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Choosing a refractor: Looking for advice and orientation

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#1 Fernando134

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Posted 18 August 2014 - 10:03 AM

Dear all,

 

I am considering purchasing a second telescope (refractor) mainly for astroimaging. I already have a C11 EDGE HD foi seeing, that are used for

all family; I can not be that much selfish and use the C11 for astrophotography with the rest of the family looking at me:))). Within my budget, I am considering the WO 132 mm triplet, the Stellarvue 115 triplet and the SW 120 mm Esprit. Right now, I am more on the SW, the less expensive and with the possibility of techincal assistance in Brasil, where I live.

 

All three report in their specifications "FPL-53", and the SW adds "Schott BK-7". What this adds considering optical quality?

Still, the WO reports a 4" focuser, the SV 3" and the SW a 3.3" focuser. Is there a big difference considering quality and astrophotography (I will use a Canon 50D)?

 

Moreover, I would be grateful if you guys could indicate some literature (not overly technical) on the topics above.

 

Thanks, Fernando

 



#2 maadscientist

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Posted 18 August 2014 - 05:58 PM

Attached File  Esprit120m8dav10min07262014DanL2.jpg   481.86KB   1 downloadsFernando,

 

I am  testing the Skywatcher Esprit 120 Triplet. So far, an excellent astro imaging scope. I have attached a single 10 minute exposure of m8.

 

Dan L



#3 Fernando134

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Posted 19 August 2014 - 06:33 AM

Hi Dan,

 

Thanks for your answer. A great picture. Are you going to be publishing your review?

 

Thanks in advance, Fernando



#4 MrJones

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Posted 19 August 2014 - 01:22 PM

This is a lower cost alternative to the SV115 and supposedly the same as the Meade 115mm ED triplet that I've been considering: http://www.altairast...productid=16380. There is a positive review from 2011 to click on in the middle.

 

The SV115T allegedly has a FPL-53 center element, the SV115EDT has FPL-51 and the Meade and Altair have FK-61 that is very similar to FPL-51. Hope that helps some.



#5 jrbarnett

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Posted 19 August 2014 - 04:10 PM

Greetings Fernando.

 

Glass type and glass brand are almost totally irrelevant in assessing the quality of a refractor.  Companies use these terms misleadingly to imply that use of a particular brand or type of glass means their scope is better than others using other brands or types of glass, but it's total baloney.  Don't be mislead by these marketing tactics.

 

Every glass maker or "brand" (Schott, Hoya, CDGM, Hikari, Ohara, etc.) makes many different types or models of optical glass (FCD1, HFK-61, FPL-53, etc.).  They offer every type of glass type they make in different quality levels.  The very best grade would cost far too much to use in amateur telescopes, but would be guaranteed homogeneous and also free of striae, occlusions and other quality defects.  The very poorest grades would have significant numbers of such defects, but would be cheap.  Note how none of the telescope re-branders who share the brand and type of some of the glasses used advertise the grade of the glass they use.

 

Beyond the total irrelevancy of glass brand and type, and the more important but still not crucial detail of quality grade, all of the listed characteristics are raw blank characteristics.  Telescopes used worked glass, not raw glass, and irrespective of glass brand, type and quality grade used, the hard part is in the working of the glass (figuring) to manufacture as perfect a lens as economically possible using the chosen glasses.  Rarely will any re-brander share optical group figure quality with you.  Most of them warrant only that the optic is "diffraction limited" which is a meaningless standard.

 

You'll also see claims that scopes using "FPL-53" have better color correction that scopes using "FPL-51" and the like.  This, too, is baloney.  The low dispersion glass type ("ED") used does not determine the level of color correction.  Rather the design, focal ratio and properties of all glass types used in the optical group determine color correction.  You can make a scope using FPL-53 (or even fluorite) that has poor color correction.  You can make a scope using FPL-51 that is virtually color free.  It all comes down the design (lens prescription, interplay of all glass types used, focal ratio and optical figure quality).

 

In general, re-branded scopes (those sold by firms like Meade, William Optics, Stellarvue, Altair, TS, etc.) are of similar quality.  Decent but not top of the heap.  I wouldn't worry too much about glass brand or type if buying any of these re-brander scopes.  Instead I'd look at things like price, accessories included and quality and load capacity of the hardware like the focusers.

