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Zeiss AS80/1200 versus SV80S

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#1 Sasa

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Posted 11 August 2011 - 04:13 AM

Last night I put those two telescopes side by side on the same mount. I was curious if I could see any difference between those two excellent 80mm refractors. SV80S is 80/480mm (f/6) apochromat with well regarded LOMO triplet. I use this scope mostly for astroimaging. Zeiss AS80/1200 (f/15) is semi-apochromatic Steinhel doublet with reduced chromatic error with respect to classical achromat (due to KzF2+BK7 glass combination). Both telescopes star tests very well and with green filter I'm not able to trace any significant deviation from textbook paterns (it does not mean that they aren't there, I'm just learning about star testing).

Main object of study was Moon. It was quite low and seeing was strongly affecting the views already at about 100x magnification. I used Pentax XO5.1 (94x) for SV80S and CZJ ortho O-12.5 (96x) for AS80/1200. When I started, the difference between the telescopes was huge. SV80S came nowhere near the performance of Zeiss. Although the image in Zeiss was boiling in this magnification, minute details were still clear, the image was very sharp, very tonal and 3D-like. SV80S was a little bit brighter and may be it had a little more color neutral image but details were washed out. At this point the scopes were outside only for about 15 minutes. I store them at home and the temperature drop was about 10C. I repeated the observation after one hour and SV80S performance greatly improved, the image was more closer to Zeiss but still Zeiss was obviously better. Not that I could find some detail which I could not observe in SV80S, but they were popping out to the eye in Zeiss and once I know about them I was able to find them also in SV80S. In general, I had a feeling that I was observing with SV80S at lower magnification. The visibility of details was similar when I switched with Zeiss to CZJ O-16 (56x) eyepiece.

Similar performance could be seen on delta Cygni. AS80/1200 was showing the two components clearly for 100% of time at 171x (TMB Mono 7mm). Where as in SV80S at 186x (Pentax XO2.5) the second component looked in the begining only like a brighter part in the first diffraction ring. This was after the telescopes were 30 minutes outside. After another 30 minutes the image in SV80S greatly improved and the double star was an easy catch as well, although the image in Zeiss was still more steady.

I could see similar behavour on Izar. Here I noted slightly bigger color contrast between the two components visible in SV80S. Again Zeiss was showing the double a little bit more clearly and sharply - the arcs of the first diffraction ring could be seen all the time (Pickering 6/10) while I would judge the seeing in SV80S at least one step smaller 5/10 (arcs were seen onlu occasionally).

Well, in summary, all it just shows that even this small triplet cools down really slowly and it takes about one hour with 10C temperature difference before one can start to do really critical observations at high powers. But even after one hour, the long Zeiss was still better, both on Moon (this I would call pretty visible difference) and double stars (here I would call it a tiny difference). Either the triplet was not fully thermalized even after more than one hour or there is indeed some adventage in average seeing conditions for long doublets.

#2 TPMack

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Posted 11 August 2011 - 05:55 AM

Very interesting report. The Zeiss AS series were wonderful lenses.

Tom Mack

#3 hpw

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Posted 11 August 2011 - 06:06 AM

Hi Sasa,

great report! That`s -exactly- the same experience I had with my TAK FS60 and my Zeiss AS-63! I don`t have the FS any more ;)

Peter

#4 TPMack

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Posted 11 August 2011 - 06:19 AM

About 3 months ago on the spur of the moment, I purchased a little Zeiss 50mm/F11 scope. This lens does not use the short flint found in the AS series and is a Fraunhofer design. I did not think I would use it much but then saw how it performed. It is probably the finest sub 60mm scope I have ever used. Very crisp views.

Tom Mack

#5 astroneil

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Posted 11 August 2011 - 06:35 AM

Either the triplet was not fully thermalized even after more than one hour or there is indeed some adventage in average seeing conditions for long doublets.


Probably a bit of both Sasa. I have noted the same thing but with different long and short scopes.

Thanks for posting.

Regards,

Neil. ;)

#6 hpw

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Posted 11 August 2011 - 06:48 AM

The Zeiss E50/540 (C50/540) are perhaps the most underestimated Zeiss-lenses. As you said, the contrast is amazing. And there`s not a lot of difference (if any) between the airspaced E and the cemented C lens.

