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Equipment Discussions >> Refractors

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Doublet or Triplet?
      #5557956 - 12/06/12 05:36 PM

The seeming casting to the wayside of the doublet, whether an achromat, "semi-apochromat" or near-apochromat, has arisen out of several deciding factors, perhaps the only worthwhile of these being the triplet, and in a presently supposed banishment of spurious colour, both intra- and extra-focally even, once and for all from the refractive design.

Though, given that any and all things manipulated and crafted by man in his resourcefulness are just that, thus the maxim: the fewer the parts, the lesser the maintenance, that is, if the extra trouble is worthwhile, and aside from a comparison between a medieval ox cart and a Maserati.

Consider the quadruplets...

http://www.takahashijapan.com/ct-products/products/FSQ-106ED.html#

...and in competition...

http://www.google.com/imgres?q=np-101&um=1&hl=en&sa=N&tbo=d&a...

...with both being imaging-specific, or astrographic refractors, both claiming photo-visual dexterity and the optics made in Japan.

However, especially in the case of the latter, and for those who prefer minimal maintenance, a live, dynamic, moderate-to-high-powered view and a comparatively carefree pride in ownership, then remain with either a doublet or a triplet.

Of doublets and triplets, which do you prefer for the finest observing experience in your leisure or retirement?

Cheers,

Alan


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Sasa
professor emeritus


Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5557978 - 12/06/12 05:50 PM

For visual doublet, for astroimaging triplet (see my signature...). I observe mostly in short, last moment decision sessions from my backyard in a small town just on the border of 1.5 milion city. I store my telescopes at room temperature and I need to use full resolving power as quickly as possible.

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DaveJ
Carpal Tunnel
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Reged: 01/07/05

Loc: NE Ohio
Re: Doublet or Triplet? new [Re: Sasa]
      #5557996 - 12/06/12 05:57 PM

Quote:

...I store my telescopes at room temperature and I need to use full resolving power as quickly as possible.




And that's exactly why I prefer my TEC 140 triplet! It's oil-spaced (only 2 air-to-glass surfaces) and has the fastest cooldown time of any telescope I've ever owned or viewed through.


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Sasa
professor emeritus


Reged: 11/03/10

Loc: Ricany, Czech Republic
Re: Doublet or Triplet? new [Re: DaveJ]
      #5558013 - 12/06/12 06:10 PM

Dave, I would love to gain some experience with oil triplets. So far, I just quickly looked through one TEC140. Definitely very nice telescope!

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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: Sasa]
      #5558024 - 12/06/12 06:19 PM

What are the pros and cons between air-spaced and oil-spaced triplets? Does the oil effect more of a unison between the three elements, with an improvement noticed both visually and photographically?

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DaveJ
Carpal Tunnel
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Reged: 01/07/05

Loc: NE Ohio
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5558081 - 12/06/12 06:56 PM

Quote:

What are the pros and cons between air-spaced and oil-spaced triplets? Does the oil effect more of a unison between the three elements, with an improvement noticed both visually and photographically?




I'm definitely biased, but I'll answer with a resounding YES! The first TEC 140 I looked through shattered what I'd thought a refractor was visually capable. I immediately put my prior triplet up for sale and placed a deposit with Yuri at TEC. I waited almost 9 months for it and have been giddy ever since receiving it. Please excuse me, I do kinda "go on" about it, but that's what love does to you.


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5558088 - 12/06/12 06:59 PM

Alan,

Air vs Oiled tripllets has some history and comes up frequently in very longish threads...

To your original question, my preference for visual in the following order:
1. Fluorite doublet, excellent color correction, light weight and fast cool down
2. Oil filled triplet: faster cooldown than an air spaced triplet (my experience with a 6.1" oil filled triplet)
3. Air spaced triplet (Used to own a chinese made triplet required a long time to reach optimal performance)
4. Quadruplet: Optimized for large fields and AP so only used if nothing else on hand.

For me cooldown time is the main issue since my telescopes need to be stored inside. Second comes weight, since I love to take the scopes out of the city to real dark skies.

If optical quality is the only criteria, the choice of the design for visual use is less important. Good materials and precise execution make the real difference.

best regards
Chris


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: chboss]
      #5558127 - 12/06/12 07:24 PM

"Air vs Oiled tripllets has some history and comes up frequently in very longish threads..."

Yes, as I'm well aware.

On the other hand, your ordered preferences are enlightening. Thank you, Chris.

Still, I can't help but wonder further if oil effects more of a homogeneity between the elements, and thus an advantage, however slight, for visual.

Regards,

Alan


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5558151 - 12/06/12 07:37 PM

Alan,

An oil filled design limits the freedom of the designer since the radii have to be very close or equal. In theory an air spaced design offers more freedom and could be better corrected. (I read that in an earlier thread so its just heresay)
The oil filled optics do not need coatings on the internal lens surfaces since the oil "removes" the glass-air surface. So there are no reflections that could have a negative influence.