 

Good luck!

 

- Jim 


Edited by jrbarnett, 19 August 2014 - 04:11 PM.

  • MrJones and cwilson like this

#6 Dakota1

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Posted 19 August 2014 - 04:47 PM

Jim; On what he is looking for where does the doublet or triplet fall in to the scope he is looking for as far as using it for AP. I may also be looking to the same thing next spring. If I do it will be AP and visual. This may change everything for my use.    Thanks

----------------------

Bill



#7 Jon Isaacs

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Posted 19 August 2014 - 06:53 PM

 

You'll also see claims that scopes using "FPL-53" have better color correction that scopes using "FPL-51" and the like.  This, too, is baloney.

 

Oh Jim, oh Jim.. 

 

It's time for the other half of the story, the facts, the reality..

 

 FPL-53 is more expensive and has superior optical properties that allow for better color correction within a particular set of design constraints. An 80mm F/7 FPL-53 doublet can have and most likely will have significantly better color correction than an 80mm F/7 doublet based on FCD1, HFK-61 or FPL-51.  You could make an 80mm F/10 using FPL-51, no one does, and achieve similar color correction to an 80mm F/7 FPL-53 doublet but it would be F/10 and not F/7.  

 

And too, the manufacturer can be most important. At the top of the heap you will find a company like Astro-Physics where each batch of glass is measured for homogeneity as well as it's optical properties. If the glass is not of the highest quality, it is discarded. The design of the lens is altered based on the measured properties of the individual melt of glass. The optics are all done in house from the design to the final hand figuring of the objective which is done by the owner himself on an interferometer.  

 

There is a lot to know about refractors, not only optical but also mechanical design.. in general, you get what you pay for..  The key is knowing what you want and what you are willing to pay.  Most are quite good, some are better values for essentially the same product, the balance between mechanical and optical quality varies..  Companies like Astro-Physics, Tec, Takahashi and TeleVue are manufacturers, they do the design, they may have an optical house oversees fabricate the optical elements to the specs and then the assemble the scope. Most other brands are, as Jim says, really vendors, they buy from a manufacturer who has a product line, they might spec out a scope slightly differently than the next guy... 

 

Jon Isaacs 



#8 drollere

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Posted 19 August 2014 - 07:37 PM

the point jim and jon both seem to be making is that abstract or conceptual criteria, whether in materials or optical performance, are not the best data points for choosing a telescope.

 

i'd add that reviews (assuming they're not just golly gee testimonials) are also hard to relate to your personal criteria. there is a favorable review for almost every telescope in the market.

 

my suggestion is that you consider your choices in terms of which scope satisfies the largest number of desired qualities that you can explicitly define. once you narrow things to "the best" optically then considerations come in that i feel will be irrelevant to your satisfaction with the instrument.

 

it's true that side by side, you might see some difference in the images put up by competing scopes. it's also true that, having just one scope to work with, your eye will adapt to what you have and you'll find the images quite satisfactory, especially as the quality differences in most commercial optics are getting smaller all the time. where quality really makes a difference is in the build quality of everything else. a slack and sloppy focuser, a finder scope you can't get in focus, a dust cover that keeps falling off the lens ... these things get old very quickly.



#9 areyoukiddingme

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Posted 19 August 2014 - 07:50 PM

I did it the other way round. I started with an 80mm refractor, and went with an FPL-53 apo, the Orion 80mm triplet (next I got the 11 Edgehd). 

 

I think the general sentiment, which I agree with, is that while the fancy kinds of glass CAN really produce the best views, that requires some serious attention to detail in the engineering. My fancy but very competitively priced apo does not do well with high magnifications, and it shows color on bright objects that I don't see in my Televue 101. It can be a difficult thing to figure out the source of the color if you don't have another scope to do a side-by-side comparison with (i.e., atmospheric refraction; color in eyepieces). It's quite possible to get a well engineered scopewith lesser glass that can produce a better result. So, yes, FPL-53, great stuff, but be careful. Spend some good time reading reviews on different scopes and don't be swayed just by claims about types of glass.



