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#1 Voyager 3

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Posted 10 September 2020 - 08:58 PM

What's the difference between a FPL-53 doublet and a FPL-51 triplet ? How will they perform against CA ? Or which one will control CA to a higher level ? These sort of questions rise all the time but can you help out a newbie whose entering refractor world step by step . Conditions (humidity, transparency) do not play a part I guess .

#2 ButterFly

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Posted 10 September 2020 - 09:10 PM

They are just types of glass.  How each is used in the overall design is more important.  Compare comparable scopes as much as you can.

 

As an example, I have a SW120ED with FPL-53.  I also have an ES80ED with FCD-01, similar to FPL-51.  One is f/7.5 at 900mm, the other f/6 at 480mm.  Is that a fair comparison?  It takes quite a bit of power to see color fringing (from other than the atmosphere) on either.



#3 Xyrus

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Posted 10 September 2020 - 09:16 PM

FPL-53 glass has a lower dispersion than FPL-51. However which one controls CA better entirely depends on how well the objective is made.

 

Glass types are not the end all be all determining factor of how well a scope controls CA.


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#4 vtornado

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Posted 10 September 2020 - 09:29 PM

What do you want to do with this scope?  Visual, AP or EAA?

 

Besides better or worse CA, A doublet is generally lighter than a triplet.  A triplet Is more fragile than a doublet, (The element positioning is critical)  A triplet has longer cool down than a doublet.

 

If the refractor is only for visual a fpl 53 doublet is adequate.  I have a 100 f/9 and 80 f7.5 and you have to look hard for the CA to see it.

It only is visible on the brightest objects (Moon, Venus, Vega) and only at high power. 


Edited by vtornado, 10 September 2020 - 09:30 PM.

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#5 bobzeq25

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Posted 10 September 2020 - 10:23 PM

What's the difference between a FPL-53 doublet and a FPL-51 triplet ? How will they perform against CA ? Or which one will control CA to a higher level ? These sort of questions rise all the time but can you help out a newbie whose entering refractor world step by step . Conditions (humidity, transparency) do not play a part I guess .

Depends on the specific lens design/manufacture.   Either one could be "better" than the other, depending on those.

 

Your best bet is reading user reviews on _specific_ scopes.



#6 gnowellsct

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Posted 10 September 2020 - 11:15 PM

These are pretty rarefied considerations, but if you're going to pay top dollar you likely want FPL 53, FPL 55, or fluorite.  Leave aside the religious wars over whether any one of those three is better than the other.

 

But like just about everyone else you're concentrating on the glass because you see a refractor and you see glass and you read about glass and everything is about glass.  And manufacturers will go out of their way to give you the glass that will make you pull the trigger and buy.

 

But the glass is only a part of the quality of the refractor which lies in:  quality of the lens cell; quality of the focuser; quality of the tube including its paint job, its baffles (or lack of same); its dew shield, its overall machining.  See here.  And of course quality of the figure on the lens, quality of the lens once the glass is selected (inclusions, or lack thereof, etc.).  Because refractors are different from rival telescope designs.  The light must pass entirely through the glass, through a minimum of two, and sometimes three, and even four, lenses.  With reflective surfaces you are concerned with only the surface of the glass.  The quality demands of a refractor are much higher.  The curves have to be calculated, the lenses manufactured, and mounted in a cell that will keep them all precisely aligned.  If you cheap out on the lens cell you compromise the optic.  But no one ever says "which lens cell should I get."  

 

It's safe to say that the highest end manufacturers are operating at five or more levels above their buyers in terms their awareness of what goes into making a good refractor.  All the customers want to talk about is glass.

 

One answer to your question is that there is no difference between FPL 51 and FPL 53 if the scopes are on an inadequate vibrating mount.  They will just be shaking scopes on a crummy mount and no one will give a fig what glass they have.

 

A telescope is a complete system, all the parts work together.  

 

Now: let me hasten to say, there is what is good, what is great, and what is good enough, and what is bad.  Opinions vary.  But it is safe to say that most of the time, when they see prices, people decide to go for good enough.    FPL 51 certainly meets that standard.  

 

And understand that the purpose of a scope is often to be the precursor to the next scope.  A mediocre telescope gives you its lessons: it may teach you that in fact you really are willing to spend $900 on a focuser, for example.  You just didn't know it till you had used the mediocre telescope for a while.

 

Or you may find that you are happy with mid-level gear and can get it to work for you.  In which case the FPL 51 is perfect.  I have a lot of mid-level gear.  Much of it is top tier, but much of it is also mid-level.  You work with your budget, you work with your experience, you work with what you can get to work out in the field.  And realize that telescopes can be bought used, and they can be resold after they have taught you their lessons.  Because they have lessons to teach, and they are perfectly willing to move on to the next owner to teach those lessons again.

