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Zambuto/Royce vs Synta/GSO

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#26 Starman81

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Posted 31 July 2018 - 11:04 AM

And of course, if you're using night-vision gear the perfection of the mirror becomes irrelevant, only high powers will reveal any imperfections. You might as well get any old light bucket. (And this isnt a criticism of NV!)

 

EDIT: Is this nonsense? Is NV done at high magnifications sometimes?

 

Generally speaking, this is correct. For most objects, you would want to stay at a faster focal ratio (i.e. lower magnification) to preserve image brightness but it does depend where you start at, with regards to focal ratio, and balancing the need for image scale vs your personal tolerance for scintillation. 

 

You may want to use higher powers on object like planetary nebulae, but even then the powers you would use would be lower than normal visual use, something that any diffraction-limited or nearly so mirror would be able to do. 


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#27 Jon Isaacs

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Posted 31 July 2018 - 11:14 AM

>Because of diffraction, resolution in inversely proportional to aperture because the diameter of the airy disk is inversely proportional to the aperture.

 

... pardon me, but that explanation isn't complete without also explaining that

the actual size of the star image will be enlarged by atmospheric effects

so that the wavefront error scales with (aperture/Fried parameter)^{5/6}.

 

   As many textbooks state, the Fried parameter is not so very small,

3/4 inch for 2 arc seconds seeing, so there is a penalty factor 1.7 for a

12 inch telescope with respect to a 6 inch telescope if I'm not making a mistake

in my math. The effective, time-averaged Strehl value for the 12 inch then deteriorates

to 0.25, getting progressively worse for longer exposure times of the detector. This penalty

will steadily increase as the seeing got progressively worse. At a certain point, a

perfect 6 inch telescope which is more rapidly acclimatized can in certain, possibly

rare cases outperform a poorly figured 12 inch telescope, particularly if the observer

is in a hurry. Not to say the Synta or GSO telescopes are bad, their quality can be quite good.

 

   I would personally keep the 6 inch. One would want the best possible optics

at a given aperture. There are so few examples nowadays where an artisan

can produce an industrial product that can outperform a computerized mass-produced

one. I figure there is intrinsic value in that fact.

 

If you are going to quote me, quote everything I wrote that is relevant:  I did say this:

 

"Getting the performance possible from a larger aperture is more difficult because of thermal issues and as aperture increases, seeing becomes more and more of an issue. "

 

The question I was asked was to explain to about why the wave front error is scaled relative to the aperture, why larger scopes have greater resolution.  That's pure optics, there are times when the seeing will support the larger optic.  

 

If the best you ever see is 2" and all you observe is the planets and double stars, first, you have my sympathy.. And in that case, the 6 inch might be a good fit but as I recall, Vlad's simulation said a larger scope was still advantageous. The actual diameter of the first minimum of the Airy disk of a 12 inch scope is twice the Rayleigh Criterion, that's about 2.2 arc-seconds.  And as experienced planetary observers like Alan French will tell you, even in poor seeing, there are moments of stable seeing where the large aperture can be used advantageously.  

 

In my world, 2" is rather poor seeing for viewing the planets, being south of the jet streams close to the Pacific ocean in one of the worlds mildest climates has it's advantages.  The greater resolution and contrast possible with a 10 inch, 12 inch, 16 inch scope can be used to a good advantage.  

 

Last night I spent some time on the star Jabbah, with my 10 inch. It's a pair of doubles, one is 1.3", the other 2.2" , both were wide clean splits, the seeing was well under 1". The views of Jupiter and Saturn were quite nice in my 10 inch.. Viewing the planets, I generally stop at 410x, the 0.6mm exit pupil is quite dim. That's where I stopped last night. 

 

I did start the scope cooling with the fan running about an hour and a half before sunset.  The scope just has generic Taiwanese optics, decent optics, I've split doubles slightly under 0.5", that won't be happening in a 6 inch.  

 

Jon Isaacs. 


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#28 X3782

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Posted 31 July 2018 - 01:43 PM

If you are going to quote me, quote everything I wrote that is relevant:  I did say this:

 

"Getting the performance possible from a larger aperture is more difficult because of thermal issues and as aperture increases, seeing becomes more and more of an issue. "

 

The question I was asked was to explain to about why the wave front error is scaled relative to the aperture, why larger scopes have greater resolution.  That's pure optics, there are times when the seeing will support the larger optic.  