#10 maadscientist

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Posted 19 August 2014 - 07:50 PM

Hi Dan,

 

Thanks for your answer. A great picture. Are you going to be publishing your review?

 

Thanks in advance, Fernando

 

Fernando,

 

I might submit it here if I can get some more clear sky. A rarity at present.  :(

 

For Astro Imaging, all 3 scopes you mention will work well. The Esprit 120 has 2 things going for it. First, it comes with a dedicated flattner with purchase. Second, the newer ones have the new dual hybird crayford rack and pinion focuser. It is superior to traditional linear power crayfords.

 

I really think you cannot go wrong with either of the three, so take the pressure off yourself and pick the one that strikes your fancy.  :)

 

Dan L



#11 jrbarnett

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Posted 19 August 2014 - 11:59 PM

 

 

You'll also see claims that scopes using "FPL-53" have better color correction that scopes using "FPL-51" and the like.  This, too, is baloney.

 

Oh Jim, oh Jim.. 

 

It's time for the other half of the story, the facts, the reality..

 

 FPL-53 is more expensive and has superior optical properties that allow for better color correction within a particular set of design constraints. An 80mm F/7 FPL-53 doublet can have and most likely will have significantly better color correction than an 80mm F/7 doublet based on FCD1, HFK-61 or FPL-51.  You could make an 80mm F/10 using FPL-51, no one does, and achieve similar color correction to an 80mm F/7 FPL-53 doublet but it would be F/10 and not F/7.  

 

And too, the manufacturer can be most important. At the top of the heap you will find a company like Astro-Physics where each batch of glass is measured for homogeneity as well as it's optical properties. If the glass is not of the highest quality, it is discarded. The design of the lens is altered based on the measured properties of the individual melt of glass. The optics are all done in house from the design to the final hand figuring of the objective which is done by the owner himself on an interferometer.  

 

There is a lot to know about refractors, not only optical but also mechanical design.. in general, you get what you pay for..  The key is knowing what you want and what you are willing to pay.  Most are quite good, some are better values for essentially the same product, the balance between mechanical and optical quality varies..  Companies like Astro-Physics, Tec, Takahashi and TeleVue are manufacturers, they do the design, they may have an optical house oversees fabricate the optical elements to the specs and then the assemble the scope. Most other brands are, as Jim says, really vendors, they buy from a manufacturer who has a product line, they might spec out a scope slightly differently than the next guy... 

 

Jon Isaacs 

 

Actually Roland says that he cannot afford to use the highest grade glasses from Ohara.

 

He also agrees that focusing on the ED element alone is pointless.

 

"There is no other glass readily available and in general production that would be superior to these two. They are both very close in terms of optical characteristics with an ultra-slight edge going to the FPL53. The only other material that is superior in a very small way is Fluorite. All three can produce triplets of essentially zero color error. They can also produce zero color error doublets, but the focal ratio must be considerably longer because no suitable mates exist to provide fast focal ratios and zero color both at the same time in a doublet.

 

It is senseless to argue about these glasses because the overall correction is due to the mating element, not the ED. Why not lets argue about suitable mates, yes?

 

Rolando"

 

http://www.astromart..._post_id=496242

 

More on the importance of the non-ED glasses used, from Roland:

 

">>Would you post a sample recipe for objectives in this design class --- for

ATMOS? I am playing around, but not really hitting anything that has this
performance. It would be fun to take such a design and see how decentering
or changing melt characteristics changes it in comparison to changes in a
flint doublet.>>
Sure, can do. In fact, I take it you will be starting a new Atmos Yahoo group,
in which case we can exchange some intersting ideas.
In the case of this particular fluorite design, the color correction depends
entirely on the mating element. Traditionally, one would choose a mate that had
the same partial dispersion. Until recently, the only viable mates were ZKN7
and KZFSN2.
While ZKN7 is readily available from Schott and other sources, KZFSN2 (or the
new N-KZFS2) have some undesireable weathering problems. The surface is highly
affected by moisture, and even if coated, the moisture will penetrate through
the pinholes that all coatings have and eventually cause opaque spots on the
glass. One can place the fluorite out front in the hopes of shielding the KZ
glass in back, but anyone who has ever seen a refractor lens the morning after
a moist night will know that the back of the glass will have condensate on it.
Two other approaches are possible. One could use a Lanthanum glass, which
results in quite low sphero-chromatism, as well as good color correction, and
the other would be to use a glass not quite a perfect match, but at least twice
as good as the one that Tak uses. This would be Shott K10. The result is quite
a nice level of color correction with almost perfect spherical using an
airspace of a few millimeters.
There are other glasses such as Scott K4, which are out of production, but may
be poured by the Chinese for some special purpose. I am in the process of
inquiring what materials are available from the Chinese glass makers for some
future products. It seems that both Schott and Ohara, for environmental
reasons, have shied away from making those glasses that could produce really
good results with fluorite or FPL53, and perhaps the Chinese are now stepping
into the void.
Roland Christen"

