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Doublet vs. Triplet

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#26 Jon_Doh

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Posted 11 August 2021 - 08:30 AM

Depends on the doublet.  You can see lots of photos taken with doublet ED's on Astrobin.  



#27 John Huntley

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Posted 11 August 2021 - 11:07 AM

Triplet vs doublet for visual. I can see the colour difference between a SD103S and LZOS 130. I’m not to concerned about the triplet showing no colour between being in and out of focus, the image just looks film like in the triplet...

 

I've noticed a similar difference between my Vixen ED102SS and my Tak FC100-DL, both of which are doublets:

 

https://www.cloudyni...ief-comparison/

 

My TMB/LZOS 130mm F/9.2 performs like a 5.1 inch version of the FC100-DL.


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#28 SeattleScott

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Posted 11 August 2021 - 01:28 PM

Hello Kei123,
Why is it that doublets are adequate for visual, but not for AP? My guess is that the spot size is too large in the blue and red ends of the spectrum in most doublets compared to triplets and this shows up in AP, because of increased sensitivity of the camera photo sites due to their ability to count photons over time. This means that faint unfocused blue and far red light can't be detected by the human eye, but the camera can detect and record it, causing reddish and bluish stars to appear unrealistically large, whereas to the human eye, it might diminish contrast a wee bit in the vicinity of such stars, but the star would not appear broader.
Is my supposition correct?
Thanks for bringing this up.
BTW, I think one would expect to detect brighter images in a 130 mm scope over a 120 mm by 17%. The OP will at least gain that by moving up to the SV scope.

Thanks for this post,

Larry

Yes this is basically right. For imaging a camera accumulates photons over time, making them far more sensitive to CA than the human eye.

Scott
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#29 25585

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Posted 11 August 2021 - 04:10 PM

I've noticed a similar difference between my Vixen ED102SS and my Tak FC100-DL, both of which are doublets:

 

https://www.cloudyni...ief-comparison/

 

My TMB/LZOS 130mm F/9.2 performs like a 5.1 inch version of the FC100-DL.

DLs are F9 so I'm not surprised. Those SS Vixens are great scopes though with Canon lenses, want one!


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#30 HowardSD

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Posted 11 August 2021 - 04:20 PM

Depends on the doublet.  You can see lots of photos taken with doublet ED's on Astrobin.  

I agree, i've a WO Zenithstar 103 & an AT 72EDII both doublets with FPL53 glass and i'm very happy with the images they both produce, maybe a tad better in the WO scope. I may eventually move up to a Triplet however with my age and health the weight of a scope is a major consideration, that besides it would have to be really good to pry my (work of art aesthetically, have the blue trim version) WO scope out of my hands. Also both are enjoyable for visual as well, no complaints here about nice doublets!


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#31 sunrag

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Posted 12 August 2021 - 11:54 AM

In my triplet (TS Photoline APO 130/F7), i find that it snaps into focus. If i turn the focus knob just a bit on either side, it is fuzzy. I have observed similar effect on my Meade 125/F15 ETX125 MAK and my 12” reflector. But on my Doublet Achromats, there is a much larger range of focus. This tells me that APO or anything else that has a tight focal range is putting all the available light in one focal plane, rather than spreading it over a range (spherical or chromatic aberrations). 



#32 BarrySimon615

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Posted 15 August 2021 - 09:13 AM

I just scanned thru the responses to the lead off post in this thread, stopping here and there to read the posts from people who are frequent contributors to get a feel for the responses.

 

In the first post you say that you are visual only.  In my mind a very good doublet is all you need.  A premium triplet apo is optimized for photography which you are not doing.  A triplet apo refractor is typically heavy plus it is front heavy.  This changes the balance point of the tube assembly so you will either have more tube assembly showing on the focuser side of the rings or clamshell vs what you see on the objective side.  You are moving the focuser toward the ground and you likely will need a larger mount.  Plus, as implied, you are buying improvement that will not likely be seen by you as you will not be using the scope for photography.

 

Stick with what you have and if you do venture into photography you may find that your scope does just fine.

 

Barry Simon


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#33 25585

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Posted 15 August 2021 - 03:52 PM

I just scanned thru the responses to the lead off post in this thread, stopping here and there to read the posts from people who are frequent contributors to get a feel for the responses.

