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Doublet vs. Triplet

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#1 wrvond

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Posted 24 July 2021 - 08:28 PM

I have a Sky-Watcher ED120 that performs very well, especially since I am a visual only observer. No complaints whatsoever. I came across a Stellarvue SVA130T for sale.

 

I tell myself the 120 is a great scope - it's outfitted with a Feather Touch focuser and is, as far as I know, flawless.

But I keep circling back around to the Stellarvue. I have never looked through any refractor more premium than my S-W so really wouldn't know what to expect using a triplet for visual. 

 

Would there be a significant improvement moving from the 120 to the 130?

 

 

Thanks for your thoughts.


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#2 ngc7319_20

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Posted 24 July 2021 - 09:20 PM

A 130 triplet would have better color correction (sharper images) in the far red and blue / violet than the 120 doublet... but your eye is not so sensitive at those wavelengths.  The 120 probably cools much faster than the 130, as the later has a lot more glass to cool.  I'd probably stick with the 120.


Edited by ngc7319_20, 24 July 2021 - 09:22 PM.

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#3 SilverLitz

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Posted 24 July 2021 - 09:21 PM

There will be a big upgrade with the SVA130T.  SW gives you a lot for the money, but it is a mass produced product.  With SW, the Esprit line is a significant bump up, especially for AP.  The high end SV scopes are not mass produced, but are hand built.  SV's Access line were Chinese built, but the SVA130T would have been built in California.  In the last few years, SV has taken their game up to a higher level with the SVX line, with Strehl's of 0.99 +.  It seems like Strehl's of the SVA130T would probably be in the 0.95-96 range, which is still very good.  Fit, finish, and tolerances will also be noticeable improvements.

 

The SVA130T will be much heavier than your SW ED120, my SVX130T is ~33# when fully loaded for AP, and the front lens cell is very heavy.  So it will want a beefier mount, especially for AP.


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#4 jeremiah2229

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Posted 24 July 2021 - 10:35 PM

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...


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#5 vtornado

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Posted 24 July 2021 - 11:10 PM

I have never viewed through a triplet either but ...

I would strongly suggest you go to a star party or club and look through someone else's before

sinking a lot of cash on a scope that will exceed yours by 5%.

Additionally your scope will cool faster and be easier to mount.


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#6 wrvond

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Posted 24 July 2021 - 11:43 PM

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.  wink.gif

 

 

Peace...

There it is. Emotional me wants the 130 very much, but logical me says I better stick with what I’ve got.

waytogo.gif


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#7 betacygni

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Posted 25 July 2021 - 12:01 AM

 

There it is. Emotional me wants the 130 very much, but logical me says I better stick with what I’ve got.

Just to play devils advocate here, there is nothing wrong with going with the emotional side (assuming you can afford it without major sacrifice). I ended up buying a tak tsa120 even though I knew rationally the 120mm doublets are 95% the performance, at a fraction of the cost. However, I still smile when I take it out of the case and mount it, I don’t think I would have that feeling with the Chinese doublets (even though they are great scopes). I suppose I appreciate the aesthetic of it, the craftsmanship and care that went into it, etc. Not rational, but very real. I totally agree the performance upgrade alone would likely not be worth it, but the joy of owning a hand crafted Stellarvue? That might be.


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#8 Nippon

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Posted 25 July 2021 - 12:52 AM

I think what really makes the most difference is accurate figure and smooth polish. I've had three triplets and several doublets. The best performing refractor out of all of them is my current Vixen ED103s. I had it before any of the triplets and one by one the Vixen said bye bye to the triplets. The reason is the Vixen has an excellent FPL53 doublet objective. It edged out the triplets just a wee bit due to excellent figure and polish. I see no reason to buy a triplet for visual use. Imaging is another matter. If I were to decide to move up to a 5" refractor I would look for a used well kept Takahashi FS128. Failing finding one of those I would really take a hard look at Stellarvue's SVX 125 doublet.  


