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Premium refractor vs. normal: visual only quality

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#1 mark8888

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Posted 11 June 2011 - 03:10 AM

Hi,

In browsing the refractor forum, I've often noticed comments which imply that although there is a large difference in the quality of photo images you can produce when you use a premium refractor (like a Takahashi TSA-120, TOA-130, or TEC 140) vs. a much cheaper mass market one (like an Orion or Celestron of similar 120-140mm aperture), mainly because of the relative lack of CA with the premium scopes... if you ignore photography and stick to visual viewing only, simply looking through the eyepiece or binoviewers, the view through a premium refractor and a mass market one, at the same aperture, is very similar. The point seems to be that difference for visual is much narrower for photography, so if someone is buying a refractor for visual only, he or she should strongly consider buying an Orion or similar rather than a TEC/TAK, because the difference for visual is so exceedingly tiny, so barely noticeable to anyone but the most dedicated and seasoned observer, that it just isn't worth the extra $3000.

So, I'm wondering if someone who has experience looking through both a premium refractor and a mass market one of the same or very close aperture, under similar conditions, could comment on how large the difference really is or isn't. Contrast... CA... how much detail you can see on planets... sharpness... etc... is what you see through a premium vs. mass market refractor pretty different (is there clearly more of a WOW factor looking through a premium refractor?), or is it extremely similar?

Thanks for any replies. :grin:
 

#2 JT5

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Posted 11 June 2011 - 06:21 AM

I have a 70mm Televue Pronto that I have owned for over 10 years. Price at the time was just a little over $1,700 and included the scope, star beam pointer and Televue tripod. The views in my scope are outstanding with a little false color around the Orion Nebula. Saturn and rings are good, but larger aperture is needed for more detail. This little scope does a tremendous job for its size and is my scope of choice for "Grab and Go".

I'll leave it to others to comment on lesser small scopes and larger inexpensive brands.

John
 

#3 roscoe

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Posted 11 June 2011 - 07:00 AM

Mark,
I have a 120mm scope with very good Japanese optics, and compared to medium-quality 120's it is clearly sharper and has less purple fringing, enough that others - like the owner of the other 120, could see the difference, but on the other hand, it takes a very good night for the differences to really show up. In this case, we're talking about a $500 vs a $1000 scope. A $3000 APO would be another clearly visible step up. If money was no object, I'd maybe own a big APO, more likely I'd own a big D&G or similar long-tube. My farm's too dusty for a reflector, but my experience is the same, better mirrors give better views, but either way, here in not-so-special new england skies, does $3000 look three times as good as $1000? I personally don't think so.
Russ
 

#4 jgraham

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Posted 11 June 2011 - 07:06 AM

Some people seem to be more sensitive to CA than others so this may be a bit of a personal issue. There's also a technical side; CA is certainly more noticeable as the object becomes brighter and as the f/ ratio becomes shorter. If you spend a lot of time observing the moon you would probably benefit from a premium refractor. However, for bright targets you hsve a lot of light to work with and filters like the semi-apo may be effective. In any case, I would tend to stay away from short focal length achros unless I were primarily a DSO observer, in this case CA is essentially a non-issue.

As a footnote, I've found that CA can be easily managed in imaging using standard RGB techniques.
 

#5 Tony Flanders

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Posted 11 June 2011 - 08:15 AM

In browsing the refractor forum, I've often noticed comments which imply that although there is a large difference in the quality of photo images you can produce when you use a premium refractor (like a Takahashi TSA-120, TOA-130, or TEC 140) vs. a much cheaper mass market one (like an Orion or Celestron of similar 120-140mm aperture), mainly because of the relative lack of CA with the premium scopes... if you ignore photography and stick to visual viewing only, simply looking through the eyepiece or binoviewers, the view through a premium refractor and a mass market one, at the same aperture, is very similar.


I own three achromats: one that's too weird to describe, a 70-mm f/6.9, and a 100-mm f/6. All of them have pretty good optics as achromats go, and the 70-mm f/6.9 has excellent optics aside from its false color.

For deep-sky observing, there's precious little difference between these achromats and apochromats with identical apertures and f/ratios. That's because color vision doesn't kick in at the light levels used for most deep-sky observing. Also, the acuity of the eye is poor at low light levels.

For daytime observing, the achromats look just great -- until you compare them side-by-side with equivalent apochromats. Then you realize the color fidelity that you were missing. Also, things like power lines have obvious colored fringes -- if you look for them.

It's on planets and the Moon, viewing at high power, where you really notice the difference. It's not just that the color isn't faithful, but also that the fringes visibly blur the view. Venus -- a special case -- is surrounded by a huge, garish, purple halo.

