ED Glass

[pbyrne]

Hi all

I am in the market for a 4" ED refractor for double star work, I want something small and portable that will sit nicely on my Skytee II mount.  I was drawn to Altair Astro, I saw the review of their 102mm Starwave Ascent in Sky at Night magazine, now I am a little skeptical about magazine reviews, are they going to risk potential advertisement revenue by stating that a product is nothing better than a doorstep?  Or are they going to be glowing in their praise even if that is not the case?  No matter, maybe I am too much of a cynic.  The Ascent 102 is sold at a fantastic price, £465, €516, WOW!.  They also sell the Starwave 102ED-R for £899, €997.  The ascent uses FPL-51 glass while the 102 ED-R uses FPL-53 glass.  My question is this, at the eyepiece, will there be any noticeable difference between the FPL-51 glass and the FPL-53 glass?  We are talking about the FPL-53 being almost twice the price.

One thing I would like to note about the Altair Astro website, they also sell a 110mm ED-R scope and in the description they are extremely honest, stating that there will be a slightly better view over the 102, slightly brighter and offering slightly better contrast.  I find this honesty refreshing, it would be easy to state that the 110 will perform far better than the 102 in the hope that the buyer purchases the bigger and more expensive scope.  Kudos, to all at Altair Astro.

[DirkSteele]

Assuming the optical specification (mating element, intrinsic optical design - radius of curvature of the lens elements etc, quality of the respective ED glass - i.e inclusions, bubble sizes etc, and the actual execution of the scope lens - polish quality etc) is the same, then the R model will perform slightly better, with a touch better colour correction, better control of sphero-chromatism, which will lead to slightly better contrast, but certainly not 2x better to reflect the doubling of the price.  Unfortunately, in the astronomy equipment world, the cost performance curve is not a straight line.

 

Not that is particularly relevant for this discussion, but I have written a few reviews for astronomy mags, and I have always been paid per word, not for the word written, so I have always been honest in my apprasial, and never experienced any push back on my submitted copy.  So hopefully you can take some comfort from the magazine reviews as well.

Edited July 2, 2020 by DirkSteele
typo

[John]

One of the the reasons for the price difference is that FPL-53 glass costs about twice as much as FPL-51.

[bomberbaz]

This price/performance difference is similarly reflected in the eyepiece world. 

EG: The morpheus performance is easily twice that of the cousin hyperion (caveat fast scope) and is about twice the price. (Mid range from lower range eyepieces)

Where this rule of thumb formula starts to slip and is well documented is the next move into higher end such as pentax/televue etc where the jump in price far exceeds the jump in performance. 

However what you do get imho is something with the edge over it's lower costing rival but also you know has a superb quality feel to it with an excellent resale prospect. 

[newbie alert]

I used to think that fpl53 was the only glass id want on a scope until a friend told me his triplet was fpl51...

So now I'm not so sure

[Tenor Viol]

FPL51 is an older formulation of ED glass. FPL53 is newer and has better performance, and it's more expensive. But, it is not the only ED glass there is also FCD100 by another maker (Chinese?). There are options.

[robhatherton]

I can’t make a comparison but I have recently bought the Ascent 102ed with the fp51 glass. I can believe the fp53 glass is better but it may be hard to tell the difference. The build quality and focuser on the Ascent are superb and identical to their considerably more expensive scopes. As to optical quality I can endorse the Sky at Night review though the weather has been poor since I got it so it’s not been out much yet but, referring to a previous thread of mine, splitting Izar has become noticeably easier than with my 120 achro. Can’t see CA focussing on distant trees against the skyline at x150 or on the moon so most people, I’m not saying everyone, wouldn’t notice it.

A year or 2 ago it was priced at £100 more than now.

[rockinrome]

Hi Paul
I have the Altair 72ED-R with the FPL51 glass and it is superb.
Had it a while but been using it in ernest now for over a year for astrophotography with the 0.8x reducer.

All the best.

[John]

FPL-53, paired with the correct mating element glass (vitally important) can produce colour correction about 2x better than FPL-51, all other things being equal.

Some of the triplets using FPL-51 actually produce more colour than the Skywatcher 120mm ED doublets which use FPL-53 of course.

 

 

[mikeDnight]

I'm not sure how helpful this will be, but here's a comparison between FPL51 and FPL53 in a ~70mm apo.  But keep in mind this is an image, where as visually the colour fringing will be significantly less noticeable. In the second image my friend Paul stands proudly next to his relatively old (presumably FPL51 or equivalent) AT 102mm ED. This scope was visually superb and didn't show any meaningful CA on either stellar or lunar and planetary targets. CA may have been present, but it certainly didn't jump out at me or detract from a quality apo view. However, if I had the available funds and was choosing between FPl51 and FPL53,  I'd choose the latter. Sometimes we can get too bogged down with CA and miss the countless hours of joy such great refractors can offer us.

[paulastro]

I would confirm what Mike says about  my ex Astro -Tech above.  I would also say similar about my present SW 72ED which isn't fpl53 either.  I also have a SW 80ED and 120ED with fpl53 of course.  All of these scopes are excellent and there is little, if anything, to judge between them.