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#9 Jon_Doh

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Posted 26 July 2021 - 08:45 AM

For visual I think you would be hard pressed to see much difference, perhaps if you had them side by side.  The 120 has excellent optics and punches above its weight.  A triplet is going to be heavier, front heavy in balancing and take longer to cool.  But, the Stellarvue's are a work of art and are well built so it's hard to argue against them.  Unless you're doing AP I think the 120 will continue to serve you well.


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#10 25585

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Posted 26 July 2021 - 09:59 AM

I have a 120 Equinox with Ohara & FPL53 lens, and a Takahashi TSA-120 triplet which is better, but only at high magnitudes. For pure whites & cooler colours the Tak is best. For lunar detail, not much in it. The Equinox is nice for warmer colours; Mars, Jupiter & G, K, M, R spectral stars. 

 

The Tak 120 is better at splitting close doubles I find.

 

I recommend either for their lenses. The Tak wins on weight for what it is.


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#11 Simoes Pedro

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Posted 26 July 2021 - 11:47 AM

You are suffering from GAS, Gear Acquisition Syndrome.

 

But just to clarify: Triplet are not better than doublets by default.

 

If you are happy with your current scope why trade it? 130 is not that much larger than 120, and yet being significantly heavier.


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#12 Simoes Pedro

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Posted 26 July 2021 - 11:50 AM

Here you can find interferometry tests for 4 SW ED120's. All excellent.

 

http://r2.astro-fore...lles-ueber-apos


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#13 SeattleScott

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Posted 26 July 2021 - 12:01 PM

Ok folks let’s be clear here. He is NOT talking about a handcrafted SVX130. He is talking about a $3k SVA130 from China. Now I’m sure these are very nice scopes with great build quality and very nice optics. But most likely the optical quality will probably be closer to his existing Chinese SW120 than a Tak or SVX.

I concur that it probably doesn’t make logical sense to upgrade. To get a noteworthy improvement, I think one needs a larger increase in aperture, or a considerable improvement in optical quality (which might not be realistic even with a Tak or SVX given how happy he seems to be with his SW120).

Scott
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#14 SeattleScott

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Posted 26 July 2021 - 12:10 PM

Here you can find interferometry tests for 4 SW ED120's. All excellent.

http://r2.astro-fore...lles-ueber-apos

I have said it before and I will say it again. Using these third party tests invites statistical sampling bias. People who buy a budget Chinese Apo don’t expect it to be outstanding, but if they feel like they really scored and got a premium lens for a budget price, they may pay to have it tested (improve resale value if nothing else). Now I think this is useful information, and worth evaluating. But in my opinion these are probably statistically biased.

That being said, the overall reputation of the SW120s is quite good. Granted many feel their optical performance isn’t on par with top tier brands, as expected. But for most it is close enough.

Scott
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#15 F.Meiresonne

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Posted 26 July 2021 - 12:11 PM

You are suffering from GAS, Gear Acquisition Syndrome.

 

But just to clarify: Triplet are not better than doublets by default.

 

If you are happy with your current scope why trade it? 130 is not that much larger than 120, and yet being significantly heavier.

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


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#16 Simoes Pedro

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Posted 26 July 2021 - 01:13 PM

I have said it before and I will say it again. Using these third party tests invites statistical sampling bias. People who buy a budget Chinese Apo don’t expect it to be outstanding, but if they feel like they really scored and got a premium lens for a budget price, they may pay to have it tested (improve resale value if nothing else). Now I think this is useful information, and worth evaluating. But in my opinion these are probably statistically biased.

That being said, the overall reputation of the SW120s is quite good. Granted many feel their optical performance isn’t on par with top tier brands, as expected. But for most it is close enough.

Scott

I would disagree. There are terrible telescopes posted as well.


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#17 betacygni

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Posted 26 July 2021 - 05:04 PM

 

Ok folks let’s be clear here. He is NOT talking about a handcrafted SVX130.