One other aspect, I prefer a longer focal ratio say f8 and higher.
High magnifications can be reached with longer focal length eyepieces which have usually better eye relief. Furthermore at f8 even simpler eyepieces perform well.

best regards
Chris


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: chboss]
      #5558215 - 12/06/12 08:24 PM

Then the extra effort required in mating the elements with oil, if done expertly, should result in a more finely-figured objective, ideally, yes?

Would the difference be noticeable visually?

I consider my air-spaced doublet at f/8 to be the fastest I'd prefer, certainly no faster than f/7, and a mirrored arrangement......no faster than f/5.

Regards,

Alan


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Mark9473
Postmaster
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Reged: 07/21/05

Loc: 51N 4E
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5558650 - 12/07/12 03:06 AM

Quote:

mating the elements with oil, if done expertly, should result in a more finely-figured objective, ideally, yes?




It's the opposite, actually. The inner glass surfaces do not require a high degree of figuring since the interface will all but disappear when the oil is filled in.


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5558728 - 12/07/12 07:01 AM

Quote:


However, especially in the case of the latter, and for those who prefer minimal maintenance, a live, dynamic, moderate-to-high-powered view and a comparatively carefree pride in ownership, then remain with either a doublet or a triplet.




Personally I am fine with doublets, triplets and Petzvals. One cannot make blanket statements without considering the specifics, the right design for the job. I find a 80mm F/7 FPL-53 doublet to be a good performer, a 102mm Hk-61 F/7 doublet to me an adequate but not stunning performer. Bigger apertures, faster scopes, require better glass and/or more elements. A 6 inch F/7 better be a triplet if one wants reasonable color correction, a 4 inch F/9 can provide color free views as a doublet.

If one is afraid of the workings of their scope, it is probably best to stick with a doublet, triplets can go out of collimation and that may well require a trip back to the factory. If one reads the forums here, doublets can require collimation but confident/competent owners generally are able to collimate them.

As far as Petzvals are concerned, I do not dismiss them because of their added elements any more than I dismiss the 31mm Nagler because of it's 6 element design. In my particular case, the previous owner of my NP-101 messed with the collimation, I collimated it myself, it was easy but I am comfortable working with my scopes.

At the eyepiece, it provides live, dynamic views at any magnification within the range of a 4 inch telescope, 15x-300x+. Wandering around the Milky Way with the 31mm Nagler + 31mm Nagler from a dark, clear site, it doesn't get any dynamic, any more live than that except maybe for wandering around the Milky Way with a 12.5inch or 16 inch RFT.

I am glad I am blessed with the frequent opportunity to view under clear, dark skies with a variety of telescopes, both achromats and apos, with doublets, triplets and Petzvals as well as with reflectors of a variety of sizes.

The pleasures and joys of observing the night sky have little to do with the particular equipment, some is a little better, some is a little worse, what is important is the sky and what I bring inside myself...



Jon

Edited by Jon Isaacs (12/07/12 07:52 AM)


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PhilCo126
Post Laureate


Reged: 01/14/05

Loc: coastline of Belgium
Re: Doublet or Triplet? new [Re: Jon Isaacs]
      #5558955 - 12/07/12 10:07 AM

As said in numerous other " Apo-topics "
Basically, the quality of any apochromatic refractor depends on three items: (1) aperture, (2) glass quality and (3) lens configuration.


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sg6
professor emeritus


Reged: 02/14/10

Loc: Norfolk, UK.
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5559031 - 12/07/12 10:47 AM

Doublet or triplet to me depends mainly on the f number and to a smaller extent the glass types used.

A doublet with ED glass then I would be happy if the f number were 7.5 or slower, I would think about f/7 if fpl-53, although I would prefer 7.5.

At the f/6 area then a triplet, I note that WO say for their short (f/6 and f/5.9) doublets, they say CA is well controlled, not eliminated. And I respect their honesty, not many are these days.

For the f/6 to f/7.5 area, if triplet then OK, if doublet then it would depend on the use I intended and the cost.


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walt99
super member
****

Reged: 08/23/08

Loc: Kentucky
Re: Doublet or Triplet? new [Re: Jon Isaacs]
      #5559800 - 12/07/12 06:46 PM

Quote:

Quote:



The pleasures and joys of observing the night sky have little to do with the particular equipment, some is a little better, some is a little worse, what is important is the sky and what I bring inside myself...



Jon




Agreed ! To see the incredible work of the greatest artist of all is often a religious experience for me . . .

Me , I like my FS 128 flourite doublet . Showed color on Saturn for the first time . My TOA 130 triplet never did that .


