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Triplet's V Doublets


carastro

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I posted this in the imaging discussion forum because my question is related to imaging.

I have never owned a triplet (always doublets), but have often wondered whether there would be any advantage in getting a triplet.  I know the colours are supposed to focus better on a triplet, but are there any other advantages  I am unaware of that makes it worth spending the extra money?

One thing in particular that bothers me is how some people manage to get pinpoint stars or even "very few" stars in their images.  It this due to the scope, the filters or the camera, or some processing Wizardry?

Thanks in anticipation of some valuable answers.

Carole 

Edited by carastro
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Good luck with this one, I really hope you get some good replies because I searched this forum and some well known others with the same question and never really saw a proper conclusion. So may different answers and probably 20% saying Doublets better for imaging and another 20% saying Triplets are better and the rest sort of saying that not all doublets are equal and also not all triplets are equal, which is pretty much assured to be is true but then when some of the more knowledgeable one went into the reasons why it kind of got above my understanding and all I really understood was that price wise doublets are cheaper because they are obviously easier to make but a good doublet is far better than a poor triplet.

Sorry I cannot answer your question but will watch your thread to see if I get a better understanding myself 🙂

Steve

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It depends on what you are imaging and what you are imaging with.  

If you are using a colour camera and want a 'fast' refractor and/or wish to image broadband objects then you would want a triplet  A fast doublet refractor would not bring enough of the wavelength range to focus at the same point and hence stars would bloat badly.  On the other hand a slow refractor would have less of this issue as the best focus range is much larger and it is much better at focussing a longer range of wavelengths - you can offset the decrease f ratio by binning the camera.  

If you want to solely focus on narrowband objects then strictly speaking (and assuming all optics are made equally) then a doublet should be better.  As you are working at narrowband then all of that narrow range would be focussed equally so bloat can be avoided.  You would have to focus each time you changed narrowband range though (e.g. SII, HII, OIII etc).  The doublet would then be better as it should cool down faster and be more temperature stable (less glass) and have less surfaces so should result in less scatter so stars should be smaller and light throughput should be higher.  In principle it should also be easier to manage as it would be lighter as well.

As for the size of stars that is probably a combination of optics, aperture, seeing and image processing.  With no seeing (i.e. in space) then the size of a star is limited by the aperture (assuming no central obstruction) - in a perfect world a larger aperture would result in a smaller star.  However, seeing, poor focus (perhaps from temperature changes) optical imperfections and so forth generally scatter enough light so they bloat to some extent.  Many then use software tools to shrink down the stars.  In the UK I would guess most stars are fatter then other images (from say Chile) because of seeing assuming both parties have equally understanding of imaging (e.g. can find the point of best focus).  

 

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In theory the more glass types and free surfaces (air spaced 2 v cemented 1) the more options the optician has to reduce the aberrations. However, the more glass the longer the cooldown time and the more complex the cell and alignment.

It often comes down to the final design and especially the execution.

Regards Andrew 

Edited by andrew s
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My take has been that for refractive optics, the more the surfaces, the more options the optician has for minimising aberrations, one of the more noticeable ones being chromatic aberration.  Given optics of the same aperture, focal length, quality of figure and best glass for the purpose, a triplet should have the edge for imaging. The downside of the triplet, apart from the higher cost, is cool down time and sensitivity to alignments of the optical components.  Visually, the difference is unlikely to mirror the cost difference.  Speaking of mirrors, it's remarkable that a one surface optic can achieve most of what a sophisticated multi element refracting objective can do.    🙂

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The obvious improvement in colour correction was once the exclusive sphere of the triplet.
However the introduction of special ED glasses has reduced their advantage.

When I dabbled in geometric ray tracing one could obtain 1/10th as much false colour with a triplet.
But, then only in comparison with a doublet of common glasses like BK7, F4 at longer focal lengths.

The triplet APO still needed very exotic and expensive glasses at that time.  e.g. Roland Christen's early 'AP' APOs.
Special glasses are now much more widespread in larger aperture blanks thanks to the Chinese popularising affordable "short" APOs.
Not to mention binoculars and spotting 'scopes.

None of which answers your question. Triplets do have the reputation for much slower cool down. [As already discussed above.]

Peter's mention of mirrors is certainly true. The ultimate "planetary" telescope in amateur sizes is/was probably still an optimised Newtonian of 12-15" aperture and longer focus.
However, the corrective elements now available to combat coma are even allowing short focus, large aperture telescopes to produce superb visual images.
But only in excellent seeing conditions. Like Florida?  Or at high altitudes. Yet another red herring to avoid giving a direct answer? :wink:

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Thanks every-one for your replies, so it does look as though the main advantage is in the chromatic aberration area as I thought.  The other differences IMO are disadvantagous.

I am pretty happy with my Doublets, I was just making sure I wasn't missing out on something.

Carole 

Edited by carastro
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22 minutes ago, carastro said:

Thanks every-one for your replies, so it does look as though the main advantage is in the chromatic aberration area as I thought.  The other differences IMO are disadvantagous.

I am pretty happy with my Doublets, I was just making sure I wasn't missing out on something.

Carole 

It is not just chromatic aberration that can be reduced but all the seidel aberrations can be reduced/optimised. E.g distortion, coma and field curvature.