 

http://sci.tech-arch...04-11/3519.html

 

It really is all about the mating glass.  Truly.

 

There are great triplets being made with FPL-51, too.  The TEC 160EDs and 200EDs, for instance.  I'd much rather have one of these than a Chinese FPL-53 triplet such as the Sky Watcher 150mm.  Sure, the TECs are slower.  And for visual that's a GREAT thing.

 

Jared Wilson along these lines:

 

"Speaking generally, refractors made with this material (it's an Ohara catalog number by the way--other manufactures may have very similar materials) have a very good reputation for quality. This is for two reasons:

1) It has the potential, when coupled with the right mating element, to produce extremely good color correction
2) Because it is intrinsically expensive, most manufacturers only use it when they are producing fairly high-end (or very high-end) refractors. So while it doesn't make a telescope perform well by itself, it is often taken as an indication of overall quality. If you weren't setting out to make a top notch scope, why would you bother to spec in such an expensive glass (or so the argument goes)?

The problem with the emphasis on FPL-53 or any other uber glass is that the heavy marketing emphasis can cause the material to take on a mystique of its own. Just as fluorite did in the past. Then people start to believe that you can't make a top quality refractor unless it includes FPL-53, and that simply isn't the case.

Why should you care about FPL-53? Because scopes that use it are usually of very good quality and have the potential to show very little chromatic aberration. Just don't fool yourself into thinking this is the only way to achieve good performance in a refractor. There are other materials that work very well also."

 

http://www.cloudynig.../183234-fpl-53/

 

Slower scope, too, have significant advantages; most importantly greatly reduced spherochromatism.  Fast doublets and triplets have reduced maximum potential polychromatic Strehls.  Fast scopes are beneficial for imaging.  For visual, they are actually something of a liability.  Sure, portability is improved, but the reality is a scope up to 4" is not much less portable at f/10 than it is at f/7.

 

So, again, whenever a manufacturer or OEM (i.e., re-brander) advertises only the type and brand of ED glasses used, they are telling you nothing important or of real value with respect to the quality of the instrument they are hawking.  It's meaningless marketing drivel intended to mislead the uninformed.  Buy quality, not glass type (or is that hype?).

 

- Jim



#12 Derek Wong

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Posted 20 August 2014 - 01:26 AM

<Roland said>

 

While ZKN7 is readily available from Schott and other sources, KZFSN2 (or the new N-KZFS2) have some undesireable weathering problems. The surface is highly affected by moisture, and even if coated, the moisture will penetrate through the pinholes that all coatings have and eventually cause opaque spots on the glass. One can place the fluorite out front in the hopes of shielding the KZ glass in back, but anyone who has ever seen a refractor lens the morning after a moist night will know that the back of the glass will have condensate on it.

 

Wow, I shouldn’t have stopped reading s.a.a. so long ago!

 

A couple Japanese astronomers have told me about “fogging” of the old Takahashi doublets.  The fogging happened after 30 years to the oldest lenses that had an uncoated KZFSN2 element, and there were Japanese shops that repolished the lenses.  The Vixen objectives and newer Tak objectives had an uncoated fluorite element but a coated second element and didn’t get the fogging.  I was skeptical because I never heard this happening in the US, but this may be an explanation for the phenomenon.  If I ever get an FC-65, it will go out on dry evenings...