Peter

#7 vahe

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Posted 11 August 2011 - 07:20 AM

Most amateurs and pros are of the opinion that resolution and overall performance are strictly a function of the aperture and that F ratio simply does not matter, add some Barlows to a fast airline portable scope and bingo you are there.

In this case all else equal when it comes to the high power use an F15 is always going to outperform an F/6, I do not know why, seeing is believing, long focus instrument are simply better for high power use and being a Zeiss makes it just that much better.

Vahe

#8 Max Lattanzi

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Posted 11 August 2011 - 08:35 AM

Since I've had the pleasure of using for a number of years the whole Zeiss AS series (63 to 200), I can testify that their quality is on par with the ***very best*** of today's apochromatic production, leaving behind any cheaper alternative.

In some fields (double stars, sun, moon and planets come to mind), the Zeiss AS can actually be preferable, delivering an incredible contrast at the eyepiece. A possible explanation for this could be, beside the obvious high correction of all aberrations (and here the long f/ratio obviously helps), the notorious very long and accurate polishing of their optical surfaces.

Roland Christen himself brought up this point more than once to justify Zeiss performances (and price).

-- Max

#9 astroneil

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Posted 11 August 2011 - 08:58 AM


In this case all else equal when it comes to the high power use an F15 is always going to outperform an F/6, I do not know why, seeing is believing, long focus instrument are simply better for high power use and being a Zeiss makes it just that much better.

Vahe


In my case, I compared a 80mm f/11 achromat to a 76mm f/6.3 ED. Both are doublets. The delta T was nearer 20C.

Regards,

Neil. ;)

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#10 jrbarnett

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Posted 11 August 2011 - 10:06 AM

Alexander:

Your results don't surprise me at all. Though your findings are much more detailed than ours, a club mate and I experienced something very similar last year performing comparative observations of Delta Cygni with two 4-inch refractors.

http://www.cloudynig...5/o/all/fpart/1

The differences between the two scopes in our evaluation were not as great (i.e., both were air-spaced doublets, neither was super-fast), but the results on Delta Cygni were quite similar. The longer, slower refractor displayed the separation more cleanly and more constantly than did the faster, shorter refractor.

Anecdotally, the 4" f/14.4 is the most consistent 4-incher I've owned at pulling out all six Trapezium components visible in that aperture, and showing intra-belt details such as festooning and barges on Jupiter.

I'm leaning toward your second option: "...or there is indeed some adventage in average seeing conditions for long doublets." The proposition would certainly be worth more field testing, I think. If I could convince myself just a little more firmly, I would probably go off the deep end and look for a custom "super 4" (i.e., 110mm to 115mm), slow, conventional doublet at a very inconvenient f/20 or so. :lol:

Regards,

Jim

#11 Sasa

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Posted 11 August 2011 - 12:40 PM

Neil and Jim, yes I know your posts about long achromats. In fact, they were part of my decision to buy AS80/1200 when I came across one couple of months ago (the other part was just sentimental, Zeiss scopes were one the first that I looked through as a kid - there were not too many commercially available scopes here in our country during communism except from Carl Zeiss Jenna). I had to try by myself.

BTW, I also made couple of weeks ago a direct comparison with my ED100. This was more boring in a sense that bigger diameter refractor clearly outperformed AS80/1200 in all aspects (only in the beginning of the session, when the ED was not fully thermalized the two scopes were performing about the same). ED100 was showing more details on Moon (for example more craterlets in Plato) and it was splitting tighter doubles, and details on Saturn were better visible in ED100. It is also lighter, shorter, and gathering power for DSO is visibly larger. So even Zeiss can't break physics laws...

#12 jrbarnett

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Posted 11 August 2011 - 12:52 PM

"So even Zeiss can't break physics laws..."

Shhh! Don't let the AS80/1200 hear you say that!

:lol:

- Jim

#13 ukcanuck

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Posted 11 August 2011 - 02:18 PM

I'd really like to get my hands on a Zeiss AS lens. In my time spent with a Telementor, it really made me think again what a small lens can do. However, all the AS lenses I've seen for sale have been just too costly. :rainbow:

#14 hpw

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Posted 11 August 2011 - 03:18 PM

A possible explanation for this could be, beside the obvious high correction of all aberrations (and here the long f/ratio obviously helps), the notorious very long and accurate polishing of their optical surfaces.


