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Doublet and Triplet refractors


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Ignoring áll the arguements about how well designed/made the triplet should outperform the doublet.

As the question had "triplet" in it I am presuming that you are refering to an ED doublet.

A lot comes down to visualising a parabolic graph and a cubic graph.

A doublet will force 2 wavelengths to the same focal plane, other wavelengths will therefore be off this plane. Therefore you can have red and blue at the same plane but not green.

A triplet forces 3 wavelengths to be one the same focal plane, so you can get red, blue/violet and green at the same plane, yellow and sort of blue/green therefore being off it. However while being "off" the focal plane they cannot really get far away from it. So what chromatic aberration there is will be very small and generally not detectable by the human eye.

An ED is better then a achro since the ED lens although putting the red+blue at the same focal plane the green cannot actually deviate as far from "ideal" then an achro will allow. A sort of shallower curve to the parabola.

What makes one triplet better then another is likely to be the wavelengths chosen that come together at the same focal plane. Do you design for 400, 550, 700nM or 420, 530, 650nM

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Some good advice above. The only thing I'd add is that making a low cost triplet apochromat refractor is not going to be automatically better in terms of colour and spherical correction than a decent doublet that uses ED glass for one of it's components.

Creating a objective lens with three lenses, from properly match glass types, that are accurately figured and polished, precisely mounted, spaced, baffled and aligned is a time consuming and expensive business. 

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I believe the fourth element is, as you say, normally a flattener.

Another variation is the Petzval design such as in my Genesis. Air spaced achro doublet of around f10 as the objective, then a cemented doublet just ahead of the focuser which reduces the focal ratio to f5 and flattens the field. Interesting design.

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I see they make quadruplets also....does the forth lens focus the color also or is it used as a field flattener?

It depends on the design. I've used a six element apochromat which used a single lens at the top end and then two groups of lenses down the body of the scope. The control of chromatic aberration (CA) was excellent even though no ED glass was used in the design. The downside was that the scope would be a real challenge to collimate if it was knocked.

There are designs where the main objective lens is a doublet and then there is a further doublet near the focuser end to correct CA and to deliver a flat field. The 101mm and 127mm Tele Vue refractors use this design and there are others too.

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I guess like everything it's a trade off. The more elements the better the potential colour correction but the more surfaces to scatter light and of course the more cost! So a camera lens that requires excellent colour correction and flat field may well have a dozen elements in it. They don't make particularly great visual telescopes because the contrast is lowered by all the reflected light from 24 surfaces. The CCD doesn't care of course, you can post-process it back. 

If you're making a 6" f25 refractor, then it's possible to achieve superb colour correction with just two lenses - especially if one is ED glass. But a 6" f4 will almost certainty have to be a quadruplet or more to achieve the same result. 

Best rule of optics I've come across is that if you have a scope that you love and that you use all the time, it's the best optical design in the world  :)

all the best

Tim

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Some great advise here , awesome stuff guys .

I have only yesterday received a $499aud Long Perng ( see my thread here ) 60mm f7 triplet using FPL53 as its sandwich ( centre ) lens element and I am in the process of testing right now , 2nd light tonight , so I am lucky to have 2 good nights in a row , yippe ! .

I am finding the new 60mm triplet to be a very , very competent APO , its impressive what can be done today for little money .

Anyway mate up until November 2013 I owned and loved a Takahashi FS60C f5.8 Flourite doublet APO and used and loved her for many years , but sold it in a fit of madness !! Grrr what WAS I THINKING ! .

OK , I will say the Takahashi 60mm Fluorite doublet was better than the Long Perng 60mm FPL53 triplet , but not by the $1000 odd will suggest , yes the Tak was worth $1500 with all extras to allow normal ( finder , adaptors , diagonal ,,,,) visual observing , the LP is $499 complete , so as it stands the LP triplet has over 95% of the Tak's visual abilities and the same in build quality at 1/3 the price ,, go figure ( pun intended ) so for me I find the LP a better scope , but its a triplet and performs almost as good as my now sold Takahashi , but this is only me and for us average ( eg. poor ) amateurs these new breeds of Chinese triplets are very good in all ways , Performance/Value .

Read my review if you want .

ps. then there is my Takahashi sky90 f5.5 Fluoirite doublet ,, get a look thru one of these and you don't need a triplet , the best 90mm G&G scope on the planet .

Brian.

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