 

Derek



#13 Fernando134

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Posted 20 August 2014 - 09:11 AM

Dear all,

 

Thank you very much for your time answering my questions (Dan, Jim, Jon, Bruce...)

Learned a lot about refractors glasses!

 

Actually, there was a recent review on the SW 120 mm Esprit published in the Canadian magazine "Sky News". A very positive one,

written by Alan Dyer. Dan, I am looking forward to your review :waytogo:

 

I would be grateful if you guys could indicate some literature on refractors glasses, not overly technical... :)

 

Regards, Fernando



#14 Jon_Doh

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Posted 20 August 2014 - 09:27 AM

Here are a couple of articles you might find helpful:

 

http://www.cloudynig...d-schott-glass/

 

http://www.opticstar...sp?p=0_10_1_3_7  (scroll to the bottom to the lens tests)



#15 MrJones

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Posted 20 August 2014 - 11:01 AM

http://www.astronomy...7239fcaeaa16056



#16 maadscientist

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Posted 20 August 2014 - 11:47 AM

Dear all,

 

Thank you very much for your time answering my questions (Dan, Jim, Jon, Bruce...)

Learned a lot about refractors glasses!

 

Actually, there was a recent review on the SW 120 mm Esprit published in the Canadian magazine "Sky News". A very positive one,

written by Alan Dyer. Dan, I am looking forward to your review :waytogo:

 

I would be grateful if you guys could indicate some literature on refractors glasses, not overly technical... :)

 

Regards, Fernando

 

Fernando,

 

You are quite welcome! With CCD imaging with triplet refractors, it is important to have a flattner that works with it. A matched flattener, I.E. one designed to work with that particular telescope is ideal. After market flattners may, or may not work to your liking. It is not one size fits all. The focuser is also criticlal. If you are going to load it up with weight, a hybrid rack and pinion is a must. Using a one shot color, or DSLR, you will be fine with a regular crayford for now. What camera are you going to use?

 

At this point, your possible scope selections are fine and will serve you as long as you want to image. There is nothing limiting about your selection of optics for imaging, top notch pictures have been made with all three. It is also good that you want to educate yourself on refractor glass. However, I must give you a heads up that APO refractor performance is a rabbit hole that is deep and wide. It is at the top of my big 4 list of controversial topics that are discussed that can lead amateur astronomers to fistacuffs:

 

1) Discussing APO refractors color correction and performance vs. much larger scopes.

2) Discussing whether color cameras can produce as good images as mono cameras plus filters.

3) Discussing the F ratio myth in Astro Imaging.

4) Discussing performance and collimation of big newtonian mirrors.

 

Since you are imaging, number 2 and 3 might eventually come into play. My best advice is to star test your scope upon arrival, and take some test shots to check for color correction, The CCD is unforgiving on this and you will be able to tell quickly if the scope is operating up to spec. Also, use some high power observing on the moon and planets, If seeing is average to above average, you should be able to use 200-250x and have a sharp image. 

 

Dan L



#17 Fernando134

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Posted 20 August 2014 - 12:26 PM

Dan, thanks again.

 

I have an unmodified Canon 50D. As far as I progress with imaging, I may modify this one.

For the near future, I will not use a CCD.

 

Also, I got the Backyard EOS 3.0 Premium Edition. I am getting familiar with the software while practicing with the Canon, by now on terrestrial pictures.

 

Regards, Fernando

 

 

 



#18 maadscientist

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Posted 20 August 2014 - 12:36 PM

We just had Hap Griffin speak at our Atlanta Meeting. I have had him as a speaker at the PSSG. He modifies DSLR's and I highly recommend him if you choose to modify.

 

http://www.hapg.org/camera%20mods.htm

 

If you want a one shot color cooled CCD, look on the classifieds here or on  Astromart for a Starshoot Pro II. Usually $750 or so.

 

Dan L



#19 Fernando134

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Posted 21 August 2014 - 12:25 PM

Dan, thanks again!.

 

In another thread I read your recomendation of a Baader clickclok for the SW 100 Pro. I have one Baadder diagonal in my C11 and it is just great! Do you think I will need a Baader clickclock, or it is worth to have one, in the SW 120mm Esprit for holding the Canon 50D?

 

Thanks in advance, Fernando








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