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Equipment Discussions >> Refractors

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JimP
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Reged: 04/22/03

Loc: South Carolina
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5869775 - 05/18/13 05:32 PM

I agree with you hfjacinto. There are some really nice scopes to choose from these days and some come with 2 lenses and others with 3 lenses. Pick one and head out and enjoy!
The only rfractors available when I started out were F/15 achromats. We have come a Long way from those days.


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audioaficionado
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Reged: 05/24/12

Loc: Medford, Orygun, USA
Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5869805 - 05/18/13 05:48 PM

The best scopes are the ones you have in hand and use frequently.

There's a price point and quality level for everyone. It's all good IMHO.

I'd love to get one of those nice $$$$ doublet or triplet scopes (someday maybe).

For now I'm really looking forward to my bargain low $$$ scope when it arrives.

This has been a very interesting and informative thread so far.


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5870176 - 05/18/13 08:41 PM

Quote:

Again this is such a stupid argument, some people like triplets some like doublets. If you can't afford an AP, then a doublet is a really nice scope. Period end of conversation.




I believe the goal of a thread such as this is to understand the differences between doublets and triplets.

Jon


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RodgerHouTex
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Reged: 06/02/09

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Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5870349 - 05/18/13 10:00 PM

If you have the book "Telescopes, Astrographs, and Eyepieces" go to the chapter where they have on axis and off axis spot diagrams for 3 element APOs and you will see that even the highest quality triplets have CA. Even if they were made perfectly to the design, and they of course aren't. It's the nature of the beast when you bend light rays of different wavelengths through glass elements and try to bring them all to one focus. If you want CA free you need to use only reflective optical elements.

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JimP
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Reged: 04/22/03

Loc: South Carolina
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870401 - 05/18/13 10:17 PM

Well, I suppose it depends on your definition of color free. If you mean what off axis spot diagrams in a book show, you may be right. If, on the other hand, color free means seeing absolutely No extraneous color at the eyepiece you are wrong as there are apochromatic refractors, many of which I have owned and used, which show no detectable extraneous color at the eyepiece. And, for me, it is the view at the eyepiece that counts. In addition, I image planets using a Skynyx color webcam (not RGB filters)and there is no extraneous color in my images. Therefore my apos fit the definition of color free meaning no detectable extraneous color at the eyepiece

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olivdeso
super member


Reged: 02/20/11

Loc: Paris FR
Re: Triplet APO or Doublet APO? new [Re: timps]
      #5870416 - 05/18/13 10:23 PM

Quote:

I don't think the TOA130 has two FPL53 elements.

As far as I am aware it has one FPL53 element between two crown & flint elements.




The TOA has a unique design : a cooke triplet 2 x FPL53 1x BS7 and a wide air space. Thanks to this design it has almost not any spherochromatism. This is the same for te TOA150.

This is the best performer among the 130 for photographic use especially if you would kike to do some CaK sun picture: the strehl at 400nm will be much higher than any other triplet of the market of simillar F/L

At little bit overkill for visual use, and its heavy and take time to cool down.

At My side I prefered the AP130GT, much smaller, shorter F/L, faster cool down, very good for APS-C sensor or smaller, exclent for wide field visual use.

But the Toa has a better optic for photo use, wider corrected spectrum, widder corrected field thanks to its 4" focuser and flatener.

You can see here test reports and have a look to the spherochromatism graph. The flatener test is also excelent at the Tak side, showing a very low focus shift on a wide field: it is diffraction limited at least at 27mm from the center and thus over the full visual spectrum. The strehl is still higher than 0.92 in the blue channel, where other classical triplet usualy give about 0.7. In the violet/UV channel, the difference is even greater.

http://www.airylab.com/contenu/mesures/astro/rapport%202011-05002-public.pdf

http://airylab.net/contenu/mesures/astro/rapport%202011-40001-a.pdf


Olivier


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BarrySimon615
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Reged: 03/01/04

Loc: New Orleans, LA
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870490 - 05/18/13 11:21 PM

Quote:

If you want CA free you need to use only reflective optical elements.




