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Equipment Discussions >> Refractors

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Arizona-Ken
Pooh-Bah


Reged: 08/31/08

Loc: Scottsdale, Arizona
Re: Triplet APO or Doublet APO? new [Re: JoseBorrero]
      #5852833 - 05/10/13 10:05 PM

Quote:

And this sample done with the Orion ED80 Triplet. spikes were done with Starspike pro on Photoshop




Jose:

A beautiful picture and very well done. However, it is funny to me after reading all the posts about how much better refractors are than any kind of newtonian that someone puts star spikes in the picture to "fake" a spider vane. I know it is done for esthetic purposes, but it is funny nonetheless.

But if you have other pictures of this quality, please feel free to post as many of them as you like. Again, very nice job.

Arizona Ken


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Mentor
professor emeritus


Reged: 09/30/10

Loc: Ontario, Canada
Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5853376 - 05/11/13 08:42 AM

LOL now Jose is just showing off

Awesome pictures, very well done!


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kevint1
professor emeritus
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Reged: 04/19/11

Loc: West Michigan
Re: Triplet APO or Doublet APO? new [Re: Mentor]
      #5853625 - 05/11/13 11:38 AM

Those are stunning photos, Jose. Your work is amazing!

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JoseBorrero
Post Laureate
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Reged: 09/04/09

Loc: Puerto Rico Island
Re: Triplet APO or Doublet APO? new [Re: kevint1]
      #5854105 - 05/11/13 04:44 PM

Thanks Kevin, So far like both refractor

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JoseBorrero
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Reged: 09/04/09

Loc: Puerto Rico Island
Re: Triplet APO or Doublet APO? new [Re: Arizona-Ken]
      #5854110 - 05/11/13 04:45 PM

Hi Ken,
feel free to visit my link galleries on my signature.


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joelimite
Carpal Tunnel
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Reged: 09/01/08

Loc: Fayetteville, AR
Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5854163 - 05/11/13 05:27 PM

Absolutely stunning images, Jose!

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Eddgie
Postmaster
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Reged: 02/01/06

Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5855167 - 05/12/13 10:35 AM

Quote:

more glass you put in the path the dimmer the view, unless somehow the lens is amplifying the light




Not really. Modern coatings make the transmission of at Triplet very close to that of a doublet.

Even my 25 year old oil spaced triplet has 96% transmission and it only used a simple single coating.

Almost all refractors have some amount of chromatic aberration.

The only question is "How bad is it at a given wavelenth of light?"

With an achromatic doublet, you can only achieve your best color correction for two colors at a time. You can have other colors close enough that they fall inside the Airy Disk, but it is almost impossible to get the reds and blues to do so, so the designer usually allows blue/violet to be unfocused so that the energy is spread out over the field making it hard to see.

With glasses of special dispersion characteristics (ED glasses) used at small apertures and moderate focal ratios, we can greatly improve things so that more than two colors can cross best focus and most others are close enought that when used visually, we don't see these unfocused colors easily.

If the focal ratio is long, even if we use a camera, the amount of fringing may be acceptably low.

But imagers want to use a fast system to reduce exposure time, but when you do this, it makes it harder to keep all of the colors reaching the same focus.

Things that are not visable to the eye become glaring when the camera shutter is open for 20 minutes.

The solution is to use a triplet. The triplet lens can be made somewhat faster and still keep the light from the different colors well focused.

For most visual use, it simply is not necessary to go to a triplet. Performance advantages over an f/8 or f/9 ED doublet are meaningless for visual astronomy. The colors all come to a close enough focus that unless the obejct is very very bright, we see the image as color free, and the amount of contrast lost to chormatic aberration is to low to be meaningful.

But for imaging, where shorter exposure times are very important, those wishing to see stars as close to their natural color as possible, and as free as possible of any fringing even on bright stars, a triplet becomes more and more necessary.

The number of elements do nothing to lower the contrast just because of the light loss. That is meaningless today (and has been for some very long time now).

The tendency of a glass element to act like an infinite number of prisms to disburse the different colors of light coming though the objective though is the real issue, and we are talking about a huge spectrum of colors.

The more elements, the more colors you can get to be in perfect focus at the same time.

Again, unnecessary for all colors to come to the exact same focus for visual use, but the long exposure astro-camera sees things that our eyes can't see.

There are many other factors here though.

For imageing, fast scopes have more steeply curved fields, and even if the color correction is near perfect, it may still become necessary to use special lenses so that all of the stars in the field will be in focus.


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junomike
Carpal Tunnel
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Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5855485 - 05/12/13 01:24 PM

Quote:

Again, unnecessary for all colors to come to the exact same focus for visual use, but the long exposure astro-camera sees things that our eyes can't see.




Eddgie, Excellent Informative post (as per usual), but I have to disagree with the above statement.

