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Equipment Discussions >> Refractors

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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861713 - 05/15/13 09:16 AM Attachment (39 downloads)

Still get diffraction spikes on bright stars

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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861720 - 05/15/13 09:20 AM

BTW in my opinion, these doublet/triple posts get so over the top that they really are without benefit, those that believe a triplet is better will never see a doublet as a true APO and those that see no benefit from a triplet think the doublet as a good cost option. In the end, both are right.

A doublet can be essentially color free, but a triplet gives a designer more options. Both can be excellent.

As can be seen in photos, both work really well.


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5861729 - 05/15/13 09:24 AM Attachment (41 downloads)

With a 120MM ED EON.

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RodgerHouTex
Pooh-Bah
*****

Reged: 06/02/09

Loc: Houston, Texas, USA
Re: Triplet APO or Doublet APO? new [Re: Fomalhaut]
      #5861760 - 05/15/13 09:41 AM

Sure I'll tell you why I replaced the FS-102 with the TSA-102. About 4 years ago OPT offered me a very good trade-in offer for the FS-102 on a C-14. So off it went. The longer I didn't have it, the more I missed it because it really was optically and mechanically a perfect scope. Finally after regretting it long enough I started looking to buy a pristine FS-102. Two popped up on Astromart, the first seller, who shall remain nameless because he frequently contributes here, withdrew his offer. The next FS-102 had a couple of flaws which I pointed out to the seller and he replied with a nasty email saying that "...obviously you want a perfect scope." A yeah! I had mine for 7 years and it had like one really small paint chip and that was it. So anyway, I was looking at the OPT website and they had 2 TSA-102s for sale, both in pristine condition. So I bought one. It had nothing to do with the TSA-102 being a triplet. I would have been just as happy with a pristine FS-102. If you'll also notice I do still have my FS-152 which is also a magnificant scope that I probably will never sell.

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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861954 - 05/15/13 11:37 AM

I beg to differ with the poster that said there is no benefit to such posts, I think that anyone looking into buying a refractor would benefit, at the very least, from reading the various posts/ opinions CNers have... These posts shed light on some of the questions people may have about APOs.

Jon, I think you are mistaken when you say that a doublet cannot be an apo.... There are manufacturers like TS and APM/LZOS that offer True APO doublets... Adding a third element certainly does offer the optician options for correcting the light path but this can be achieved with 2 element design i.e. Schott or ok4 with FPL-53 ED... Why would reviewers and manufacturers claim zero to almost undetectable color in an f/7.5 SW 120pro ED, the obvious answer is not always the only one


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Binojunky
Carpal Tunnel
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Reged: 12/25/10

Re: Triplet APO or Doublet APO? new [Re: RodgerHouTex]
      #5861963 - 05/15/13 11:42 AM

I had two ED 80 scopes, one a triplet(Meade 5000) the other a doublet (Orion ED80, the one that started the low cost revolution)the triplet was rubbish optics wise, the Orion I still have and use,the number of lens elements do not make or break for that matter a good scope,DA.

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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? new [Re: Binojunky]
      #5861997 - 05/15/13 11:59 AM Attachment (38 downloads)

Some one asked for an UNPROCESSED image, which is imposiible as I can't load FITS files, so all are processed. But here is one that is just stretched so one can see the image, resized and converted to JPG.

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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5862000 - 05/15/13 12:00 PM Attachment (34 downloads)

After processing.

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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? new [Re: hfjacinto]
      #5862007 - 05/15/13 12:02 PM

Again the debates between which is better are really not worth it. So many scopes are killer, stop worrying about it and use the scope, most are so close you can't tell. The premiun scopes give you better harder, but a lot of the lower cost APOs work really well.

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Uela
member


Reged: 08/09/12

Loc: Sardinia Italy.
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5862075 - 05/15/13 12:41 PM

I've got a doublet apo, skywatcher 120Ed Apo black diamond, and I'm very happy.
Eyepieces celestron.
The moon is very clear in any single dettails. Saturn is perfect clear, so clear that I can see the black space between the planet and the rings!
in my opinion skywatcher 120 Ed apo is a good telescope!!!
Higly recommended !!