 

You can get nice refractors for under a grand, I would suggest you jump into the pool and see how it goes.

 

Greg N


Edited by gnowellsct, 10 September 2020 - 11:21 PM.

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#7 Jon Isaacs

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Posted 11 September 2020 - 04:20 AM

Lots of wandering around the question without actually answering it.

 

If the question were phrased as such:

 

For identical apertures, focal length/focal ratios refractors, identical except in the design, one being an FPL-51 triplet and one being an FPL-53 doublet, which provides the better color correction?

 

Vladimir Sacek's Telescope-optics website has a section on Polychromatic point spread function that addresses this issue. If you really want to get confused, this is a good read:

 

https://www.telescop...romatic_psf.htm

 

If you want to see some examples, then figure 74 is informative. 

 

There are two pairs that are of interest examples 63, 64 and 42, 44.

 

63 and 64 are 6 inch F/8s, 63 is FPL-53 doublet, 64 is a FPL-51 triplet, they essentially identical. 

 

42 and 44 are 4 inch F/7s, 42 is a FPL-51 triplet and 44 is the FPL-53 triplet. In this case, the triplet is somewhat better. 

achro_apo.PNG

 

The entire design seems to be important, not only is The ED glass important but the mating glasses make a difference.

 

But I'd say that in general, they are similar..

 

Jon


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#8 junomike

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Posted 11 September 2020 - 07:31 AM

IME the FPL-53 Triplet 120ED (SW, Orion, WO, Etc) had better CA control then the ES 127 Triplet using FCD-1 which compares to FPL-51.

 

As other stated there's no real set answer as the making and matching of the glass can be as critical as the kind of glass used.



#9 Wildetelescope

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Posted 11 September 2020 - 08:17 AM

What's the difference between a FPL-53 doublet and a FPL-51 triplet ? How will they perform against CA ? Or which one will control CA to a higher level ? These sort of questions rise all the time but can you help out a newbie whose entering refractor world step by step . Conditions (humidity, transparency) do not play a part I guess .

Jon Isaacs chart tells the tale. Basically, the differences are pretty subtle.   In another thread, I just gave a review of my astrotech 80 mm EDT that is a triplet using I BELIEVE a Chinese sourced ED glass similar to the FPL 51.   Back in the day, I had an Orion Eon 80 mm F6 Doublet that used the FPL 53 ED glass.  At magnifications up to 180X or so, the performance was pretty similar.  No color on moon or planets, very little false color on the brightest objects.  Both scopes were very well collimated and had more or less text book diffraction patterns inside and outside of focus. Both scopes will show a little bit of blue halo around bright stars in imaging applications, but nothing I would  consider obtrusive.   In my experience the Astro Tech triplet handles the "stupid Magnification" (>50X per inch) category a bit better.  In both scopes the image dims but with the Eon 80 Doublet, the sharpness of the image deteriorated quickly and CA becomes more evident.  With the Astro tech triplet, I still get a pretty sharp image around 200X  and the color correction does not seem to deteriorate as much.   The caveat is that I am going from memory on the Eon 80, which I sold probably 6 years ago or so.   I was a less experienced observer at that time, and did not appreciate the wider field scope at that time.  It was a really nice scope, but I was more focused on planets and such, so I sold it and got a TV 102.  There is a very happy fellow in Michigan that is observing away with it.  

 

Here is an additional, purely subjective observation, so take it for what it is.  In my experience poor seeing conditions conspire with the the scope's tendency for CA to make the image worse.  Independent of glass type, I find that my triplet refractors seem to handle poor seeing conditions a bit better than the doublet designs.   An example.  My TV 102 is an ED doublet, which I speculate uses FPL 53 glass, although TV will not say.  95% of the time, I notice NO color on Jupiter at any magnification.  However, on nights of rather poor seeing(I live under the jet stream), I occasionally see a flash of color.  I have never noticed a similar effect with my Pre ED glass AP triplets under any seeing conditions.   

 

Hope that helps!

 

JMD



#10 gnowellsct

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Posted 11 September 2020 - 10:17 AM

There's a lot going on in Jon's graph (thank you for posting it) that I find difficult to disentangle.  For one thing it is difficult to read.  But if I look at the 6 inch apertures and what I assume are the Point Spread Functions (standing in for Strehl, or does that work the other way around?)  I see:

 

--#63 and #64, the FPL53 being identical PSF to FPL51 below it, but #64 is a triplet and #63 is a doublet which lends credence to the observation that some doublets exceed some triplets (though here they are tied, but at what must be  a weight penalty for the triplet).

