass. The flint glass had a different refractive index to the 'crown' glass at the front of the objective. This achieved a remarkable improvement over earlier designs. In modern high quality Fraunhofer doublets the 'flint' part of the doublet is now made of extra-low-dispersion glass.

 

ED Glass

[Nightspore]

12 minutes ago, Marmot said:

Hi Nightspore

having looked at the pics you provided of your 3 differing scopes with ED glass, I believe the top one looks like an Altair Astro from the Starwave line up? Can I ask which one it is as I believe they do a couple of models using slightly different quality glass. FPL53 and then FPL51 (or similar) in the ascent series. The Altair’s scopes seem good value (Although they don’t come with as many accessories as the Skywatcher equivalents) and they appear to be of a good build with decent focuser. I would appreciate your views if you read this.

It's a 102mm Starwave ED-R

 

 

It's the biggest ED doublet I own.

 

 

 

[ribuck]

Marmot  My rule of thumb is as follows :-

 

Refractor Imaging

• Broadband Imaging(LRGB)  = Triplet, as doublets focus the RGB light differently which require extra post processing to get the colour balance sorted.

 

• Narrowband(Ha, SII, OIII) - Doublet or triple are both fine.
• Achromat = Never
• Planetary Imaging - Recommend Triplet

 

Visual

In most cases a doublet will be just fine and in some cases more preferable than a Triplet. Most modern doublets have very little CA and in a lot of cases it's barely noticeable.

 

Achromats = No, no, no

 

 

 

Edited May 14, 2021 by ribuck

[Nightspore]

2 minutes ago, ribuck said:

 

 

Achromats = No, no, no

 

 

 

 

I dunno, my ST102 is great for rich field. It was worth putting a 360 quid focuser on it. 

 

 

CA isn't noticeable below 30x.

[ribuck]

i guess it comes down to personal preference, budget and many other factors

[Marmot]

Opinions will always vary, but I thank everyone who has given me theirs on this topic: they have helped me become a lot more informed as to the benefits of ED glass in its various guises.  

I now just have to decide if an ED refractor would be right to compliment the achro I already have and weather the extra expenditure would be worthwhile.

 

 

[Nightspore]

1 hour ago, Marmot said:

Opinions will always vary, but I thank everyone who has given me theirs on this topic: they have helped me become a lot more informed as to the benefits of ED glass in its various guises.  

I now just have to decide if an ED refractor would be right to compliment the achro I already have and weather the extra expenditure would be worthwhile.

 

 

 

There are a lot of factors here. I'd always recommend a decent short tube achromat for rich field and some DSO's. As I'm physically disabled any scope that sees regular use has to be portable enough to be deployed relatively easily. This limits its size of course. And as we know; size matters. Notwithstanding achromats are generally cheaper. Small aperture slow achromats often show negligible false colour.

 

I actually considered the ST120 like yours for a while, but decided that the 102mm version was easier to pick up with one hand (I'm partially paralysed in my right arm and hand). It can also be mounted fairly confidently on a Vixen Porta II/TL130 combo. 

 

Chromatic aberration is always going to be a problem with refracting lenses, this is just physics. Basically over many decades this problem has been slowly solved to some extent. I've seen CA in triplets, so there is no perfect solution. For purely visual observing a good ED doublet has some advantages over triplet and quadruplet designs. The efficacy of ameliorating chromatic aberration has more to do with the respective lens glass quality and compatibility, than the amount of lens cells utilised.

 

The Synta ST series are actually quite decent achromatic doublets for what they cost. However, there are distinct differences in observing with a quality ED doublet. Overall colour separation, acuity, and contrast will be noticeably superior. There will be far less lateral and off-axis aberrations. Far higher magnifications can be achieved with ED doublets without aberration. 

 

A fast achromatic doublet can be very effective as a rich field or DSO scope. Unfortunately CA will be a problem for high magnification lunar/planetary observations. So the primary advantage of an ED doublet will be that it is much more versatile. 

 

 

I can take my modified ST102 out in this bag. It gets a lot of use on Moonless nights as it is the biggest refractor I can easily take out. The Starwave may have better glass but it's heavier, more difficult for me to set up, and slower. 

 

 

I can carry these two bags together. The AZ5 is light enough to be rapidly deployed easily. My Altair 60 EDF and modified ST80 are essentially grab and go scopes.

 

 

They both get out a fair bit.

 

 

So far this year I've been out forty two times with a telescope. The 60mm Altair ED doublet has been out twenty two times. It's pretty good for the Moon. Interestingly, the ST80 isn't far behind with fifteen times during Moonless nights, often with dodgy seeing. For me, the Altair is the better all-rounder because of its ED glass. The ST80's extra 20mm helps during Moonless nights of below average seeing. My point being is that they both have their uses. An achromat can be an effective scope if you accept its limitations.

Edited May 15, 2021 by Nightspore

[Marmot]

Thanks for the further info Nightspore.

i do like my ST120 and I am not after replacing it as it keeps me quite happy doing what I got it for, basically rich fields and a wide fields of view.

I have even looked at the moon through it, purely out of interest to see how it performed and yes the CA was obvious but certainly didn’t make me shy away.

your remarks re size and portability are of relevance to me, but for different reasons to your own: I have very limited space and any scope(s) have to be put away after each session and easily stored when not in use.

i look longingly at large aperture, long focal length scopes, but I am realistic as to what I can have. Also as it needs to be put away after each session, it needs to be quick to put up. So it sounds like my criteria are similar to yours for different reasons🙂

So I am seriously considering an ED refractor to cover some of the situations when my ST120 doesn’t perform at its best. All the comments in this post seem to reinforce that it may work for me.

[Nightspore]

39 minutes ago, Marmot said:

Thanks for the further info Nightspore.

i do like my ST120 and I am not after replacing it as it keeps me quite happy doing what I got it for, basically rich fields and a wide fields of view.

I have even looked at the moon through it, purely out of interest to see how it performed and yes the CA was obvious but certainly didn’t make me shy away.

your remarks re size and portability are of relevance to me, but for different reasons to your own: I have very limited space and any scope(s) have to be put away after each session and easily stored when not in use.

i look longingly at large aperture, long focal length scopes, but I am realistic as to what I can have. Also as it needs to be put away after each session, it needs to be quick to put up. So it sounds like my criteria are similar to yours for different reasons🙂

So I am seriously considering an ED refractor to cover some of the situations when my ST120 doesn’t perform at its best. All the comments in this post seem to reinforce that it may work for me.

 

Sounds like a good plan! 

 

 

I sometimes look at the Moon with my ST102. The CA isn't too bad really, I believe the ST102 and ST120 have larger gaps between their respective Fraunhofer elements specifically to lessen CA. One of my favourite anti-fringing stacks is a Lumicon #8 and a TS Optics UCF1. This gives a very natural overall colour with just a hint of yellow, which in itself can be useful for contrast. The Lumicon #8 is very subtle. I recommend this stack.