In my view, though fpl53 is the best glass, it's by no means the whole story as others have pointed out.  I would certainly buy another non-fpl53 refractor if  its performance was up to my expectations, and theres the rub.  Unless you have some experience and can check any potential purchase before hand, you cannot be sure what you are getting - even different samples of the same make and model can vary - but this is less likely in more respected models by a reputable maker that have been in production a while.

If you know personally someone experienced whose judgement is proven to you that's fine.  Alas newcomers are unlikely to have such a useful person to hand.

 

 

 

 

Edited July 3, 2020 by paulastro

[Rob]

Ah, the hot debated ED glass.. Lots that have been said here is true and accurate. That said FPL-51 is a good performer in its own right, more so if your a visual observer.

Also fair to say the focal ratio of the scope has another performance effect. My ED 80mm is indeed FL-51 with a focal ratio of f6.8 as a visual observer this max's out the performance of the 51 lens. The only slight CA (yellow fleck) that's noticeable is when I point the scope to Venus.

I post this for newcomers who are visual observers on a budget. FPL-51 is a good performer, and should not be dismissed.

 

[Paz]

I think either glass is going to be good for visual and it's just a case of the value each customer puts on quality.

The laws of diminishing returns (and the laws of physics!) apply and you don't get 2x the quality for 2x the price.

For many higher end items where I've been able to compare them, often I can't see the difference unless I look for it which means in practice I would enjoy either item just the same if I put my effort into using it rather than analysing it... but bear in mind I'm not  very experienced.

So why then buy the more expensive kit? For me because it stops me wondering if I could get a better view with better kit (that I could reasonable afford - there's always going to be something better) and this allows me to focus on enjoying the views instead.

Edited July 4, 2020 by Paz

[John]

When we start to push our kit towards the edges of it's performance capability, small differences start to show and matter a bit more

A wise man (Michael Wilkinson I think !) once posted that the time to consider an upgrade is when you try something and see a difference that matters to you.

Once you have seen an improvement it is very difficult to "un-see" it

 

 

 

 

[Adam J]

There are some designes that use two ED elements in a triplet APO, usually those are based on FLP51 or similar and they tend to perform very well, the TS 115mm triplet is a example of this. However in my opinion there will be a notable difference between the average FPL 51 doublet and and average FPL53 doublet expecually some that incluce Lanthanum doped flint elements.

Adam

[Louis D]

Here's a nice comparison of various objective designs done by Vlad Sacek on his Amateur Telescope Optics polychromatic PSF page:

As you can see, an FPL-51 triplet has roughly the same correction as an FPL-53 doublet.  Also, a moderate focal ratio FPL-53 triplet comes very close to the ideal refractor objective.

These two pages on semi-APO and APO refractor objectives and design are also a good read.

[John]

I wish I knew how to properly interpret diagrams of this type

And this (although I can make a little sense of this one):

The above objective seems to be classed as a "super apochromat" but I'm not sure what the definition of that term is

[Louis D]

1 hour ago, John said:

I wish I knew how to properly interpret diagrams of this type

I believe this shows the distance in millimeters each wavelength of light actually focuses in front of or behind the focal point as you move away from the optical axis (bottom) to the edge (top).  Thus, about 70% out from dead center to edge produces a nearly color free image when in focus.

[John]

1 minute ago, Louis D said:

I believe this shows the distance in millimeters each wavelength of light actually focuses in front of or behind the focal point as you move away from the optical axis (bottom) to the edge (top).  Thus, about 70% out from dead center to edge produces a nearly color free image when in focus.

Thanks Louis.

The number of "crossings" seems to be important. This one appears to show 4 and nearly a 5th. What is the significance of that ?

[Louis D]

4 hours ago, John said:

Thanks Louis.

The number of "crossings" seems to be important. This one appears to show 4 and nearly a 5th. What is the significance of that ?

The classic definition of achromatic is 2 crossings and apochromatic is 3 widely spaced crossings.  Superapochromatic is 4 or more.  However, many APOs have one or more crossings in the infrared rendering them less useful.  Here's a diagram from Wikipedia showing the various objective types:

Crossings refer to wavelengths of light that focus at the focal plane of the objective.  In a reflector, all wavelengths focus at the focal plane by comparison.

[GazOC]

@John I tried to find one but couldn't but if you can locate a similar diagram for an ED doublet or an achromat it may show the idea more clearly 

Depending on how the designer has chosen to set up scope the focus point for the wavelength at the blue end of spectrum will be further apart from the on the vertical scale  from where the red/ green wavelengths come to focus than it is the example above. This is the CA you see at the eyepiece 

 

Edited July 7, 2020 by GazOC

[Louis D]

Here's a collection of longitudinal aberration plots for various achromatic, semi-APO (ED?), and APO objectives analyzed by Vlad on his optics page linked above.  Note that the horizontal scale varies from plot to plot:

One of the takeaways is that there is no one singular definition of an achromatic, semi-APO (ED?), or APO objective.  Depending on the design and glass choices, the chromatic performance can vary quite a bit across the field of view within each group.  However, it is clear that each step up in general correction yields an improvement.

Edited July 7, 2020 by Louis D