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Affordable Triplet Refractors?


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Over two years ago I wrote a short piece for our website explaining why we were not sourcing budget triplet refractors. Little has changed so the article is as relevant today as it was back then, so here it is again :smiley:

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We are often asked to source and supply 'affordable' triplet refractor telescopes. It would be easy enough, FLO has access to them, so why are we not doing so?

From a purely marketing point-of-view cheap high quality triplets are the holy grail, but triplets are considerably more difficult to manufacture to a high standard than doublets. They are expensive for a reason.

In an attempt to get a 'handle' on a complex subject people concentrate on whether the design features FPL51 or FPL53 (or equivalent) glass and whether it is a doublet or triplet. But, not all FPL51/53 glass is equal and it is used only for the ED element, of equal importance is the quality and finish of the mating elements.

Then there is the lens cell - the metal structure that holds the elements in alignment. A considerable amount of thought, expertise and manufacturing know-how is necessary to produce a precise and robust cell that maintains alignment and doesn't 'pinch' on a cold winters night. Some big names have tried and failed and a poor cell will ruin an otherwise excellent optic.

When modern materials and manufacturing processes enable the mass production of affordable triplet refractors with consistently high optical performance (not just the first batch or the one supplied for reviews) then we will offer them here at FLO. Until then we'll sit tight and monitor the situation.

Hope that helps.

The original post can be found at our website.

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I just wrote something very similar ...

Excellent post :icon_salut:

A while back Steve (BinocularSky) posted saying not all Bak-4 glass (used for binocular prisms) is equal. And he is right. Similarly not all FPL51 or FPL53 glass is equal. The Japanese Ohara company have three manufacturing facilities. One in China, for the Chinese market, one in Taiwan, for the Taiwanese market, and one in Japan predominantly for their Japanese home market. Guess which one produces the highest quality FPL51/53 glass...

Designing refractor optics is relatively easy. It is the materials and manufacturing processes that determine whether a design achieves it's full potential. Unfortunately high quality materials and manufacturing is expensive.

Steve

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The above considerations lead me to get my 80mm F/6 affordable triplet from APM, because I do not believe they would let bad ones pass. I have heard of problems with the same design sourced from other dealers. APM claim to do their own QC checks, and they need to, to keep their reputation.

Note that I have not used it for imaging yet (got side-tracked down the solar route), but visually the results are very pleasing indeed.

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That captures it in a nutshell. Kit is for looking through and imaging. Brand names and specs are not important. Performance is.

Absolutely, but an understanding of how optical quality is achieved is not a bad thing :smiley:

Actually, brand names are important because not all brands are equal and there is some assurance in buying a brand that values it's reputation. You acknowledged that when you said you chose APM.

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Excellent post :icon_salut:

A while back Steve (BinocularSky) posted saying not all Bak-4 glass (used for binocular prisms) is equal. And he is right. (Steve, if you are reading this please link to your thread). Similarly not all FPL51 or FPL53 glass is equal. The Japanese Ohara company have three manufacturing facilities. One in China, for the Chinese market, one in Taiwan, for the Taiwanese market, and one in Japan predominantly for their Japanese home market. Guess which one produces the highest quality FPL51/53 glass...

Designing refractor optics is relatively easy. It is the materials and manufacturing processes that determine whether a design achieves it's full potential. Unfortunately high quality materials and manufacturing is expensive.

Steve

Thats interesting to know. I thought the Ohara FPL-3 glass for the ED doublet Skywatchers was, at least originally, sourced from Japan and then figured in China. Has that arrangement changed now ?

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Absolutely, but an understanding of how optically quality is achieved is not a bad thing :smiley:

Actually, brand names are important because not all brands are equal and there is some assurance in buying a brand that cares for it's reputation.

True, certain brands consistently supply high quality stuff, but some others have wildly differing offerings. The Meade S5K UWA I had was certainly very good, at TV levels of performance, even in Olly's 20" F/4.1 dob. However, mention the dreaded Meade QX 26mm and several people who had this foisted upon them when they bought an otherwise fine scope from Meade get hot under the collar. Celestron and SkyWatcher also have hugely varying offerings, varying from cheap and cheerful beginner's kit, to really nice optics. In those cases you need good reviews of the performance.

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Thats interesting to know. I thought the Ohara FPL-3 glass for the ED doublet Skywatchers was, at least originally, sourced from Japan and then figured in China. Has that arrangement changed now ?

I think we can safely say it is 'Ohara' glass but I cannot say which manufacturing facility it comes from.

(At our website we only quote 'Japanese Ohara' when we are confident it comes from the Japanese manufacturing facility)

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You sell the Skywatcher Espirit triplets, does that suggest the Espirits are better than other Chinese triplets?

I agree triplets are much harder to make than doublets. There are 33% more glass to grid, 50% more lens to align and each surface has to be polished to a higher precision in order to achieve the same Strehl ratio. It also explains why every Chinese quadruplets and quintuplets models had problems.

When it comes to high end optics, how it is made is much more important than what it is made of. Legends has it that top manufacturers such as LZOS tweak their lens design for every melt of glass to correct for minor variation in glass property. I doubt any Chinese manufacturers do this. Also TAL had shown you can make good apo with just regular crown and flint glass, far better than what you will get if you give an average amateur a fluorite blank to grind in his shed.