Not only that. Mr.Busch (who replaced the spacers of my As lense because they got too thin over the decades) told me that Zeiss took advantage of the tight connection between Schott glassworks and Zeiss Jena. They used the best mating melts and the purest glasses available from Schott for their objectives -especially in the early days of Zeiss Jena.

BTW: Mr. Wolfgang Busch designed an built an oilspaced triplet objective (HAB) in the 70ies. Long before Zeiss did with their APQs. Today he restores the famous Zeiss B-objectives up to 200mm - a really fascinating man. And he is over 80! I was really lucky he took care of my little AS63 :)

Peter

#15 7331Peg

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Posted 11 August 2011 - 11:01 PM

About 3 months ago on the spur of the moment, I purchased a little Zeiss 50mm/F11 scope. This lens does not use the short flint found in the AS series and is a Fraunhofer design. I did not think I would use it much but then saw how it performed. It is probably the finest sub 60mm scope I have ever used. Very crisp views.

Tom Mack


I think there were three of those Zeiss 50mm scopes for sale, Tom, and I bought one of the other two. I couldn't agree more on the crisp views. I have a Zeiss 63/840 Telementor, and even though there's a bit of clouding on the lens, it has the same crisp views.

They aren't going to defy the laws of physics, but the views are certainly brighter and - there's no other word for it - crisper. There's a certain "snap" to them that just has to be seen in order to know what I mean.

A real pleasure to look through.


John :refractor:

#16 Sasa

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Posted 12 August 2011 - 08:04 AM

About 3 months ago on the spur of the moment, I purchased a little Zeiss 50mm/F11 scope. This lens does not use the short flint found in the AS series and is a Fraunhofer design. I did not think I would use it much but then saw how it performed. It is probably the finest sub 60mm scope I have ever used. Very crisp views.

Tom Mack


I think there were three of those Zeiss 50mm scopes for sale, Tom, and I bought one of the other two. I couldn't agree more on the crisp views. I have a Zeiss 63/840 Telementor, and even though there's a bit of clouding on the lens, it has the same crisp views.


Interesting, my AS80/1200 has also little clouding when I look at the lens at certain angle. If I look straight through it, it looks perfect (except one or two small bubbles). Do you know from where this fog is coming from? Should I be concerned?

#17 Tom and Beth

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Posted 12 August 2011 - 01:25 PM

Most amateurs and pros are of the opinion that resolution and overall performance are strictly a function of the aperture and that F ratio simply does not matter, add some Barlows to a fast airline portable scope and bingo you are there.

In this case all else equal when it comes to the high power use an F15 is always going to outperform an F/6, I do not know why, seeing is believing, long focus instrument are simply better for high power use and being a Zeiss makes it just that much better.

Vahe


Could it be due to the exit pupil at comparative magnification? Heck, Vahe, isn't that why you have the F20 Tec ;)

#18 Astrojensen

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Posted 12 August 2011 - 01:30 PM

Could it be due to the exit pupil at comparative magnification?


Ehrmmm, Tom, exit pupil at identical magnifications are always the same when apertures are also identical, regardless of f/ratio. 80x on an 80mm always give a 1mm exit pupil, whether it's an f/6 or an f/15.

:o :grin:


Clear skies!
Thomas, Denmark

#19 jrbarnett

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Posted 12 August 2011 - 01:53 PM

That's true, Thomas.

One of the things we wondered was whether slight variations in aperture made it more logical to either (a) match magnification and eyepiece design as much as possible or (b) match magnification per inch and eyepiece design as much as possible.

For example consider a comparison of a 105mm (4.13") and 100mm scope (3.94"). Which is more useful; 150x in each scope or 165x in the 105mm versus 158x in the 100mm scope?

Regards,

Jim

#20 Astrojensen

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Posted 12 August 2011 - 01:58 PM

Most amateurs and pros are of the opinion that resolution and overall performance are strictly a function of the aperture and that F ratio simply does not matter, add some Barlows to a fast airline portable scope and bingo you are there.

