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Equipment Discussions >> Refractors

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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? new [Re: junomike]
      #5860038 - 05/14/13 02:14 PM

The post above is very interesting. Like Eddgie I have years of objective experience to back up my objectively based personal assessment that Eddgie's postulates are essentially true. I cannot fault them because my experience agrees with his (and I have owned 3 triplet apo's plus another half dozen or so scopes, while only doublets, considered to be apochromatic.

Experience and statements based upon that experience (without detailed test data) is by it's very nature subjective.....but have confidence that it is well meaning and honestly offered and for many, as good as fact.

Barry Simon


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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: junomike]
      #5860211 - 05/14/13 03:19 PM

so after reading the many opinions / factoids, I find myself wondering if anyone has something to say about the claims made by some reviewers that the SW 120ED Schott/FPL-53 Doublet is in-fact a true APO able to hold it's own against any similar aperture triplet for both visual and AP..

i suspect this may ignite something.. i'll find the reviews i refer to, just don't have them handy yet, but i'm sure a few of you have come across one or two..

at f/7.5 it's not so bad for AP, obviously a faster scope is highly preferred but the price makes this scope a serious contender..imho, just mho


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cjc
sage


Reged: 10/15/10

Loc: Derbyshire, England
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860271 - 05/14/13 03:56 PM

My Sky-Watcher Equinox 120mm F/7.5 doublet with S-FPL53 and a Schott manufactured crown glass of unknown type (but perhaps N-ZK7) is nearly colour free in practice. The image is sharp with only a very slight violet (or perhaps indigo) splash around bright stars. The focused image is stable and white even under unsteady atmospheric conditions (which we have often here in the UK). Out of focus, on a star test, colour can easily be seen, but with a solar continuum (540nm) filter, it shows very clean diffraction rings.

The baffles (push fit I understand) are placed correctly, so far as I can tell. It is vignetted by the very long, 205mm, 2" draw tube with a diagonal (112mm for my 2"). I estimate (misusing a diagonal calculator here) that the clear field for an eyepiece with its focal plane at the eyepiece barrel shoulder, is only about 12mm in diameter. For a field of 27mm (maximum 1.25" eyepiece field) the outer rim will be vignetted by about 0.12 magnitude and for a 45mm field (maximum 2" eyepiece) by about 0.4 magnitude, or to about 70%.

The scope cools quickly and weighs just 15lb. It is reasonably well constructed and is nice to handle. I find it a delight to use visually but that might not hold for imaging.


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RodgerHouTex
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Reged: 06/02/09

Loc: Houston, Texas, USA
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860277 - 05/14/13 03:58 PM

I own an EON 120 which is essentialy the same as the SW 120ED. Both are doublets with FPL-53 which has the same optical properties as Fluorite. In focus, my EON shows no color at all. Out of focus a little, but that would be true of triplets also and who the heck uses their scope out of focus? And don't let people tell you that an EON or equivelant can't be used for AP. There is a guy on CN who uses an EON to produce stunning deep sky and solar system photos. His user name here is hfjacinto, which if you click on it under "User List" will take you to the astrophotos that he has taken with his EON 120. They are stunning.

I also used to own a FS-102 Tak doublet. It too, was essentialy perfect.

And here was a little factoid that most people don't want to talk about. Back in the late '80s when Roland Christen was starting Astro Physics his triplets showed quite a bit of color. Now I totally respect what Roland has done for amatuer astronomy, but my point is, it is the designer who determines how good or bad a doublet or triplet is based on his design. Some doublets are excellent as are some triplets. Some doublets aren't so good as are some triplets. You have to try and rely on people who own the scopes to give you an accurate representation of how the perform. And that can't always be counted on. Also some people will try to convince you that their 5 in. Astro Physics is better that anything else. Again, not true.

Good luck in your quest.


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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? new [Re: RodgerHouTex]
      #5860312 - 05/14/13 04:21 PM

Check these out, taken with a 120 mm ED doublet


www.cloudynights.com/ubbthreads/showthreaded.php/Number/5857373

Barry Simon


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greju
Carpal Tunnel
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Reged: 10/13/05

Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5860333 - 05/14/13 04:32 PM

Someone much smarter than me once said "a good doublet is better than a bad triplet". And it used to be said that it takes three lenses to even come close to the definition ( ) of apochromatic. Unless technology has surpassed that statement doesn't it still hold true?

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CounterWeight
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Reged: 10/05/08

Loc: Palo alto, CA.
Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860356 - 05/14/13 04:44 PM

Those are beautiful images (color on the cluster a bit odd but it's clear) and show what can be done. I think in all things it's important to try these options and see for yourself were or if something has value, and to what degree. IMO for imaging things like the focuser quality, cost and availbility of flattenrs and reducers that will work with your imaging chip loom large, these IMO not same importance for visual only.

In my link below are some of those same object through a budget FPL53 triplet... IMO in ways images are more about the imager than than the object. There are difference in imaging with a doublet, maybe more critical 'OSC', but again that is my being subjective.

I don't see it as a good new / bad news scenario unless it's that bad news is there are a lot of choices in the recent years, and can make decision difficult?


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csrlice12
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Reged: 05/22/12

Loc: Denver, CO
Re: Triplet APO or Doublet APO? new [Re: CounterWeight]
      #5860364 - 05/14/13 04:48 PM

My ST80A quit showing CA when Jim's shadow passed over it as he walke by....now it wouldn't dare.....

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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: csrlice12]
      #5860457 - 05/14/13 05:13 PM

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?

