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Doublet or triplet

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#1 356als

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Posted 17 February 2019 - 02:48 PM

I am new to this site and I'm sure this topic has already been discussed.  I have had always an interest in astronomy.  Looking to purchase a 80mm refactor as my first scope.  What I want to know, is what is the difference in performance between the 2 types of lens configuration, doublet or triplet?  Is it worth spending the extra money on a triplet?  I would be using this only for viewing, no astro photo.  



#2 junomike

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Posted 17 February 2019 - 03:00 PM

At that size (80mm) IME a good ED Doublet should suffice for Visual.  An Achro will exhibit CA and a Triplet isn't necessary at that size IMO.


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#3 salico

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Posted 17 February 2019 - 03:03 PM

many will tell you, a doublet will do the job. I think they are not wrong, but incidentely I got a binocular telescope with two wonderful LOMO 80mm lenses. The clearest, sharpest views I ever had. Take more time to cool down though...


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#4 356als

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Posted 17 February 2019 - 03:24 PM

When will the doublet show color, imaging or viewing?


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#5 russell23

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Posted 17 February 2019 - 03:28 PM

I am new to this site and I'm sure this topic has already been discussed.  I have had always an interest in astronomy.  Looking to purchase a 80mm refactor as my first scope.  What I want to know, is what is the difference in performance between the 2 types of lens configuration, doublet or triplet?  Is it worth spending the extra money on a triplet?  I would be using this only for viewing, no astro photo.  

Depending upon what scopes you are looking at there may not actually be a price difference between the doublet and the triplet anyway.

 

The Stellarvue 80mm f/7 doublet is $699 + shipping.  The Astrotech 80mm f/6 triplet is $749 with shipping included and I think you would get the CN member discount so there really is negligible cost difference all things factored in and the AT triplet might even be a little less. 

 

I just went back and forth on this decision myself and finally settled on the Stellarvue doublet because there was a great deal on a Certified Pre-owned for $589 + shipping on the SV website.  If you look under the "sale" tab in their website you will see what they have.  Right now there are no Certified pre-owned scopes but they pop up fairly regularly. 

 

The 80mm f/6 is capable of giving a little wider TFOV because of the shorter FL.  For example, a 31mm Nagler would give a 5.0 deg TFOV in the Astrotech scope vs. 4.3 deg in the SV scope.   But the SV scope can more easily reach higher magnifications and will be a little easier on the edge performance of eyepieces.

 

The other thing that may play a role depending upon the climate you observe in is that a doublet will cool down faster than an equal aperture triplet.

 

You really can't go wrong with either scope. 


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#6 russell23

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Posted 17 February 2019 - 03:29 PM

When will the doublet show color, imaging or viewing?

Not sure. I was at 179x on the Moon with my 80mm f/7 SV doublet Friday night and saw zero false color.

 

But some people's eyes are more sensitive to false color than others which is one of the reasons why you get discrepant reports.


Edited by russell23, 17 February 2019 - 03:29 PM.


#7 chakel

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Posted 17 February 2019 - 03:35 PM

You might consider a used Tele Vue 85mm doublet.  One can be picked up for a little over $1k, but would satisfy you for a long time. 


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#8 zyzyz

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Posted 17 February 2019 - 03:36 PM

For your first scope there's a lot of things to consider.

 

How much do you want to spend for the mount, eyepieces, diagonal, and the telescope itself?

 

Do you want a crayford microfocuser or is the stock rack and pinion ok?

 

Do you want a rich field scope (f/5 wide view), general observing (f/7-10), or a planet killer (f/10-15 narrow view)? This is probably the hardest decision and why most members own multiple telescopes.

 

Is a little false color on bright objects objectionable? If not a good achromatic is fine. A scope with ED glass has less color but is more expensive. A triplet is color free but takes longer to cool down. They're also even more expensive.

 

It's a confusing choice but after you consider the available scopes and prices you'll find one to fit your viewing style and budget.

 

 



#9 Astro-Master

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Posted 17 February 2019 - 03:42 PM

One advantage on the Stellarvue is a better focuser.  Most focusers on telescopes from China are junk, and need to be replaced.


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#10 Mike W

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Posted 17 February 2019 - 03:42 PM

Get a Televue 85, it will last, give you years of pleasure and re-sale is very good. I have a Televue doublet and only at high power on Vega will you see any color. I don't spend much time looking at Vega so ……………...waytogo.gif 


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#11 rustynpp

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Posted 17 February 2019 - 03:45 PM

Welcome to Cloudy Nights!

 

I'm in the doublet camp, at least for visual astronomy. Due to the absence of the third element, doublets will generally be less expensive, lighter (as well as easier to balance), and reach ambient temperature more quickly (heat currents in the telescope tube resulting from a temperature difference between inside and outside the tube result in reduced image quality).