In this case all else equal when it comes to the high power use an F15 is always going to outperform an F/6, I do not know why, seeing is believing, long focus instrument are simply better for high power use and being a Zeiss makes it just that much better.




I have found the exact same thing when comparing short and long focus refractors of identical aperture but different f/ratios. I believe the explanation is a combination of several factors that add up to a final, and better, viewing experience.

1. Long focus refractors have their entrance pupil (the objective) higher above the ground and further away from the observer.

2. Long focus instruments can use longer focal length eyepieces for the same magnification, giving a more relaxed viewing experience, especially for planetary fanatics, who prefer single orthos and the like. Longer focal length eyepieces are also easier to make well, adding to the "quality stack".

3. The depth of focus is a *lot* bigger on a long-focus instrument, meaning accurate focus is easier to obtain. It has been said that just using a 1:10 microfocuser on a short scope is enough to even out this difference between them, but that is not my experience, because there's another factor in the play as well:

4. When we focus our scopes, have you noticed that it can sometimes be hard to keep your eye focused on infinity, as you rack the focuser towards the sharpest image? Our eyes want to "move ahead" and refocus themselves to give the sharpest possible image. This is likely prone to be a bigger problem for younger observers with eyes that can accomodate a very wide focus range. The problem is that on a very short f/ratio scope the focus depth is so shallow that as soon as the focus of your eyes drift off a little bit, the image is grossly defocused and the seeing will appear to be very bad. You wait and wait, but it never improves. The image seems sharp behind the waves, so you wait. This often happens to me. I then need to refocus just a bit and the image magically sharpens. In a long focus scope, the depth of focus is much bigger and the eyes can defocus a little without the image loosing sharpness. [NB! I am not sure this explanation is 100% correct, but it sounds logical to me and explains something I experience at the eyepiece]

5. Long focus scopes are easier to consistently make well and keep in collimation. If a long focus scope is made by a high-end maker, this often means incredibly high quality optics with fantastic performance.

6. Everything behind a long-focus objective just works better! Everything optical behind it doesn't need to work as hard as if behind a short focal length objective.

There are probably more reasons I have forgot (I specifically avoided mentioning color correction). I have witnessed a 108-year old 85mm f/19 uncoated Zeiss apochromat stopped to 75mm aperture completely destroy a 76mm f/6.3 TeleVue apochromat on the planets, so I know the benefits of the long f/ratio are not imaginary. It was like night and day, the difference was easily visible.


Clear skies!
Thomas, Denmark

#21 Astrojensen

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Posted 12 August 2011 - 02:20 PM

One of the things we wondered was whether slight variations in aperture made it more logical to either (a) match magnification and eyepiece design as much as possible or (b) match magnification per inch and eyepiece design as much as possible.

For example consider a comparison of a 105mm (4.13") and 100mm scope (3.94"). Which is more useful; 150x in each scope or 165x in the 105mm versus 158x in the 100mm scope?



Interesting thoughts, Jim. My solution when comparing the 85mm Zeiss and the 76mm TeleVue was to stop the Zeiss down to 75mm. You could have done the same, by stopping the 105mm to a 100mm.

BTW, was it an Antares 105/1500? If so, I've found these to be 102mm free aperture, not 105mm!

I tried matching eyepiece designs as much as possible, but I had to use a barlow to get the TV even near the Zeiss in magnification and often I was unsure of the exact power, due to the use of adapters and extension tubes, so I aimed to get as comparable images as possible and just observe and compare a lot, to get a feeling for the performance level in each instrument, always seeking maximum detail. I did aim to get magnifications as identical as possible, of course, but some uncertainty always crept in.

But on no occasion could the little TeleVue hold a candle to the planetary performance of the Zeiss. It was like the objective of the TeleVue was smeared with grease by comparison... I even tried to handicap the Zeiss as much as possible, by using a uncoated diagonal prism and 0.965" orthos and no barlows, and coated 1.25" orthos and a high-end INTES barlow on the TeleVue, but in hindsight, I might have achieved the exact opposite, since what I did on the Zeiss was minimizing the number of optical elements (including coatings, which should be considered optical elements in some cases!).