IME, I have detected CA in all the ED Doublets I've looked through. I haven't had the privilege of viewing through the Tak FS-series scope, so I can't comment on them, but I have had experience with the Orion/SW FPL-53 Doublets. The 100mm F9 was very close to being visually CA free, but at higher powers It was noticeable, although you did have to look for It.

My biggest complaint though is the tonal change(s) associated with and directly related to magnification.
Using the Moon as an example, At lower mags, the Image is near perfect, but as you move up 150X - 200X you can see two things happen:

1) The pure white color of the Moon's surface starts to change to a slight cream color. The higher the magnification, the stronger the color.
I have noticed this in my AT111EDT also, but to a much lesser degree)

2) The dark black colors (Craters) can change to a dark purple or off black.

It's easier to detect these If you have a Dob or other CA free telescope nearby for comparison.

Overall I felt that this was robbing me of potential contrast and/or detail. For this reason I upgraded to a Triplet. Was I happy about forking out an excessive amount of $$$ for a Triplet Refractor when I could've obtained a Doublet for less than half the cost? No, but I am happier than If I had used half my scope funds on a Doublet I would not have been happy with.

Mike


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terry59
Post Laureate
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Reged: 07/18/11

Loc: Colorado, USA
Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5855787 - 05/12/13 03:40 PM

What about when imaging with a mono CCD and focus is adjusted for each filter? Is there any benefit to a triplet then?

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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Triplet APO or Doublet APO? new [Re: terry59]
      #5855845 - 05/12/13 04:03 PM

Well, probably there would be very little difference between doublet and triplet when imaging with mono camera.
Regarding AP, biggest advantage that triplet has over doublet is that it's much easier to manufacture faster scope for the same color correction. IMHO


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azure1961p
Postmaster
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Reged: 01/17/09

Loc: USA
Re: Triplet APO or Doublet APO? new [Re: crazyqban]
      #5856909 - 05/13/13 07:25 AM

Jose,


Stunning photos !!!! That its through an 80mm lens is amazing.
Pete


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5857004 - 05/13/13 08:53 AM

Quote:

but I have to disagree with the above statement.




I did not say that there would be no CA.

I said that it was unnecessary to have a triplet to get good visual performance.

The amount of fringing on modern ED refractors is so small that it will likley only be noticed on bright stars.

But for visual use, the question is does that degrade the amount of detail that the visual observer will see.

Of course it does, but is it enough that the visual observer will miss detail when using a moderate focal ratio doublet vs a triplet?

In other words when using visually, will the triple allow them to see more datail?

It is not necessary to have a tripet to get accepeable visual performance. If one wishies to be able to view the brightest stars in the sky and not see any fringe, then maybe they want a triplet.

But if the goal is to see all of the detail in all of the targets one points the telescope at, a triplet is not really necessary.

And if one wants completly color free images, one can buy a reflector.

But for visual use, I stand by my belief that unless the system is very large and very fast, for visual use, there is no more detail to see using a triplet than an ED doublet.


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Eddgie
Postmaster
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Reged: 02/01/06

Re: Triplet APO or Doublet APO? new [Re: terry59]
      #5857024 - 05/13/13 09:01 AM

To be 100% honest, I don't know the answer.

I suppose it is a value proposition. Why use a filter wheel and take three exposurese rather than use a scope with a broad range of correction and take one exposure?

I have over the years seen many superb photos taken with achromats using filters, so of course it is possible.

Looking at some of the pictures taken with state of the art large aperture air spaced triplets though, you can see why people want them.

For visual use though, which is what the OP was concerned with, unless the apeture is relatively fast and large, it is doubtful that he would see any detail in a triplet that would not be visisble in an equal apeture, equal quality doulet of slightly more moderate focal ratio.

I believe that the OP is trying to determine if it is essential to have a triplet for visual use, and my own belief is that the advantage is only important for larger, fast refractors used for imaging.

For visual use, a 4" ED triplet will show you pretty much everthing that a 4" f/9 ED scope will show you.

He seemed to believe that the two elements of the ED scope would give better contrast, and of course it won't.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5857048 - 05/13/13 09:18 AM

Quote:


I believe that the OP is trying to determine if it is essential to have a triplet for visual use, and my own belief is that the advantage is only important for larger, fast refractors used for imaging.




Color correction depends on both aperture and focal ratio so I think one cannot characterize the color correction of an ED doublet based on aperture alone. Glass type matters too.

What a triplet allows for visual is a shorter, more compact scope of the same aperture with excellent color correction. A 4 inch F/9 probably has quite acceptable color correction for most observers but it's also about 3 feet long. A 4 inch F/6 or even F/7 is about 2 feet long, is handier to use, requires less substantial mount and may even be airline portable.

Jon


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bobhen
professor emeritus


Reged: 06/25/05

Re: Triplet APO or Doublet APO? new [Re: Eddgie]
      #5857092 - 05/13/13 09:51 AM

Quote:



For visual use, a 4" ED triplet will show you pretty much everthing that a 4" f/9 ED scope will show you.