Having said that, the 70-mm f/6.9 in particular is a wonderful planetary and lunar scope, given the limitations of its aperture. Definitely not as good as a 70-mm APO, but plenty good enough to show the polar caps on Mars, Syrtis Major, and considerable structure along and within the major bands on Jupiter.

False color is proportional to the square of the aperture divided by the focal length -- so a 100-mm f/9.9 would have the same amount of false color as my 70-mm f/6.9, and my 100-mm f/6 has much more -- namely, enough to make the difference between it and a 100-mm f/6 apochromat truly glaring at high power on the planets.

On the other hand, old-fashioned long-focus achromats, like an 80-mm f/11, have very little false color indeed, and make splendid planetary scopes.
 

#6 j3ffr0

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Posted 11 June 2011 - 08:54 AM

When it comes to visual observing, my 120ED doublet will hold it's own against any 120mm class doublet. Mine is a very good sample, but some might not get that lucky. Perhaps AP and Tak would be a little better in a side by side test, but it wouldn't be very obvious. My scope shows no false color on in-focus images.

I also don't believe triplets are necessary at all for visual observing. I believe 90% of visual observers who are looking for a refractors view would be happy with a sub-premium ED doublet. I know I am, and I have indeed looked through premium scopes such as Taks and APs.

For those who want the best, the premium options remain. I would love to have one, but I'm just not willing to pay for it at this time. :)
 

#7 JayKSC

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Posted 11 June 2011 - 10:35 AM

I've been a refractor guy for ages - since times when it was not too terribly difficult to get on the AP wait list. Having owned and loved many achromat and apochromat refractors I both agree and disagree with your general view about mass market versus premium refractors.

Foremost, there is a big difference between a mass market achromat and any apochromat. Unless you're looking through a classic achromat with long focal length, I've consistently and continuously found the view through any decent apochromat (yes, I include ED refractors as apos) sharper, with better contrast, and far cleaner color correction. The visual difference is non-trivial, to me. I notice that my 150mm achromat tends to give mushier views starting at around 150x or so; I find myself needing to fidget with its focus more compared to my apos which just snap to easy focus even at obnoxiously high powers. I also notice that views through all of the achromats I've owned over the years have not had the same stark sky background as the apos I've owned. There is a gain in contrast and sharpness with apos and this does make a difference for lunar and planetary viewing as well as with deep sky targets.

Now, I mostly agree with the idea that a mass market apochromat (including the ED scopes) compare well visually to expensive premium apos. When you get into this comparison, I've generally found the premium apos have had tighter quality control, usually a tad nicer focuser, and often a slight gain in optical performance. However, the difference here isn't usually stunning. I liken the difference to jumping from a dusty standard mirror diagonal one's owned for awhile to a brand new dielectric. It'll give a boost in visual performance, but if comparing a mass market apo to a premium one side by side, the boost is likely to always be quite subtle.

That said, I personally like the premium apos. For one, the companies tend to have a strong dedication to their customers, which I appreciate. I also like to know that the optics I'm using on distant galaxies, for instance, are performing at their best. Why make it any tougher for those dim and ever distant photons to register in my visual perception by using anything less than the most top notch optics? Yes, this mentality gets expensive, but one perk to going to the best for me is that I've not felt a need or much desire to upgrade equipment in a decade. I think in the long run purchasing a lifetime dream scope is no more costly than upgrading gear every year or two.

Have a good weekend all!

:) Jay
South Florida
 

#8 howard929

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Posted 11 June 2011 - 10:56 AM

Jay,

New guy here. When you say premium, do you mean Vixen/Takahashi?

Howard
 

#9 FLYcrash

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Posted 11 June 2011 - 11:03 AM

Aside from the quality of the optical components, one of the things you get with a premium scope is good build - a much smoother and stronger focuser free of slop, a better lens cell that holds collimation better and/or is user-collimatable, better mounting hardware for the scope and the accessories, more careful blackening of the tube, more beautiful fit and finish, etc.

To do a really fastidious job with these things compared with pushing good-enough components down the assembly line is a qualitatively different business. Those good CNC machines, careful workers, and scopes thrown out because they failed Q/C cost a lot of money.

I've always had the feeling that the quality of a scope (or any other piece of merchandise) is something like logarithmic in the price. Doubling the price gives a fixed improvement in the subjective experience. You're always up against diminishing returns. Where you decide to stop in terms of price and quality is up to you.
 

#10 roscoe

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Posted 11 June 2011 - 11:17 AM

Mark,
I should also mention that I have a Baader minus-violet filter semi-permanently installed to keep purple fuzzies under control and slightly sharpen up the image because it takes some of the deep red and deep purple out-of-focus light out of the mix, and I also upgraded my focuser to a good crayford. so in the end, I threw a couple of hundred more dollars into mine, things, as pointed out above,come installed on a better scope.
Russ
PS don't know if it is still available, but there's an Antares 812 for sale this morning in the classifieds! It's the first one I've seen offered in a couple of years on CN.
 