 

If the best you ever see is 2" and all you observe is the planets and double stars, first, you have my sympathy.. And in that case, the 6 inch might be a good fit but as I recall, Vlad's simulation said a larger scope was still advantageous. The actual diameter of the first minimum of the Airy disk of a 12 inch scope is twice the Rayleigh Criterion, that's about 2.2 arc-seconds.  And as experienced planetary observers like Alan French will tell you, even in poor seeing, there are moments of stable seeing where the large aperture can be used advantageously.  

 

In my world, 2" is rather poor seeing for viewing the planets, being south of the jet streams close to the Pacific ocean in one of the worlds mildest climates has it's advantages.  The greater resolution and contrast possible with a 10 inch, 12 inch, 16 inch scope can be used to a good advantage.  

 

Last night I spent some time on the star Jabbah, with my 10 inch. It's a pair of doubles, one is 1.3", the other 2.2" , both were wide clean splits, the seeing was well under 1". The views of Jupiter and Saturn were quite nice in my 10 inch.. Viewing the planets, I generally stop at 410x, the 0.6mm exit pupil is quite dim. That's where I stopped last night. 

 

I did start the scope cooling with the fan running about an hour and a half before sunset.  The scope just has generic Taiwanese optics, decent optics, I've split doubles slightly under 0.5", that won't be happening in a 6 inch.  

 

Jon Isaacs. 

 

You live in San Diego.smile.gif    My hometown was a place where you could barely see magnitude 2 stars (couldn't easily do polar alignment because Polaris can't be seen without an enhanced CCD camera and compass), getting on the train (because a lot of people don't own cars. It can cost 200-300 USD per month to rent a parking space if your home doesn't have space) for 1 hour in any direction didn't improve things much, maybe magnitude 4 could start to be seen. Most people bought refractors up to 4 inches. I admit....... I never even tried to see DSO's from my home. If you think about carrying the telescope on public transport for 1 hour, walking somewhere, setting it up, and coming back before the last train with perhaps some drunken people........ 6 inches mounted on a top-end alt-az head + photographic carbon fiber tripod like Gitzo 5533 maybe the limit. Not a few people might buy a premium portable or collapseable 6 inch American-made telescope with a good brand name attached to it.

 

The place I live now is vastly improved, cause I can see magnitude 4 on a regular basis, but still with my current lifestyle, typical seeing that I am used to is 2". So 6 inches is actually quite fine. Though I have big dobs too (it was always an ambition of mine).....  These 10-12 inch dobs are only practical in places where either most people own cars, or the light pollution is low enough to observe from one's home. You might be surprised just how many millions of otherwise wealthy people in the world live under conditions that exclude the use of 10-12 inchers. I can't say how big the 6-7 inch compact premium market might be...... but it might not be so surprising if 100 mirrors could be sold at a cost of 1000 USD each. But it has to be a recognized brand name.


Edited by X3782, 31 July 2018 - 03:16 PM.

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#29 dgoldb

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Posted 31 July 2018 - 02:33 PM

The correct answer should be whenever the market value of the mass produced exceeds the zambuto.  Because you could always sell and buy the other. 

 

But assuming you don't want that pedantic answer, I'd say probably a 12".  A 6", even if perfect, isn't going to outperform even a mediocre 12" GSO.  Plus, IMO, a 6" reflector is too close to refractor / binocular territory.  I'd rather have a 4" refractor grab-n-go plus a 12" dob.  More versatile.   


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#30 Jon Isaacs

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Posted 31 July 2018 - 05:38 PM

The place I live now is vastly improved, cause I can see magnitude 4 on a regular basis, but still with my current lifestyle, typical seeing that I am used to is 2". So 6 inches is actually quite fine. Though I have big dobs too (it was always an ambition of mine).....  These 10-12 inch dobs are only practical in places where either most people own cars, or the light pollution is low enough to observe from one's home. You might be surprised just how many millions of otherwise wealthy people in the world live under conditions that exclude the use of 10-12 inchers. I can't say how big the 6-7 inch compact premium market might be...... but it might not be so surprising if 100 mirrors could be sold at a cost of 1000 USD each. But it has to be a recognized brand name.

 

 

Please be assured that I am well aware that there are many reasons why a 6 inch scope or even smaller might be the right scope.  I really enjoyed Barbie's post because she just said, premium optics or not, a 6 inch F/8 Newtonian is the right scope for me. And I do my best to assist excited amateur astronomers who do not have the resources to acquire competent equipment.  Besides the US and Canada, I have shipped 130mm ~F/5 Newtonians to both Mexico and Iceland at my own expense just to get some good telescopes into the hands of people who want to use them. 

 

And I try to keep in mind, just how spoiled I am, how spoiled many of my fellow astronomers are.  I try to keep that in check.  A 10 inch Dob is a magnificent telescope and very capable.  To have even larger scopes and easy access to dark, clear skies as well as a place to stay, not many people have these opportunities, I try to keep in mind, just how blessed I have been.  