Regards Andrew 

Edited by andrew s
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8 hours ago, Rusted said:

However, the corrective elements now available to combat coma are even allowing short focus, large aperture telescopes to produce superb visual images.
But only in excellent seeing conditions. Like Florida?  Or at high altitudes. Yet another red herring to avoid giving a direct answer? :wink:

Nice answer, and as I suspected it is not that black and white anyone can give a one line answer to this.

In the very simplest terms then, If my pockets are not bottomless but I have a reasonable amount to spend on an imaging scope (say around £1000), and at this stage I am not after perfection as I am still trying to get to grips with imaging anyway but just want some decent images, would I be right to say I could not go far wrong getting a fairly high end doublet with flattener and that without spending considerably more I am not really gaining much.

I know I have the SW Esprit 100 but it was bought as new 2nd hand at a price I couldn't resist, if I was to progress from the WO73 and to buy new I would probably have got another doublet.  Due to weather and working in China last 8 weeks I have not had a lot of chance to use it yet but my first (and only so far) image seems better than with my WO 73 but I have no real way yet of knowing whether this is the scope or me actually improving my techniques.

Steve

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Been wondering the exact same thing as I sit here for the last week finger at the ready as to whether the 73mm WO doublet or the 71mm WO triplet will fill the gap. I have used a few quality instruments here over the last 9 years from the most costly to the no so. These have included doublets and what I will say is Triplets win on visual with object like Venus. This being one of the most difficult to tame for CA. Though even my 70mm Ed gives a reasonable disc in the centre of field. I am sure though, not having seen them, the best doublets do a fine job and whilst we do not all image Venus and Jupiter and even then try and keep the like of Vega out of view, a good doublet will do a great job. It would be the snob in me that wanted the triplet, though you have to say Carole the Tak family do look very sharp indeed.

Alan

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13 hours ago, teoria_del_big_bang said:

Nice answer, and as I suspected it is not that black and white anyone can give a one line answer to this.

In the very simplest terms then, If my pockets are not bottomless but I have a reasonable amount to spend on an imaging scope (say around £1000), and at this stage I am not after perfection as I am still trying to get to grips with imaging anyway but just want some decent images, would I be right to say I could not go far wrong getting a fairly high end doublet with flattener and that without spending considerably more I am not really gaining much.

I know I have the SW Esprit 100 but it was bought as new 2nd hand at a price I couldn't resist, if I was to progress from the WO73 and to buy new I would probably have got another doublet.  Due to weather and working in China last 8 weeks I have not had a lot of chance to use it yet but my first (and only so far) image seems better than with my WO 73 but I have no real way yet of knowing whether this is the scope or me actually improving my techniques.

Steve

Read with interest Steve, but the only thing I would say is 73mm WO and a 100mm Esprit, not really an apple with apples comparison, the ES is over an inch bigger and indeed a fine scope also more than 100mm longer F/L. I wanted that that scope but you beat me to it by about 2 minutes.

Alan

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13 hours ago, alan potts said:

Read with interest Steve, but the only thing I would say is 73mm WO and a 100mm Esprit, not really an apple with apples comparison, the ES is over an inch bigger and indeed a fine scope also more than 100mm longer F/L. I wanted that that scope but you beat me to it by about 2 minutes.

Alan

Yes I agree with that, both the apples and oranges thing and that it is a great scope, or so many on here seem to say that so I can only think it is true, and for me time will tell if it helps me to obtain better images, but at this stage most is probably down to me and how I continue to progress.

What I really meant was that if I had not seen this for sale (and sorry for whipping it from under your feet -- he says "tongue in cheek") I would probably have spent just a little less and bought another doublet as that was the way I was heading when this opportunity appeared, in essence even 2nd hand was a bit more than I had budgeted for and thankfully my very understanding wife was fine with the extra outlay 🙂 

Steve

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Is a doublet a doublet, and triplet a triplet? Of course not..you get better doublets over others, you can get better doublets over some triplets.The glass used in some doublets are better than the glass used in some triplets...

Definately a difference between fpl53 over fpl51...fluorite is a superb glass used in some doublets

I've owned 2 doublets and now a triplet, more than happy with the triplet but not because of the glass..other factors come into play also..

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The other thing of course is human nature !  ..I would say get the best you can afford , everyone I spoke to before I brought my scopes told me a triplet was the way to go , and I can’t complaint with my 130 ..

So here is where the human nature comes into it...... if you buy a Dublet will you always be thinking IS the triplet better ! 🤔

if your not that sort of person it doesn’t matter ,  but  most of us are 🤗

anyway whatever you decide , I’m sure YOU will get the best out of it ! ....  

regards Brian 

 

Edited by Brian28
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I had never heard of a triplet until I had been imaging for a few years.  So wasn't a choice for me, especially as I got my first imaging rig as a "job lot" which came with a WO Megrez 72 APO which was a doublet.  Nice scope but it was the older version where the focusser slipped so I changed that for a SWED80 which I still have.

Carole 

 

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I've had doublets and triplets, I was lucky in that I bought both my Esprits second hand in as new condition.

The most obvious difference to me was contrast, there was so much more IMHO, especially in Ha.

Not sure if that helps?

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