#11 jrbarnett

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Posted 11 June 2011 - 11:20 AM

Hi.

It's not really a question of "premium" versus "normal" but rather a question of optical quality. Through time, the average optical quality of "normal" refractors relative to their premium counterparts has improved greatly. Today, if you are speaking strictly visual use and image quality, and ignoring mechanical quality, fit and finish and the like, you have a reasonably good chance of getting optics of high enough quality when buying a "normal" refractor that you would have a very hard time detecting differences between your "normal" refractor and a "premium" one.

But...(there's always a catch, isn't there?)...let's make sure that we understand the difference between "normal" and "premium". Would you consider Televue "premium" or "normal"? I would consider it "normal". Here's why. Televue designs its refractors, subcontracts manufacturing of the optics to a third party, and then assembles the optics into tube assemblies back in the US. In my book, because Televue does not itself make the optics, by reason of business model they are no different than other rebranders like Stellarvue, Orion, Explore Scientific, etc. They are a little different than those, to be fair, in that Televue designs every scope it sells. Those others do not. In Stellarvue's case they sometimes pair market-available optics of third party design with their own tube assemblies. Orion always does this. Explore Scientific sells scopes designed and made by parent JOC, which scopes are available globally under several other brands. So Televue, in my view, straddles the true "premium" and the "normal" makers.

So what does it take to be clearly "premium"? To me a premium maker manufactures its own designs, including fabrication of the optics. Astro-Physics, TEC and Takahashi are examples of "premium" makers.

But back to the question of "is there a reason to buy premium for strictly visual use"? I think the answer is, it depends. It depends on several factors including (a) your tolerance for risk, (b) how fussy and/or knowledgeable you are about image quality and © the kind of visual observing you do.

With respect to (a) (risk tolerance), while "normal" optical quality has averaged up over the last decade, in independent interferometric testing "normal" scopes still generally (though not always) come in a little behind premium scopes in optical quality. The margin of difference, however, isn't great. The typical "premium" optic might rate 1/7 to 1/8 wave. The typical "normal" optic might instead range from 1/6 wave to 1/7 wave. The old rule of thumb is that an optic 1/4 wave or better is diffraction limited and therefore visually perfect. The old rule of thumb is wrong, but 1/4 wave is still a useful inflection point in that most observers would consistently rate images through a less than 1/4 wave optic as not being as good as those through a 1/4 wave or better optic. As optical quality improves, the detectability of differences between different optics is harder. I doubt most observers could detect the difference between a 1/7 wave and 1/8 wave optic visually, for example.

In my opinion, most folks probably could tell the difference between a 1/5 to 1/6 wave optic and a 1/7 to 1/8 wave optic at high magnification on planets, the Moon or in separating double stars near the limit for the aperture. Now, when you buy premium you have a very high chance of getting a 1/7 to 1/8 wave optic. When you buy normal, you have a decent, but less good chance of obtaining that same level of optical quality. Certain normal brands, and certain models within the lineups of certain normal brands give you better odds than others. In the end, however, you take more risk buying normal of a good but not supreme optic. You also have a higher chance of getting a truly bad optic, but even so I'd say that risk is small.

With respect to (b), if you don't know what to look for or how to look for it, it well may be that 1/5 wave is as good for your purposes as 1/8 wave. Likewise, even if you do know how to estimate refractor optical quality, you may not feel that the subtle quality difference between different qualities of optics is worth the generally large price premium demanded for a "premium" scope. For a knowledgeable and fussy observer, the answer is often "yes". For a knowledgeable but unfussy observer, it may be "no". For an un-knowledgeable observer, the answer is also likely "no".

With respect to ©, even the most knowledgeable and fussy observer who prefers DSOs, extended objects and/or lower magnification vistas, likely wouldn't benefit from the "premium" investment. An un-knowledgeable observer who nonetheless prefers mostly higher magnification lunar, planetary and stellar work, on the other hand, might benefit from paying the piper.

Note that the amount of "CA" (or "chromatic aberration") visible doesn't necessarily map to the "premium" vs. "normal" distinction. That is, there are plenty of "premium" refractors that show considerable chromatic aberration (example: Takahashi FS-152, Takahashi Sky90, Astro-Physics Starfires from the 80s and early 90s). There are "normal" scopes that control chromatic aberration exceptionally well (example: Astro-Tech AT106, Stellarvue SV105 Raptor). In my book, optical quality is a heck of a lot more important to image quality visually than is the presence or absence of false color.

Clear as mud?