 

So, my reply to your post was not to say that 6 inch telescopes do not have their place or to say that they were somehow lacking.  Rather, I had been trying to explain why wave front error in an absolute sense scales with the Airy disk.  

 

And, if you ever get close to San Diego, you are more than welcome to stop by for a visit, even some viewing.

 

Jon


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#31 CHASLX200

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Posted 31 July 2018 - 06:51 PM

So how does the square with the fact that you are selling your 4 inch TMB apo triplet refractor, arguably one of the best 4 inch refractors ever, and purchasing a 6 inch Synta doublet?  It seems to me in this instance, you are choosing the aperture of a commercial scope over the perfection of a smaller scope with near perfect optics.

 

I do expect the 6 inch Skywatcher Evostar to show more detail on Jupiter and Saturn than the 4inch, such is the nature of optics.  I would also expect a 10 inch Skywatcher Dob to show more detail on Jupiter and Saturn than a 6inch high end Newtonian.

 

Jon

The TMB is just too big for a 4".  Not a grab and go bro.  If i am going to have to use a G8 i just as well go for a 6".  The TMB is perfect.  Not sure about the 6" SW until i get it.  



#32 CHASLX200

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Posted 31 July 2018 - 06:53 PM

So how does the square with the fact that you are selling your 4 inch TMB apo triplet refractor, arguably one of the best 4 inch refractors ever, and purchasing a 6 inch Synta doublet?  It seems to me in this instance, you are choosing the aperture of a commercial scope over the perfection of a smaller scope with near perfect optics.

 

I do expect the 6 inch Skywatcher Evostar to show more detail on Jupiter and Saturn than the 4inch, such is the nature of optics.  I would also expect a 10 inch Skywatcher Dob to show more detail on Jupiter and Saturn than a 6inch high end Newtonian.

 

Jon

I have had run of the mill 10" Newts and they were never close to a Zambuto.



#33 Keith Rivich

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Posted 31 July 2018 - 07:20 PM

I'd go with the 20" and use the 6" as a finder...



#34 Adun

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Posted 31 July 2018 - 07:38 PM

So far what I can see in clear votes is:

 

12: 3 votes

10": 4 votes

Keep the 6": 3 votes



#35 Auburn80

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Posted 31 July 2018 - 08:22 PM

I have some experience with newts and I'd say a 12". If I could pick the ota/platform. 😊

#36 Don H

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Posted 01 August 2018 - 12:03 PM

I would keep the 6 inch... Having a Zambuto that I could sit on my lap and aim at the sky and just scan around would be a treat to enjoy for the rest of my life. And if mounted, the 6 inch f/6 would give fantastic little planetary views... Besides, I already have 8, 10 and 12.5 inch scopes.



#37 Adun

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Posted 01 August 2018 - 12:25 PM

I would keep the 6 inch... Having a Zambuto that I could sit on my lap and aim at the sky and just scan around would be a treat to enjoy for the rest of my life. And if mounted, the 6 inch f/6 would give fantastic little planetary views... Besides, I already have 8, 10 and 12.5 inch scopes.

 

Which you  wouldn't keep in the imaginary experiment.

 

¿would you still keep the 6" as your one and only telescope?


Edited by Adun, 01 August 2018 - 12:31 PM.

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#38 Starman81

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Posted 01 August 2018 - 12:26 PM

I would keep the 6 inch... Having a Zambuto that I could sit on my lap and aim at the sky and just scan around would be a treat to enjoy for the rest of my life. And if mounted, the 6 inch f/6 would give fantastic little planetary views... Besides, I already have 8, 10 and 12.5 inch scopes.

 

That makes sense but it does violate the restriction in the OPs opening post: 

 

 

Imagine you own one, and only one telescope: 
 



#39 Don H

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Posted 01 August 2018 - 01:41 PM

Which you  wouldn't keep in the imaginary experiment.

 

¿would you still keep the 6" as your one and only telescope?

duh, ok, so I would indeed keep the 6, as having that as I get older (and older) it will always be a treat to own. However, the notion that I would have to trade only for a mass produced scope of a larger diameter puts me into a scenario I would avoid. If I had the 6 Zambuto, I would keep it and look to supplement it with a 10 inch. I guess I really cannot imagine having to own just one scope, so perhaps my vote must be removed...