 

In the first post you say that you are visual only.  In my mind a very good doublet is all you need.  A premium triplet apo is optimized for photography which you are not doing.  A triplet apo refractor is typically heavy plus it is front heavy.  This changes the balance point of the tube assembly so you will either have more tube assembly showing on the focuser side of the rings or clamshell vs what you see on the objective side.  You are moving the focuser toward the ground and you likely will need a larger mount.  Plus, as implied, you are buying improvement that will not likely be seen by you as you will not be using the scope for photography.

 

Stick with what you have and if you do venture into photography you may find that your scope does just fine.

 

Barry Simon

I have a triplet for visual. It beats all my doublets. I am not an experienced observer but notice differences as differences. Maybe its because I don't make mental allowances but notice and judge on what is better to my eye, especially nearer maximum. 

 

At first I was sceptical but that scepticism was disproved. Seeing more and better with the same aperture size and magnification proved to me that there is a richer viewing experience from triplets. Fine doublets there are to be sure, but they are not the end-all for visual refractor astronomy.  


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#34 Deadlake

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Posted 15 August 2021 - 04:02 PM

I have a triplet for visual. It beats all my doublets. I am not an experienced observer but notice differences as differences. Maybe its because I don't make mental allowances but notice and judge on what is better to my eye, especially nearer maximum. 

 

At first I was sceptical but that scepticism was disproved. Seeing more and better with the same aperture size and magnification proved to me that there is a richer viewing experience from triplets. Fine doublets there are to be sure, but they are not the end-all for visual refractor astronomy.  

I agree, however I believe the ok1/4 material of the LZOS lens takes its part for my scope.
A fluorite doublet (FS-128) would put up more competition but these are no longer available, unless you spring for an Ageema. 
 



#35 Wildetelescope

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Posted 15 August 2021 - 04:46 PM

Below is probably the most useful comment you will get;-) it captures pretty much everything! Up to you to decide whether the advantages warrant getting the sv130.

Good luck!

Jmd

I have the Evostar 120 and a SV130EDT here. The 130 delivers a purer image (sterile) and is better corrected for SA but you do need them side by side to see the difference as it is pretty close. I always like the image from the 130 better but 9 times out of 10 I grab the 120 over the 130 and blame it on the weight and my age. I wouldn't call the difference "significant" but it is there when I use them together. I do not feel I'm missing anything when using the 120 and not the 130. Now if the clouds leave I'll have to get them out and compare again. ;)


Peace...[/quote]
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#36 teashea

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Posted 15 August 2021 - 07:32 PM

I have learnt that there is hardly any advantage to a triplet over an apo for visuals. But when it comes to AP, it can be a different story. However, at the relatively long focal length of the 120ed, there would not be significant difference even in AP.
That said, some things in this industry are really for satisfaction in the brand name and not particularly the performance. Some owners only look for reasons to justify purchases or ownership after the fact. There is a feel-good thing involved. And if it makes you happy, why not? Such is life.

The reason I purchase on Takahashi telescopes is not the brand per se.  Rather it is the extraordinary high quality of the optics, design, manufacture and finish.  To most people it does not make a difference but to me it does.


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#37 teashea

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Posted 15 August 2021 - 07:33 PM

It depends on the doublet and the triplet.  Not all doublets are created equal - same for triplets.  Some doublets are junk - some are superlative.  


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#38 kel123

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Posted 15 August 2021 - 09:44 PM

It depends on the doublet and the triplet. Not all doublets are created equal - same for triplets. Some doublets are junk - some are superlative.


Exactly. Some of the comparisons are not exactly apples to apples. You have to compare a good example to a good example.
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#39 teashea

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Posted 16 August 2021 - 07:13 PM

Exactly. Some of the comparisons are not exactly apples to apples. You have to compare a good example to a good example.

indeed



#40 vtornado

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Posted 16 August 2021 - 07:27 PM

I would guess??? that a triplet lens receives more care to avoid other errors (e.g. spherical abberation, roughness, pinching ...) than an ED  doublet.  Same goes for an ED over an achro refractor.  I had two 80mm scopes on the moon the other night and the ED was noticably sharper and it was not CA. I was observing craterlets which are high contrast and somewhat immune to CA.  Not to say every ED beats an achro, and every triplet beats a double.    