:grin:

Regards,

Jim
 

#12 jrcrilly

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Posted 11 June 2011 - 11:28 AM

To me a premium maker manufactures its own designs, including fabrication of the optics. ... and Takahashi are examples of "premium" makers.


That's not consistent with your earlier statement. Like Tele Vue, Takahashi has its optical designs rendered by a subcontractor. If, by your definition, TV is only "normal" then so is Tak.
 

#13 Waxing Gibbous

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Posted 11 June 2011 - 12:20 PM

Some achros are very good (WO Megrezes, D&Gs, I-Stars) and are nearly Apos in terms of showing minimal false colour. Others are rubbish: blue-tube Skywatchers and almost all Saxons, short-tube Orions, Tasco anything, cheapo Celestrons and Meades etc.
Some find minimal colour OK. I HATE it. I wouldn't bother with an achromat except as a guide or finder-scope.

I'll take an APO every time.
I have owned Televues, TAK, APs, TMBs, Stellarvues and an A&M.
All along-side "lesser" refractors.
There is indeed a difference in image quality (sharpness/contrast/colour correction)between say an AP 130 and an EON 120. But its not huge. Certainly not $4000 worth, unless you make your living from astrophotography, or are just plain fussy about your images.

Of all the refractors I've used in the last 10 years I found the following to be the 'best':
AP 155
TMB 152
TMB 130 f9
AP 130 f6
SV105 / 6.2
AT106
SV LOMO 80/480
TV101

These are, for the most part, different scopes for different viewing styles so they are not directly comparable, but I found it made no difference whether the scope was made entirely "in-house" or built from 'sourced' optics.
Its the final quality control that matters.
My SV105 had its optics sourced from Russia and produced a higher Strehl ration than the AP130 (.989 vs .983 if I recall). If the company is commited to producing a fine scope, it doesn't really matter where its components come from.

At present, IMHO, the best value-for-money scopes are the Orion EON series, the WO Fluoro-stars and the Astro-Tech lensed scopes in their various guises (Sharpstar, Astrotech, Teleskope Service, APM etc,).

Typically their build quality, at least in terms of materials, is not as high as AP, TEC, Stellarvue or others in the 5K+ bracket, but there optics are VERY close.
Between a TAK120 and an EON 120, I'd choose the EON - it's that good.
Between a LOMO 80/480 and the offerings from TS or APM? Hard choice, but I'd take the cheaper scopes as the difference is, again, not $1000 worth.

So to re-iterate: yes, there is a difference between premium and 'work-a-day' scopes (particularly apo vs achro), but if you choose your brand carefully, its rarely as much as the price difference suggests.
 

#14 Otto Piechowski

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Posted 11 June 2011 - 12:42 PM

Mark,

In response to your excellent question, I will share an expert opinion and then my own experiential data confirming his opinion. The opinion comes from Dr. Mike Palermiti who among many other things, spent time with the US Navy ascertaining ideal apertures, magnifications, etc. for observing instruments. His quote goes as follows:

“While many refractive designs exist, many of the designs represent gross "over kill" for the marketplace that they are introduced into. A well made achromatic refractor will show every bit as much planetary, lunar, and deep space detail as the most exotic systems produced today. The notion of "false color" is often misrepresented. Our atmospheric conditions often change the transmission, and color yielded from an astronomical subject. The choice of eyepiece and use of filters will most certain change the way we observe a subject. Finally, the observer's experience and their own eyes play the final role on what is perceived.”

(Dr. Palermiti has three advanced degrees in; Physics, Optics and Astronomy and 44 years experience with optical astronomy, laboratory testing and imaging systems. He's owned and operated several development companies doing optical and systems design work for institutions, agencies and private groups.)


I have owned and used about a dozen achromats and three EDs and two apochromats. From using these and taking notes on my experiences, and reflecting on those notes, I have observed the following:

1. According to my notes using two world class quality apochromats compared to my more recent observations using an achromat of the same aperture and focal length (each in the low 90mm and around F5.5.) and all having well made optics which were near perfectly collimated: (a) all that could be seen in the apos could be seen in the achromat (e.g. close disequal double stars, lunar features); (b) the achromat combined with a Vixen Porta Mount was truly a one-hand grab and go; whereas the apos on AZ3 or VersaGo mounts were more clumsy to do with one hand; © Low power, wide field views were equally aesthetically pleasing in all of these scopes.