#40 X3782

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Posted 01 August 2018 - 01:50 PM

Please be assured that I am well aware that there are many reasons why a 6 inch scope or even smaller might be the right scope.  I really enjoyed Barbie's post because she just said, premium optics or not, a 6 inch F/8 Newtonian is the right scope for me. And I do my best to assist excited amateur astronomers who do not have the resources to acquire competent equipment.  Besides the US and Canada, I have shipped 130mm ~F/5 Newtonians to both Mexico and Iceland at my own expense just to get some good telescopes into the hands of people who want to use them. 

 

And I try to keep in mind, just how spoiled I am, how spoiled many of my fellow astronomers are.  I try to keep that in check.  A 10 inch Dob is a magnificent telescope and very capable.  To have even larger scopes and easy access to dark, clear skies as well as a place to stay, not many people have these opportunities, I try to keep in mind, just how blessed I have been.  

 

So, my reply to your post was not to say that 6 inch telescopes do not have their place or to say that they were somehow lacking.  Rather, I had been trying to explain why wave front error in an absolute sense scales with the Airy disk.  

 

And, if you ever get close to San Diego, you are more than welcome to stop by for a visit, even some viewing.

 

Jon

Thanks! The invention or gradual development of the Dobsonian telescope by various people in the US was a huge technological innovation, so now 6 inches seems like nothing. But this innovation doesn't translate equally to every part of the world, in my home city they keep coming out with more and more expensive ($5-13k.....) apos in the 4-5 inch range. I sarcastically asked the T****** salesperson, "who buys this?", and he surprised me by answering that actually some months the 7k models outsell the 2.5k ones.


Edited by X3782, 01 August 2018 - 02:32 PM.


#41 MitchAlsup

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Posted 01 August 2018 - 03:06 PM

I am going to pick the best everytime.

I would also,

not just in optical quality,

but mechanical quality, 

and also artistic impressions of metal sculpture and wood work and finishing.



#42 CHASLX200

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Posted 01 August 2018 - 06:08 PM

I would also,

not just in optical quality,

but mechanical quality, 

and also artistic impressions of metal sculpture and wood work and finishing.

I would much rather have a used Starmaster than some cheap made Dob from a 5 and dime.



#43 barbie

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Posted 01 August 2018 - 10:02 PM

The "Cheap Dobs" from the 5 and dime have gotten to be quite excellent, at least in sizes 12" and under and I've seen them personally give the "premium" dobs a run for their money!!


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#44 X3782

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Posted 02 August 2018 - 01:17 AM

The premium Dobs just need to stay very slightly better than the mass-produced ones and innovate from time to time, offering some new feature, and there will still be a market for people willing to shell out 5x or 10x or 20x the cost for years to come. Just like for apos, audio equipment, cameras..... Wristwatches are an extreme example, the mechanical movement devices are thousands of times worse in accuracy compared to a solar-battery-powered radio-synchronized one with a lot less features, some of them force the user to regularly wind them up just to keep telling the (maybe inaccurate) time, can cost a fortune to maintenance the moving parts, everyone knows this, but the Rolexes "certified chronographs" are making a killing also in China, whereas the cheaper more precise mass-market commodity producers are operating with razor-thin profit margins and hurting a lot.

 

The fact that Dobs look like scientific instruments with rational discussions of aperture, contrast, and resolution etc. confuses us, but actually the whole dynamic works like any other consumer market. People want to own things that make them feel special. A premium telescope with a person standing behind it is special. Some company with c.a. 50 employees looked hard at the numbers and introduced a 5-inch refractor for 13000 USD a few years back, and....... apparently it's selling.


Edited by X3782, 02 August 2018 - 01:57 AM.

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#45 MDavid

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Posted 02 August 2018 - 08:50 AM

The premium Dobs just need to stay very slightly better than the mass-produced ones and innovate from time to time, offering some new feature, and there will still be a market for people willing to shell out 5x or 10x or 20x the cost for years to come. Just like for apos, audio equipment, cameras..... Wristwatches are an extreme example, the mechanical movement devices are thousands of times worse in accuracy compared to a solar-battery-powered radio-synchronized one with a lot less features, some of them force the user to regularly wind them up just to keep telling the (maybe inaccurate) time, can cost a fortune to maintenance the moving parts, everyone knows this, but the Rolexes "certified chronographs" are making a killing also in China, whereas the cheaper more precise mass-market commodity producers are operating with razor-thin profit margins and hurting a lot.

 

The fact that Dobs look like scientific instruments with rational discussions of aperture, contrast, and resolution etc. confuses us, but actually the whole dynamic works like any other consumer market. People want to own things that make them feel special. A premium telescope with a person standing behind it is special. Some company with c.a. 50 employees looked hard at the numbers and introduced a 5-inch refractor for 13000 USD a few years back, and....... apparently it's selling.