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? *DELETED* *DELETED* new [Re: Jon Isaacs]
      #5560178 - 12/07/12 11:45 PM

Post deleted by Sky Muse

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KWB
Postmaster
*****

Reged: 09/30/06

Loc: Westminster,Co Elev.5400 feet
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5560371 - 12/08/12 03:02 AM

Whether simplicity as to choosing a telescope based on the fewest number of optical elements as the best choice is just one of many subjective opinions posted on this forum and can no way substantiated as a basis in fact,just as any other opinion as to advocating another choice of a telescope design with more or less elements as being the "better one"cannot be proved,either. One size doesn't fit all when it comes to choosing a particular telescope,whether it be based on price or sophistication. The object behind any forum is the free exchange of ideas and opinions in a friendly and respectful manner. This thread has the makings of an excellent one with valuable information for the newer observers,provided we refrain from equipment bashing which will not be tolerated here.

I will post this from our TOS for our newer members to consider:

The Basics:

As the TOS has grown over the years of forum operation, it has been decided it might be helpful to include a short summary of the expectations of the users - an executive summary if you will. Simply put, the following behavior is expected from users:

* Play Nice
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Above all, do unto others as you would have them do unto you.

Feel free to discuss things here, but please don't beat it to death. Stating your viewpoint once, on-topic and in a civil manner, in a thread is sufficient.

Any disrespectful or trollish postings will be removed immediately and without warning.


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Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Doublet or Triplet? new [Re: Sky Muse]
      #5560405 - 12/08/12 04:41 AM

Quote:


It is not that anyone should be afraid, or frightened; rather, one would simply exist in the realisation that they are not as mechanically-inclined as others.




Something to consider:

Nearly any refractor can shift collimation, doublets, triplets, and Petzvals. If one is not mechanically inclined, then nearly any refractor may require a trip to the factory to be collimated. Most but not all can be collimated by an owner with basic optical and mechanical skills. An example:

Help Collimating a Takahashi FS-102

As far as doublets being the best.. they have significant field curvature and less than perfect color correction though in the smaller sizes/slower focal ratios it may not be apparent. They may be the best compromise for certain observers, but triplets and Petzvals can provide more perfect views. In fact, Takahashi no longer makes larger doublets, the 102mm and larger scopes are all FPL-53 Triplets and Petzvals.

And I must say, the characterization concerning lost observing time is off the mark... I collimate my Newtonians each and every time I use them. It takes a few minutes, normally done before sunset. No eyepiece time lost there. I collimated my NP-101 two years ago. Since then it has traveled thousands of miles, been used hundreds of times, it's still just fine. In those two years, I have lost far more observing time finding dropped glasses than I ever spent startesting and collimating it.

I asked you once how much eyepiece time you have looking through an NP-101. Rather than secondhand links, I would like to hear your experiences looking through an NP-101, an NP-127 or other Petzvals. What has been your personal experience under dark skies with the essentially perfect 4+ degree views the NP-101 provides? Have you had any collimation issues or do you know a friend who has? Have you observed Venus at 200x and noted that there is no chromatic aberration even out of focus?

Doublets like the Takahashi FS-102 or the TV-102 are fine scopes and may suit the requirements of many observers. For me, the 800+ focal length represents a deal breaker. I know that many observers enjoy these scopes and are not particularly interested in those big views, that is fine for them, not for me.

Jon Isaacs


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: KWB]
      #5560584 - 12/08/12 09:47 AM

...done.

Regards,

Alan


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Sky Muse
sage


Reged: 10/26/12

Loc: De Soto County, MS
Re: Doublet or Triplet? new [Re: Jon Isaacs]
      #5561519 - 12/08/12 09:14 PM

I've collimated my 6" f/5 Newtonian, though not the refractor, for there's been no need, yet.

Incidentally, when I was two, I was placed in a wooden playpen, passed down from elder cousins, and against my will, of course. I reached up, grabbed a flatware knife from the counter above and adjacent to my "prison", then proceeded to unscrew and disassemble one corner, and until I escaped. Two to four screws were discovered afterwards at the scene. There were witnesses......and to another adroit incident...

"As far as doublets being the best.. they have significant field curvature..."

I haven't noticed that in mine, but don't mind me.

"In fact, Takahashi no longer makes larger doublets..."

I understand that it would cost at least three times the price I paid for my fluorite, a doublet, in 2003, to exactly replicate it today, and for the price of the current TOA-130, a triplet.

"I asked you once how much eyepiece time you have looking through an NP-101."

None, for I bought an FS-102 instead, and for visual only. The NP-101 is a "photo-visual" petzval, with "photo" being its strength and intent per design. If I want low-powered views, there are perhaps oculars to accomplish same with the F/8. Besides, I have my eyes, my binoculars and a 6" f/5 Newtonian to enjoy low-powered views when desired. Then, I have my Parks 8" f/5 Newtonian, purchased just after the refractor, at which time I had comet-hunting in mind, and do still, and long before personal knowledge of David H. Levy's 6" f/5 "Comet Hunter".

If I ever did desire a quadruplet, perhaps in another life, I'd save my pennies for Takahashi's.

Sometimes, not often, I look at my refractor, at f/8, and think...

"Is is too fast?"

...but that's just me.

Cheers,

Alan


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