Edited by vtornado, 16 August 2021 - 07:27 PM.


#41 Tom Masterson

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Posted 18 August 2021 - 09:27 AM

Emotional vs logical, oh the agony. . . .

 

There have been many times I've given in to the siren's song, and others when I stayed lashed to the mast. Often when caught in an endless decision loop, I'll let my emotions split the hair. Done it many times with scopes, accessories, cameras, sports cars, and motorcycles. In a couple cases I've stayed firmly lashed to the mast. There have been many times since the mid 80s I've coveted newer APOs and considered selling my trust old AP 6" 6/8 to fund the upgrade, same with my no-quite-so-old Brandon. For those two scopes my lashings remain tight - oddly enough for emotional reasons. I think of how both scopes still show me absolutely wonderful things and how I still LOVE looking through them. Would a new modern completely color free APO do the same or better? Without a doubt, but if I'm absolutely happy with what I have now, why give in to the urge? Am I chasing perfection? If 36 years later my current scopes still make me say WOW, don't I already have it? In other cases I've made the jump simply because "I wanted it." At those times it just feels just so darned good to be seduced - logic be ****!

 

But hey, that's me. As they say YMMV.


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#42 25585

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Posted 18 August 2021 - 03:03 PM

I've noticed a similar difference between my Vixen ED102SS and my Tak FC100-DL, both of which are doublets:

 

https://www.cloudyni...ief-comparison/

 

My TMB/LZOS 130mm F/9.2 performs like a 5.1 inch version of the FC100-DL.

DL's are so good!


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#43 Jarno

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Posted 19 August 2021 - 06:13 AM

Something else that has only been peripherally mentioned is the weight of a triplet, and in particular the weight distribution. A triplet will be much heavier at the front so when it's balanced the focuser end will be (much) lower than with the doublet. For AP that's not much of an issue but for visual use it might change your observing position from "awkward" to "chiropractor business enhancer".

 

And then there's the cooldown time, all that extra glass at the front needs to cool down too and the middle element is effectively insulated by the outer elements so it might take longer than you'd expect.

 

So at the very least you're looking at extra cost, extra weight and extra cooldown time for maybe a bit better visual performance. Unless you can test the scope to see how much of an improvement you'll get I wouldn't do it.

 

Jarno


Edited by Jarno, 19 August 2021 - 06:14 AM.

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#44 wrvond

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Posted 19 August 2021 - 06:54 AM

Something else that has only been peripherally mentioned is the weight of a triplet, and in particular the weight distribution. A triplet will be much heavier at the front so when it's balanced the focuser end will be (much) lower than with the doublet. For AP that's not much of an issue but for visual use it might change your observing position from "awkward" to "chiropractor business enhancer".

 

And then there's the cooldown time, all that extra glass at the front needs to cool down too and the middle element is effectively insulated by the outer elements so it might take longer than you'd expect.

 

So at the very least you're looking at extra cost, extra weight and extra cooldown time for maybe a bit better visual performance. Unless you can test the scope to see how much of an improvement you'll get I wouldn't do it.

 

Jarno

I have no idea what the Strehl value of my Sky-Watcher 120 ED doublet is. I do know it provides excellent views. The assumption is that any telescope with a Strehl approaching 1.0 would provide a better image, but how much better is the question.

Ultimately it was the physical aspects - weight and cool down time - combined with the fact that I've never been unhappy with the S-W that led me to decide to stick with the S-W.  waytogo.gif


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#45 spot37

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Posted 25 August 2021 - 11:09 PM

I am trying to decide whether to order a Tak FC-100DF, primarily for imaging. And all this talk about how doublets are not as good as triplets for AP has me taking a long pause to think about it. I have a FC-100DL and while fantastic for visual observing it's a bit slow for imaging. I ponied up for the FC-35 0.66x reducer, which brings it to f/5.9. But I struggle with my OSC imaging to get really sharp tight stars with the 100DL/reducer setup. For example: https://flic.kr/p/2m8FiRE

I do better on that score with my much less expensive SharpStar 76 triplet. https://flic.kr/p/2mfLnm6

(Of course there could be other factors causing the slightly soft stars in the Tak images...)