Hell yeah!, if our imaginary 6" Zambuto had gold mirror coatings like the James Webb's mirrors, I'd definitely switch my vote from the 10" GSO to the Golden 6" Z. And then I'd buy that night vision gear...hmm.gif


Edited by MDavid, 02 August 2018 - 09:03 AM.


#46 George N

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Posted 02 August 2018 - 07:57 PM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

 

For what it's worth -

 

A good friend owns a "premium Dob" company - and will put whatever optics you want in your telescope. His personal rather large Dob has a primary by one of the commonly noted premium mirror makers (not Z or R). He has seen them all and can have whatever he wants - and picked what he wanted. I've observed with this scope - and it performs!

 

He also sells Dobs with GSO mirrors - if that is what the customer wants. He has said that the recent GSO primaries he as gotten have been quite good. From my experience - this guy knows what he is talking about - and can tell a great mirror from a fair one.

 

While I suppose you can get a less-than-wonderful mirror from GSO - there seems every reason to expect many current ones will be very fine. If it is 'junk' - return it - or have it re-figured.


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#47 Smug

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Posted 02 August 2018 - 09:08 PM

I could never own only 1 scope, but if I had to it'd be a lot bigger than 6". I can't see much around here with a scope that small.

 

So I'd trade for anything 12" or larger, but only if it had GOTO.


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#48 waso29

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Posted 02 August 2018 - 09:08 PM

The scope just has generic Taiwanese optics, decent optics, I've split doubles slightly under 0.5", 

taiwan, nice!!waytogo.gif

 

as educated consumers with ethics, given a choice, always choose products made by the free


Edited by waso29, 02 August 2018 - 11:10 PM.

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#49 X3782

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Posted 02 August 2018 - 10:23 PM

For what it's worth -

 

A good friend owns a "premium Dob" company - and will put whatever optics you want in your telescope. His personal rather large Dob has a primary by one of the commonly noted premium mirror makers (not Z or R). He has seen them all and can have whatever he wants - and picked what he wanted. I've observed with this scope - and it performs!

 

He also sells Dobs with GSO mirrors - if that is what the customer wants. He has said that the recent GSO primaries he as gotten have been quite good. From my experience - this guy knows what he is talking about - and can tell a great mirror from a fair one.

 

While I suppose you can get a less-than-wonderful mirror from GSO - there seems every reason to expect many current ones will be very fine. If it is 'junk' - return it - or have it re-figured.

 

The reason for the improvement here is that computerized grinding and polishing machines can now handle large diameter optics with acceptable precision, these devices used to be "astronomically" expensive but have come down in price, so telescope companies in the Far East can afford to install them. But still highly skilled humans figuring and polishing by traditional methods can produce better quality optics on a more consistent basis. The whole rationale for introducing automation of course is to lower production costs, so one wants to produce a lot of mirrors rapidly without spending too much on one optic. Then depending on the price point, the quality control decides what fraction of low-quality copies to discard, and ships the rest to the customers. Some of these shipped optics would be good, some mediocre.

 

Be it either human manual labour or computerized automatic figuring, by producing enough mirrors you can "happen" to produce a high-quality copy. Already at the time of Galileo Galilei in the 17th century, the opticians could hit 1/2 lambda for 80 mm lenses (they measured the figure of these priceless museum pieces using a Zygo interferometer) using presumably a turning lathe. The thinking is that they must have made many many lenses, and Galileo selected the best one that happened to be so good.

 

I'd like to add that also in the Far East, there are a few artisan opticians who produce high-quality optics, but the price is often even more expensive than in the US. There is not enough demand, though as e.g., populous countries like China or India prospers in the future, there could be a significant number of customers wanting high quality custom optics. The point is that mass production and high-end "manual" optics will always coexist.


Edited by X3782, 02 August 2018 - 10:59 PM.


#50 barbie

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Posted 05 August 2018 - 02:03 PM

Why go manual when you can have high end mass production!lol.gif

I was observing Jupiter and Saturn at 250x with my 6" F8 'Mass produced" optics Saturday night and they didn't even break a sweat!!  I could have gone higher but didn't have the eyepiece/barlow combination available to do so.grin.gif  I've also had high end "hand figured" optics and mass produced optics at the same time and compared them side by side and found(after 50 years of observing and testing) that there is little to no difference between TODAY's mass produced optics and the more expensive "hand figured" optics, other than cost and bragging rights!!grin.gif grin.gif  Hand figured, mass produced, if it shows me what I want to see, then I'll buy it!!


Edited by barbie, 05 August 2018 - 02:30 PM.



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