But even if the result is perhaps not representative of what is *truly* possible with a short-focus objective, it was indeed a very representative test, as it took place under real-world conditions and with real-world limitations and as such was truly representative of the performance level of each objective and its strength and weaknesses, which can be summed up as such:

TeleVue: Good performance and extreme portability. Hard to focus accurately.

Zeiss: Mind-blowing performance. Very easy to focus. NOT so portable!


Clear skies!
Thomas, Denmark

#22 jrbarnett

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Posted 12 August 2011 - 02:51 PM

Thomas:

Perhaps our next order of business is to compare a 4" scope stopped down to 80mm to an 80mm achromat with the same native f/11 focal ratio. If I stop the 102mm f/8.6 Televue down to 80mm, it will have a focal ratio of f/11. Will the "80mm" TV-102 and an 80mm f/11 (say Vixen) achromat perform comparably on the image stability front, or will the native 80mm f/11 achromat still have some advantage in this regard?

On one hand, perhaps 80mm f/11 is 80mm f/11. On the other, due to the differences in materials and (presumably) radii of curvature between the ED doublet and the C&F doublet, could there be "more to the story"?

- Jim

#23 Astrojensen

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Posted 12 August 2011 - 02:54 PM

Jim, that would be a *most* interesting experiment! Please do it!

I have some thoughts about it that I want to share, but it's getting late and I must go to bed now. Will be back tomorrow.


Clear skies!
Thomas, Denmark

#24 vahe

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Posted 12 August 2011 - 03:07 PM

Couple of comments on your comments:

The position of the objective, being higher above the ground in long focus instruments does not matter all that much. I have two F/20 Maks, these fat cats always sit much lower than my F/9 refractor, I see no adverse effect due to them being closer to the ground.

Long focus instrument produce larger image scale at the instrument’s focal plane, this requires less magnification from an eyepiece, unlike faster scopes where more magnification is required from the eyepiece to achieve the same power.
Anytime you are able to move the job of the image magnification from the eyepiece to the objective you are way ahead in quality.

Long focus instruments produce longer cone of light resulting in better control of various image aberrations, an F/6 apo, no matter who makes it, will suffer from the 5th order aberration, it is the nature of the beast, other aberrations are normally controlled, but are right at their limit. Collectively these things add up and you can see the result at the eyepiece.

Bottom line, if you prefer short and fast system you should recognize their tradeoffs, as they say, there is no free lunch.

Vahe

#25 astroneil

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Posted 12 August 2011 - 03:19 PM

Thomas:

Perhaps our next order of business is to compare a 4" scope stopped down to 80mm to an 80mm achromat with the same native f/11 focal ratio. If I stop the 102mm f/8.6 Televue down to 80mm, it will have a focal ratio of f/11. Will the "80mm" TV-102 and an 80mm f/11 (say Vixen) achromat perform comparably on the image stability front, or will the native 80mm f/11 achromat still have some advantage in this regard?

On one hand, perhaps 80mm f/11 is 80mm f/11. On the other, due to the differences in materials and (presumably) radii of curvature between the ED doublet and the C&F doublet, could there be "more to the story"?

- Jim


An interesting proposition Jim. I've often thought about that very question:

Do achromats give more stable images because they are achromats?:thinking:

Let's think basics here. In physics, when a system loses energy it becomes more stable. Think of an ion - a charged atom - combining with an electron say. When they recombine, they release a photon and become more stable.

Is there an optical equivalent?

An achromat has less energy in its image because not all the light is focused within the Airy disk. I've often wondered whether the lowest contrast features i.e the features best seen by an Apochromat are the most affected by seeing. Because the achromat shows these features less well, it APPEARS more stable at the eyepiece.

Another analogy: suppose you have a room full of children at a party. Some are well behaved and some are unruly. Imagine that you want to concentrate your gaze on a single child and/or feature in the room. If you subdue the most unruly children darting about the room, you can better study the subject of interest i.e it doesn't distact you so much.

Think of it as 'signal' and 'noise'.

Of course, Vladmir Sacek has already uncovered a novel aspect of long focus achromats ie. they transfer less energy to the diffraction rings than a short focus Apo. Less prominent rings means that they are not pushed around by the seeing, creating the impression that the image is more stable.

Details in my article here:

http://www.cloudynig...5/o/all/fpart/1

Right, that's me off my soap box. :lol:

Regards,

Neil.


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