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KaStern
scholastic sledgehammer
*****

Reged: 04/18/06

Loc: InTheDark
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5862176 - 05/15/13 01:12 PM

Hi Joe,

Quote:

with zero CA




there is no apochromat with zero colour aberration.
Neither triplett, nor doublet.
If you want zero ca you have to buy a pure mirror scope.

The number of lenses is only one point, glas pairing is another one.
There are tripletts with more ca than doublets.
If glass pairing is the same the triplet has the same longitudinal ca
but lesser spherochromatism.

Cheers, Karsten


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JoeM101
scholastic sledgehammer


Reged: 04/09/12

Loc: 45.66086, -73.54702
Re: Triplet APO or Doublet APO? new [Re: KaStern]
      #5862990 - 05/15/13 07:40 PM

I just want to point out i'm that i'm not motivated by any particular reason but to know why one would opt for a triplet over a doublet... but i do thank you all for your opinions, i just wanted to see what informed CNers had to say.

i'm sure more opinions will follow, but i'd like to see a real star test side-by-side.. will try to get to a local star party and see what i can peep through


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BarrySimon615
Pa Bear
*****

Reged: 03/01/04

Loc: New Orleans, LA
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5863297 - 05/15/13 10:06 PM

Here is a post of mine from 2009 in which I report about my comparison of my Orion ED 100 to other scopes and finally to my new (at that time) Takahashi TSA 102 triplet apo. My conclusions as reported may seem contradictory in view of what I have written about doublets vs triplets in this thread. I would argue however that what I wrote then does not contradict what I have recently written. In spite of the differences seen I would still consider the Orion ED to be an apo and a very good scope at that. Now having said this, the Tak TSA, at least mine, was superior to my Orion ED 100 in several ways. Those ways included -
1) build quality
2) sharpness/resolution
3) ability to take magnification
4) apparent feeling that it could reach fainter magnitudes, a function I think of tighter stars with concentrated light, perhaps a function of better sharpness/resolution

I know some will read my post from 4 years ago as an endorsement of triplets over doublets and I can see why. Having said that I still feel strongly that in many instances a moderately priced doublet can be competitive with a more expensive triplet (witness my comparison of my Orion ED 100 with my Takahashi FCT 76 triplet. Anyway, here is what I wrote 4 years ago:

The Orion ED100 is a great entry level ED tube assembly that approaches true apochromatic performance and meets it if you go by many definitions.

I got mine within a day or two of when they were first announced (summer of 2004). Mine essentially replaced a Vernonscope Brandon 130 mm f/8 triplet apo tube that was sold due to non-use.

The Orion ED 100 had the advantage of light weight and a tube diameter of 100 mm that could easily be gripped and carried with one hand. I found the build quality to be generally quite good. I was ok with the non-retractable dew shield as I have never been a big fan of retractable dew shields. My main quibble was with the focuser which was quite rough and sloppy in mine, requiring some major surgery on the roller surface before the scope could even be used. I quickly replaced the focuser with a FeatherTouch which put the scope in a totally different league. My tube assembly initially cost about $1000 back in 2004. Add the FeatherTouch and it's interface and you now have a $1500 tube assembly. With today's price and a ready to fit Moonlite focuser, I believe you can do essentially the same thing for about $1000.00

In respect to performance the ED 100 did very well during a refractor shoot out that was done in 12/04. The other participant scopes included a TeleVue NP101, TeleVue 102, Takahashi FC100N, Vernonscope Brandon 94 Apo, and an Orion-Vixen 102 f/6.5 ED. We found that the top 3 scopes listed plus the ED100 were very close with the NP101 and the Tak FC100N being slightly better. The TV 102 and Orion ED100 were virtually the same optically with the FeatherTouch on the ED100 blowing away the standard TV focuser. The Brandon and the Vixen-Orion 102, being faster scopes could not keep up in respect to color correction, but they were clearly better than achromats.