 

Modern, mid and high-end doublets will generally use "synthetic fluorite" glass in an attempt to approach the color correction of a well-made triplet. You might've seen glass types like FPL-53 or FPL-51 in telescope specifications, and this refers to the type of synthetic fluorite used in the objective lens. Where you might see color in doublets is when manufactures use inferior glass types, or design the objective in such a way that is less than optimal for color correction. That is to say, not all doublets are created equal. Objects most likely to show color are really bright stars, planets, and the moon. Basically, more light & higher contrast = more opportunity for color. 

 

Well made doublets will show essentially zero color in almost every situation. Astrophotography is more demanding than visual astronomy, and in general any imperfection will be magnified by in the process of collecting lots of photons. Even so, many people use high-end doublets for photography and obtain terrific results. I'm no expert here though, so I won't get any deeper into this.

 

Long story short, my advice is get a doublet. 


Edited by rustynpp, 17 February 2019 - 03:58 PM.

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#12 nicoledoula

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Posted 17 February 2019 - 03:47 PM

Stellarvue doublet for a lot of reasons



#13 SeattleScott

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Posted 17 February 2019 - 04:14 PM

Doublets are usually preferred for visual as they are lighter, cheaper and cool faster. Either a doublet or a triplet could be color free or show CA depending on the model and F ratio, glass used, etc. With a triplet you can achieve lower F ratio while keeping CA at bay if the right glass is used, so if you want maximum FOV with no false color then you are looking at a higher end triplet, and lots of money. If you can be satisfied with a FOV wide enough to fit say nine full Moons across, you can get by with a cheaper, lighter doublet that will cool faster.

Scott

#14 russell23

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Posted 17 February 2019 - 04:15 PM

You might consider a used Tele Vue 85mm doublet.  One can be picked up for a little over $1k, but would satisfy you for a long time. 

The last used TV85 listed on Astromart was listed for $1050 but the previous 5 or 6 were between $1250 and $1450.   That seems a bit pricey considering the SV Access is $699 new and the Astrotech Triplet is $749 new.   Brand new the TV85 is $2125.

 

That difference in price could buy a lot of other items the OP will eventually want - eyepieces, filters, etc.   It's not like the SV Access and AT triplet are placeholder junk.  They are excellent scopes.  


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#15 SeattleScott

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Posted 17 February 2019 - 04:52 PM

For someone starting out I agree, a Chinese Apo makes sense, and with the savings they can get a 10” Dob later. For an experienced stargazer with multiple scopes a TV makes sense as a lifetime refractor, if their budget allows it. With 81mm Vixens at half the price of the TV 85 with similar optical quality, the TV is really for those who want the heirloom quality and pride of ownership.

Scott

#16 Jeff B

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Posted 17 February 2019 - 05:08 PM

Welcome to CN's!  Another person from the "Best Seeing Capitol of the World", Ohio. lol.gif

 

I would pick up a used Orion 80ED doublet, good 2" star diagonal with a 1.25" eyepiece adapter, a nice barlow and some good use Plossl eyepieces.  You can talk about the mounting in that forum

 

I'd budget $350 for the 80ED, $75 for the diagonal & adapter, $50 for the barlow and $40 for each of the eyepieces.  Assuming 3 eyepieces, that would get you in the door under $600.

 

The Orion 80ED are typically of first rate optical quality, mine certainly is, with basically no false color visually or even photographically.  

 

Now for about another $100 or so more, you can step to its 4" F9 cousin and the mounting requirements are not all that much different.  Both are excellent doublets.

 

Good luck and welcome!

 

Jeff


Edited by Jeff B, 17 February 2019 - 05:09 PM.


#17 25585

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Posted 17 February 2019 - 05:14 PM

I love my TV85 (and Genesis). But both the Vixen mentioned above and Sky Watcher Esprit (a triplet) were tempting, but lost out.


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#18 gnowellsct

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Posted 17 February 2019 - 05:20 PM

I wouldn't get a Chinese 80mm but to be sure they are 90% of the offerings on the market.

 

An 80 mm scope has many potential applications.  One of them is to ride on top of a larger scope--a five or six inch refractor, or an 8 inch or larger SCT.  In that particular application, reducing weight solves a lot of problems, and so I would go for the ED.  

 

Having evaluated (visually) an SV LOMO triplet and a Vixen ED doublet at nearly identical magnifications on both deep sky and daytime, I'd say that under the conditions of my night time astro testing the two scopes were at the same level of performance.  Sometimes I would think one had the advantage but then later it would switch.

 

In daytime applications there seems to be a clear advantage to the triplet, which makes me wonder whether the night time views were simply an absence of opportunity for the LOMO to pull ahead.  But I'm not likely to continue these experiments.    Also, since I did those experiments, the Vixen went to Japan for cleaning and collimation check, and it's been very well behaved since its return.  So....I'm sort of agnostic.