2. A leisurely and careful comparison of the achromat to a well figured and collimated maksutov of the same aperture revealed (a) that one never revealed details not visible in the other. The objects compared included Saturn, a half dozen well picked subtle features on the moon, and the double star porrima. Only in the case of Porrima was the view of the double better in the mak than it was in the achromat, but that is totally attributable to the characteristics of an obstructed to non-obstructed system. (b) The color tones and saturation seemed identical at low and high magnifications. My guess here would be that the unfocused violet does not significantly contribute to the overall color palette, and that the older eye (i.e. 56 years) is less sensitive to violet. © Using the hackneyed formula of the unobstructed aperture equivalency of the diameter (D) of the front element in the maksutov system minues the diameter (d) of the secondary mirror spot, and since the maksutov is nearly completely free of chromatic aberration, in terms of resolution I was in effect comparing the 90mm achromat to a 72mm apochromat. There was no difference in the detail seen or in the contrast quality of the detail.

3. Subtle details, examined on the moon using the same achromat without a minus-violet filter and with a high quality minus-violet filter, were equally visible in the achromat both with and without the minus-violet filter.
 

#15 jrbarnett

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Posted 11 June 2011 - 01:06 PM

John:

Are you certain? I thought Takahashi had its fluorite blanks made by a subcontractor (Canon) but actually ground (at least some of) its own lenses. I know they grind and polish all of their own mirrors, but have wondered about the lenses.

Rick at JapanAstro who sold a lot of used Takahashi and lives in Japan, for example, said:

"All the Fluorite was/is sourced from Canon Optron while the current Super ED glass should be from Ohara. It is not clear if this is raw blanks or finished lenses.

I was told that one of the reasons from the switch from Flourite to ED is it can be sourced from several producers as production budgets change, so there is no assurance Ohara's FPL53 is used across the line.

With Fluorite, there is only one source in Japan and the minimum production order required large inventory storage costs. More than 5yrs of production in the case of the larger apertures and it was no longer economically viable.

Rick"

http://www.cloudynig...9/o/all/fpart/2

It would be interesting to know for certain whether Takahashi buys finished elements (making them more like Televue) or just blanks (making them more like Astro-Physics).

Regards,

Jim
 

#16 Otto Piechowski

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Posted 11 June 2011 - 01:12 PM

Mark,

Thank you for your question, I will share an expert opinion and then my own experiential data confirming his opinion.

Expert Opinion: “While many refractive designs exist, many of the designs represent gross "over kill" for the marketplace that they are introduced into. A well made achromatic refractor will show every bit as much planetary, lunar, and deep space detail as the most exotic systems produced today. The notion of "false color" is often misrepresented. Our atmospheric conditions often change the transmission, and color yielded from an astronomical subject. The choice of eyepiece and use of filters will most certain change the way we observe a subject. Finally, the observer's experience and their own eyes play the final role on what is perceived.”

(Dr. Palermiti has three advanced degrees in; Physics, Optics and Astronomy and 44 years experience with optical astronomy, laboratory testing and imaging systems. He's owned and operated several development companies doing optical and systems design work for institutions, agencies and private groups. Among these accomplishments, Dr. Palermit spent time with the US Navy ascertaining ideal apertures, magnifications, etc. for observing instruments used by everyday observers in normal observing conditions in situations where observation of detail was critical.)


Experiential Observations: I have owned and used about a dozen achromats and three EDs and two apochromats. From using these and taking notes on my experiences, and reflecting on those notes, I have observed the following:

1. According to my notes using two world class quality apochromats compared to my more recent observations using an achromat of the same aperture and focal length (each in the low 90mm and around F5.5.) and all having well made optics which were near perfectly collimated: (a) all that could be seen in the apos could be seen in the achromat (e.g. close disequal double stars, lunar features); (b) the achromat combined with a Vixen Porta Mount was truly a one-hand grab and go; whereas the apos on AZ3 or VersaGo mounts were more clumsy to do with one hand; © Low power, wide field views were equally aesthetically pleasing in all of these scopes.

2. A leisurely and careful comparison of the achromat to a well figured and collimated maksutov of the same aperture revealed (a) that the one never revealed details not visible in the other. The objects compared included Saturn, a half dozen well picked subtle features on the moon, and the double star porrima. Only in the case of Porrima was the view of the double better in the mak than it was in the achromat, but that is totally attributable to the characteristics of an obstructed to non-obstructed system. (b) The color tones and saturation seemed identical at low and high magnifications. © Using the hackneyed formula of the unobstructed aperture equivalency of the diameter (D) of the front optical element (i.e. the corrector plate, sometimes called "the meniscus") in the maksutov system minus the diameter (d) of the secondary mirror spot, and since the maksutov is nearly completely free of chromatic aberration, in terms of resolution I was in effect comparing the 90mm achromat to a 72mm apochromat. There was no difference in the detail seen or in the contrast quality of the detail seen.

3. Subtle details, examined on the moon using the same achromat without a minus-violet filter and with a high quality minus-violet filter, were equally visible in the achromat both with and without the minus-violet filter.