--#61 with FPL 53 and clearly pulling distinctly ahead of 63 and 64 with a near perfect PSF; this is the scope that is saying if you want near perfection you need FPL53 (or fluorite? or FPL 55?)

--#62 which in spite of being f/6.5 is ahead of both #63 and #64, and so delivering a better image while delivering a wider field, which we all know is a feat that costs money

--#65 an FPL51 "apo doublet" which is far enough behind this pack that it clearly is one of the scopes contributing to giving FPL 51 a bad name

--#66 and #67, f/15 and f/8 achromats that are in the category of "if you use these things please don't ever lecture me about central obstruction again".  

 

What's diffiicult disentangle here is that there is apparently some room for design and execution of the scope to operate independently of what you might predict.  On the basis of focal ratio one would expect #62 to be handicapped relative to #s 63 and #64, especially since we're talking about color correction and glass, and f/8 has an intrinsic advantage over f/6.5 with regard to color correction.  But #62 is overcoming that handicap.

 

So on the one hand we could just say well almost all of these scopes are diffraction limited >.80, and in fact clustering above .87, so they're all very close.    But on the other hand I see evidence for wanting a very well made scope with FPL 53 and I see evidence that design and execution matters (the f/6.5).  

 

So one way to read this is if you're going to pay for top end glass make sure it is in the hands of someone who knows how to bring it to a good figure and put it into a cell where it will stay put.

 

In other words I read the chart both ways, there is a reason to say most people most times most applications will be happy with FPL 51 and on the other hand the perfectionists who want FPL 53 in the hands of a master tech probably aren't wrong either.

 

Greg N


Edited by gnowellsct, 11 September 2020 - 10:30 AM.


#11 gnowellsct

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Posted 11 September 2020 - 11:23 AM

They are just types of glass.  How each is used in the overall design is more important.  Compare comparable scopes as much as you can.

 

As an example, I have a SW120ED with FPL-53.  I also have an ES80ED with FCD-01, similar to FPL-51.  One is f/7.5 at 900mm, the other f/6 at 480mm.  Is that a fair comparison?  It takes quite a bit of power to see color fringing (from other than the atmosphere) on either.

People want ultra-corrected glass in their refractors in order to have a razor sharp color saturated presentation of atmospheric refraction.  


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#12 Wildetelescope

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Posted 11 September 2020 - 11:27 AM

There's a lot going on in Jon's graph (thank you for posting it) that I find difficult to disentangle.  For one thing it is difficult to read.  But if I look at the 6 inch apertures and what I assume are the Point Spread Functions (standing in for Strehl, or does that work the other way around?)  I see:

 

--#63 and #64, the FPL53 being identical PSF to FPL51 below it, but #64 is a triplet and #63 is a doublet which lends credence to the observation that some doublets exceed some triplets (though here they are tied, but at what must be  a weight penalty for the triplet).

--#61 with FPL 53 and clearly pulling distinctly ahead of 63 and 64 with a near perfect PSF; this is the scope that is saying if you want near perfection you need FPL53 (or fluorite? or FPL 55?)

--#62 which in spite of being f/6.5 is ahead of both #63 and #64, and so delivering a better image while delivering a wider field, which we all know is a feat that costs money

--#65 an FPL51 "apo doublet" which is far enough behind this pack that it clearly is one of the scopes contributing to giving FPL 51 a bad name

--#66 and #67, f/15 and f/8 achromats that are in the category of "if you use these things please don't ever lecture me about central obstruction again".  

 

What's diffiicult disentangle here is that there is apparently some room for design and execution of the scope to operate independently of what you might predict.  On the basis of focal ratio one would expect #62 to be handicapped relative to #s 63 and #64, especially since we're talking about color correction and glass, and f/8 has an intrinsic advantage over f/6.5 with regard to color correction.  But #62 is overcoming that handicap.

 

So on the one hand we could just say well almost all of these scopes are diffraction limited >.80, and in fact clustering above .87, so they're all very close.    But on the other hand I see evidence for wanting a very well made scope with FPL 53 and I see evidence that design and execution matters (the f/6.5).  

 

So one way to read this is if you're going to pay for top end glass make sure it is in the hands of someone who knows how to bring it to a good figure and put it into a cell where it will stay put.

 

In other words I read the chart both ways, there is a reason to say most people most times most applications will be happy with FPL 51 and on the other hand the perfectionists who want FPL 53 in the hands of a master tech probably aren't wrong either.