This statement begs the question: So how much then does the eyepiece influence chromatic aberration? (Given a pure image, because it was reflected by a mirror, does it remain pure going thru 3 or more elements in an eyepiece?)

Barry Simon


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5870889 - 05/19/13 08:48 AM

Quote:

If you want CA free you need to use only reflective optical elements.




Reflective optics have their own set of aberrations and difficulties. Personally, I find refractive optics are best suited for small scopes, reflective optics for larger scopes. Each represents compromises, optimizations.

Jon


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RodgerHouTex
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Reged: 06/02/09

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Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5871326 - 05/19/13 12:54 PM

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5871584 - 05/19/13 02:15 PM

Quote:

The answer is yes eyepieces introduce their own CA. In "Telescope Optics" by Rutten and VanVerooj they raytrace a few designs for three colors and clearly show how eyepieces behave from a CA standpoint.




On axis, eyepieces are essentially color free, off-axis lateral color, we know about.

Jon


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RodgerHouTex
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Reged: 06/02/09

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Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5871586 - 05/19/13 02:16 PM

You say that many of the scopes you have looked though are color free. What you are ignoring is the response of the detector, your eye. Eyes are notoriously insensitive to wavelengths away from yellow-green, particularly red and blue. Everyone's eyes are different. However if you can't see color using triplets, you must also allow that some folks don't see it with doublets. I was only saying that when you look at a cold, hard ray trace analysis neither triplets nor doublets are color free.

As it relates to a one shot color camera, all CCDs are monochrome. To make a "color" camera the manufacturer inserts red, blue, and green color filters between the source and the ccd. So what you are recording is a pseudo-color image based on a summation of adjacent red, green, and blue pixels. And since you are only looking at three specific wavelengths it is possible for it to be color free. More likely is that the red and blue signals are just much weaker than the green and therefore don't show up in the final image.


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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5871590 - 05/19/13 02:18 PM

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Clear skies!


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RodgerHouTex
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Reged: 06/02/09

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Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5871599 - 05/19/13 02:22 PM

I believe that is what I am attempting to do. Empirically some people don't see color in doublets or triplets. Some do. When you look at it from a strict analytical standpoint, neither are color free.

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JimP
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Reged: 04/22/03

Loc: South Carolina
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5871841 - 05/19/13 04:08 PM

Thanks Rodger. Actually we agree. When I owned a Brandt 8" doublet F/13 achromat andthe color was "impressive". White stars appeared yellowish to me. After a while using the scope white stars appeared white to me! Many people would look through one of today's modern doublets and what little color they saw might "disappear" in time. Plus many people are far more color sensitive than others. My feeling is that if I see no extraneous color at the eyepiece the lens is color free. Of course I also feel if the double star companion of Antares looks green to me it's green (although we know it is blue and looks that way as a CONTRAST effect).
So, my opinion, based on almost 48 years of observing is that many of today's doublets, and I will use the Takahashi FS series (Fluorite) as an example, are as color free as you need for visual observations and they get more and more color free the longer you use them. Today's triplet apos like the Takahshi TOA and TSA models along with my favorite, Astro-Physics (and others)represent the height of the maker's art. They don't get any better. Is it necessary to own a triplet apo if you want a fine telescope to observe with? No. If you want the very best example of what can be done today I would say yes. Just my opinion based on experience.
I hope this is helpful.

best,

Jim

Edited by JimP (05/19/13 04:09 PM)


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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5872320 - 05/19/13 07:52 PM

Quote:

I believe that is what I am attempting to do. Empirically some people don't see color in doublets or triplets. Some do. When you look at it from a strict analytical standpoint, neither are color free.




I think there is more to it than just whether a scope is a doublet or a triplet, more to it than whether a scope uses FPL-53 or FPL-51.... The aperture and the focal ratio are important in determining the level of color correction in apo/ED scopes just as they are in determining the color correction in achromats and singlets.

Jon


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olivdeso
super member


Reged: 02/20/11

Loc: Paris FR
Re: Triplet APO or Doublet APO? new [Re: JimP]
      #5872325 - 05/19/13 07:53 PM

Concerning color correction there are 2 differences between triplets and doublets.