But owning the priceyTak reducer, I feel compelled to look at the idea of combining it with the FC-100DF, which gets me to f/4.8 and of course wider views on the chip. Tak recommends the combo for imaging, and I see some good images on Astrobin... but there aren't many Astrobin members using the combo. Not a popular choice for imaging, apparently.

If I get the FC-100DF, am I likely to see the same issues I'm having when imaging with the 100DL?

If doublets are not good at focusing all 3 color bands in the same plane, that would seem to demand a mono camera and refocusing for each color filter. I don't think I'm ready for that leap in complication. (I've only been doing this for about one year.)

Appreciate any insights!

Jeremy


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#46 photoracer18

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Posted 26 August 2021 - 02:17 PM

Remember his SW120 does have a FT focuser.

And that 95% Strehl is based on a vendor provided test. From what I understand the full aperture isn’t tested. There are other games you can play with Strehl tests, such as testing in red instead of green.

Granted I get the impression SV had a little higher standards for their Chinese optics than SW, so there very well could be an improvement in optical quality. To go along with a little better correction from being a triplet. So you could be right in that maybe the Chinese SV would be nearly indistinguishable from a Tak or SVX. Or there might be a reason the SVA was a $3k scope and the SVX is a $5k scope.

Ultimately it seemed like there were about three posts fawning over the SVA like it was a top of the market brand, like Tak or SVX. I just wanted to clarify. I mean, at the time the SVA was offered, it was $3,200 for a 130mm doublet. Versus the SW120 at $1600-1700. If you call it $2,000 for the SW120 with FT focuser, that is still over $1,000 more for the SV, which is a triplet with 10mm more aperture. Being a triplet and the extra aperture are probably going to eat most of that but it seems like there is some margin left for improved build quality or QA. So I would like to think the SV would be a bit higher quality, but overall the cost structure is more similar than different, so it is probably unfair to expect a noteworthy improvement in optical quality compared to the SW.

Bottom line, I had a SW100 and wanted to see what a premium apo was all about. I went with Vixen 103. I could see the difference. If you already have a good, well-regarded Chinese apo and want to see the difference a premium apo makes, I’m not sure if it makes sense to get a different Chinese apo that maybe has a reputation for a little better QA.

Scott

I have one of the first SVA scopes from Vic, #0002. And I used to work for a dealer (who sold TAK also). Yes the SVA line were Japanese glass finished in China and mounted in SV hardware. However unlike the other venders Vic did not just receive completed scopes and then ship them out to customers. They tested them and rejected anything under .950 Strehl. Mine is exactly .95 (100% also) so you know where the limit was established.


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#47 Jarno

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Posted 07 September 2021 - 07:21 AM

I have a 130 mm TS triplet, nice scope but indeed very top heavy. Your really have to aware on how you carry it.

Speaking of carrying, by pure coincidence I discovered that the Benro Tripod Bag AC3770A is a near perfect fit for the TS130. Not only does it physically fit well but the bags carrying handles are off-center and almost perfectly above the scopes center of gravity. For carrying the scope when it's out of the bag I put a Vixen dovetail on the top of the rings. It does double-duty as a carrying handle and mounting point for the guide scope.

 

Jarno



#48 F.Meiresonne

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Posted 07 September 2021 - 11:49 AM

I have mine in a case, but i meant carrying whilest placing it on the mount...it does not feel like other scopes cause it is really top heavy. One has to be a bit carefull. And also due to its weight you really have to tighten the knobs on the mount darn carefully too...



#49 Alan French

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Posted 07 September 2021 - 12:03 PM

I don't know if it has been mentioned, but a triplet opens more glass options for the designer. A doublet limits the choices. 

 

Since most modern triplet designs feature only one expensive Extra-low Dispersion element, whether fluorite (CaF2) or a fluor-crown (FPL55, FCD100, etc.) adding a third does not add exceptionally to the cost of material and provides additional degrees of freedom to correct aberrations.

 

Clear skies, Alan 


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