That statement is at the heart of a lot of these posts. Is “pretty much” good enough? And, I presume, is what the OP is asking.

Why spend $4,500 on a Takahashi 120 when you can get a SkyWatcher 120 ED for less that $2,000. And, of course, then why would you spend for a SkyWatcher 120 ED when you can get a Celestron 120 mm achromat for a few hundred dollars?

Why pay Zambuto for one his 10-inch mirrors when you can get a 10-inch Orion Dobsonian for less?

The answer is that telescopes are interments that are pushed and used at the LIMIT of their performance. The objects observed are dim and very far away and “fine” details can be fleeting. High quality optics will ALWAYS show you more under ANY conditions (all else being equal).

If you only ever look at Saturn at 100x then you might not need a top quality optic (of any design). But if you are always pushing your scope to the limit, then higher quality optics will reward.

Those detail differences can be subtle, fleeting, and for many may not be worth the cost. BUT when pushed, the higher quality optic will be able to go the extra mile.

Bob


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PhilCo126
Post Laureate


Reged: 01/14/05

Loc: coastline of Belgium
Re: Triplet APO or Doublet APO? new [Re: bobhen]
      #5857143 - 05/13/13 10:17 AM

Summary: Apochromatic doublets are few and far between.... Apo = triplet

Basically, the quality of any apochromatic refractor depends on four items: (1) aperture, (2) glass quality , (3) lens configuration and (4) Focal length.

Astrophotography = an ED doublet will work but preferably get a true APO (air-spaced triplet).
Visual observations = an ED doublet will do very fine!


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: PhilCo126]
      #5857183 - 05/13/13 10:43 AM

Quote:

Summary: Apochromatic doublets are few and far between.... Apo = triplet




"APO" is a definition and both doublets and triplets are capable of meeting the definition.

It's worth noting that a doublet can have better color correction than a triplet... of course the focal length and glass type may not be identical...

Jon


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BarrySimon615
Pa Bear
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Reged: 03/01/04

Loc: New Orleans, LA
Re: Triplet APO or Doublet APO? [Re: PhilCo126]
      #5857298 - 05/13/13 11:47 AM

Summary: Apochromatic doublets are few and far between.... Apo = triplet

Not all doublets are mere achromats and not all triplets are automatically apochromats.

Basically, the quality of any apochromatic refractor depends on four items: (1) aperture, (2) glass quality , (3) lens configuration and (4) Focal length.

.....and the optician's skill, some do it better than others.

Astrophotography = an ED doublet will work but preferably get a true APO (air-spaced triplet).

Define "true apo" AP labels their doublets like my Star 12ED from the early 90's as apochromats. Are we now looking at a different definition where this is no longer true?

Visual observations = an ED doublet will do very fine!

Correct, but they, at least some of the better ones, do just dandy for astrophotography as well.

(Responses in italics and color are made to each statement from a previous post.)

Barry Simon


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audioaficionado
scholastic sledgehammer


Reged: 05/24/12

Loc: Medford, Orygun, USA
Re: Triplet APO or Doublet APO? [Re: Jon Isaacs]
      #5857459 - 05/13/13 12:59 PM

Quote:

...A 4 inch F/9 probably has quite acceptable color correction for most observers but it's also about 3 feet long. A 4 inch F/6 or even F/7 is about 2 feet long, is handier to use, requires less substantial mount and may even be airline portable.

Jon




This might be the direction I go in the future upgrading my refractor. My 3' scope isn't very heavy, but it is somewhat unwieldy to handle at times.


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junomike
Carpal Tunnel
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Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? [Re: Eddgie]
      #5859908 - 05/14/13 01:19 PM

Eddgie, Your posts have been informative to me in the past and I always enjoy the way you fully explain yourself, but I am disappointed in you putting your opinion across as fact.

Quote:

I said that it was unnecessary to have a triplet to get good visual performance.




Subjective! For you yes, for others maybe not!

Quote:

The amount of fringing on modern ED refractors is so small that it will likley only be noticed on bright stars.




Again, subjective? "likely only" is an uncertainty! If one doesn't see anything, you'd be correct, If one does see something, you'd be wrong.

Quote:

But if the goal is to see all of the detail in all of the targets one points the telescope at, a triplet is not really necessary.

And if one wants completly color free images, one can buy a reflector.




I find this particular statement ridiculous! "Not really" is "grey area" (again) and the mention of a reflector is largely out of category, unless you know of a Doublet or Triplet Reflector?

Quote:

But for visual use, I stand by my belief that unless the system is very large and very fast, for visual use, there is no more detail to see using a triplet than an ED doublet.




Ah, ok, now some realism. Although you have forgotten more than I'll even know about this Hobby, "your belief", although strongly respected is not fact. It's an opinion and no more correct than mine.

Mike

(no emoticons used and no harsh words, just a friendly rebuttal!)


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