- ED triplets have lower spherochromatism than ED doublet of same diameter and same focal ratio. This means triplets are corrected on a wider spectrum, at least from blue to red and sometimes from violet to deep red, where ED doublet are corrected from light blue to light red.

All the refractors are optimized (i.e. "centered")at a single color, usually the green, or yellow-green. The red and blue channels are less corrected (one is over corrected and the other under corrected)

There is an exception : the takahashi TOA series that have almost no spherochromatism. The blue is as good corrected as the red and green.

For visual use, ED doublets are usually good enough, but the sperochromatism still quickly rises together with the diameter.

For CCD that are sensitive on a wider spectrum than eyes, the triplet makes more sense.

At the end, a triplet of same F ratio and same diameter and same ED glass focuses more energy in the diffraction pattern than a ED doublet resulting in an higher contrast.

The difference is marginal for long focal ratio and small diameter, like the TSA102 vs the FS102 for instance, but for larger diameter and shorter focal ratio, the triplet can take the advantage even for visual use.


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Fomalhaut
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Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? [Re: olivdeso]
      #5873093 - 05/20/13 04:50 AM

Other than achros, apos produce much less longitudinal chromatic aberration. OTOH, as a result of their usually much shorter focal lengths they tend to be plagued by spherochromatism (=variation of spherical aberration with wavelength), first of all.
As can be seen on Wolfgang Rohr's Website "astro-foren.de", despite the TOA's virtually complete freedom from spherochromatism, they are OTOH, as far as longitudinal chromatic aberration is concerned, not better color-corrected than other top-notch apochromats! - Yes, they are good performers in that latter discipline as well, but with an RC-index of ~0.5 are no better than Tak-TSAs, for example...
Plus in this contex, their Strehl is distinctly lower in blue as the ones in green, yellow and red.

Chris


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ken svp120
Carpal Tunnel


Reged: 12/19/04

Loc: Ohio
Re: Triplet APO or Doublet APO? [Re: JoeM101]
      #5873217 - 05/20/13 08:25 AM

Quote:

If any of you feel this is a stupid argument, I ask that you try to understand that the original intent was to understand the differences between the two and to get better informed.... Many people on here really don't understand what some of the glass types are and how they're arranged affects performance, let alone trying to grasp the, what seems to ne, loosely defined APO designation

If you have nothing empirical to add other than to express your opinion, try to make it constructive so that the readers can benefit somehow

Clear skies!




Joe,

I too continue trying to understand this subject. Call me old-school but I believe in defining a word and then sticking to that definition. Once you start chipping away at or broadening a definition, the next thing you know the original word no longer means anything at all. That being said, I am of the understanding that the original definition of apochromatic was established by Abbe and that (in addition to spherical aberration and coma) it requires there be correction for 3 widely spaced wavelengths...i.e. bringing three colors to focus at the same point.

So the way I look at answering this question of "can a doublet be apochromatic" simply comes down to answering the question of "can two pieces of glass accomplish bringing 3 widely spaced wavelengths to focus at the same point while correcting for spherical at 2 and coma as well."

If the answer is Yes, then a doublet has the ability to be apochromatic.

If the answer is No, then they can't.

As an aside, I'm also not sure that bringing three colors to focus necessarily means color-free....does it? Or is that just a convention we're assuming to be meant by apochromatic but really isn't necessarily the case?

Anyway - good topic!


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wh48gs
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Re: Triplet APO or Doublet APO? [Re: Fomalhaut]
      #5873222 - 05/20/13 08:28 AM Attachment (7 downloads)

Don't know where the Rohr's numbers come from, but Takahashi China says 130TOA has 0.01mm focus shift between g and C lines (F to C is only slightly less). That's like 1/50 of the C-F shift in an achromat. The graph they show implies more like 0.02mm, but it's still far in the apo land.

Vla


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Fomalhaut
scholastic sledgehammer


Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? [Re: wh48gs]
      #5873252 - 05/20/13 08:48 AM

Just have a look yourself, for example (!) here:
http://www.astro-foren.de/showthread.php?11522-Takahashi-TOA-130-S&p=4604...
(scroll up and then down)
P.S. Measured and not calculated, plus focused on green!

Chris


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