Conclusions:
1. Achromats and Apochromats of equal aperture and equal focal length, with equally well made and well collimated optics perform identically or nearly so (i.e. the average amateur stargazer will not notice any diffrerence) in terms of details seen in bright extended objects at high magnifications.

2.Achromats of significantly longer focal length than apochromats of the same aperture, will show the same details on extended bright objects with the same level of contrast.

3. At low magnification and wide field views of terrestrial scenes (e.g. a flower bed) or of a stra field will be identically aesthetically pleasing in both having the same aperture and focal length.

4. Possibly, a well made ED and a well made achromat of the same aperture and focal length will perform equally well in terms of observing fine detail on extended objects. However, I cannot corroborate this as I never had a comparative test.

5. However, I feel confident in saying that a well made achromat of 90mm and the same focal length as a well made ED of 72mm will show the same amount and quality of detail.

6. In terms of normal observation, e.g. enjoying the sheen on the features of a dove, taking in a flower bed or mountain range panorama, looking at an extended star field, one will not notice violet in the lower wide field views through an achromat. At higher powers (30X to 60X per inch of aperture, one will see violet on the edges or around some objects in some circumstances.

7. If the above observations, based on expert opinion (Palermiti and others) and my experiences, are correct, the reasons might include (a) that the color violet does not significantly contribute to the overall brightness, color tone and saturation of what is viewed or (b) that as eyes age, becoming less sensitive to the effects of violet. Older eyes viewing an extended terrestrial or celestial scene will see the object as violet-deficient in the apo (not be aware of the influence the violet is having in the overall integrated color view) and will not be aware of the absence (distortedness) effect of the violet in the achromat.
 

#17 Jawaid I. Abbasi

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Posted 11 June 2011 - 02:26 PM

A difference between a premium and Archo. is the spectrum of light do not at the point in archo. Hence the image either in visually or in photography shows false colour and soft image.

The image breakdown in archro. is much faster because of mentioned above.

As an example: I had a Vixen ED scope and an archo with same aperture. I used to go more higher in magnification in ED lens then my archo. On Jupiter at 180x; the image still look sharp and minimal colour and the image do not show shofter in ED lens then using the same magnification using my Archo.
 

#18 Mark Costello

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Posted 11 June 2011 - 02:49 PM

Hi,

In browsing the refractor forum, I've often noticed comments which imply that although there is a large difference in the quality of photo images you can produce when you use a premium refractor (like a Takahashi TSA-120, TOA-130, or TEC 140) vs. a much cheaper mass market one (like an Orion or Celestron of similar 120-140mm aperture), mainly because of the relative lack of CA with the premium scopes... if you ignore photography and stick to visual viewing only, simply looking through the eyepiece or binoviewers, the view through a premium refractor and a mass market one, at the same aperture, is very similar. The point seems to be that difference for visual is much narrower for photography, so if someone is buying a refractor for visual only, he or she should strongly consider buying an Orion or similar rather than a TEC/TAK, because the difference for visual is so exceedingly tiny, so barely noticeable to anyone but the most dedicated and seasoned observer, that it just isn't worth the extra $3000.

So, I'm wondering if someone who has experience looking through both a premium refractor and a mass market one of the same or very close aperture, under similar conditions, could comment on how large the difference really is or isn't. Contrast... CA... how much detail you can see on planets... sharpness... etc... is what you see through a premium vs. mass market refractor pretty different (is there clearly more of a WOW factor looking through a premium refractor?), or is it extremely similar?

Thanks for any replies. :grin:



Hi Mark

In the end, this will come down to your personal decision. Questions include:

1) How much chromatic aberration (CA) has to be there to annoy you. Do you plan to use the scope mainly to observe the planets (my experience is that CA is most noticeable here), the moon (my experience is that CA is not discernible except on the rim) and/or deep sky objects (DSOs - I'm not hampered by CA except in looking at bright stars and in particular observing uneven double stars)?

2) How into astronomy are you? Do you have other avocations or passions? This translates into: Do I spend my discretionary money on this hobby or share it with other pursuits? As for me, I am more of a casual observer and I have other avocations. The $5,000 that I could spend on a telescope also might go into financing a dream vacation out west or in particular taking Laura and kids out to the Los Angeles area to visit her youngest brother (this could be turned into a dream vacation :)).

I own and observe with a fast (F6.5) achromatic refractor (achro). I use it on everything. The CA to me is noticeable in observing planets but in spite of this I can pick out details on both Jupiter and Saturn (haven't tested it on Mars and don't do Venus). I plan on prolonged observations of the lunar surface with it tonight at 206X and from past experience I'll tell you that CA is not going to be an inhibition there. As mentioned, it is not a problem to me on DSOs except maybe it limits my ability to observe tight uneven double stars.

