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Doublet APO? How does that work?


FZ1
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Quick question:

 I thought that an APO was a device that can bring all parts of the visible spectrum to focus at the same point. Received wisdom suggests that this can only be done with a triplet lens.  However, I've seen adverts for Doublet APO refractors, so how can a doublet achieve Apochromatic  results  rather than the more usual Achromatic for a two lens system?  And if its genuine  APO performance, why does anyone bother buying a triplet?

Any enlightenment will be much appreciated.  Thanks!

Jon 

P.S.  I'm  not buying, just interested.

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Even triplets don't bring whole spectrum to same focus point - they bring only 3 points or rather three wavelengths of light to exact focus vs 2 points doublets bring together.

If you want whole spectrum in focus - need to use pure reflection systems.

APO performance is somewhat loose term - it should read as no chromatic aberration perceivable by observer. That is the key - there is no color aberration that you can see.

How much of color aberration will you see depends on defocus for particular wavelength but also for eye sensitivity at that particular wavelength.

Have a look at this graph:

image.png.578b52f504f18f3b1842f6c29c475ad8.png

Blue line is simple lens - or a singlet - it just brings one wavelength at any one time to focus. Next is green dotted graph - that is doublet lens (this particular one is achromat) - it brings two wavelengths to focus at the same time.

Orange and red are APO and four lens combination (optical quadruplet lens - but not photo quadruplet). Amount of color that you will see depends on how much depicted curve lies far away from central 0 axis (how much defocus and related blur there is). Thing with ED doublets is that they produce much "tighter" curve than green achromat one. It will be of same shape, but it will be very close to 0 at 400-700nm range.

If you get really good ED doublet you will virtually see no color aberration.

So why do people purchase APO triplets at all?

One obvious reason is astro photography. CCDs are much more sensitive than human eye across larger range of frequencies, and while human eye can see color aberration - CCD will be able to record it.

Other reasons might include - sensitivity to color (some people can see residual chromatic blur because they are more sensitive to it), just wanting higher class instrument (if instrument is of higher cost to manufacture - there is higher profit margin and incentive for maker to be extra careful of how they figure lens and QA things and so....). There is also issue of F/ratio - ED doublet can be corrected only at F/ratios from 7 or 8 (depending on scope size) and upward. If you want F/6 or F/5 instrument that is free of color - you need APO triplet to be able to do it.

But for all other purposes (casual visual without color) - ED doublet is simply sufficient.

 

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From the original definition, the lens must be corrected for longitudinal chromatic aberration in three wavelengths, and spherical aberration in two. to do this you need three distinct glass types. ED doublets can only bring two wavelengths to a common focus,as the chart Vlaiv posted above shows. Calling an ED doublet "apo" is nobut marketing guff.

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Thanks very much for the info Dave and John and thanks for the detailed explanation Vlaiv; very helpful

I learn loads from reading all the posts in these forums and its rare that I end-up having to ask my own questions (and that's usually because I haven't spotted what I need in the search results)

Thanks again gents, much appreciated.

Jon

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20 minutes ago, DaveS said:

From the original definition, the lens must be corrected for longitudinal chromatic aberration in three wavelengths, and spherical aberration in two. to do this you need three distinct glass types. ED doublets can only bring two wavelengths to a common focus,as the chart Vlaiv posted above shows. Calling an ED doublet "apo" is nobut marketing guff.

Doublets using a fluorite element seem to do a pretty good job. I don't see any false colour in my Tak FC-100DL at focus and either side of it as well.

 

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Also remember a doublet will cool down much faster than a triplet too and a triplet can be a lot more expensive. Even a doublet 53 scope to a brand new person costs alot to some too much.

An example a 4 inch f 10 acro could be $400  a skywatcher 100f9 evostar costs $1400 with tax. My takahashi 102 TSA triplet cost $5000 with the cradle bar and finder bracket.

A long doublet can give the same level of color  correction as a shorter triplet. 

So a doublet can give u near perfect images and for visual only theres no need for triplet when your eyes cant tell much difference and it cost 3x to 4x more.

But if u image then the camera could pick up that small amount of false colour so then triplet is reccomend. 

Depending what u want to do and spend

Joe

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On 15/12/2019 at 19:37, FZ1 said:

Quick question:

 I thought that an APO was a device that can bring all parts of the visible spectrum to focus at the same point. Received wisdom suggests that this can only be done with a triplet lens.  However, I've seen adverts for Doublet APO refractors, so how can a doublet achieve Apochromatic  results  rather than the more usual Achromatic for a two lens system?  And if its genuine  APO performance, why does anyone bother buying a triplet?

Any enlightenment will be much appreciated.  Thanks!

Jon 

P.S.  I'm  not buying, just interested.

I think this is a really good question. There's so much talk about colour correction these days that many might conclude that it alone determines a refractors performance, and even put some off ever choosing a refractor. Also, the high demands of imagers, who naturally want the purest image from their scopes seems to have somehow leached into the visual observers realm, where miniscule levels of residual CA is by no means as important. The human eye is nowhere near as sensitive to CA as todays imaging cameras, and in most good refractors is barely noticeable, if at all. At times it seems that all astronomers look for in a refractor is false colour and they will use everything in their arsenal to find it. The truth is that even a good achromat can give any modern apo a serious run for its money, if its made well. A good doublet ED or fluorite doublet will bring all the colours so closely together that the image can be termed apochromatic. Many older apochromatic triplets, even from top end manufacturers such as Astro Physics, show more CA than many of today's ED Apo doublets, so the fact a scope is a triplet is no gaurantee of Apo performance. Most of today's ED doublets deliver such great views they are entirely capable of giving a lifetime of flawless enjoyment.