 

Greg N

That would be my interpretation as well.  waytogo.gif  Perhaps the hardest part of figuring out what is the right piece of gear to get for a person new to the hobby, is understanding ones own expectations.  This is true for scopes, mounts etc...   How to decide whether you REALLY want/Need Exceptional performance, as opposed to very good.   There are very good scopes made using FPL 51 glass types.  The triplet design helps balance against a slightly lower Abbe number for the ED glass type.

 

To the OP,  I would make the following observation.  Jon Isaacs who has posted above is one of the folks here whose comments I always give serious attention.  He has quite a collection of very nice optics ranging from refractors to big dobs.  I think he talks as much about the pleasure he derives from observing with his little ST80 achromat as he does about any of his other scopes.  It is about how he uses that scope, as an ultra wide field  portal to the sky that he can't get any other way.    Until you get started, it is difficult to know what you know and like.   Starting out don't get too hung up worrying about things like glass types, etc.. Even Achromats will show you more than your naked eye.   If the Hobby bug bites you, then you will likely owning more than one scope the same way a carpenter carries a tool box full of tools.   It is the nature of the hobby.  As you gain experience, you will be able to carefully choose your subsequent purchases in a targeted fashion. The first step in learning to swim is to jump in the pool and getting wet.  Everything progresses from there:-) 

 

Cheers!

 

 

JMD


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#13 Wildetelescope

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Posted 11 September 2020 - 11:28 AM

People want ultra-corrected glass in their refractors in order to have a razor sharp color saturated presentation of atmospheric refraction.  

Lol, that made my day.  lol.gif

 

JMD


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#14 ButterFly

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Posted 11 September 2020 - 04:12 PM

People want ultra-corrected glass in their refractors in order to have a razor sharp color saturated presentation of atmospheric refraction.  

Getting the ADC to work visually brought home how much I used to rely on eyepiece lateral color for correction.



#15 Voyager 3

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Posted 12 September 2020 - 07:26 AM

Oops it will b used for visual use only. And the scope is TS optics 102mm F/7 FPL-53 doublet . I don't know anything abt refractors but I'm learning quickly . Any reviews abt that scope ? I think they aren't made in china but Germany . So how is that scope in relation to it's price ? The focuser is a 2.5" rack n pinion focuser .


For those who are interested I'm sharing the link .... https://www.teleskop...-Objective.html

#16 25585

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Posted 12 September 2020 - 07:48 AM

Better lenses can allow higher useful magnifications per mm2 of lens surface area, to a limit.



#17 howardcano

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Posted 12 September 2020 - 11:40 AM

Oops it will b used for visual use only. And the scope is TS optics 102mm F/7 FPL-53 doublet . I don't know anything abt refractors but I'm learning quickly . Any reviews abt that scope ? I think they aren't made in china but Germany . So how is that scope in relation to it's price ? The focuser is a 2.5" rack n pinion focuser .


For those who are interested I'm sharing the link .... https://www.teleskop...-Objective.html

I believe that is made by the same company that made my Stellarvue SV102 Access, but with the same focuser as my AstroTech AT102ED.  If so, that would be a wonderful scope!


Edited by howardcano, 12 September 2020 - 11:40 AM.


#18 SeattleScott

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Posted 12 September 2020 - 12:45 PM

They are made in China for a German company. The German company just outsourced production to China for this model, much like most companies.

The German company is a telescope store. They don’t generally make the scopes they sell. Sometimes they order scopes sold under their own brand, much like Astronomics or OPT. But make no mistake, the scopes are made in China, just like with Astronomics or OPT.

In general this model has a good reputation in the various iterations under different brand names that it has been sold under. Excellent CA control, good to great optics depending on how lucky you get. Certainly an excellent value at that price.

Scott
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#19 stevew

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Posted 12 September 2020 - 01:45 PM

Oops it will b used for visual use only. And the scope is TS optics 102mm F/7 FPL-53 doublet . I don't know anything abt refractors but I'm learning quickly . Any reviews abt that scope ? I think they aren't made in china but Germany . So how is that scope in relation to it's price ? The focuser is a 2.5" rack n pinion focuser .


For those who are interested I'm sharing the link .... https://www.teleskop...-Objective.html

I have the Stellarvue branded version of this scope.

Get it!

It has great color correction and has an excellent optical figure. It's also small and compact which means it gets used a lot.

If your looking for reviews google Stellarvue 102 Access.



#20 Voyager 3

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Posted 15 September 2020 - 09:44 AM

Thank you I'll look in it ... The reviews looks cool for ite price 😀


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