Now all of the above is based greatly on my personal tolerance for CA. Things may be different with you and the best way to tell is to point an achro you'd consider getting at a planet (Saturn's up) and see how badly CA is to you. A lot of good people here can't stand CA and so they buy apos. Here I'll throw out one final thing. You mention a difference of $3,000. Lets say you have a budget that goes up to $5,000. That could buy you a nice 4.5" or 5" apochromat (apo) refractor or either a "not-top-o'-the-line" 4-5" apo or 5" achro and a big reflector (10" newtonian or Schmidt Cassegrain - SCT) and maybe leave you with money in your pocket.

All success in your choice.... :)
 

#19 Jon Isaacs

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Posted 11 June 2011 - 03:16 PM

A few thoughts, experiences.

Otto:

When you quoted the good Doctor in the refractor forum it was pointed out that he was only discussing long focal length, slow focal ratio achromats. Is this not the case? I did not see a link this time. Quotes need links.

You comment about the low power views on a bright day being clean and crisp was explained by me, I will do again. Essentially a fast achromat on a bright day acts as a small long focal length scope at low powers.

During the day, you pupil is contracted to 2 mm or less. An 100 mm f/5 telescope at 20x has a 5 mm exit pupil. You pupil only let's in 2mm so you now have a 40 mm f/12.5 achromat, this free from false color as long as the view is centered. If you move your eye around, so you are not looking the the center of the objective false color is visible.

False color is out of focus light, whether you see it as purple, it still smears the fine detail and reduces the contrast. As others have pointed out, larger apertures, faster focal ratios result in more color fringing.

Regarding the premium manufacturers, my understanding is the same as John Crilly's. A-P and Tec are the two that do everything in-house. TVs are designed by TV, the optics are manufactured to their proprietary and secrets design in Japan. The mechanical components are designed by TV and manufactured in the US. The Scopes are assembled and tested buy TV in the US.

Tak is very similar but do not have the singular figure of Al Nagler who looks through each TV , a bit more corporate.

Others have a variety of business models but generally use packages designed and manufactured elsewhere and available to any vendor willing to pay the price.

All that said... I have a Televue NP 101. It is one of the finest 4 inch refractors one can buy. It's 4 element Petzval design provides essentially perfect views from 15x to 300x. No field curvature, no false color.. New, out the door it costs close to $4000. It does the best about the best job possible for a 4 inch scope.

However, 4 inches is 4 inches. Setup alongside my 10 inch F/5 GSO/zhumell Dob, the Dob shows more planetary detail, splits tighter double stars, shows fainter galaxies, resolves globular clusters to a greater extent. The difference is not subtle, it is major.

So buying a premium apo should wait until one is experienced and knows exactly what they want. It's one of those, "If you need to ask, now is not the time" things. One should be prepared to setup their $4000 rig next to $500 Dob and know that the Dob will "blow away" the 4 inch.

Jon Isaacs
 

#20 Jawaid I. Abbasi

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Posted 11 June 2011 - 03:45 PM

Wow,
Jon very well explained and Cover do's and don't.
 

#21 mark8888

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Posted 11 June 2011 - 04:03 PM

Thank you everyone for the great replies. A lot to chew on at 5:32 local time (oy!), but extremely interesting and helpful, everything...


However, 4 inches is 4 inches. Setup alongside my 10 inch F/5 GSO/zhumell Dob, the Dob shows more planetary detail, splits tighter double stars, shows fainter galaxies, resolves globular clusters to a greater extent. The difference is not subtle, it is major.

So buying a premium apo should wait until one is experienced and knows exactly what they want. It's one of those, "If you need to ask, now is not the time" things. One should be prepared to setup their $4000 rig next to $500 Dob and know that the Dob will "blow away" the 4 inch.




I know it'll do the rest, but... will that $500 dob also blow away the 5.5 inch TEC 140... specifically on planetary detail, and the "crispness" you can see in the planetary detail?



Between a TAK120 and an EON 120, I'd choose the EON - it's that good.


Wow!

Basically, my novice summary of the posts so far in this thread is:

Yeah, there's a pretty sizable difference between achromats and apochromats, in particular when looking at the moon and planets, bright objects.

Comparing "premium" APOs (which can essentially be defined as AP, TEC, Takahashi, and Tele Vue, peeeerhaps even in that order, based on all the threads I've read...) to very well made newer mass market APOs like some of the Orions, is a bit of a game of chance.

Premium: excellent fit and finish, excellent service, virtually 100% chance of getting an excellent optic.

Higher end mass market: good fit and finish but not excellent. Maybe a... 55% ? chance of getting an excellent optic which most people would actually not be able to distinguish from the excellent optic in a premium scope, a 40% ? chance of getting a good optic but you might actually see a bit of CA, blurring of details on Jupiter and Saturn, some false color on the moon which might disturb you if you're sensitive to it ... and a 5% chance that you'll get a below average/poor optic.