Regardless of country of origin, I’d argue a certified .95 strehl or higher triplet refractor built to Stellarvue’s specs, sporting a 2.5” or 3.0” feather touch focuser no less, qualifies as handcrafted. I would venture to say it would give even their latest SVX line a run for its money (if not be indistinguishable at the eyepiece), or any other premium scope for that matter. This of course is not to diminish the 120mm doublets, but the scopes would be different in design and levels of execution.


Edited by betacygni, 26 July 2021 - 05:05 PM.


#18 SeattleScott

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Posted 26 July 2021 - 06:31 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
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#19 SeattleScott

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Posted 26 July 2021 - 06:46 PM

I would disagree. There are terrible telescopes posted as well.

That can be the other side of bias. Someone who buys an excellent premium apo has little reason to pay to have it tested. They don’t need a test report to sell it. They can sell it for a premium based on brand name.

However if they buy a subpar premium Apo, they will pay for a test report so they can go back to seller and demand money back, exchange, etc.

Someone with a budget Apo is typically cost conscious. Paying for a test will not convince SW or ES to swap out their 91% Strehl Apo. So why bother? Well if you think it is 95%+ then you can get it tested and now you improved the resale value.

So there is a tendency on these sites to see some surprisingly poor premium Apos and some surprisingly good budget Apos. Certainly you can find subpar premium Apos as well as excellent budget Apos “in the wild.” I just suspect the ratios on these sites isn’t representative of the population as a whole.

That being said it is good information to share, if nothing else to show just how good these budget Apos can be. Just keep sampling bias in mind. They won’t necessarily all be that good.

Scott
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#20 John Huntley

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Posted 26 July 2021 - 07:18 PM

I have a Skywatcher ED120 which sits amongst a Vixen ED102SS F/6.5, a Takahashi FC100-DL F/9 and a TMB/LZOS 130mm F/9.2 triplet. While the build quality of the more expensive premium brands is better than that of the Skywatcher ED120, optically the ED120 compares very well indeed with it's illustrious stablemates - it's not outclassed at all.

 

Maybe I've got a particularly good ED120 shrug.gif

 

ed120ercole01.JPG


Edited by John Huntley, 26 July 2021 - 07:20 PM.

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#21 betacygni

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Posted 26 July 2021 - 07:34 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


Don't see any post in this thread that has been fawning unrealistically over the SV, in fact quite the opposite all have said to expect minimal difference. But again, that’s not the entire equation. We all likely happily own scopes or eyepieces that don’t make a lot of rational sense (in fact most refractors don’t visually just as a design, but here we all are). Observing is about the experience for many, and that includes owning scopes that you appreciate, entirely rational or not. But I agree it’s very good to always keep your equipment choices in perspective.

#22 teashea

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Posted 26 July 2021 - 07:55 PM

Ok folks let’s be clear here. He is NOT talking about a handcrafted SVX130. He is talking about a $3k SVA130 from China. Now I’m sure these are very nice scopes with great build quality and very nice optics. But most likely the optical quality will probably be closer to his existing Chinese SW120 than a Tak or SVX.

I concur that it probably doesn’t make logical sense to upgrade. To get a noteworthy improvement, I think one needs a larger increase in aperture, or a considerable improvement in optical quality (which might not be realistic even with a Tak or SVX given how happy he seems to be with his SW120).

Scott

very good points



#23 kel123

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Posted 26 July 2021 - 08:49 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.
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#24 lwbehney

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Posted 10 August 2021 - 10:12 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


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#25 Deadlake

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Posted 11 August 2021 - 02:21 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 suspect that is coming down to FPL glass versus OK1/4 glass (fluorite like).

While the triplet is heavier it’s shorter, swings and roundabouts.

I think this really comes down to aperture, if you go to 130 mm view doublets around.

Another option is Tecnosky has a 130 mm triplet with a guarantee Strehl of 0.95, best value I’ve seen.


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