Edited by JoeM101 (05/14/13 05:16 PM)


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JimP
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Reged: 04/22/03

Loc: South Carolina
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860515 - 05/14/13 05:24 PM

I don't know and I have been using refractors since the early 80's. I think, in other words it is my opinion, that doublet ED scopes can be great scopes, especially for visual observations. Takahashi FS fluorite doublets have a wonderful reputation that I believe is well deserved. Having said all that, I believe the best of the best refractors are the apochromatic refractors from the top apochromatric triplet makers like AP, TEC, LZOS and Takahashi. And the best views the observer can get will come from these scopes. My Opinion.

JimP


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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: JimP]
      #5860641 - 05/14/13 06:12 PM

Here is something to consider in being well informed: A little overview of the subject (with a shifting definition). Apparently if an apo doublet does not quite meet the definition, neither would most "apo" triplets that are marketed as apos.

Additionally with a somewhat less stringent and perhaps more realistic definition, doublets do exist that do meet the criteria to be considered apos (as do triplets) in the context of the current discussion.

http://rohr.aiax.de/chapter%204a.htm

I find the discussion much like the purchase of the very best golf clubs or tennis rackets. Even the very best equipment in the hands of virtually all the players really does not improve your "game" all that much. It might look nice, but how much do you really gain?

Barry Simon


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junomike
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Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860671 - 05/14/13 06:30 PM

Quote:


I find the discussion much like the purchase of the very best golf clubs or tennis rackets. Even the very best equipment in the hands of virtually all the players really does not improve your "game" all that much. It might look nice, but how much do you really gain?




Barry, I agree! One has to decide where the want to get off of the "performance to cost" train.
But there's more to It than just improving one's game.
We're talking about something other than visual appearance. And how much one feels they "really gain" is a personal preference related to the cost/performance train.

Some people will spend small fortune on Premium eyepieces that eek out that last 5% - 10% detail. Others would never spend that and rather upgrade the scope.

For me the difference between a Doublet Apo and Triplet Apo above 4" is detectable and sometimes obvious.
Others appear to be quite content with Doublets and all the power to then!

One question for the Imagers of the 120ED's. Are those pics "raw" or processed? Curious?

Mike

Note: I really wish I knew someone with a Tak FS as I'd love to see what the views through one of them are like?


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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5860677 - 05/14/13 06:32 PM

well put Barry, how much of a difference is there really?

Has anyone had the chance to do a side-by-side? Has a serious comparison been done?

Edited by JoeM101 (05/14/13 07:05 PM)


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SteveG
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Reged: 09/27/06

Loc: Seattle, WA
Re: Triplet APO or Doublet APO? new [Re: greju]
      #5860768 - 05/14/13 07:20 PM

Quote:

Someone much smarter than me once said "a good doublet is better than a bad triplet". And it used to be said that it takes three lenses to even come close to the definition ( ) of apochromatic. Unless technology has surpassed that statement doesn't it still hold true?




No not true. You can come close to an APO with 2 lenses.


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JoeM101
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Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: SteveG]
      #5860779 - 05/14/13 07:25 PM

wasn't it established that you can have a 2 lens APO?

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RodgerHouTex
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Reged: 06/02/09

Loc: Houston, Texas, USA
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5860894 - 05/14/13 08:31 PM

The answer is no. Given a top notch designer a triplet adds 4 more degrees of freedom which can be used to reduce color error, field curvature, etc. There are two new radii of curvature, another glass type, and the spacing. The key is the top notch designer. And a triplet will most definitely not be cheaper than a doublet if they are both manufactured to the same standards.

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Fomalhaut
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Reged: 08/16/08

Loc: Switzerland
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861424 - 05/15/13 03:11 AM

Quote:

I also used to own a FS-102 Tak doublet. It too, was essentialy perfect.





Rodger,

May I ask you for what reasons you seem to have replaced your essentially perfect FS-102 Tak doublet by a TSA-102 Tak triplet? Is it also "essentially perfect", or is it sth. more?

Chris


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ValeryD
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Reged: 11/26/05

Loc: Kherson, Ukraine.
Re: Triplet APO or Doublet APO? [Re: JoeM101]
      #5861490 - 05/15/13 05:26 AM

Quote:

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?




Yes. But not much cheaper. In the reality can be even somewhat more expensive than a triplet.
And only fluorite doublets.
All depends of the designs of a doublet and a triplet. As farther from F/7 to slower F/D, as doublet becomes more and more advanced vs triplet.


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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? [Re: JoeM101]
      #5861576 - 05/15/13 07:32 AM

Quote:

In an effort to come back to my original question, taking all into account, would it be fair to say that an f/7 doublet with the highest quality glass, i.e.LZOS, Schott, etc., with test report showing minimum standard of 95% Strehl, etc, maybe throw in a field reducer like a Riccardi (f/5.2), would be just as good as a premium triplet of the same aperture for both visual and AP? oh, and much cheaper?




I think that you will find that at F/7, for a scope 100mm or larger, there are no high quality doublets using the highest quality glasses, (OK-4, FPL-53, Fluorite) they are all triplets. Valery could probably make you one but as he said, it would represent a cost savings.

It probably doesn't make sense to invest the time and expense fussing with the ED glass and not make it a triplet.

Jon


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hfjacinto
I think he's got it!
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Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? [Re: Jon Isaacs]
      #5861712 - 05/15/13 09:16 AM Attachment (25 downloads)

I have an ED doublet and love it, I see no reason to spend so much for a triplet when most doublets work so well.

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