I know those %'s are really rough, but... does the above description, and even the percentages, sound in the ballpark?
 

#22 hfjacinto

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Posted 11 June 2011 - 04:51 PM

I have an EON 120MM and an 80MM. I also have access to a TEC 140 and Tak 130 and purely from VISUAL standpoint the EON is optically as good as these 2 scopes.

For imaging its no slouch either.

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#23 hfjacinto

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Posted 11 June 2011 - 04:54 PM

Even bright globulars looks good.The build quality in the TEC and TAK are superior, but the optics, I haven't seen it.

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#24 hfjacinto

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Posted 11 June 2011 - 04:55 PM

And they look as cool as a TEC or a TAK (actually better as I like black scopes)

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#25 Otto Piechowski

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Posted 11 June 2011 - 05:43 PM

Thank you for sharing your analyses and comments with us on this topic, Jon. I can tell you have some expertise; experiential and perhaps cognitional on this topic.

Part 1:
In my response to these musings of yours on the other forum, I had asked if the central part of the field of view, even in daylight, is the full beneficiary of the entire aperture of the scope. I don't know how to ask the question clearly. Permit me the slow way of asking a question by using examples.

Let's say we have a 100mm F5 refractor. Let us suppose we are using a 25mm eyepiece. The magnification being 20X, the exit pupil will be 5mm. Now, let's assume it is a dark transparent moonless night, and we are dealing with someone who eyes can still open to 5mm. As I understand it, this means that the star visible at the center of the field of view (FOV) is receiving photons gathered from the full 100mm of aperture. Similarly, stars visible near the edges are also receiving photons gathered from the full 100mm of aperture.

Now, let us suppose the full moon is out sometime later and his/her pupil is reduced to four millimeters because of the moon's glare. He looks through his 100mm F5 scope with a 25mm eyepiece. The exit pupil is still 5mm but his own pupil is 4mm. 4 divided by 5 is 80%. Here is where the math gets a bit finicky to talk about. Let's now look at the star visible at the center of the field of view. Is that star collecting light from only 80 of the 100mm, or from only 80% of the area of the objective (which corresponds to 89.4mm of the 100mm). Or, a third option, is the star at the center collecting light from the entire 100mm of aperture? Now, let's ask the same set of questions from a star visible near the edge; is its light being collected from 80 of the 100mm of aperture, or from 89.4mm of aperture?


Part 2:
I am pretty sure most people who, like Dr. Palermiti, speak of achro/apo equivalency, are referring, as you say, to apo and achros of similar aperture but of different focal rations. To me it is not absolutely clear that Dr. Palermiti is also making the same assertion because later in the article he offers a qualification; i.e. "The first choice is not often a practical one, since a F/20 or F/30 refractor presents a large system with other considerations beyond which most amateurs want to deal with. Using smaller aperture systems is not a preference, either". Having said that, if I were betting on it, I would assume though that Dr. Palermiti means different focal ratios being assumed.

What I have added to his opinion is my own experiential evidence that the performance of two well figured and well collimated refractors; one apo and one achro, of similar aperture and focal length perform nearly the same to my older eyes at anywhere from 20X to 200X in terms of detail seen. Similar performance. But, without qualification or equivocation, I state that a 90mm F5.5 achro shows as much detail on extended bright objects as a 72mm F6 ED refractor.


Part 3:
One of the reasons I got rid of the world class apochromat is that it was too heavy for one hand portability. The ST90 is not too heavy. Another reason I did not like the world class apo was that its triplet design required too long to thermally equalize. The ST90 seems to equalize much sooner. Of course, a similar aperture similar focal length ED scope or Flourite (apo) scope might be equivalent in time needed for thermal equalization. I would guess this is probably the case. The weight difference could be removed by using some lighter OTA material; say carbon fiber, or just ST90 type thin aluminum. Then, of course, there is the matter of cost; the ST90 costs, when one can find it, about 1/10th of the world class apo triplet and about 1/5th of the well made ED. And since in my eyes, the performance between the $200 scope, the $1000 scope and the $2000 scope are identical in all ways at lower magnifications and identical in detail seen at higher magnification, this adds a factor in the achro's favor. For me, this reasoning applies, a fortiori if one is thinking of taking the scope on a trip where damage or theft are more likely.


Would enjoy hearing your feedback...especially to part 2, Jon.

Otto

P.S. What I would also enjoy is if Dr. Palermiti, Chris Lord, Mardina Clark, and/or Christen Roland would share their thoughts about the comparative performance of similar aperture and focal length apos, EDs, and achros.
 






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