 

But this thing for sure:  The SV LOMO is just not practical for me to mount on either of my SCTs with my current hardware, it's just too heavy.  And even if the hardware was perfect there is the issue of manipulating the OTA with the heavier refractor on it.  It passes some kind of threshold for me and it's not a good one.

The Vixen ED, on the other hand, is very manageable.

 

The debate is further complicated by the fact that some of the mainland China triplets aren't as well color corrected as the very best made doublets.  Some of them wouldn't stand a chance, I suspect, against the new Agema line, or even against the Tak FS fluorite doublet series.   This generalization is a moving target though because what was true five or ten years ago may not be true of what's currently being sold.

 

So anyhow I would tend to an f/7 or f/7.5 ED 80mm (or the very wonderful Vixen SD81s f/7.7).  And I would encourage you to keep in the back of your mind two add-on visual applications

 

1.  Mounting on an eventual SCT or larger refractor (I like using the 81mm on my 130 mm apo as well as my SCTs)

2.  Solar use.  This is something of a separate distinction but eventually you may want to invest in solar h-alpha viewing with a Daystar quark.  With this gear your night scope can be a solar scope.  Daystar quarks and h-alpha generally for some reason of physics that escapes me need to be at f/7 to f/7.5 which with a 2x telecentric barlow yields f/14 to f/15 and a 4x telecentric yields f/28 to f/30.  This gives you the best performance out of your daystar quark.

 

You would be using the daystar quark for visual but you'll still want to make sure you have a good R&P focuser (not a crayford) because h-alpha solar makes for a tail heavy set up.  It's really equivalent to an imaging train. 

 

So anyhow buying "for the longer term" rather than just getting a few photons in an inexpensive refractor may impose considerations of which you are not aware.  An 80mm is a versatile instrument and a good one can be a part of your viewing repertoire for a long time.  For photography an Esprit 80 mm f/5 is the scope they recommend--well a lot of them anyhow--at the NEAF imaging conference.

 

Greg N 



#19 Geo.

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Posted 17 February 2019 - 06:04 PM

OK, I just sold an Orion 110 ED doublet to fund a nice used 115 EON triplet; thinking AP. But for tossing sum'tin on the AltAz fur viewing, the $225 90mm f/5.5 Yuyao Yuzhong Optical "Aquila" (seen them sell for $150 used), is better than most  conditions warrant. I'd call it a semi APO , At least side by side with a non-APO WO Zenithstar 80, it performed as well or better as far as color went. So much so I sold the WO. 



#20 Mr. Mike

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Posted 17 February 2019 - 06:05 PM

The last used TV85 listed on Astromart was listed for $1050 but the previous 5 or 6 were between $1250 and $1450.   That seems a bit pricey considering the SV Access is $699 new and the Astrotech Triplet is $749 new.   Brand new the TV85 is $2125.

 

That difference in price could buy a lot of other items the OP will eventually want - eyepieces, filters, etc.   It's not like the SV Access and AT triplet are placeholder junk.  They are excellent scopes.  

I agree.  While the TV 85mm is a wonderful scope, I can’t see paying quite a bit more for it than either the SV or the ES.  I scored a deal on my ES102mm triplet and and thrilled.  I suppose the TV used at the right price is certainly intriguing but it’s an older design, I believe and I just couldn’t part with $1200 bucks for that scope when that much gets you  a 102mm stellarvue or even a 115 triplet from explore sci or Astrotech.  Nah, just couldn’t do it.


Edited by Mr. Mike, 17 February 2019 - 06:06 PM.


#21 Steve Allison

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Posted 17 February 2019 - 06:09 PM

Quality comes at a price.



#22 Nippon

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Posted 17 February 2019 - 06:09 PM

I have two excellent 4" apo refractors. one is an f/7 triplet and the other an f/7.7 doublet. Visually there is little difference to be seen. The doublet may have a slightly warmer view but that's about it.



#23 barbie

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Posted 17 February 2019 - 07:08 PM

I would go with a Skywatcher 80mm or 100mm doublet. They are excellent scopes.

#24 russell23

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Posted 17 February 2019 - 07:21 PM

Quality comes at a price.

You don’t need to spend $2200 to get a quality 80mm APO.  



#25 Richard Whalen

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Posted 17 February 2019 - 07:56 PM

I would suggest more aperture for a first scope, 80mm with a beginner can be pretty disappointing. If you are set on a refractor I would be looking at 100mm minimum. 110 to 115mm even better.

 

Otherwise a used C8 SCT is probably a better choice for a first scope. You can find them as cheap as a 80mm APO. For visual use only it will show a lot more.


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