I later compared my ED100 to my Tak FCT76 and found the resolution and color correction to be virtually the same on these scopes. The later sale of the FCT76 and it's P2Z mount had me contemplating the purchase of a new TAK TSA 102 (fall 2007). As good as the ED100 was I wondered if the TSA could be better? I did buy that TSA 102 and a shootout between it and the ED100 confirmed that it was no contest. Stars that appeared to be color free thru the ED100 suddenly had color when compared to the TSA 102. In addition the superior triplet objective concentrated light into smaller brighter points which allowed one to go .1 to .2 magnitude fainter. The tighter stars with the TSA 102 made the stars look a litte mushy with the ED100.

The question then becomes - Is the quality difference worth the current $2000+ differential in price ($680 vs $2699)? For some it is, and for others, it is not.


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: BarrySimon615]
      #5863379 - 05/15/13 10:36 PM

Thanks Barry... most informative!

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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Triplet APO or Doublet APO? new [Re: JoeM101]
      #5863846 - 05/16/13 07:27 AM

Quote:


Jon, I think you are mistaken when you say that a doublet cannot be an apo.... There are manufacturers like TS and APM/LZOS that offer True APO doublets... Adding a third element certainly does offer the optician options for correcting the light path but this can be achieved with 2 element design i.e. Schott or ok4 with FPL-53 ED... Why would reviewers and manufacturers claim zero to almost undetectable color in an f/7.5 SW 120pro ED, the obvious answer is not always the only one




Joe:

I think there is some confusion here.

- I did not say a doublet could not be an apo, in fact I had earlier said that it could be. You had asked about high quality F/7 doublets using the highest quality glasses and I had pointed out:

"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."

The important factor here is the F/7 focal ratio.. I don't know of any F/7, FPL-53 or OK-4 doublets that are 100mm or greater. I was not stating an opinion, I was simply stating a fact.

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.

- 80mm F/7 with FPL-53, to my eye, it is possible to hide the chromatic blur behind the Airy disk. The 80mm Synta ED-80 was F/7.5 and is considered color free by most. The 100mm Synta ED-100 was slowed to F/9, a little math shows this is consistent with maintaining the same level of color correction as the ED-80. With the ED-120, the designers faced a dilemma. If they slowed the ED-120 to maintain that same level of color correction, the scope would have to be F/11.2 with a 1320mm focal length. That would be a large, heavy, unwieldy scope. So, I think made they decision to compromise the color correction and go with the lighter, more manageable 900mm focal length.

- Color correction is a function of both focal length and aperture. Historically, 4 inch FPL-53, OK-4 and fluorite doublets have been F/8 to F/9, not F/7. Smaller doublets can be faster, larger doublets need to be slower.

Valery D. worked some magic with his 7 inch F/8 fluorite doublet by using an exotic mating element and according to Valery, it did not make the optics any less expensive than they would have been if it had been a triplet. One cannot use Valery as a valid data point, he works outside the normal envelope, whether it's the Chromacorr, ion milling 1 meter optic sets for Ritchey–Chrétien telescopes or designing the optics for the legendary Astro-Physics 10 inch Mak, he does things that others can't do.

Jon Isaacs


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junomike
Carpal Tunnel
*****

Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? new [Re: BarrySimon615]
      #5863860 - 05/16/13 07:48 AM

Quote:

Stars that appeared to be color free thru the ED100 suddenly had color when compared to the TSA 102.




This was my experience as well. If one doesn't compare against a better color corrected scope, then It doesn't become obvious. Once you do see the difference It is hard to forget it.

Quote:

In addition the superior triplet objective concentrated light into smaller brighter points which allowed one to go .1 to .2 magnitude fainter.




I found that my slower Apo Tiplet seems to go deeper than my fast Achro Doublet while still giving up almost 10mm Aperture.

Quote:

The tighter stars with the TSA 102 made the stars look a little mushy with the ED100.




This again is really only seen when one compares two scopes. I was quite happy with mine Achro until.......