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You sell the Skywatcher Espirit triplets, does that suggest the Espirits are better than other Chinese triplets?

Yes, the Esprit series are significantly better than budget Chinese triplets.

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I think we can safely say it is 'Ohara' glass but I cannot say which manufacturing facility it comes from.

Maybe it has changed then or maybe it varies batch to batch. The OVL catalogue from 2007 says ".... the second element is made of the highest grade extra-low dispersion Japanese FPL-53 flourite glass...." My ED120 Pro is a gold tube one from that era.

Wherever it's sourced from the scope performs very well, which is what counts :smiley:

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Maybe it has changed then or maybe it varies batch to batch. The OVL catalogue from 2007 says ".... the second element is made of the highest grade extra-low dispersion Japanese FPL-53 flourite glass...." My ED120 Pro is a gold tube one from that era.

I know you know this John but an FPL53 element is not the same as a 'fluorite' element. I think the copywriters got a little carried away when writing that product description :smiley:

Wherever it's sourced from the scope performs very well, which is what counts :smiley:

Indeed it does. They certainly know how to make affordable quality doublets :glasses2:

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I think we can safely say it is 'Ohara' glass but I cannot say which manufacturing facility it comes from.

(At our website we only quote 'Japanese Ohara' when we are confident it comes from the Japanese manufacturing facility)

If the glass is from Ohara, is glass from their Chinese facilities go to be much difference from ones made in Japan? I assume we are talking about Ohara's own factory here and not subcontractors.

Also, is there different grades of FPL53? The will be slight differences in every melt and I would assume Ohara will sell good ones (higher purity, less bubbles etc...) for more than the bad ones. High end optics manufacturers will probably go for a higher grade glass than low end manufacturers

A lot of budget triplets are made with FPL51, CGDM FK61 and Hoya FCD-1. Obviously the preformance of those will be different

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About a year or so back i was looking to get a Vixen 80ED doublet - with Ohara glass. But also was looking at the Vixen 81EDs which was over twice the price, from this thread i would assume that the vixen/synta would have been Chinese Ohara glass and the 81ED would be a totally made in Japan product?

andrew

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If the glass is from Ohara, is glass from their Chinese facilities go to be much difference from ones made in Japan? I assume we are talking about Ohara's own factory here and not subcontractors.

Also, is there different grades of FPL53? The will be slight differences in every melt and I would assume Ohara will sell good ones (higher purity, less bubbles etc...) for more than the bad ones. High end optics manufacturers will probably go for a higher grade glass than low end manufacturers

The Ohara company can manufacture to whatever grade is required so the answer to your question depends on whether the Chinese, Taiwanese and Japanese markets have different requirements. I personally believe the Japanese market insist on only the best.

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About a year or so back i was looking to get a Vixen 80ED doublet - with Ohara glass. But also was looking at the Vixen 81EDs which was over twice the price, from this thread i would assume that the vixen/synta would have been Chinese Ohara glass and the 81ED would be a totally made in Japan product?

andrew

Yes, the f7.5 Vixen 80ED is essentially the Skywatcher 80ED but the f7.7 Vixen ED81 S is a wholly Japanese-made product :smiley:

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I have been reading this with interest - I am intrigued by how you determine an 'affordable' triplet? Under £500, under £1000?

It depends on the aperture :smiley:

I am not saying you cannot achieve good performance with a budget triplet (the ones sent to Olly for review are clearly good) but I am concerned with batch-to-batch consistency and performance at low temperatures. Michael.h.f.wilkinson mentioned the need to choose your supplier carefully. He is right. My advice to anyone wanting a budget triplet is to buy it from Ian King Imaging (FLO doesn't offer budget triplets). Ian knows the score and is a gent' so will do the right thing if you find you have purchased a lemon.

I fully understand someone wanting to buy one, they promise a lot for the price.

HTH,

Steve

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4 months FLO! I've only used it 3 times but the cursed weather and lighter nights is just giving me the withdrawl symptoms!! I must admit that the engineering quality is superb and it looks very similar to the Altair Astro range! The 3" Crayston Focuser is good and smooth with no slippage, and I like how it rotates 360 as well, the scope as retractable dew shield the only qruims I have on the TS 115mm Triplet is the stupid the vixen dovetail which is not great, but I quickly replaced with a Losmandy one (much better and studier) The Triplet lens cell is a good quality FPL-51 I believe with good multi-coat! It's delievered a excellent images so far with no false colour that I can detect! The 3 Element Riccardi Focal reducer and field flattener is very handy device which speeds up my Refractor F7 down to F5.2 which seems to perform quite well for CCD imaging! So far this scope is so much better at producing high quality images than my Lunt 80 ED F7 Refractor, where the Lunt shows hint of blue false colour when imaging!! It just shows that the qualty does count more on APOs!!! Hence the reason the more you buy the expensive scope better the quality will be. But unfornately my budget does stretch that far for a TAK or a Televue! Shame!!! If only if I won the Lottery????? :shocked: :shocked: :shocked:

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