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In practice visual observers and narrowband imagers may find so little false colour in a really good doublet for it not to matter to them. Exacting deep sky imagers will be very unlikely to be entirely satisfied by a doublet, though.

Vlaiv mentioned the fact that faster F ratios are harder to correct. I think it's also true that increasing aperture makes colour correction harder.

Some doublets will beat some triplets, such is the overlap in real performance.

Olly

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A few years ago I carried out some head to head visual tests between my FLT98 f6.3 triplet and my Carton 100 f13 achro for lunar / planetary observing. The Carton won hands down with sharper images and better contrast and the added depth of field was a factor as well. The FLT98 would be the better imaging scope and all rounder but for visual lunar/planetary use the Carton showed what an achro from the days when they made great achros could do. 

No longer have the FLT98 but Big Red the Carton 100f13 is not going anywhere and should look very nice on the Rowan AZ100 mount when it arrives, hopefully in time for Xmas. 🙏🏼🙏🏼🙏🏼

Edited by johninderby
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3 minutes ago, ollypenrice said:

In practice visual observers and narrowband imagers may find so little false colour in a really good doublet for it not to matter to them. Exacting deep sky imagers will be very unlikely to be entirely satisfied by a doublet, though.

True enough for broad band imaging involving use of a clear IR blocking filter for a luminence channel.  However, for those swathed in light pollution and restricted to narrow band imaging an apochromat is simply unnecessary.  However, high quality, apochromatic scopes generally excel for other reasons - free of distortions, flat field, fast and with a good focuser.  Triplet apochromats aren't easy to put together, beware the cheap apochromat, sometimes a good quality doublet might be the better option.

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On 15/12/2019 at 19:58, vlaiv said:

APO performance is somewhat loose term ... 

^ What he said 🙂 

There is no industry standard for what is/isn't 'apochromatic' so manufacturers and end-users are left to decide for themselves what is/isn't acceptable.

Design, materials and manufacture determine the performance of a telescope. The latter is more important than most people realise because, sadly, it is where most cost-cutting happens. Possibly because it is difficult for an end-user (you & me) to assess and quantify manufacturing quality. I.e. specifying FPL53 glass ticks a box that pleases and assures most people but FPL53 refers only to the glass composition, not the grade, the accuracy of the optical surface or the optical coatings. For this reason the manufacturers reputation is arguably at least as important as the telescope's design and specification. It is also why a well manufactured doublet from a respected brand will outperform a budget triplet. (We first wrote about this back in 2011). 

HTH, 

Steve 

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On 15/12/2019 at 20:29, John said:

Doublets using a fluorite element seem to do a pretty good job. I don't see any false colour in my Tak FC-100DL at focus and either side of it as well.

 

Yes, and higher end units with FPL53 glass give a remarkable performance too, if the f ratio is sensible. I have reviewed a couple or three telescopes with good doublet lenses that colour-wise, gave views equal to my triplet. Perfect focus has been a key element though, a fraction of a millimetre in or out can alter things and this makes them more suited to observation than photography, the camera is a lot less forgiving than the eye, and tube length can alter by a millimeter or two on a long session with altering temperatures.

In actual fact, I might go as far as to say I prefer planetary views with a good doublet over those of a good triplet. Maybe.....

Tim

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I always took apo to mean bringing 3 wavelengths to focus at the same point and achromat to mean 2 wavelengths. Apart from that everything else is up for grabs!

For visual a doublet has the benefit of lighter weight, faster cool down, lower cost, and cheaper mounting requirements. 

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1 hour ago, John said:

Isnt there a "super-apo" as well which gives 4 colour "crossings" as they are known ?

I dont know about that, but when I made an inquiry about the colour correction in Vixens little VSD100, the data that came back included correction for yellow light, that was a bit different to the dominant green.

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From a visual observers point of view, the SW 100ED when in focus is pretty hard to distinguish from a good triplet or fluorite doublet. Around ten years ago my now late friend Phil almost begged me to swap my SW Equinox 120 ED for his essentially perfect triplet. The views between the two were to all intents and purposes identical when in focus. The Equinox was considerably lighter though, and more easily manageable. I loved my Equinox and so declined his offer. I loved Phil like a brother, and whenever we were together it took only a few minutes before we were laughing our sides out, usually over the silliest little things. I would have liked to have helped him out but from experience, I knew that the bigger the refractor gets the less I'm likely to want to use it. I'm probably the odd one out on SGL in this regard!

1707347547_2018-01-0523_31_53.thumb.jpg.7fbc877f804a2fc92494161032691aa8.jpg58626435a25c0_2016-11-3021_27_02.jpg.d1318aa789ed3d79551be8bafb8d773d.thumb.jpg.97deefa7d9fe8e51eeb7647c32224119.jpg Phil & his 127mm triplet

 

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I have a 130mm triplet and an ED120 doublet. The triplet is quite a bit larger and heavier than the the doublet - much more so than the 10mm aperture difference would suggest. Both excellent scopes though :smiley:

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Thanks for all the info folks.  The tech info is really what I was after, and I got that in spades, but discussion about different scopes is always interesting (Like I said in the OP, I'm not looking to buy, I'm just interested in the science, terminology and theory)

In summary [1] for anyone browsing this thread in future:

In terms of use, it seems that there's not a lot of difference in visual observing between a a good Achro doublet and an Apo triplet., so the definitions of each are less relevant or meaningful.  However,  for  astro photography, long exposures with CCD or CMOS sensors are  more sensitive than our eyes, so GENERALLY a triplet Apo will be better colour-corrected and therefor help give better photos.

[1] - This is a generalisation of all input above.  Everyone knows someone who paid 30-bob for doublet that takes better photos than the Hubble or knows of a £5,000 Quad that is rubbish .... but this is a summary! 

Cheers and Happy Christmas

Jon

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