Mike


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junomike
Carpal Tunnel
*****

Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? new [Re: Jon Isaacs]
      #5863883 - 05/16/13 08:19 AM

Quote:

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.




Jon, I have viewed thru the 120ED's and I agree. I feel the relative claims of CA (or there lack of) is one of the issues with this scope. The problem is not only the fact we all experience CA differently (as well as specimen variation), but also the "pawning off" of the visible CA as minor or non-Intrusive. IMO, If It's there, It's there. If It isn't, It Isn't. For me personally there's little room for the "grey area", but I'm a sensitive stickler when It comes to CA.

Quote:

- 80mm F/7 with FPL-53, to my eye, it is possible to hide the chromatic blur behind the Airy disk. The 80mm Synta ED-80 was F/7.5 and is considered color free by most. The 100mm Synta ED-100 was slowed to F/9, a little math shows this is consistent with maintaining the same level of color correction as the ED-80. With the ED-120, the designers faced a dilemma. If they slowed the ED-120 to maintain that same level of color correction, the scope would have to be F/11.2 with a 1320mm focal length. That would be a large, heavy, unwieldy scope. So, I think made they decision to compromise the color correction and go with the lighter, more manageable 900mm focal length.




This is exactly how I see the SW/Orion/ WO scopes as well. I've viewed thru them all and consider the 80mm and 100mm CA free as well (YMMV ). The trade off with the 120ED is "bang on" IMO and IME.

Although I personally would have preferred the longer, thus better color corrected version of the 120ED, I believe the designers made the correct choice (trade off) as It has to be one of the best selling Refractors I know of. Most owners are quite pleased with It's performance.

Mike


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? [Re: junomike]
      #5863890 - 05/16/13 08:24 AM

Jon,

Quote:

- The Synta 120mm F/7.5 is close to F/7 but it does not fall under what I consider high quality and having had experience reading reviews for the past 20 years, I take "zero to almost undetectable false color" claims with the entire salt shaker. I have read reviews of fast achromats that made similar claims.




Have you actually looked through one?

I think my pictures pretty much show my sample is essentially color free.


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junomike
Carpal Tunnel
*****

Reged: 09/07/09

Loc: Ontario
Re: Triplet APO or Doublet APO? [Re: hfjacinto]
      #5863944 - 05/16/13 09:02 AM

Helder, I have actually looked through two Skywatcher 120ED's. The first one showed quite a bit of CA on the Moon (mainy dark crators). The second one was much better, but did reveal a little CA when pushed in Magnification.

Quote:

I think my pictures pretty much show my sample is essentially color free




No offense Helder, but here we go again! "essentially", "basically", "relatively".

For most that's fine, but It's not the same as CA free. I strongly feel I can see that last little bit of unfocused color.

I agree, your pics are fantastic and I was the one asking for a "raw" image as I was curious If the CA was somehow cancelled out (not saying It was or wasn't, just wondering)

Mike


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hfjacinto
I think he's got it!
*****

Reged: 01/12/09

Loc: Land of clouds and LP
Re: Triplet APO or Doublet APO? [Re: junomike]
      #5863988 - 05/16/13 09:23 AM

Mike,

No offense taken but look at the moon image, they have no color, I have used over 300 power before I noticed some CA and that was with a Nagler. With an Ortho I could see no false color. The camera is much more sensitive than the eye and it essentially captures no CA.

My friend had a TAK 130 and an FSQ 106. The FSQ106 is color free, but the TAK 130 was as color free as was the 120MM. After looking through my 120, he sold the TAK 130 and brought a mount and a 9.25 as we all know aperture rules on DSO

I know some people are sensitive to CA but for some of us, its not that big deal and we don't see it.

I also have expeirenced the "I paid a lot for my NP101, AP, TEC, etc so of course its better than your 18" reflector".

What I am saying is that while a triplet can be good, a lot of people use doublets and they work really well also. I have also looked through a lot of doublet and triplets and the only pure color free refractor that I ever looked through is an FSQ.


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